Saturday, February 24, 2024

Repairing a dead Kenwood TS-850S

Recently, a Kenwood TS-850S - a radio from the mid-early 1990s - crossed my workbench.  While I'm not in the "repair business", I do fix my own radios, those of close friends, and occasionally those of acquaintances:  I've known this person for many years and we have many mutual friends.

If you are familiar with the Kenwood TS-850S to any degree, you'll also know that they suffer from an ailment that has struck down many pieces of electronic gear from that same era:  Capacitor Plague.

Figure 1:
The ailing TS-850S.  The display is normal - except
for the frequency display showing only dots.  This error is
accompanied by "UL" in Morse.
Click on the image for a larger version.
This isn't the same "Capacitor Plague" of which you might be aware where - particularly in the early 2000s - many computer motherboards failed due to incorrectly formulated electrolytic capacitors, but rather early-era (late 80s to mid 90s) surface-mount electrolytic capacitors that began to leak soon after they were installed.

The underlying cause?

While "failure by leaking" is a common occurrence in electronics, this failure is somewhat different in many aspects.  At about this time, electronic manufacturers were switching over to surface-mount devices - but one of the later components to be surface-mounted were the electrolytic capacitors themselves:  Up to this point it was quite common to see a circuit board where most of the components were surface-mount except for larger devices such as diodes, transistors, large coils and transformers - and electrolytic capacitors - all of which would be mounted through-hole, requiring an extra manufacturing step.

Early surface-mount electrolytic capacitors, as it turned out, had serious flaws.  In looking at the history, it's difficult to tell what aspect of their use caused the problem - the design and materials of the capacitor itself or the method by which they were installed - but it seems that whatever the cause, subjecting the capacitors themselves to enough heat to solder their terminals to the circuit board - via hot air or infrared radiation - was enough to compromise their structural integrity.

Whatever the cause - and at this point it does not matter who is to blame - the result is that over time, these capacitors have leaked electrolyte onto their host circuit boards.  Since this boron-based liquid is somewhat conductive and mildly corrosive in its own right, it is not surprising that as surface tension wicks this material across the board, it causes devastation wherever it goes, particularly when voltages are involved.

The CAR board - the cause of "display dots"

In the TS-850S, the module most susceptible to leaking capacitors is the CAR board - a circuit that produces multiple, variable frequency signals that feeds the PLL synthesizer and several IF (Intermediate Frequency) mixers.  Needless to say, when this board fails, so does the radio.

They most obvious symptom of this failure is when damage to the board is so extensive that it can no longer produce the needed signals - and if one particularly synthesizer (out of four on the board) fails, you will see that the frequency display disappears - to be replaced with just dots - and the letters "UL" are sent in Morse Code to indicate the "Unlock" condition by the PLL.

Figure 2:
The damaged CAR board.  All but one of the surface-mount
electrolytic capacitors has leaked corrosive fluid and damaged
the board.  (It looked worse before being cleaned!)
Click on the image for a larger version.
Prior to this, the radio may have started going deaf and/or transmitter output was dropping as the other three synthesizers - while still working - are losing output, but this may be indicative of another problem as well - more on this later.

Figure 2 shows what the damaged board looks like.  Actually, it looked a bit worse than that when I first removed it from the radio - several pins of the large integrated circuits being stained black.  As you can see, there are black smudges around all (but one) of the electrolytic capacitors where the corrosive liquid leaked out, getting under the green solder mask and even making its way between power supply traces where the copper was literally being eaten away.

The first order of business was to remove this board and throw it in the ultrasonic cleaner.  Using a solution of hot water and dish soap, the board was first cleaned for six minutes - flipping the board over during the process - and then very carefully, paper towels and then compressed air was used to remove the water.

Figure 3:
The CAR board taking a hot bath in soapy water in an
ultrasonic cleaner.  This removes not only debris, but spilled
electrolyte - even that which has flowed under components.
Click on the image for a larger version.
At this point I needed to remove all of the electrolytic capacitors:  Based on online research, it was common for all of them to leak, but I was lucky that the one unit that had not failed (a 47uF, 16 volt unit) "seemed" OK while all of the others (10uF, 16 volt) had disgorged their contents.

If you look at advice online, you'll see that some people recommend simply twisting the capacitor off the board as the most expedient removal procedure, but I've found that doing so with electrolyte-damaged traces often results in ripping those same traces right off the board - possibly due to thinning of the copper itself and/or some sort of weakening of the adhesive.

My preferred method of already-leaking capacitors is to use a pair of desoldering tweezers - which are more or less a soldering iron with two prongs that will simultaneously heat both pins of the part simultaneously, theoretically allowing its quick removal.  While many capacitors are easily removed with this tool, some are more stubborn:  During manufacture, drops of glue were used under the part to hold it in place prior to soldering and this sometimes does its job too well, making it difficult to remove it.  Other times, the capacitor will explode (usually just a "pop") as it is being heated, oozing out more corrosive electrolyte.

With the capacitors removed, I tossed it in the ultrasonic cleaner for other cycle in the same warm water/soap solution to remove any additional electrolyte that had come off - along with debris from the removal process.  It is imperative when repairing boards with leaking capacitors that all traces of electrolyte be completely removed or damage will continue even after the repair.

At this point one generally needs to don magnification and carefully inspect the board.  Using a dental pick and small-blade screwdriver, I scraped away loose board masking (the green overcoating on the traces) as well as bits of copper that had detached from the board:  Having taken photos of the board prior to capacitor removal - and with the use of the Service Manual for this radio, found online - I was confident that I could determine where, exactly, each capacitor was connected.

When I was done - and the extent of the damage was better-revealed - the board looked to be a bit of a mess, but that was the fault of the leaking capacitors.  Several traces and pads in the vicinity of the defunct capacitors had been eaten away or fallen off - but since these capacitors are pretty much placed across power supply rails, it was pretty easy to figure out where they were supposed to connect.

Figure 4:
The CAR board, reinstalled for testing.
Click on the image for a larger version.
As the mounting pads for most of these capacitors were damaged or missing, I saw no point in replacing them with more surface-mount capacitors - but rather I could install through-hole capacitors on the surface, laying them down as needed for clearance - and since these new capacitors included long leads, those leads could be used to "rebuild" the traces that had been damaged.

The photo shows the final result.  Different-sized capacitors were used as necessary to accommodate the available space, but the result is electrically identical to the original.  It's worth noting that these electrolytic capacitors are in parallel with surface-mount ceramic capacitors (which seem to have survived the ordeal) so the extra lead length on these electrolytics is of no consequence - the ceramic capacitors doing their job at RF as before.  After successful testing of the board, dabs of adhesive were used to hold the larger, through-hole capacitors to the board to reduce stress on the solder connections under mechanical vibration.

Following the installation of the new capacitors, the board was again given two baths in the ultrasonic cleaner - one using the soap and water solution, and the other just using plain tap water and again, the board was patted dry and then carefully blown dry with compressed air to remove all traces of water from the board and from under components and then allowed to air dry for several hours.

Testing the board

After using an ohmmeter to make sure that the capacitors all made their proper connections, I installed the board in the TS-850S and... it didn't work as I was again greeted with a "dot" display and a Morse "UL".

I suspected that one of the "vias" - a point where a circuit traces passes from one side to another through a plated hole - had been "eaten" by the errant electrolyte.  Wielding an oscilloscope, I quickly noted that only one of the synthesizers was working - the one closest to connector CN1 - and this told me that at least one control signal was missing from the rest of the chips.  Probing with the scope I soon found that a serial data signal ("PDA") used to program the synthesizers "stopped" beyond the first chip and a bit of testing with an ohmmeter showed that from one end to the other, the signal had been interrupted - no doubt in a via that had been eaten away by electrolytic action.

Figure 5:
Having done some snooping with an oscilloscope, I noted
that the "PDA" signal did not make it past the first of the
(large) synthesizer chips.  The white piece of #30 Kynar
wire-wrap wire was used to jump over the bad board "via"
Click on the image for a larger  version.

The easiest fix for this was to use a piece of small wire - I used #30 Kynar-insulated wire-wrap wire - to jumper from where this control signal was known to be good to a point where it was not good (a length of about an inch/two cm) and was immediately rewarded with all four synthesizer outputs being on the correct frequencies, tuning as expected with the front-panel controls.

Low output

While all four signals were present and on their proper frequencies - indicating that the synthesizers were working correctly - I soon noticed, using a scope, that the second synthesizer output on about 8.3 MHz was outputting a signal that was about 10% of its expected value in amplitude.  A quick test of the transmitter indicated that the RF output was only about 15 watts - far below that of the 100 watts expected.

Again using the 'scope, I probed the circuit - and comparing the results with the nearly identical third synthesizer (which was working correctly) and soon discovered that the amplitude dropped significantly through a pair of 8.3 MHz ceramic filters.

The way that synthesizers 2 and 3 work is that the large ICs synthesize outputs in the 1.2-1.7 MHz area and mix this with a 10 MHz source derived from the radio's reference to yield signals around 8.375 and 8.83 MHz, respectively - but this mix results in a very ugly signal spectrally - full of harmonics and undesired products.  With the use of these ceramic bandpass filters - which are similar to 10.7 MHz filters those found in analog AM and FM radios - these signals are "cleaned up" to yield the desired output over a range of the several kiloHertz that they vary depending on the bandpass filter and the settings of the front panel "slope tune" control.

Figure 6:
The trace going between C75 and CF1 was cut and a bifilar-
wound transformer was installed to step up the impedance
from Q7 to that of the filter:  R24 was also changed to 22
ohms - providing the needed "IF-7-LO3" output level at J4.
Click on the image for a larger version.

The problem here seemed to be that the two ceramic 8.3 MHz filters  (CF1, CF2) were far more lossy than they should have been.  Suspecting a bad filter, I removed them both from the circuit board and tested them using a temporary fixture on a NanoVNA:  While their "shape" seemed OK, their losses were each around 10dB more than is typical of these devices indicating that they are slowly degrading.  A quick check online revealed that these particular frequency filters were not available anywhere (they were probably custom devices, anyway) so I had to figure out what to do.

Since the "shape" of the individual filter's passbands were still OK - a few hundred kHz wide - all I needed was to get more signal:  While I could have kludged another amplifier into the circuit to make up for the loss, I decided, instead, to reconfigure the filter matching.  Driving the pair of ceramic filters is an emitter-follower buffer amplifier (Q7) - the output of which is rather low impedance - well under 100 ohms - but these types of filters typically "want" around 300-400 ohms and in this circuit, this was done using series resistors - specifically R24.  This method of "matching" the impedance is effective, but very lossy, so changing this to a more efficient matching scheme would allow me to recover some of the signal.

Replacing the 330 ohm series resistor (R24) with a 22 ohm unit and installing a bifilar-wound transformer (5 turns on a BN43-2402 binocular core) wired as a 1:4 step-up transformer (the board trace between C75 and CF1 was cut and the transformer connected across it) brought the output well into the proper amplitude range and with this success, I used a few drops of "super glue" to hold it to the bottom of the board.  It is important to note that I "boosted" the amplitude of the signal prior to the filtering because to do so after the filtering - with its very low signal level - may have also amplified spurious signals as well - a problem avoided in this method.

Rather than using a transformer I could have also used a simple L/C impedance transformer (a series 2.2uH inductor with a 130pF capacitor to ground on the "filter side" would have probably done the trick) but the 1:4 transformer was very quick and easy to do.

With the output level of synthesizer #2 (as seen on pin CN4) now up to spec (actually 25% higher than indicated on the diagram in the service manual) the radio was now easily capable of full transmit output power, and the receiver's sensitivity was also improved - not surprising considering that the low output would have starved mixers in the radios IF.

A weird problem

After all of this, the only thing that is not working properly is "half" of the "Slope Tune" control:  In USB the "Low Cut" works - as does the "High Cut" on LSB, but the "High Cut" does not work as expected on USB and the "Low Cut" does not work as expected on LSB.  What happens with the settings that do NOT work properly, I hear the effect of the filter being adjusted (e.g. the bandwidth narrows) but the radio's tuning does not track the adjustment as it should.  What's common to both of these "failures" is that they both relate to high frequency side of the filter IF filters in the radio - the effect being "inverted" on LSB.

I know that the problem is NOT the CAR board or the PLL/synthesizer itself as these are being properly set to frequency.  What seems to NOT be happening is that for the non-working adjustments, the radio's CPU is not adjusting the tuning of the radio to track the shift of the IF frequency to keep the received signal in the same place - which seems like more of a software problem than a hardware problem:  Using the main tuning knob or the RIT one can manually offset this problem and permit tuning of both the upper and lower slopes of of the filters, but that is obviously not how it's expected to work!

In searching the Internet, I see scattered mentions of this sort of behavior on the TS-850 and 950, but no suggestions as to what causes it or what to do about it:  I have done a CPU reset of the radio and disconnected the battery back-up to wipe the RAM contents, but to no avail.  Until/unless this can be figured out, I advised the owner to set the affected control to its "Normal" position.  If you have experienced this problem - and especially if you know of a solution - please let me know.

Figure 7:
The frequency display shows that the synthesizer is now
working properly - as did the fact that it outputs full power
and gets good on-the-air signal reports.
Click on the image for a larger version.

Final comments

Following the repair, I went through the alignment steps in the service manual and found that the radio was slightly out alignment - particularly with respect to settings in the transmit output signal path - possibly during previous servicing to accommodate the low output due to the dropping level from the CAR board.  Additionally, the ALC didn't seem to work properly - being out of adjustment - resulting in distortion on voice peaks with excessive output power.

With the alignment sorted, I made a few QSOs on the air getting good reports - and using a WebSDR to record my transmissions, it sounded fine as well.

Aside from the odd behavior of the "Slope Tune" control, it seems to work perfectly.  I'm presently convinced that this must be a software - not a hardware - problem as all of the related circuits function as they should, but don't seem to be being "told" what to do.

* * * * *

This page stolen from


Sunday, January 14, 2024

Reducing RFI (Radio Frequency Interference) for a POE (Power Over Ethernet) camera or wireless access point

One of the (many) banes of the amateur radio operator's existence is often found at the end of an Ethernet cable - specifically a device that is being powered via "Ethernet":  It is often the case that interference - from HF through UHF - emanates from such devices.

Figure 1:
POE camera with both snap-on ferrites installed -
including one as close to the camera as possible -
and other snap-on/toroids to suppress HF through VHF.
Click on the image for a larger version.

Why this happens

Ethernet by itself is usually relatively quiet from an (HF) RF standpoint:  The base frequency of modern 100 Megabit and gigabit Ethernet is typically above much of HF and owing to the fact that the data lines are coupled via transformers making them inherently balanced and less prone to radiate.  Were this not the case, the integrity of the data itself would be strongly affected by the adjacent wires within the cable or even if the cable was routed near metallic objects as it would radiate a strong electromagnetic field - and any such coupling would surely affect the signal by causing reflections, attenuation, etc.

This is NOT the case with power that is run via the same (Ethernet) cable.  Typically, this power is sourced by a switching power supply - too often one that is not filtered well - and worse, the device at the far end of the cable (e.g. a camera or WiFi access point - to name two examples) is often built "down to a cost" and itself contains a switching voltage converter with rather poor filtering that is prone to radiation of RF energy over a wide spectrum.  Typically lacking effective common-mode filtering - particularly at HF frequencies (it would add expense and increase bulk) - the effect of RF radiating from the power-conducting wires in an Ethernet cable can be severe.

Even worse than this, Ethernet cables are typically long - often running in walls or ceilings - effectively making them long, wire antennas, capable of radiating (and intercepting) signals even at HF.  The "noisy" power supply at one or both ends of this cable can act as transmitters.

What to do

While some POE configurations convey the DC power on the "spare" conductors in an eight conductor cable (e.g. the blue and brown pairs), some versions use the data pairs themselves (often using center-tapped transformers in the Ethernet PHY) meaning that it may not be easy to filter just the DC power.

While it is theoretically possible to extract the power from the Ethernet cable, filter it and and reinsert it on the cable, the various (different) methods of doing this complicate the matters and doing so - particularly if the DC is carried on the data pairs - can degrade the data integrity by requiring the data to transit two transforms incurring potential signal attenuation, additional reflection and affecting frequency response - to name just a few.  Doing this is complicated by the fact that the method of power conveyance varies as you may not know which method is used by your device(s).

It is possible to subject the entire cable and its conductors to a common-mode inductance to help quash RFI - but this must be done carefully to maintain signal integrity.


Some POE cameras also have a coaxial power jack that permits it to be powered locally rather than needing to use POE.  I've observed that it is often the case that using this local power - which is often 12-24 volts DC (depending on the device) - will greatly reduce the noise/interference generated by the camera and conducted on the cable - provided, of course, that the power supply itself is not a noise source.  Even if a power supply is used near the camera, I would still suggest putting its DC power cable through ferrite devices as described below to further-reduce possible emissions.
There are some devices (such as those sold by DX Engineering) that are essentially back-to-back signal transformers that can reduce radiation of signals from Ethernet cable, but these typically do not permit the passage of power and are not candidates for use with POE devices.

Ferrite can be your friend

For VHF and UHF, simple snap-on ferrites can significantly attenuate the conduction of RF along, but these devices are unlikely to be effective at HF - particularly on the lower bands - as they simply cannot add enough impedance at lower frequencies.

To effectively reduce the conduction of RF energy at HF, one could wrap the Ethernet cable around a ferrite toroidal core, but this is often fraught with peril, particularly with cable carrying Gigabit Ethernet - as tight radius turns can distort the geometry of typical CAT-5/6 cable, affect the impedance and cause cross-coupling into other wire pairs.  If this happens, one often finds that the Ethernet cable doesn't work reliably at Gigabit speeds anymore (being stuck at 100 or even 10 Megabits/second) or starts to "flap" - switching between different speeds and/or slowing down due to retransmissions on the LAN.

One type of Ethernet cable that is quite resistant to geometric distortion caused by wrapping around a toroidal core is the flat Ethernet cable (sometimes erroneously referred to as "CAT6" or "CAT7").  This cable is available as short jumpers around 6 feet (2 meters) long and, with the aid of a female-female 8P8C (often called "RJ-45") coupler can be inserted into an existing Ethernet cable run - just be sure that it is from a reputable source and rated for "Gig-E" service.  As it is quite forgiving to being wrapped around ferrites, this flat cable can be pre-wound with such devices and inserted at the Ethernet switch end and/or the device end at a later time.  I have found that with reasonable quality cable and couplers that this does not seem to degrade the integrity of the data on the LAN cable - at least for moderate lengths (e.g. 50 feet/15 meters or less) - your mileage may vary with very long cable runs.

As the flat cable and female-female Ethernet coupler are to be inserted into the cable run, they must be of known, good quality so it is best to test the couplers and cable that you obtain prior to installation to be sure that their use doesn't cause a reduction in signal quality/speed.

Practical examples

Best attenuation across HF

Figure 2:
Three toroids wound on "flat" Ethernet cable.  An FT114-43
is used on each end with an FT114-31 in the middle.
Click on the image for a larger version.
Using a test fixture with a VNA, I determined that for best overall attenuation across the entire HF spectrum I needed three ferrite toroids on the 2 meter long flat Ethernet jumper.  All three of these were FT-114 size (1.14", 29mm O.D.) with the first and last being of material type 43 and the center being type 31:  Both types 31 and 43 offer good impedance to low HF but 43 is more effective on the higher bands - namely 10 and 6 meters - and types 52 and 61 may be useful at even higher bands:  The three toroids, separated by a few inches/cm, offer better all-around rejection from 160 meters through 10/6 meters than just one.  Having said this, it is unrealistic to expect more than 20dB or so of attenuation to be afforded by ferrite devices at high HF/low VHF - "because physics".

One might be tempted to use the more-available FT-240 size of toroids (2.4", 60mm O.D.) but this is unnecessarily large, comparatively fragile and expensive:  While you can fit more turns on the larger toroid, one hits the "point of diminishing returns" (e.g. little improvement with additional turns) very quickly owing to the nature of the ferrite and coupling between turns.  Using the FT114 or FT140 sizes is the best balance as it may be much less expensive than a larger device, it can accept 6-8 turns with the cable's connector installed, and more than 6-8 turns is rapidly approaching the point of diminishing returns for a single ferrite device, anyway.

In bench testing with a fixture, it was found that three toroids on a piece of flat Ethernet cable provided the best, overall attenuation across HF and to 6 meters - significantly better than any combination of FT114, FT140 or FT240 toroids of either 43 or 31 mix alone:  Figure 2, above, shows what this looks like.  Two FT114-43 and one FT114-31 toroid were used - the #31 toroid being placed in the center, providing the majority of series impedance at low HF and a #43 at each end being more effective at higher HF through 6 meters.

To construct this, the flat Ethernet cable was then marked with a silver marker in the center and four turns were wound from each end, in turn, for a total of eight turns on the FT114-31.  Placing an FT114-43 at 12 inches (25cm) and winding seven turns puts the FT114-43 fairly close to each connector, allowing the installation of one or two snap-on ferrites very close  to the connector if it is determined that more suppression is required to suppress radiation at VHF frequencies.  Small zip-ties (not shown in Figure 2) are used to help keep the turns from bunching up too much and also to prevent the start and stop turns from getting too close to each other:  Do not cinch these ties up enough to distort the geometry of the Ethernet cable as that could impact speed - particularly when using Gig Ethernet.

It is important that, as much as possible, one NOT place a "noisy" cable in a bundle with other cables or to loop it back onto itself - both of which could cause inadvertent coupling of the RFI that you are trying to suppress into the other conductors - or to the far side of the cable you are installing.

Best attenuation at VHF and HF

If you are experiencing interference from HF through VHF, you will need to take a hybrid approach:  The use of appropriate snap-on and toroidal ferrite devices.  While snap-on ferrite devices are not particularly useful for HF - especially below about 20 MHz - they can be quite effective at VHF, which is to be expected as that is the purpose for which they are typically designed.  Similarly, a ferrite toroid such as that described above - particularly with type 43 or 31 material - will likely have little effect on VHF radiation - particularly in the near field.

Figure 3:
A combination of a snap-on device with an extra turn looped
through it and two ferrites to offer wide-band suppression
from HF through VHF.
Click on the image for a larger version.

Figure 3 shows such a hybrid approach with a snap-on device on the left and two toroids on the right to better-suppress a wider range of frequencies.  In this case it is important that the snap-on device be placed as close to the interference source as possible (typically the camera) as even short lead lengths can function as effective antennas at VHF/UHF.  You may also notice that the snap-on has two turns through its center as this greatly improves efficacy at medium/low VHF frequencies but may be counter-productive at high VHF/UHF frequencies owing to coupling between turns.

Doing this by itself is not likely to be as effective in reducing radiation at VHF/UHF from the cable itself, often requiring the placement of additional ferrite devices.  Figure 1 shows the installation of several snap-on devices placed as close to the POE camera as physically possible - mainly to reduce radiation at VHF and UHF as at those frequencies where even a few inches or centimeters of cable emerging from the noise-generating device can act as an effective antenna.

Determining efficacy

During the installation of these devices on my POE cameras I was interested in how much attenuation would be afforded at VHF:  Since I'd already used the "chokes on a flat cable" approach like that in Figures 2 and 3 I knew that this would likely be as effective as was practical at HF - but because the VHF/UHF noise could be radiated by comparatively short lengths of "noisy" cable - and that the 43 and 31 mix ferrites were probably not as effective at those frequencies - I needed to be able to quantify that what I did made a difference - or not.

Figure 4:
The cable in Figure 3 installed, but not yet
tucked into place as depicted in Figure 1.
(This does not show the snap-on ferrites installed
where the wire exits the camera housing.
The female-female RJ45/8P8C "splice" can be
seen in the upper-left corner of the picture.
Click on the image for a larger version.

For HF this was quite simple:  I simply tuned my HF receiver - connected to my main antenna - to a frequency where I knew that I could hear the noise from the cameras and compared S-meter readings with the system powered up and powered down.  This approach is best done at a time during which the frequency in question is "dead" or at least weak (e.g. poor propagation) - 80/40 meters during the midday and 15/10 meters at night is typical.

For VHF this required a bit more specialized equipment.  My "Go-To" device for finding VHF signals - including noise - is my VK3YNG DF sniffer which has extremely good sensitivity - but it also has an audible "S-meter" in terms of a tone that rises with increasing signal level:  This allowed an "eyes and hands off" approach in determining efficacy of the installation of a ferrite device simply by hearing the pitch of the tone..  Switching it to this mode and placing it and its antenna at a constant distance fairly close to the device being investigated allowed me to "hear" - in the form of a lower-pitched tone - whether or not the application of a ferrite device made a difference.

Slightly less exotic would be an all-mode receiver capable of tuning 2 meters such as the Yaesu FT-817, Icom IC-706, 703 or 705.  In this case the AM mode would be selected and the RF gain control advanced such that the noise amplitude audibly decreased:  This step is important as not doing this could mean that if the noise decreased, the AGC in the receiver would simply compensate and hiding the fact that the signal level changed.  By listening for a decrease in the noise level one can "hear" when installing a snap-on ferrite made a difference - or not.

One cannot use a receiver in FM mode for this as an FM detector is designed to produce the same amount of audio (including noise) at any signal level:  A strong noise source and a weak one will sound exactly the same.  It's also worth noting that the S-meter on a receiver in FM mode - or an FM-only receiver - are typically terrible in the sense that their indications typically start with a very low signal and "peg" the meter at a signal that isn't very strong at all which means that if you try to use one, you'll have to situate the receiver/antenna such that you get a reading that is neither full-scale or at the bottom of the scale to leave room for the indication of change.

Of course, a device like a "Tiny SA" (Spectrum Analyzer) could be used to provide a visual indication, using the "Display Line", markers and stored traces to allow a quick "before and after" determination.  As mentioned above, one would want to place the antenna and the receiving device (an actual receiver or spectrum analyzer) at a fairly close distance to the device being investigated - but keep it and its antenna in precisely the same location (or connected to a fixed-location antenna) during the entire time so that one can get meaningful "before and after" readings.


With the use of ferrites alone, one should not expect to be able to completely suppress radiation of RF noise from an Ethernet cable - the typical maximum to be reasonably expected is on the order of about 20dB (a bit over 3 "S" units) and this can vary wildly with frequency.  In a situation where the POE device is very close to the antenna, it may not be possible to knock the interference down to the point of inaudibility in which case relocation to place the two farther apart - or trying similar devices of different models/brands to try to find a combination to reduce it..

The most effective use will be for noise sources will be at some distance from the receive antenna - particularly if a long cable is used that may act as an antenna.  Additionally, these measures can be effective in situations where your transmitter causes problems with the device itself due to ingress of RF energy along the Ethernet cable.

Be prepared to install appropriate ferrite devices at both ends of the cable as it's often the case that not only does the POE device itself (camera, wireless device) radiates noise but also the POE switch itself:  No-name brand POE power supplies and Ethernet switches are, themselves often very noisy and the proper course of action would be to first swap out the supply or POE switch with a known quiet device before attaching ferrite.

As every interference situation is unique, your mileage may vary, and the best road to success is being able to quantify that changes you have made made things better or worse.

This page stolen from


Wednesday, December 27, 2023

Remote (POTA) operation from the Conger Mountain BLM Wilderness Area (K-6085)

It is likely that - almost no matter where you were - you were aware that a solar eclipse occurred in the Western U.S. in the middle of October, 2023.  Wanting to go somewhere away from the crowds - but along the middle of the eclipse path - we went to an area in remote west-central Utah in the little-known Conger Mountains.

Clint, KA7OEI operating CW in K-6085 with Conger
mountain and the JPC-7 loaded dipole in the background.
Click on the image for a larger version.

Having lived in Utah most of my life, I hadn't even heard of this mountain range even through I knew of the several (nearly as obscure) ranges surrounding it.  This range - which is pretty low altitude compared to many nearby - peaks out at only about 8069 feet (2460 Meters) ASL and is roughly 20 miles (32km) long.  With no incorporated communities or paved roads anywhere nearby we were, in fact, alone during the eclipse, never seeing any other sign of civilization:  Even at night it was difficult to spot the glow of cities on the horizon.

For the eclipse we set up on BLM (Bureau of Land Management) land which is public:  As long as we didn't make a mess, we were free to be there - in the same place - for up to 14 days, far more than the three days that we planned.  Our location turned out to be very nice for both camping and our other intended purposes:  It was a flat area which lent itself to setting up several antennas for an (Amateur) radio propagation experiment, it was located south and west of the main part of the weather front that threatened clouds, and its excellent dark skies and seeing conditions were amenable to setting up and using my old 8" Celestron "Orange tube" C-8 reflector telescope.

(Discussion of the amateur radio operations during the eclipse are a part of another series of blog entries - the first of which is here:  Multi-band transmitter and monitoring system for Eclipse monitoring (Part 1) - LINK)

Activating K-6085

Just a few miles away, however, was Conger Mountain itself - invisible to us at our camp site owing to a local ridge - surrounded by the Conger Mountain BLM Wilderness Area, which happens to be POTA (Parks On The Air) entity K-6085 - and it had never been activated before.  Owing to the obscurity and relative remoteness of this location, this is not surprising.

Even though the border of the wilderness area was less than a mile away from camp as a crow files, the maze of roads - which generally follow drainages - meant that it was several miles driving distance, down one canyon and up another:  I'd spotted the sign for this area on the first day as we our group had split apart, looking for good camping spots, keeping in touch via radio.

Just a few weeks prior to this event I spent a week in the Needles District of Canyonlands National Park where I could grab a few hours of POTA operation on most days, racking up hundreds of SSB and CW contacts - the majority of being the latter mode (you can read about that activation HERE).  Since I had already "figured it out" I was itching to spend some time activating this "new" entity and operating CW.  Among those others in our group - all of which but one are also amateur radio operators - was Bret, KG7RDR - who was also game for this and his plan was to operate SSB at the same time, on a different band.  As we had satellite Internet at camp (via Starlink) we were able to schedule our operation on the POTA web site an hour or so before we were to begin operation.

In the late afternoon of the day of the eclipse both Bret and I wandered over, placing our stations just beyond the signs designating the wilderness study area (we read the signs - and previously, the BLM web site - to make sure that there weren't restrictions against what we were about to do:  There weren't.) and several hundred feet apart to minimize the probability of QRM.  While Bret set up a vertical, non-resonant end-fed wire fed with a 9:1 balun suspended from a pole anchored to a Juniper, I was content using my JPC-7 loaded dipole antenna on a 10' tall studio light stand/tripod.

Bret, KG7RDR, operating 17 Meter SSB - the mast and
vertical wire antenna visible in the distance.
Click on the image for a larger version.
Initially, I called CQ on 30 meters but I got no takers:  The band seemed to be "open", but the cluster of people sending out just their callsign near the bottom of the band indicated to me that attention was being paid to a rare station, instead.  QSYing up to 20 meters I called CQ a few times before being spotted and reported by the Reverse Beacon Network (RBN) and being pounced upon by a cacophony of stations calling me.

Meanwhile, Bret cast his lot on 17 meters and was having a bit more difficulty getting stations - likely due in part to the less-energetic nature of 17 meter propagation at that instant, but also due to the fact that unlike CW POTA operation where you can be automatically detected and "spotted" on the POTA web site, SSB requires that someone spot your signal for you if you can't do it yourself:  Since we had no phone or Internet coverage at this site, he had to rely on someone else to do this for him.  Despite these challenges, he was able to make several dozen contacts.

Back at my station I was kept pretty busy most of the time, rarely needing to call CQ - except, perhaps, to refresh the spotting on the RBN and to do a legal ID every 10 minutes - all the while making good use of the narrow CW filter on my radio.

As it turned out, our choice to wait until the late afternoon to operate meant that our activity spanned two UTC days:  We started operating at the end of October 14 and finished after the beginning of October 15th meaning that with a single sitting, each of us accomplished two activations over the course of about 2.5 hours.  All in all I made 85 CW contacts (66 of which were made on the 14th) while Bret made a total of 33 phone contacts.

We finally called it quits at about the time the sun set behind a local ridge:  It had been very cool during the day and the disappearance of the sun caused it to get cold very quickly.  Anyway, by that time we were getting hungry so we returned to our base camp.

Back at camp - my brother and Bret sitting around
the fake fire in the cold, autumn evening after dinner.
Click on the image for a larger version.

My station

My gear was the same as that used a few weeks prior when I operated from Canyonlands National Park (K-0010):  An old Yaesu FT-100 equipped with a Collins mechanical CW filter feeding a JPC-7 loaded dipole, powered from a 100 amp-hour Lithium-Iron-Phosphate battery.  This power source allowed me to run a fair bit of power (I set it to 70 watts) to give others the best-possible chance of hearing me.

As you would expect, there was absolutely no man-made noise detectable from this location as any noise that we would have heard would have been generated by gear that we brought, ourselves.  I placed the antenna about 25' (8 meters) away from my operating position, using a length of RG-8X as the feedline, placing it far enough away to eliminate any possibility of RFI - not that I've ever had a problem with this antenna/radio combination.

I did have one mishap during this operation.  Soon after setting up the antenna, I needed to re-route the cable which was laying on the ground, among the dirt and rocks, and I instinctively gave it a "flip" to try to get it to move rather than trying to drag it.  The first couple of "flips" worked OK, but every time I did so the cable at the far end was dragged toward me:  Initially, the coax was dropping parallel with the mast, but after a couple flips it was at an angle, pulling with a horizontal vector on the antenna and the final flip caused the tripod and antenna to topple, the entire assembly crashing to the ground before I could run over and catch it.

The result of this was minor carnage in that only the (fragile!) telescoping rods were mangled.  At first I thought that this would put an end to my operation, but I remembered that I also had my JPC-12 vertical with me which uses the same telescoping rods - and I had a spare rod with that antenna as well.  Upon a bit of inspection I realized, however, that I could push an inch or so of the bent telescoping rod back in and make it work OK for the time-being and I did so, knowing that this would be the last time that I could use them.

The rest of the operating was without incident, but this experience caused me to resolve to do several things:

  • Order more telescoping rods.  These cost about $8 each, so I later got plenty of spares to keep with the antenna.
  • Do a better job of ballasting the tripod.  I actually had a "ballast bag" with me for this very purpose, but since our location was completely windless, I wasn't worried about it blowing over.
  • If I need to re-orient the coax cable, I need to walk over to the antenna and carefully do so rather than trying to "flip" it get it to comply with my wishes.

* * *

Epilogue:  I later checked the Reverse Beacon Network to see if I was actually getting out during my initial attempt to operate on 30 meters:  I was, having been copied over much of the Continental U.S. with reasonably good signals.  I guess that everyone there was more interested in the DX!

P.S.  I really need to take more pictures during these operations!

This page stolen from


Wednesday, December 13, 2023

"TDOA" direction finder systems - Part 2 - Determining signal bearing from switching antennas in software.


This is a follow-up to a Part 1 blog post on this topic where we discuss in general how "rotating" (or switched) antennas may be used to determine the apparent bearing of a transmitter.  It is recommended that you read Part 1 FIRST and you can find it at:  "'TDOA' direction finder systems - Part 1 - how they work, and a few examples." - LINK.

In part 1 (linked above) we discussed a simple two-element "TDOA" (Time Difference Of Arrival) system for determining the bearing to a transmitter.  This method takes advantage of the fact that - under normal conditions - one can presume the incoming signal to be a wave "front", which is to say like ripples in water from a very distant source, they "sweep" over the receiver in lines that are at a right-angle to the direction from the transmitter.  Note that in this discussion, most of the emphasis will be placed on how it is done in the analog domain with switching antennas as this can help provide a clearer picture of what is going on.

Why this works

If we are using a two-antenna array, we can divine a difference between the arrival time of the two antennas as this drawing - stolen from part 1 of this article - illustrates:

Figure 1:
A diagram showing how the "TDOA" system works.
Click on the image for a larger version.


As illustrated in the top portion of the above illustration, the wave front "hits" the two elements at exactly the same time so, in theory, there is no difference between the signal from each of these elements.  In the bottom portion of the illustration, we can see that the wave front will hit the left-most element first and the RF will be out of phase at the second element (e.g. one element will "see" a the positive portion of the wave and the other will see the negative portion of the wave).

If we constrain ourselves with having just ONE receiver to use, you might ask yourself how one might use the signal from two antennas?  The answer is that one switches between the two antennas electronically - typically with diodes.  If the two signals are identical in their time of arrival - and the length of coaxial cable between the antenna and when one switches "perfectly" between the two antennas and there is no disturbance in the received signal, we know that the signal is likely to be broadside of our two-antenna array.

If the signal is NOT broadside to the the array, there will be a "glitch" in the waveform coming out of our receiver when we switch our antenna.  Because we are using an FM receiver - which detects modulation by observing the frequency change caused by audio modulation - we can also detect that "glitch".  To understand how this works, consider the following:

Recall the "Doppler Effect" (Wikipedia article - link) where the pitch of the horn of a car increases from its original when it is moving toward the observer - and it is lower in pitch when it moves away from the observer:  It is only at the instant that the car is closest to the observer that the pitch heard is the actual pitch of the horn.

Now, consider this same thing when we look at the lower diagram of Figure 1.  If we switch from the left-hand antenna to the right-hand antenna, we have effectively moved away from the transmitter and for an instant the frequency of the received signal was lower because - from the point of the receiver on the end of the coax cable - the antenna moved away from the transmitter.  Because changes in frequency going up and down cause the voltage coming out of the receiver to go up and down by the same amount, we will get a brief "glitch" from having changed the frequency for a brief instant when our antenna "moved".

If we then switch back from the right-hand antenna to the left-hand antenna, we have suddenly moved it closer to the transmitter and, again, we shift the frequency - but in the opposite direction, and the glitch we get in the receiver is opposite as well.

We can see the glitching of this signal in the following photo, also stolen from "Part 1" of this article:

Figure 2:
Example of the "glitches" seen on the audio of a receiver connected to a TDOA system that switches antennas.

The photo in Figure 2 is that of an oscilloscope trace of the audio output of the FM receiver connected to it and in it, we can see a positive-going "glitch" when we switch from one antenna to the other, and a negative-going glitch when we switch back again.

If we have a simple circuit that is switching the antennas back-and-forth - and it "knows" when this switch happens, we can determine several things:

  • When the two antennas are broadside to the transmitter.  If we have the situation depicted in the top drawing of Figure 1, both antennas are equidistant and there will be NO glitches detected.
  • When antenna "A" is closer to the transmitter.  If we arbitrarily assign one of the antennas as "A" and the other as "B", we can see - by way of our "thought experiment" above - that if antenna "A" is closer to the transmitter than "B", our frequency will go DOWN for an instant when we switch from "A" to "B" - and vice-versa when it switches back.  Let us say that this produces the pattern of "glitches" that we seen in Figure 2.
  • When antenna "B" is closer to the transmitter.  If we take the above situation and rotate our two-antenna array around 180 degrees, antenna "B" will be closer to the transmitter than "A" and when our switch from "A" to "B" happens, our frequency will go UP for an instant when it does so - and vice-versa.  In that case, our oscilloscope will show the glitches depicted in Figure 2 upside-down.

In other words, by looking at the polarity of the glitches from our receiver, we can tell if the transmitter is to our left or to our right.  We can also infer a little bit about how far to the left or right our transmitter is by looking at the amplitude of the glitches:  If the signal is off the side of the antenna as depicted in the lower part of Figure 1, the glitches will be at the strongest - and the amplitude of the glitches will diminish as we get closer to having the two elements parallel as depicted in the top part of Figure  1.

There is an obvious limitation to this:  Unless we sweep the antenna back and forth, all we can do is tell if the antenna is to our left or right.

Walking about with an antenna like this it is easy to sweep back and forth and with some practice, one can infer whether the the transmitter is to the left or right and in front or behind - but if you have a fixed antenna array (one that is not moving) or if you are in a vehicle where their orientation is fixed with respect to the direction of travel, this becomes inconvenient as you cannot tell if it is in front or behind.

Adding more antennas

Suppose that we want to know both "left and right" and "front and back" at the same time - and in that case, you would be correct if you presumed that you were to be able to do this by adding one more antenna and - and then did some switching between them.  Consider the case in Figure 3, below:

Figure 3:
A 3-antenna vertical array, with elements A, B and C.  A right-angle is formed between antennas "A" and "B" and "A" and "C".   Also see Figure #4.
Click on the image for a larger version.

In Figure 3 and 4 we have three vertical antennas - separated by less than 1/4 wavelength at the frequency of interest 1 and we also have two transmitters located 90 degrees apart from each other.  Note that these antennas are laid out in a "three-sided square" - that is, if you were to draw lines between "A" and "B" and "A" and "C" they would form a precise right angle.

We know already from our example in Figure 1 that if we are receiving Transmitter #1 that we will get our "glitch" if we switch between antenna "A" and "B" - but since antennas "A" and "C" are the same distance from Transmitter #1, we will get NO glitch.

Similarly, if we are listening to Transmitter #2, if we switch between antenna "A" and "C", we will get a glitch as "C" is closer to the transmitter than "A" - but since antennas "A" and "B" are the same distance, we would get not glitch.

From this example we can see that if we have three antennas, we can switch them alternately to resolve our "Left/Right" and "Front/Back" ambiguity at all times.  For example, let us consider what happens in the presence of Transmitter #2:

  • Switch from antenna "A" to antenna "B":  The antennas are equidistant from Transmitter #2, so there is no glitch.
  • Switch from antenna "A" to antenna "C":  We get a glitch in our received audio when we do this because antenna "C" is closer to Transmitter #2 than antenna "A".  Furthermore, we can tell by the polarity of the glitch that antenna "C" is closer to the transmitter.

Let us now presume that our array in Figure 3 and 4 was atop a vehicle and the front of the vehicle was pointed toward the left - toward Transmitter #1:  With just the above information we would know that this transmitter was located precisely to our right - and that if we wanted to drive toward it, we would need to make a right turn.

Figure 4:
A 3-antenna vertical array, with elements A, B and
C as viewed from the top.
Click on the image for a larger version.

Bearings in between the antennas

What if there a third transmitter (Transmitter #3 in Figure 4) located halfway between Transmitter #1 and Transmitter #2 and we were still in our car pointed at Transmitter #1?  You would be correct in presuming that:

  • Switching between Antenna "A" and "B" would indicate that the unknown transmitter would be to the front of the car.
  • Switching between Antenna "A" and "C" would indicate that the unknown transmitter would be to the right of the car.
  • We get "glitches" when switching between either pairs of antennas (A/B and A/C) - but these "glitches" are at lower amplitude than if the transmitter were in the direction of Transmitter #1 or Transmitter #2.

Could it be that if we measured the relative amplitude and polarity of the glitches we get from switching the two pairs of antennas (A/B and A/C) that we could infer something about the bearing of the signal?

The answer is YES.

By using simple trigonometry we can figure out - by comparing the amplitudes of the glitches and noting their relative polarity - the bearing of the transmitter with respect to the antenna array - and the specific thing we need is the inverse function "ArcTangent".

If you set your "Wayback" machine to High School, you will remember that you could plot a point on a piece of X/Y graph paper  and relative to the origin, use the ratio of the X/Y values to determine the angle of a line drawn between that point and the origin.  As it turns out, there is a function in many computer languages that is useful in this case - namely the "atan2()" function in which we put our "x" and "y" values.

Figure 5:
Depiction of the "atan2" function and how to get the angle, θ.
This diagram is modified from the Wikipedia "atan2"
article - link.

Click on the image for a larger version.
Let us consider the drawing in Figure 5.  If you remember much of your high-school math, you'll remember that if straight-up is zero degrees and the right-pointing arrow is 90 degrees that the "mid-point" between the two would naturally be 45 degrees.

What you might also remember is that if you were drop a line between the dot marked as (x,y) in Figure 5 and the "x" axis - and draw another line between it and the "y" axis - those lines would be the same length.

By extension, you can see that if you know the "x" and "y" coordinates of the dot depicted in Figure 5 - and "x" and/or "y" can be either positive or negative - you can represent any angle.

Referring back to Figure 2, recall that you will get a "glitch" when you switch antennas that are at different distances from the transmitter - and further recall that in Figures 3 and 4 that you can use the switching between antennas "A" and "B" to determine if the transmitter is in front or behind the car - and "A" and "C" to determine if it is to the left or right of the car.

If we presume that the "y" axis (up/down) is front/back of the car and the "x" axis is right/left, we can see that if we have an equal amount of "glitching" from the A/B switch ("y" axis) and the A/C switch ("x" axis) - and both of these glitches go positive - we would then know that the transmitter was 45 degrees to the right of straight ahead.

Similarly, if we were to note that our "A/B" ("y" axis) glitch was very slightly negative - indicating that the signal was behind and and that our "A/C" glitch was strongly negative indicating that it was far to our left:  This condition is depicted with the vector terminating in point "A" in Figure 5 to show that the transmitter was, in fact, to the left and just behind us - perhaps at an angle of about 260 degrees.

Using 4 antennas

The use of three antennas isn't common - particularly with an "L" (right-angle) arrangement - but one could do that.  What is more common is to arrange four antennas in a square and "rotate" them using diode switches with one antenna being active at a given instant - and having more antennas and more switching between antennas to create our glitches gives us more data to work with which can only help reduce the uncertainty of the bearing.  Consider the diagram of Figure 6.

Figure 6:
A four antenna arrangement.
Click on the image for a larger version.

In this arrangement we have four antennas arranged in a perfect square - and this time we are going to switch them in the following pattern:


Now let us suppose that we are receiving Transmitter "A" - so we would get the following "glitch" patterns on our receiver:

  • A->B:  Positive glitch (A is closer to TX #1 than B so the the source is seen to move farther away)
  • B->C:  No glitch (B and C are the same distance from TX #1)
  • C->D:  Negative glitch (D is closer to TX #1 than C so the source is seen to move closer)
  • D->A:  No glitch (A and B are the same distance from TX #1)

As expected, going from "A" to "B" results in a glitch that we'll call "positive" as antenna "B" is farther away from the transmitter than "A" - but when we "rotate" to the other side and switch from "C" to "D" - because we are going to an antenna that is closer, the glitch will have the opposite polarity as the one we got when we switched from "A" to "B" - but both glitches will have the same amplitude.

Since antenna pairs B/C and A/D are the same distance from the transmitter we will get no glitch when we switch between those antennas.

As  you can see from the above operation, every time we make one "rotation", we'll get four glitches - but they will be in equal and opposite pairs - which is to say the A->B and the C->D are one pair with opposite polarity and B->C and D->A are the other pair with opposite polarity.  If we take the measured voltage of these pairs of glitches and subtract each set, we will end up with vectors that we can throw into our "atan2" function and get a bearing - and what's more, since we are getting the same information twice (the equal-and-opposite pairs) this serves to increase the effective amplitude of the glitch overall to help make it stand out better from modulation and noise that may be on the received signal.

Similarly, if we were receiving a signal from Transmitter #3 (in Figure 6) we could see that being at a 45 degree angle, each of our four glitches would have the same strength but differing polarities - with the vector pointing in that direction.  What's more, the magnitude of those glitches will be a bit lower than our example with Transmitter #1, above:  Since Transmitter #3 is shifted 45 degrees, this means that the apparent distance between any antenna switch will be about 71% as great as it would have been had it been Transmitter #1 or #2.  If you recognized that 71% - or 0.707 is the sine (or cosine) of 45 degrees, you would be exactly right!

A typical four-antenna ARDF unit will "spin" the antenna at anywhere between 300 and 1000 RPM - the lower frequencies being preferable as it and their harmonics are better-contained within the 3 kHz voice bandwidth of a typical communications-type FM receiver.

Figure 7:
Montreal "Dopplr 3" with compass rose,
digital bearing indication and adjustable switched-
capacitor band-pass filter running "alternate"
firmware (see KA7OEI link below).
Click on the image for a larger version.

Improving performance - filtering

As can be seen in the oscillogram of Figure 2, the switching glitches are of pretty low amplitude - and they are quite narrow meaning that they are easily overwhelmed by incidental audio and - in the case of weaker signals - noise.  One way to deal with this is to use a very narrow audio band-pass filter - typically something on the order of a few Hz to a few 10s of Hz wide.

In the analog world this is typically obtained using a switched-capacitor - the description of which would be worthy of another article - but it has the advantage of its center frequency being set by an external clock signal:  If the same clock signal is used for both the filter and to "spin" the antenna, any frequency drift is automatically canceled out.

It is also possible to use a plain, analog band-pass filter using op amps, resistors and capacitors - but these can be problematic in that these components - particularly the capacitors - are prone to temperature drift which can affect the accuracy of the bearing, often requiring repeated calibration:  This problem is most notable during summer or winter months when the temperature can vary quite a bit - particularly in a vehicle.

By narrowing the bandwidth significantly - to just a few Hz - it is far more likely that the energy getting through it will be only from the antenna switching and not incidental audio.

There is another aspect related to narrow-band filtering that can be useful:  Indicating the quality of signal.  In the discussions above, we are presuming that opposite pairs of antennas will yield equal-and-opposite "glitches" (e.g. A->B and C->D are mirror images, and B->C and D->A are also mirror images) - but in the case of multipath distortion - where the receive signal can come from different directions due to reflection and/or refraction - this may not be the case.  If the above "mirroring" effect is not true, this causes changes in the amplitude of the tone from the antenna spin rate (the "switching tone") which can include the following:

  • The switching tone can decrease overall due to a multiplicity of random wave fronts arriving at the antenna array.   If multipath is such that one or more of our antennas gets no signal - or they get a delayed bounce that "looks" like one of the other antennas, you might get a missing glitch or one that has the wrong polarity.  A signal distorted in such a manner probably won't make it through our very narrow band-pass filter very well at all.
  • The switching tone's frequency can double if each antenna's slightly-different position is getting a different portion of a multipath-distorted wave front.  If the multipath is such that every antenna as a different version of the bounced signal it may be that you don't get the "equal and opposite" glitches that you expect.  Again, if our switching tone is doubled, it won't make it through the band-pass filter.
  • The switching tone can be heavily frequency-modulated by the rapidly-changing wave fronts.  Remember that Frequency Modulation is all about the rapid phase changes of the carrier with modulation - but if you are driving through an area with a lot of reflections, this can add random phase shifts to the received signal which can cause the switching tone of our antennas' rotation to be seemingly randomized.  Because the randomization will likely appear as noise, this will likely "dilute" our switching tone and there will be less of it to be able to get through our narrow band-pass filter.
If you have ever operated VHF/UHF from a moving vehicle, you have experienced all three of the above to a degree:  It's likely that you have stopped at a light or a sign, only to find out that the signal to which you were listening faded out and/or got distorted - only to appear again if you moved your vehicle forward or backwards even a few inches/centimeters.  Similarly, you've likely heard noise (e.g. "Picket Fencing") as you have driven through an area with a lot of clutter from buildings and/or terrain:  Imagine this happening to four antennas in slightly different locations on the roof of your vehicle, each getting a signal that is distorted in its own, unique way!

Each of the above cause the switching tone in the receiver to be disrupted and with the worse disruption, less of the signal will get through the narrow filter.  Of course, having a good representation of the antenna's switching tone does not automatically mean that it is going to indicate a true bearing to the transmitter as you could be receive a "clean" reflection - but you at least you can detect - and throw out - obviously "bad" information!

Improving performance - narrow sampling

In addition to - or instead of narrow-band sampling - there's another method that could be used and that is narrow sampling.  Referring to Figure 2 again, you'll note that the peaks of the glitches are very narrow.  While the oscillogram of Figure 2 was taken from the speaker output of the receiver, many radios intended for packet use also include a discriminator output for use with 9600 baud and VARA modes which has a more "pristine" version of this signal.

Because we can know precisely when this glitch arrives (e.g. we know when we switch the antenna - and we can determine by observation when, exactly, it will appear on the radio's output) we can do a grab the amplitude of this pulse with a very  narrow window (e.g. "look" for it precisely when we expect it to arrive) and thus reject much of the audio content and noise that can interfere with our analysis.  

Further discussion of this technique is beyond the scope of this article, but it is discussed in more detail here.

Improving performance - vector averaging

If you have ever used a direction-finding unit with an LED compass rose before, you'll note that in areas of multipath that the bearing seems to go all over the place - but if you look very carefully (and are NOT the one driving) you may notice something interesting:  Even in areas of bad multipath, there is likely to be a statistical weight toward the true bearing rather than a completely random mess.  This is a very general statement and it refers more to those instances where signals are blocked more by local ground clutter rather than a strong reflection from, say, a mountain, which may be more consistent in their "wrongness".

While the trained eye can often spot a tendency from seemingly-random bearings, one can bring math to the rescue once again.  Because we are getting our signal bearings by inputting vectors into the "atan2" function, we could also sum the individual "x" and "y" vectors over time and get an average.  
This works in our favor for at least two reasons:
  1. It is unlikely that even multipath signals are entirely random.  As signals bounce around from urban clutter, it is likely that there will be a significant bias in one particular direction.
  2. Through vector averaging, the relative quality of a signal can be determined.  If you get a "solid" bearing with consistently-good signals, the magnitude of the x/y vectors will be much greater than that from a "noisy" signal with a lot of variation.

In the case of #1, it is often that, while driving through a city among buildings that the bearing to a transmitter will be obfuscated by clutter - but being able to statistically reduce "noise" may help to provide a clue as to a possible bearing.

In the case of #2, being able to determine the quality of the bearing can, through experience, indicate to you whether or note you should pay attention to the information that you are getting:  After all, getting a mix of good and bad information is fine as long as you know which is the bad information!

Typically one would use a sliding average consisting of a recent history of samples.  If one uses the "vector average" method described above it is more likely that poor-quality bearings will have a lesser influence on the result. 

Antenna switching isn't ideal

Up to this point we have been talking about using a single receiver with a multi-antenna array that sequentially switches individual antennas into the mix - but electronic switching of the antennas is not ideal for several reasons:

  • The "modulation" due to the antenna switching imparts sidebands on the received signals.  Because this switching is rather abrupt, this can mean that signals 10s and 100s of kHz away can raise the receive system noise floor and decrease sensitivity.
  • The switching itself is quite noisy in its own right and can significantly reduce the absolute sensitivity of the receive system.  For this reason, only "moderate-to-strong" signals are good candidates for this type of system.
  • In the presence of multipath, the switching itself can result in the signal being more highly disrupted than normal.  This isn't too much of a problem since it is unlikely that one could get a valid bearing in that situation, anyway, but it can still be mitigated with filtering as described above.
If one is actively direction-finding with gear like this, it should not be the only tool in their toolbox:  Having a directional antenna - like a small Yagi - and suitable receiver (one with a useful, wide-ranging signal level meter) is invaluable both for situations where the signal may be too weak to be reliably detected with a TDOA system and when you are so close to it that you may have to get out of the vehicle and walk around.

Doing this digitally

There is something to be said about the relative simplicity of an analog TDOA system:  You slap the antennas on the vehicle, perform a quick calibration using a repeater or someone with a handie-talkie, and off you go.  To be sure, a bit of experience is invaluable in helping you to determine when you should and should not trust the readings that you are getting - but eventually, if the signal persists, you will likely find the source of the signal.

These days there are a number of SDR (Software-Defined Radio) systems - namely the earlier Kerberos and more recent Kraken SDRs.  Both of these units use multiple receivers that are synchronized from the same clock and use in-built references for calibration.

The distinct advantage of having a "receiver per antenna" is that one need not switch the antennas themselves, meaning that the noise and distortion resulting from the electronic "rotation" is eliminated.  Since the antennas are not switched, a different - yet similar - approach is required to determine the bearing of the signal - but if you've made it this far, it's not unfamiliar:  The use of "atan2" again:  One can take the vector difference of the signal between adjacent antennas and get some phasing information - and since we have four antennas, we can, again, get two equal and opposite pairs (assuming no multipath) of bearing data.

If you have two signals from adjacent antennas - let's say "A" and "B" from Figure 6 - we already know that the phasing will be different on the signal if the antenna hits "A" first rather than "B" first and this can be used in conjunction with its opposite pair of antennas ("C" and "D") to divine one of our vectors:  A similar approach can be done with the other opposite pairs - B/C and D/A.

This has the potential to give us better-quality bearings - but the same sorts of averaging and noise filtering must be done on the raw data as it has no real advantage over the analog system in areas where there is severe multipath:  It boils down to how it does its filtering and signal quality assessment and, more importantly, how you, the operator, interpret the data based on experience gained from having used the system enough have become familiar with it.

As far as absolute sensitivity goes between a Kerberos/Kraken SDR and an analog unit - that's a bit of a mixed bag.  Without the switching noise, the absolute sensitivity can be better, but in urban areas - and particularly if there is a strong signal within the passband of the A/D converter (which has only 8 bits) the required AGC may necessarily reduce the gain to where weaker signals disappear.
There are other possibilities when it comes to SDR-based receivers - for example, the SDRPlay RSPduo has a pair of receivers within it that can be synchronous to each other:  Using one of these units with a pair of magnetic loops can be used to effect the digital version of an old-fashioned goniometer!  This has the advantage of relative simplicity and can take advantage of the relatively high performance of the RSP compared to the RTL-SDR. 

Finally, there exist multi-site TDOA systems where the signals are received and time-stamped with great precision:  By knowing when, exactly, a signal arrives and then comparing this with the arrival time at other, similar, sites it is (theoretically) possible to determine the location of origin - a sort of "reverse GPS" system.  This system has some very definite, practical limits related to dissemination of receiver time-stamping and the nature of the received signal itself and would be a topic of of a blog post by itself!

Equipment recommendations?
My "go to " ARDF unit for in-vehicle use is currently a Montreal "Dopplr 3" running modified firmware (written by me - see the link to the "KA7OEI ARDF page, below) with four rooftop antennas.  Having used this unit for nearly 20 years, I'm very familiar with its operation and have used it successfully many times to find transmitters - both in for fun and for "serious" use (e.g. stuck transmitter, jammer, etc.) 
This unit has the advantage of being "grab 'n' go" in that it takes only a few seconds to "boot up" and it has a very simple, intuitive compass rose display. I believe that its performance is about as good as it can possibly be with a "switched antenna" type of ARDF unit:  For the most part, if a signal is audible, it will produce a bearing.

A disadvantage of this unit to some would be that it's available only in the form or a circuit board (still available from FAR circuits - link ) which means that the would-be builder must get the parts and put it together themselves.

"Pre-assembled" options for this type of unit include the MFJ-5005 which can sometimes be found on the used market and several options from the former Ramsey Electronics - along with the Dick Smith ARDF unit:  Information on these units may be found on the K0OV page linked below.
Comment:  Do NOT try to use ANY ARDF gear with inexpensive Chinese radios like BaoFengs.  The reason for this is that owing to their "receiver on a chip" having its own DSP processor, there are variations on how long the audio is delayed with respect to when the signal arrives at the antenna and this will certainly wreck any attempt at doing anything that requires consistent timing - which is true for all systems that use multiple antennas.  You will be much better off using a "conventional" (non-DSP) receiver:  Radios that are decades old - particularly if they don't have any features - are often ideal as they are typically robust and can be bought inexpensively.

Another possible option is the "Kraken SDR":  I have yet to use one of these units, but I'm considering doing so for evaluation and comparison - which I will report here if I am actually able to do so.

Final words

This (rambling) dissertation about TDOA direction finding hopefully provides a bit of clarity when it comes to understanding how such things work - but there are a few things common to all systems that cannot really be addressed by the method of signal processing - analog or digital:
  • Bearings from a single fixed location should be suspect.  Unless you happen to have an antenna array atop a tall tower or mountain, expect the bearing that you obtain to be incorrect - and even if you do have it located in the clear, bogus readings are still likely.
  • Having multiple sources of bearings is a must.  Having more than one fixed location - or better yet having one or more sources of bearings from moving vehicles is very useful in that this dramatically decreases the uncertainty.
  • The most important information is often just knowing the direction in which you should start driving.  Expecting to be able to located a signal with a TDOA system with any reasonable accuracy is unrealistic.  It is often the case that when a signal appears, the most useful piece of information is simply knowing in which direction - to the nearest 90 degrees - that one should start looking.
  • The experience of the operator is paramount.  No matter which system you are using, its utility is greatly improved with familiarity of its features - and most importantly, its limitations.  In the real world, locating a signal source is often an exercise in frustration as it is often intermittent and variable and complicated by geography.  No-one should reasonably expect to simply purchase/build such a device and have it sit on the shelf until the need arises - and then learn how to use it!

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  1. On systems like this where one switches between (or uses) multiple antennas - it is necessary that adjacently-compared antennas be less than a quarter wave apart at the highest operational frequency.  While it is possible to get better resolution by increasing the spacing between antennas, the directional response will have multiple lobes meaning that there can be an uncertainty as to which "lobe" is being detected.
Having more than 1/4 wavelength spacing can be useful if you have means of resolving such ambiguities.  Spacing antennas closer than 1/4 wavelength can work, but the phase difference also decreases meaning that differences between antennas reduces making detection of bearing more difficult and increasingly susceptible to incidental signal modulation and the uncertainty that those factors imply.  From a purely practical stand point, the roof of a typical vehicle is only large enough for about 1/4 wavelength spacing on 2 meters, anyway.

Related links:

  • K0OV's Direction Finding page - link - By Joe Moell, this covers a wide variety of topics activities related to ARDF. 
  • WB2HOL's ARDF Projects - link - This page has a number of simple, easy to build antenna/DF projects.
  • KrakenSDR page - link - This is the product description/sales page for the RTL-SDR based VHF/UHF SDR.


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