Sunday, February 14, 2021

The appearance of the "Chinese Woodpecker" on the HF bands

Listening about on 40 meters this morning I heard a familiar sound - the "putt-putt-putt" of what sounded very much like the infamous "Russian Woodpecker" Over The Horizon Radar (OTHR) of the 1970s and 1980s - even having the same 10 Hz repetition rate that was common for the woodpecker.

Just by looking at the waterfall display, I could see that this signal was quite different:  Rather than taking 100s of kHz of bandwidth, this signal seemed to be fairly well contained within a bandwidth of a few 10s of kHz implying techniques unlike those of the signal from the 70s.

In other words:  It sounded like the bad old Russian Woodpecker, but it clearly was not.

Pulse versus chirp:

In the "old" days, a lot of RADAR systems simply blasted out a pulse of RF energy and then listened for the echo.  "Because physics", there is a 4th power distance relationship of reflected RF (e.g. doubling the distance causes a signal to decrease by a factor of 16) so tremendous radiated power levels were required to receive enough energy from the return pulse - which had to be intercepted over a wide bandwidth to get precise timing - from the object off which it had bounced.  Distance may be ascertained by timing the delay between the transmit and return pulse, often integrating this information over many pulses.

Audio clip of possible Chinese OTHR as heard on the KFS WebSDR system on
40 meters.  Significant backscatter is apparent in this recording, but the signal was
very much stronger and "cleaner" on remote receivers in Asia.
(Another type of signal - the "buzz-buzz" - was heard mid-recording and is not related.)

Many modern RADAR systems transmit a CW (continuous) signal that, instead of being pulsed, is swept in frequency.  Rather than relying solely on the time between the transmit and return pulse, one can measure the difference in frequency between the transmit signal and received (reflected) signal because the transmitter will have shifted frequency by the time the reflected signal arrives, and the greater the round-trip distance, the greater the frequency difference.  In other words, instead of timing the pulse directly - because there isn't one - the frequency difference, using a receiver that has a local oscillator that effectively tracks the transmitter's frequency, is what indicates distance.

Because the latter case uses a CW signal and a tracking receiver, one may use narrowband techniques (anything from a simple, narrow filter or an FFT with multiple "narrow" bins) on the receive end, potentially obtaining 10s of dB of processing gain.  In other words, to obtain the same Direction, Range and Distance information, far lower power may be required than with the old-fashioned pulse-type RADAR for comparable results.

It is likely because this "new" OTHR radar is chirp - that is, a swept-frequency transmitter - that its spectrum is far-better contained than the "bad old" Russian Woodpecker, and it's likely that the effective radiated power - while still quite high - is far lower.

A bit of sleuthing:

In the past, amateur radio operators could ascertain the location of the Russian Woodpecker only by obtaining antenna headings from multiple stations around the world and then compiling the data to determine a likely location of the transmitter.  These days, we have other methods at our disposal - and as readers of this blog will be aware, one of these is the KiwiSDR "TDOA" network.

In short, the TDOA network consists of a number of participating KiwiSDR receivers around the world that, when commanded to do so via the software extension's control panel, will record a GPS time-stamped audio file from the selected receivers and send them to a server that will analyze this data and determine the apparent location of the received signal.

This, I have done several times over the past several weeks, and today I had the opportunity to do it several times more - the results appearing in the maps below:

KiwiSDR TDOA results from the "Woodpecker" signal heard on 40 meters from remote receivers.

The above maps show the results of several TDOA sessions using several receivers scattered across the Pacific and Australasia - the lower image being derived from higher-quality data as propagation improved - and, possibly, as the transmitted beam was better-oriented in the direction of the receivers, reducing the appearance of backscatter.  Because these transmissions are (apparently) rapid frequency sweeps, they are nearly ideally suited for the type of analysis needed to determine the Time-Difference Of Arrival (TDOA) techniques employed - in other words, a RADAR in reverse.

Due to the vagaries of ionospheric propagation - and as should be apparent from the roughness of the numbers given - the absolute location of the transmitter is likely to be accurate within only about 100km at best for this sort of exercise - but something is clear:  It is probably not likely a "Russian Woodpecker", but more likely something akin to the " 啄木鸟   中国人 " - which Google tells me means something roughly akin to "Woodpecker from China".


There is a known Chinese OTHR transmit site near Nanjing (approx. 32.05°N, 118.78E) but that location does not correlate with the results above.  It's very possible that the TDOA error is quite large, but the coordinates on the maps shown above were similar for several runs:  More TODA runs, over time, may help to resolve this uncertainty.

"Will my radio's noise blanker help?"

Probably not!

Unlike the bad old Russian Woodpecker that consisted of narrow (and very broadband) pulses, this appears to be a swept carrier, meaning that unlike the old, Russian variant, a pulse-type noise blanker is unlikely to work well at all:  Rather than the pulse being "everywhere" within a few 10s (or 100s) of kHz of the desired receive signal simultaneously in the case of the Russian Woodpecker - and other impulse noises like vehicle ignition, electric fences and lightning - this signal is only on one frequency at any given instant and the wideband amplitude detector comprising a standard impulse-type noise blanker will likely be ineffective.  This "new" signal sounds like a pulse only because it spends a small amount of time in an SSB receiver's passband during each sweep.

In theory, it should be possible to design a software-based filter that will remove this signal as it is very repeatable, but it's likely that no software-defined receivers in common use at this time (e.g. as of the original posting of this article) will have anything at all that will touch it!

* * * * *

This is not the first time that a Chinese OTHR has appeared on the ham bands:  For years, now, one can hear what sounds like a loud "buzz" that comes and goes as described on some of the links below.  The appearance of the "10 pps" version - possibly a refinement for longer-distance observation - seems to be comparatively new.

* * * * *

Related links about the Chinese OTHR:

* * * * *

This page stolen from


Sunday, February 7, 2021

RFI (Radio Frequency Interference) sleuthing with the TinySA - Part 1: The gear

Although focused on the use of the TinySA, the techniques and equipment described in this article may be applied to using any other type of Spectrum analyzer - or even a simple receiver - for the detection of interference/noise sources or general-purpose direction-finding.

Figure 1:
The Tiny SA, connected to an outdoor HF antenna during
daylight hours.  The displayed span is 2-22 MHz.
Click on the image for a larger version

About the TinySA:

The "TinySA" is a small spectrum analyzer with a 2.4" touch screen, costing approximately U.S. $50.  Capable of operating from below 100 kHz to as high as 950 MHz (the optimal range being between 100 kHz and 350 MHz) its minimum resolution bandwidth (RBW) is about 2.6 kHz.  This resolution bandwidth is too wide for precise analysis of the components of an SSB or NBFM signal, but it is useful for general RF surveying - including the measurement of harmonic and off-frequency spurious components of a transmitter in addition to the detection of low-level signal sources.

While it may seem silly to use a spectrum analyzer for the detection of signals, the TinySA, at about U.S. $50,  is only about 2-3 times the cost of the least expensive, battery-powered shortwave receiver that you'll find online - and it has the advantage of "seeing" a large swath of spectrum in a single view, allowing detection of signal sources that may otherwise go unnoticed.

The usable sensitivity of the TinySA in the range of 1 MHz to 350 MHz (e.g. the "Low" input) is approximately -152dB/Hz meaning that its sensitivity is slightly poorer - but roughly comparable to - "real" spectrum analyzers with expensive-sounding names:  This value is quite a bit poorer than a typical communications-grade receiver which will likely have a sensitivity better than -160dB/Hz - often much better.

The TinySA comes with a telescoping antenna, but being only about 12" (25cm) long it's usefulness extend below VHF frequencies (e.g. 50-100 MHz):  At HF, the combination of the 50 ohm input impedance of the analyzer and the phenomenally poor mismatch of the small telescoping whip results in uselessly-poor sensitivity, meaning that one must be nearly atop a signal source before it may even be seen on the analyzer.  Clearly, more help is needed here!

The TinySA is NOT the NanoVNA!

The TinySA is NOT electrically similar to a NanoVNA, despite physical similarities:  It has completely different circuitry and works like a "real" spectrum analyzer in that it's a proper, swept frequency, narrowband logarithmic receiver-detector.
While a NanoVNA does have a detector that tracks its built-in signal generator, it makes a terrible spectrum analyzer!

Where to get a TinySA?

The TinySA is widely available - but there are apparently many inferior clones out there (e.g. lack of shielding, poorer quality or omitted components, etc.)  

For the U.S. readers, I suggest R&L Electronics as a source (see the link HERE).  I have no connection to R&L and suggest them only because they are supplying the genuine article and they are an established equipment dealer (e.g. more likely to help if you have any problems with the unit), and they are offering it for a decent price.

* * * * *

Active antennas to the rescue:

Adhering to the theme of portability, a useful companion to the TinySA would be some sort of small, active antenna - and two of the most common types are the active (E-field) whip and the electrically-small loop.

The Active Whip:

A popular active whip is of the "PA0RDT" design - the so-called "Mini-Whip".  It is the intent that this antenna be mounted outside and in the clear with a quiet "local" ground - that is, well away from noise sources, grounded to a mass of metal (or a ground system) that is isolated from the (noisy!) shack ground with a common-mode choke or two - so using it as a "sense" antenna for sniffing RF sources isn't exactly what the designer had in mind.

Figure 2:
A homebrew E-field whip using the PA0RDT design.
This unit is built into a piece of 1/2" PVC irrigation pipe.
A BNC connector was chosen over an SMA for durability.
Click on the image for a larger version
The reason for this specification of a "clean" ground and being placed outside, in the clear is that this antenna - being a tiny fraction of a wavelength on the highest frequency for which it is to be used - is essentially a capacitor that couples into the "æther" (e.g. free space).  To accomplish this, the whip is just a Hi-Z probe that has no gain of its own, but rather a circuit that matches the impedance of the "antenna" portion (usually a piece of wire or a plate of copper on a circuit board) to 50 ohms typically using JFET and bipolar follower circuits, avoiding what would otherwise be tremendous losses due to mismatch - but this is only half of the antenna:  The feedline - and the gear to which it is connected - is the other half - and this is the basis of the recommendation of a "local" ground with the feedline being decoupled via a common mode choke to minimize the conduction of RF energy from a "noisy" shack ground.

As a "sense" antenna in this application, its purpose is largely to determine the presence of RF energy:  Sources of this energy may be located by noting the amplitude increase as one moves the antenna nearer to the potentially-noisy device - or near a conductor (e.g. power cord, cable, etc.) that is conducting or radiating this RF energy.  Because the antenna will be carried by the user, the "ground" portion of the antenna will be nothing more than the hand capacitance of the user holding the equipment and the connecting cables - and it is imperative that these not be a source of interference in their own right or connected to something that could be a source of such signals.

Figure 3:
Inside the E-field whip of Figure 2. The "antenna" is the
copper foil "plate" with the electronics having been built
on an SMD prototype board.  This version uses the
BF862 JFET as the source-follower and a BFU589GX as
 the emitter-follower.
Click on the image for a larger version.
Because of the vagaries of coupling energy into this "whip" - which include the inconsistencies mentioned above - it's difficult to make absolute signal level measurements, but relative amplitude measurements can be made by noting signal levels as it's moved about.  Because it lacks directionality and is relatively insensitive to polarization, proximity is indicated mostly by relative amplitude:  If one happens to be close to the suspected device or interference source, this is easy to discern as even small changes of distance to the source can markedly change amplitude, but if you are somewhat distant from it this will not be the case and the lack of a directional response can be a challenge as one may not know in which direction one should move to get closer.

A few more articles about the small whip - its construction and its operation:   - The diagram of this ubiquitous circuit is that in Figure 7.

And an "improved" version:


Where to get a mini-whip?

If you choose not to build one, the so-called "Mini-Whip" and its clones may be found on EvilBay and Amazon - to mention but two places - typically for $25 U.S. or less.  All of these devices work reasonably well - the ones from Eastern Europe often performing slightly better and being more consistent in quality than those from Asia - and they come with a "power inserter" (a.k.a. "Bias Tee") - a device that injects DC power for the amplified antenna onto its coaxial cable.

These antennas are also easy to construct, the details being found - and the theory of operation - in the above links.

* * * * *

The Small Loop:

This name refers to an "electrically small" loop - that is, one with a circumference that is a small fraction of the wavelength of the highest frequency for which it to be used.  For LF, MF and HF purposes, we are referring to a receive-only loop that is typically 1-3 feet (25-75cm) in diameter.

Figure 4:
The completed shielded loop.  The gap in the shield is on
the top with the transformer and amplifier in the box at the
bottom.  The loop itself is 3/8" "Heliax" supported by a
frame made of 3/4" PVC irrigation pipe, using a "cross" in
the center and "tees" on the sides.
Click on the image for a larger version
Unlike the active whip, this antenna can be entirely "self contained" in the sense that the amount of energy impinging on the loop that is being conveyed to the receiving device and the cable itself has little effect on this level.  Additionally, this type of antenna exhibits a degree of directionality with sharp, deep nulls being present normal to the plane of the loop - which is another way of saying that if you were to hold the loop flat against your chest, the nulls would be directly in front of and behind you with very broad peaks to your left and right.

This loop, being electrically small for the intended wavelengths, has negative gain compared to a full-sized antenna, but even a chunk of coaxial cable connected directly to the loop with no attempt at matching will work "less badly" than a piece of wire or telescoping antenna - with no amplification or matching - of similar size.  Having said this, including a modest amount of gain in the loop's signal path is extremely helpful.

Having consistent "gain" lobes and nulls means that the amplitude readings from this type of antenna are more predictable and it is possible to divine the apparent bearing of the noise source by exclusion.  Because this type of antenna has two nulls (and two broad peaks) one must move about to resolve this ambiguity, noting the geometry of the direction of the these null and your surroundings to determine if it is very close or distant - and whether it is in front of or behind you!

Where do I get a loop like this?

Unfortunately, these loop antennas are nowhere near as ubiquitous as the "Mini Whip" - and those that are available tend to cost about an order of magnitude more than a mini-whip.  Fortunately, these types of antennas are not particularly difficult to construct - and such a loop is described below.

While this type of antenna may be constructed using just  loop of wire - and the techniques shown in Figure 7, below, will work fine with just a piece of wire, the preferred version is the "shielded loop" as it is somewhat less sensitive to very nearby E-field interference and its pattern of broad peaks and nulls can be more symmetrical and of better quality - as discussed here:


 * * * * *

Which type of antenna should I use?

If I had to choose just ONE of these types of antennas, I would pick the loop antenna due to its directional properties using the nulls.

Having said that, I would prefer to have both on hand:  The whip is much smaller and can easily be held near suspect devices and conductors to aid in detection/exclusion.

* * * * *

Constructing a shielded, symmetrical, untuned loop:

This portion of the article will described the "untuned, balanced, shielded" loop, and adding a simple amplifier to extend the noise floor of the TinySA.  There are many ways to construct this type of loop, but what follows is a rather simple and effective version that will tolerate component variations quite well.

In addition to locating sources of noise and interference, it provides reasonable performance for general listening on frequencies from below the AM broadcast band to the top of HF, and its ability to cast a null toward a noise source may prove to be useful.  While lacking the apparent "gain" response of the resonance peak of a tuned loop at a specific frequency, an untuned loop is easier to build and more convenient to use in a broadband application.

What to use for the loop itself?

It common to use "Heliax" (tm) coaxial cable (50 or 75 ohm is fine) to construct the loop itself - typically of the 1/2 inch or 3/8 inch variety as it is fairly rigid and can support itself fairly well, mechanically.  Aluminum-jacketed CATV (cable TV) "hardline" will work as well - although unlike the Heliax - which has a copper jacket - making connection to the outside shield can be a challenge.  

"I don't have any hardline/Heliax - can I use 'normal' coax?"

Barring the availability of "scraps" of Heliax cable, ordinary coaxial cable will also work quite well, but it must be mechanically supported as it may not be able to hold any shape -  The frame depicted in Figure 4, above, can do this.  If you are using "ordinary" coaxial cable, practically any coax will work, from RG-58 to RG-11 (the impedance doesn't really matter):  Even inexpensive RG-6 cable will work if one uses standard "F" connectors  - both on the cable itself and on the box containing the electronics - to make reliable connections to the aluminum shield found on most cables of this type.

Figure 5:
Details of the gap in the middle of the shield - the dielectric
foam being retained for additional support.  This gap
should be covered with heat-shrinkable tubing or electrical
tape to protect it from moisture.  Sliding a piece of rigid
tubing over the gap is recommended to immobilize the cable
at the gap in the shield.  A small piece of metal tubing may be
used for this as long as it's insulated from the shields.
Click on the image for a larger version.
For the loop described, I used a 66" (167cm) long scrap of 3/8" Heliax (FSJ2 type) which would correlate with a loop diameter of approximately 21" (53cm) - but the precise length and diameter is unimportant in this case:  While a larger loop (with a greater area inside the loop) will intercept more signal, a loop that is between 12" and 24" (25-50cm) diameter will work.

In the center of the loop, as depicted in Figure 5, there must be a small gap where the shield is opened, symmetrical about the loop's circumference:  Were a gap not present, this would simply be a loop of coax and a signal could not be excited on its center conductor with respect to the shield portion.  This type of loop has the advantage that an electrostatic field cannot easily excite the inner conductor, but the lack of the "shorted turn" provided by the gap in the shield means that the magnetic field portion of the signal is unimpeded.    (Hint:  Determine the center of the length of coaxial cable that you plan to use for your loop before you form it into a loop!)

Feeding the loop:

Figure 6:
Inside the box - the transformer and loop amplifier.
The transformer (T101 in Figure 7, below) may be seen in the
middle.  The circuitry is built "dead bug" on a piece of glass-
epoxy circuit board material bolted to the bottom of the box.
The shields of the halves of the loop are connected with
heavy wire to the circuit board - both for electrical bonding
and mechanical support - and wire (a.k.a. "zip") ties are used
to prevent the cable from being pulled out through the sides
of the box.  The RF out/DC in is via a BNC connector
mounted to the side of the box - this being preferable to
the bottom of the box to help protect it from damage in
Click on the image for a larger version.
The two open ends of the loop are brought together opposite the shield gap and it is there that the shields are connected to each other at a common signal "ground" - and this is typically done in a small enclosure:  While a metal box is preferred, it's not absolutely necessary.  The box shown in Figure 6 is a die-cast aluminum enclosure approximately 5-3/4" x 2-1/2" and 1-1/2" deep (about 122 x 65 x 40mm).

While one may simply connect a 50 ohm coax to either side of the center conductor of the loop - and connecting the shield of that cable to the shield of the loop itself - this will cause a bit of asymmetry.  Practically speaking, this isn't going to cause much of an issue in terms of directionality, but it is preferred that balance be maintained to provide the best symmetry in its response.  While there are many designs out there that include differential amplifiers, the use of a simple, ferrite transformer will be just fine for this - and most - applications.  Remember:  We are looking for noise sources!

For this, a small transformer must be constructed.  A good choice for this is a toroid - either an FT37-75 or FT50-75.  Experimentally I determined that either 2 or 3 turns on the primary and 3 or 4 turns on the secondary will provide good response across the HF spectrum (I used 3 turns on each, for a 1:1 turns ratio).  The gauge of wire is not critical and something in the range of 18-26 is suggested with a preference on the larger size.


Because of its small size, the gain of the loop will be very low compared to a full-sized antenna and to bring the signal level up to where weaker signals may be "seen" by the TinySA, a simple amplifier is used, depicted schematically in Figure 7, below.  This amplifier could be considered to be a general-purpose "gain block" as it is quite well-behaved in terms of stability and input and output impedance - and it is very forgiving in terms of component variation.

Circuit description:

Figure 7:
Schematic of the loop/amplifier and power inserter (a.k.a. "Bias Tee").
Click on the image for a larger version

The shielded loop is schematically depicted in Figure 7, above, along with the transformer T101, which I found to work reasonably well with a 1:1 turns ratio.  Using a VNA and with a bit of empirical testing, this configuration was optimized for low-mid HF (e.g. 3-15 MHz or so) but it seemed to work quite well through the AM broadcast band and below.  Like many similar loops - and the Mini-Whip - its performance will start to drop off at higher frequencies (above 20 MHz) and a bit of circuit redesign would be required to optimize for these higher frequencies.  As noted, the circumference of the loop must be a small fraction of the wavelength at the highest frequency at which it will be used in order to maintain the directionality of the nulls, so a loop of approximately 18" diameter will work well throughout HF, but not offer the desired properties on, say, 2 meters where its circumference would be approximately a quarter wave!

T101 does a reasonable job of maintaining the symmetry of the loop itself - although purists would insist on a somewhat different topology to eke out every bit.  As described, the symmetry is quite good and it is possible to completely null local 50 kW AM broadcasts stations.

Amplifier portion:

Q101 and associated components form a simple feedback-type RF amplifier.  This basic circuit is well-behaved and has reasonable input and output matching to 50 ohms.  Shown is the use of the common 2N3904 transistor which is perfectly acceptable for this sort of use - and similar devices, such as the 2N2222 and 2N4401 - work pretty well.  A "better" device would be the 2N5109, 2N3866 or similar RF amplifier which will offer a bit better performance in terms of intermodulation distortion and gain at higher frequencies, but they are more expensive and harder to find - and are likely overkill for a "sniffing" device.  This amplifier is not the penultimate in performance (e.g. IP3, P1dB, etc.) but it has very good performance and fairly low noise (6-8 dB noise figure) considering its simplicity.

To a degree, the gain of this stage may be adjusted by varying the value of feedback resistor R101 - resistances over the range of 330 to 680 ohms being useful, with higher gain (roughly 18 dB) being associated with higher resistance.  The purists will note changing devices or feedback resistance will alter the properties of the amplifier (e.g. input/output impedance, etc.) but one can generally ignore this in all but the most critical applications - such as matching to an impedance-sensitive filter network.

Although a battery could be placed within the loop's enclosure - in which case L101 could be eliminated and the V+ lead connected to the junction of C104/R104 - it is common to use a power inserter (a.k.a. "Bias Tee") that couples DC onto same coaxial cable that conducts receive RF from the loop, making one bias tee useful for multiple antennas!

How much RF noise does the TinySA produce?

Because it's a computer with a display, one might wonder how much RF noise the TinySA itself produces.
The designers of the TinySA appear to have been very careful about this - for example, critical components are shielded and they to have chosen to use linear voltage regulators instead of more-efficient switching-type regulators.  (I can't speak to the construction of the many clones out there!)
In testing, the TinySA was held up to the E-field whip and placed inside (and moved around) inside the circumference of the loop:  Through the range of 2-22 MHz, there are no obvious "spikes" or lumps of noise that appear on the display - at least above the ambient RF noise floor in my ham shack.
Clearly, one isn't likely to walk around with the TinySA held against the antenna - but the initial glance shows that you probably could get away with it!

The power for the amplifier is picked off the coaxial cable by decoupling choke L101.  The value of this choke is not critical - and anything above about 100uH will work fine to a bit below the AM broadcast band.  

Molded chokes may work, but they tend to have quite high internal resistance and the current consumption of the amplifier itself (50-80 milliamps) can cause a significant voltage drop.  An alternative is to use the same core as that used for T101 (you did get several FT50-75 toroids, didn't you?)

Power inserter (a.k.a. "Bias Tee")

The "power inserter" (a.k.a. "Bias Tee") is also included on the diagram and its job is to combine the RF and DC onto the same cable, using the same type of choke (L201) as on the amplifier portion.  Optional enhancements to this device would be a diode (D201) to protect against accidental reverse-polarity application to the antenna and an LED to show that power is turned on (LED201) with its current intentionally set low (e.g. a 10k limiting resistor for about 1mA) to minimize battery drain.  The addition of a fuse - preferably of the self-resetting thermal type (F201, with a current rating of 100-300 mA) is a good idea as well to prevent damage to the blocking choke, L201 should the output be accidentally shorted and to limit current into protection diode D201 should reverse polarity be applied.

If you happen to get a "Mini Whip" from one of the online sources, it will likely come with a power inserter/bias tee that is electrically very similar to that depicted above - and that device will work just fine with the loop/amplifier depicted in Figure 7.

* * * * * 

Using a shortwave receiver for RFI sensing and source identification:

Up to now we haven't mentioned one RFI-locating tool that may be already in your possession  - a portable shortwave receiver.

Figure 8:
A small selection of inexpensive portable shortwave
receivers.  Upper-right:  A DAK MR-101 - an inexpensive
receiver from the early-mid 90s.  Lower-right:  A Grundig
YB-400PE - the only one shown that is capable of CW/
SSB reception.  On the left:  A $15 receiver (labeled
LCJ 310) - one of many nearly-identical units available from.
Amazon capable of tuning from 5.9-21.85 MHz on SW and
on the AM and FM broadcast bands.  Of the receivers
pictured, only the Grundig and DAK have external
antenna connections (a 3.5mm "phono" connector) which
would be required to use the directionality of the loop antenna.
This connector would be needed to be added to the "LCJ"
receiver to take advantage of this antenna property.
Click on the image for a larger version.

While one can only "hear" a small bit of spectrum with a receiver rather than "see" a larger slice with an analyzer and increase the likelihood of detecting an otherwise-unnoticed interference source, being able to tune into an interference source and listen to it can give a clue as to the source of this signal and type of device.  If your portable shortwave receiver has an external antenna jack - especially one that automatically disconnects the built-in antenna when a connection is made - you can use the same antennas (whip, loop) described above and take advantage of any directional/nulling properties.

For example, a mains-frequency "hum" in the noise implies a switching power supply while a sharp "buzz" might indicate a triac light dimmer.  If your receiver has a BFO, one may be able to hear the harmonics of a switching supply and be able to specifically identify it by it's unique "sound" as it's powered on and off, being able to distinguish it from other devices.

A caveat with the use of a receiver:  Other than "missing" interference sources on frequencies other than that to which the receiver is tuned, a receiver will have an AGC which, by its nature, will adjust the internal gain to keep the volume constant.  When trying to locate a signal or noise source, this can work against you as it may be difficult to determine if the signal is moderately weak, strong, or very strong unless the AGC can be defeated and a manual RF gain control be operated - something that cannot usually be done with very inexpensive receivers.  Even if this is the case, the "sound" of the interference can still be useful in providing a clue as to the type of device that may be causing the interference.

* * * * *

Again, there are many possible ways to do this, but the gear described above has been proven to be useful.  The next installment of this two-part series will include details on how it has been used and what to expect when doing so.

* * * * *

This page stolen from


Thursday, January 28, 2021

Using surplus GE Mastr II and Exec II transmitters as amateur radio beacon transmitters

Back in 2000, a friend of mine (Glen, WA7X) wanted to place VHF and UHF propagation beacons at his cabin located in remote central Utah.  On-hand were used GE MASTR II Exec FM transceivers:  These radios - similar to the GE Mastr II - are crystal-controlled transceivers that date from the mid-late 70s and into the mid 80s and are still available surplus.  Available in "low", "high" and "UHF" band versions all amateur bands from 6 meters through 70cm may be covered (including 220 MHz with a bit of modification.)

For the purposes of beacon operation, it does not matter if the Mastr II or the Exec II (a slightly simpler, lower-cost version) is used as they share many of the same parts and some of the modules.  Of course, the receiver portion of the radio (front end casting, RF and IF boards) are not needed for beacon operation, but at least on the MVP, the interface board (on the "bottom" side) contains needed voltage regulators and the like.

To this end, three beacons - one each for 6 meters, 2 meters and 70cm - were constructed using modified (by me) GE MASTR II Exec radios and installed at a remote site belonging to WA7X.

Keying the transmitter:

The most obvious way to key the transmitter would be to use the PTT line - and this would work... sort of - but there are problems with doing it this way.

  • The PTT line keying the transmitter is fine for FM, but for keyed CW, the attack/decay waveform leaves much to be desired:  Severe "key clicks" are the result.
  • The crystal oscillator is actually keyed.  While you might get away with this on 6 meters, turning on and off the oscillator itself will likely result in an audible "chirp" - especially on 70cm!

What this means is that one needs to keep the oscillator running all of the time so that it remains stable and key farther down the signal path.  Fortunately, there is another way to key the radio.

Using the power control for keying:

These transmitters have a power control, of sorts.  In the PA (Power Amplifier) module there are a number of amplifier stages to take the 200-400 milliwatt signal from the exciter up to the rated output power of the final amplifier, typically 35 or 100 watts, depending on the type.  Typically the "pre-pre-driver" and "pre-driver" have their collector voltages fed via a series transistor and this voltage is made adjustable to set the amount of drive to the driver and output transistors.

While one could simply key this voltage, there is a problem:  Because all of the stages are "Class-C" type (e.g. non-linear) key clicks would surely result if the pre-pre-driver and pre-driver voltages were simply turned on and off.  What's more is that with this non-linear RF circuitry one will, as the drive power is increased by adjusting that voltage upwards, get no RF output at all - but very suddenly, the RF output will appear and increase very rapidly with respect to voltage - and then, suddenly, the rate of increase starts to drop again very quickly.  In other words, over a very small adjustment range one will go from no power at all to full power.  If we want both "clean" keying signals and to be able to select a given power level, things get a bit more complicated.

What this means is that you really can't use the original power control circuit for keying, either, but another, fairly simple circuit may be substituted, described below:

Figure 1:
Beacon power controller schematic.  This circuit not only provides keying shaping, but allows one to select three pre-set output power levels.  The PTT line of the transmitter itself is asserted all of the time causing the oscillator to run continuously.
Click on the image for a larger version

Explanation of the keying/power control circuit:

A sample of the RF voltage is provided (the terminal "From RF Power Detector") and applied to U1A, which is a unity-gain follower.  This voltage, from the RF detector, is then applied via U1D, wired as a unity-gain, inverting amplifier:  If the RF output of the amplifier - which is the voltage from U1A - drops below that of that applied to its inverting input, its output will go higher which, buffered by U1C, will turn on the output 2N3904 stage some more, causing the modified RF amplifier (described below) to produce more power.  The non-inverting input of U1D is provided from the "1 watt adj." potentiometer via U2A and in this way, the output power can be made variable by its setting.

The above circuit controls the amplifier power output via a closed-loop servo - but keying it while minimizing key "clicks" must still be done.  The keying input (active high - that is, ground = un-keyed, voltage = keyed)  is applied via U2B, wired as a comparator:  Its noninverting input is supplied from the output of U2A only because it was a convenient voltage somewhere between 2 and 4 volts.

When the transmitter is keyed, U2B's output goes low, but the minimum voltage is set to be three diode drops below the output of U2D and this discharges the 0.47 capacitor on the non-inverting input of U2C slowly through the 220k resistor.  Conversely, when unkeyed, the output of U2B goes high and that same 0.47uF capacitor is charged more quickly via the 22k resistor and its higher voltage is ilmited by the single diode "pointing" to the output of U2D.  The ultimate result of this is a voltage-limited keying waveform being applied to the non-inverting input of U2C that has carefully-controlled rise and fall times.

When the voltage applied to the noninverting input of U2C rises ("unkeyed") its output voltage also rises and is conducted into the same signal line as the "Power Detector" via a diode:  This high voltage - seemingly from the RF power detector - signals as if the RF power output is too high and the RF output is dutifully reduced to zero in response.  Conversely, when the transmitter is keyed, the output of U2C drops and its output diode no longer conducts, the RF output rising to that set by the power control.  The rise/fall of the signal being applied to U2C minimizes key clicks.

In reality, there is only one power level setting - the "1 watt adj" - and this is, by far, the most sensitive, requiring the greatest amount of control.  To effect higher power settings (e.g. 10 watts and 100 watts) the beacon controller turns "on" one of two N-channel FETs with series potentiometers in the drain lead:  When the respective FET is turned on, this parallel resistance shunts the output of the RF detector, requiring more RF power to be output to achieve the same voltage as before, increasing the RF output power as appropriate.  Of course, this means that when calibrating everything, the "1 watt" power setting must be done first!

Modification to the GE RF amplifier module:

Power control:

The GE MASTR power amplifiers' output power was originally controlled by a module that either did so by sensing the RF output from a power sensor, or "open loop" using a thermistor to try to compensate for the change in amplifier gain with temperature.  In either case an NPN emitter-follower transistor was placed in series with the supply voltage for the first two driver stages:  The higher the voltage on the base of this NPN transistor, the more voltage applied to these stages - and the higher the driver (and output) power.

Figure 2:
Modifications to the GE amplifier board
Click on the image for a larger version
This NPN transistor was replaced with a PNP or P-channel FET that allows the output power to be controlled by pulling the control pin (base or gate, as appropriate) low (e.g. toward ground):  It is this line that is pulled toward ground by the control circuit depicted in Figure 1.

The center-left insert of Figure 2 depicts this modification using a PNP Darlington transistor:  It simply replaces the original NPN follower - a task that requires a bit of rewiring and the addition of the two transistors shown.  (A standard non-Darlington PNP was originally tried, but it proved difficult to turn it "on" enough to provide 100 watts of RF output.)

A somewhat better option is the use of a P-channel power FET:  The same combinations of 1k resistors (gate-source, gate-control) are used:  The "loop gain" of the FET circuit is somewhat lower than that of a bipolar Darlington pair but being a FET, it is very easy to drive.

RF Sensing:

Figure 3:
The RF power sense circuit, coupled to the RF output
While some versions of the GE amplifier modules have built-in RF sensing, some (e.g. the MASTR Exec II) do no, so the circuit depicted in the upper-left corner of Figure 2 is required.  This is simply a small piece of wire laid atop the trace that goes from the output of the RF amplifier to its low-pass filter as depicted in Figure 3.  The desired amount of coupling is that which will yield approximately 2.5 volts of DC output across a 47k resistor at 1 watt of RF output.

Conversely, a small-value (e.g. 2-50pf) ceramic variable capacitor could be used to couple to the detector diode rather than the piece of wire:  It must have a 100 volt rating, minimum, and initial adjustment would start from the lowest-capacitance.

Temperature stability:

As expected, the power will vary slightly with temperature - but between summer and winter, only about 5% power variance has been noted.

Beacon keying:

The keying for this beacon is provided by a simple PIC-based beacon transmitter.  The only departure from a standard device - like a "WinKeyer" - is that it has outputs to select 1, 10 or 100 watts for different parts of the message.  The majority of the beacon message is transmitted at the 10 watt level to reduce overall power consumption, but it contains an embedded 5-second key-down (and accompanying message) at all three power levels.

The code for this keyer is very simple (if you wish to have a copy, send me an email) and could be easily implemented on about any similar device:  An Arduino mini would be a more contemporary choice.

As mentioned above, the transmitter's original PTT line is keyed all of the time that the beacon is in operation, causing the oscillator to be continuously running.  The only exception to this is that a remote (IP-controlled relay) device is connected to the PTT lines of these radios allowing the transmitters to be disabled remotely.

* * *

These beacons have been in operation at the WA7X site since late 2000 - over 20 years at the time of writing:  They have been extremely reliable - the only issues occurring fairly early-on and being due to random component failures, and their signals have been heard far and wide.

For more information about this beacon and its history, see the "WA7X Beacon Technical page".

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