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

Note:

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:

    A->B->C->D->A

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!

 * * *

Footnote:

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:

• Part 1 of this article -  "'TDOA' direction finder systems - Part 1 - how they work, and a few examples." - LINK.

• KA7OEI ARDF page - link - This page deals mostly with "Doppler" (TDOA) direction-finding gear.
  

• 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.
  

 

This page stolen from ka7oei.blogspot.com

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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".

Comment:

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:

• On "SigIDWiki"
• Predicted capabilities of the Chinese OTH RADAR  
• A YouTube video on this topic.  (What's with the obviously-computerized voice narration?)
• ARRL News item about the Chinese OTH RADAR (This article does not refer to the "10pps" rate on the signals discussed above, so this is clearly a different mode/system.)
  

* * * * *

This page stolen from ka7oei.blogspot.com

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Interesting signals on the 20 meter band: Probable Radio Habana Cuba transmitter malfunction - not jamming

 I happened to be looking at the various receivers at the Northern Utah WebSDR - as I'm wont to do (since I maintain them!) and noticed a few strange-looking signals that hadn't been there before:

Figure 1: 
Obvious QRM (interference) in the 20 meter amateur band.  The signal repeated every 66 kHz or so, allowing its source - below the 20 meter band - to be easily divined. 
Click on the image for a larger version.

The first thing that I did was to check other receivers - both on-site and across the U.S. - to make sure that this wasn't some sort of local problem (overload, image, nearby source) and found it elsewhere - but the selective fading visible in the waterfall display made me quite sure that this was ionospherically propagated and not local.  The errant signal was practically nonexistant in the Eastern U.S. - but with the known skip distance of 20 meters, that might have meant that those receivers were closer to the source, geographically.

When tuned in using AM, there was a very obvious audio tone (approximately 363 Hz) associated with the signal with a vestige of distorted speech underneath and the RF signal itself wasn't stable frequency-wise.  The tell-tale sign that this was more likely a spurious signal of some sort was the fact that this seemed to appear at intervals - roughly 65-70 kHz - so I decided to "follow the money", tuning lower in frequency and finding stronger and stronger instances.

Figure 2:
YouTube clip with audio from the errant spurious signal.  This clip - from one of the instance of spurious signal "nearby" the original - clearly contains Spanish-language audio - a clue as to a possible source!

Adjacent to the 20 meter amateur band is the 22 meter Shortwave Broadcast Band, and there I found the culprit:  A Radio Habana Cuba signal with the same sort of tone on it, symmetrically flanked by the same sidebands.  Using the TDOA feature of the KiwiSDR network clinched the diagnosis:  I tuned to one of the lower-frequency components of this signals, ran the analysis and came up with the results, below:

Figure 3:
  Several TDOA runs on the WebSDR network yielded the same results:  The errant signal appeared to be coming from western Cuba.  The main signal was not actually on 13563 kHz:  It was slightly higher up the band (probably 13700 kHz) - I just picked this particular spurious component because it was one of the strongest ones and "in the clear" - not atop another signal. 
Click on the image for a larger version.

Clearly, the program material matched the location!

While writing this, the spurious signal suddenly disappeared at around 1503 UTC:  Perhaps someone noticed the problem and switched the errant transmitter off (or fixed something) - or maybe whatever it was that had been failing finally gave up the ghost?

Interesting!

Update:

The same problem was noted again on 18 November (during the 1500 UTC hour) with spurious signals appearing on the 22 and 19 meter shortwave broadcast bands with interference again appearing in the 20 meter amateur band.  Again, the KiwiSDR TDOA network showed the likely source of the signal to be Cuba.

Either the folks at Radio Habana Cuba are unaware of the problem, or don't care enough to fix it/curtail transmissions to avoid causing issues across the HF spectrum!

The most likely source of the interference is the transmitter on 13700 kHz as it is symmetrically flanked with spurious signals above and below, spaced about 68 kHz (variable).  There is clearly something wrong with the 11760 kHz transmitter as well based on its long-term issues of very poor audio quality.

This page stolen from ka7oei.blogspot.com

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"TDOA" direction finder systems - Part 1 - how they work, and a few examples.

Next to using a directional antenna, one of the simpler ways to determine the direction of a received signal is to use what is often referred to as the "TDOA" system which stands for Timed Direction Of Arrival.

One method of implementation involves the use of two separate antennas, switched at an audible rate, and connected to a narrowband FM receiver.  In its simplest form the antenna switch signal could be produced by anything from a 555 timer to an oscillator made from logic gates to one made using an op-amp:  All that is necessary is that the duty cycle of the driving (square) waveform me "near-ish" 50% (+/- 30% is probably ok...) and be of sufficient level to adequately drive the switching diodes on the antenna.

See the diagram below for an explanation of how this system works:

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

In short, if both antennas - which are typically half-wave dipoles - are exactly the same distance from the signal source, the RF waveform on each of the two antennas will have arrived at exactly the same time.  If we electronically switch between the two antennas, nothing will happen because both signals are identical.

If one of the two dipoles in our DF antenna is closer to the transmitter than the other, switching the between the two antennas will cause the receiver to see a "jump" in the RF waveform:  Switching from, say, A to B will cause it to jump forward while switching from B to A will cause it to switch backward.

The switching, causing the RF waveform to "jump", is seen by the FM receiver as phase shift in the received signal - and being an FM receiver, it detects this as a "glitch" in the audio as depicted in Figure 2:
 

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

Because the discontinuity in the RF waveform caused by the antenna switch is abrupt, the "glitch" in the audio waveform is transient and occurring only when the antenna is switched.  You might notice something else in Figure 2 as well:  If we assume that a the first glitch is from switching from antenna "A" to antenna "B" and that it causes the glitch to be positive-going,  the switch from antenna "B" back to antenna "A" is going to be negative-going. Ideally, the glitches are going to be equal and opposite but sometimes - such as the case in Figure 2, they are not exactly the same and this is usually due to multipath distortion at the DF antenna array.

At this point, several things may have already occurred to you:

• If switching from antenna "A" to antenna "B" causes a positive glitch and vice-versa, we know that the antenna array is not broadside to the transmitter.
• If we rotate the antenna so that switching from antenna "B" to antenna "A" now causes a positive glitch and from "B" to "A" causes a negative one, we can reasonably assume that if antenna "A" were closer to the transmitter before we rotated it, that the direction of the transmitter is somewhere in between the two antenna positions.
• If the two antennas are equidistant from the transmitter, the glitches will go away entirely.  At this point, the antenna will be oriented directly broadside to the transmitter, indicating its bearing.
• The magnitude of the glitches provides some indication of the error in pointing:  If the antennas are equidistant with the two-element array perfectly broadside to the transmitter, the amplitude of the glitches will be pretty much nonexistent, but if the antenna is 90 degrees off (e.g. with the boom "pointed" at the transmitter as one would a normal Yagi antenna) the glitches - and the audible tone - will be at the highest possible amplitude.

Detecting the glitches by ear:

If the antennas are switched at an audible rate, these glitches will be clearly audible as a tone superimposed on the received signal.  While it is not possible to determine the phase relationships of these glitches by ear to determine whether the transmitter is to the left or right of our signal, by sweeping it back and forth and mentally noting where the "null" (e.g. the point at which the tone disappears) is, we can infer the direction of the transmitter by doing so - and that is exactly how the simplest of these TDOA systems work.

For an example of a simple TDOA system using a 555 timer, see the following web page:

WB2HOL 555 Time Difference-of-Arrival RDF - link

This circuit is about as simple as it gets:  A 555 timer that generates a square-ish wave.  It is up to the user to move the antenna back and forth, note the null and infer from that the direction of the signal.

It should be noted that in this case the transmitter could be behind the user, but with a bit of skill and practice one can resolve this 180 degree ambiguity - particularly if one is fairly close to the transmitter - by noting how the apparent bearing changes with respect to the relative locations of the user and the transmitter.

Detecting the glitches electronically:


To electronically determine if the signal is to the left or right of our position we must be able to determine if the glitch that occurs when switching from on antenna to the other is positive-going or negative-going and to do this, we need some simple circuity and one way to do this is to include a "window" detector - that is, a detector that "looks" at the receive audio for a brief instant, just after the antenna switch occurs and here are two circuits that do just that:

WB2HOL's Simple Time Difference-of-Arrival RDF - link
and
The WA7ARK TDOA units - link  (Some of the circuits on this page are described below. )

In looking at the WA7ARK pages, we find two units that indicate left/right in different ways:

1. In the "Aural" unit, a 565 chip - which is a PLL (Phase Locked Loop) - is used to both generate the signal for switching the antennas and also to determine if the signal indicated is to the left or right by changing the pitch of the tone.
2. In the "Metered" unit described below, an approach is taken very similar to that of the WB2HOL Simple Time Difference-of-Arrival RDF circuit in that a "snapshot" of the audio is taken an the appropriate time to determine the polarity of the glitches and display this as a left/right indication on a meter movement.

In both of these circuits one still hears the tone, but there is an additional input to the user - the pitch of the tone in the case of #1 and the movement of the meter for #2 - to indicate that the transmitter is left/right of the current antenna orientation.  Having this additional information also helps the user more-easily resolve the 180 degree ambiguity because, if the transmitter is behind, the left-right indications will be reversed.

Finding the glitches:

At this point in the discussion I would like to redirect your attention back to Figure 2, above.  You'll notice that these glitches are really quite brief:  They don't last very long at all - and most of the space between glitches is empty - at least if there is no other audio on the transmitted signal.

What about if the signal being received is heavily modulated with voice or noise?  That glitch can be easily lost amongst the clutter - but we have advantage:  We can know precisely when that glitch is going to occur and look for it only then, ignoring everything else.  In selectively looking for that glitch, much of the effect of modulation on that signal that would serve to "dilute" the signal that we want is reduced and this method is used in the "Metered" version of the WA7ARK circuit, reproduced below:

Figure 3:  The WA7ARK "Metered" circuit.
The "X" and "Y" taps are always "5" apart (0 and 5, 2 and 7, etc.) and are selected either with an oscilloscope or
experimentally, using a "clean" signal from a known-good receiver.
Click on the image for a larger version.

This circuit works as follows:

• U3C, an op amp, is wired as an oscillator with the frequency selected as being in the neighborhood of 10 kHz.  The precise frequency really isn't critical, but it should be stable:  Just make sure that you don't use a ceramic capacitor for C4.
• U2 is a 4017 CMOS divide-by-10 counter.  The "Cout" pin has a square wave at 1/10th of the frequency of the U3C oscillator (approximately 1 kHz) and this signal, buffered by U3D, drives the switched antennas.
• The FM receiver is connected via J1 and this contains the audio with the "glitches" in it.
• For every 10 count made by U2, there are two glitches:  One occurs when the square wave output from U3D goes from high-to-low, and another when it goes from low-to-high.  During that time, the "0-9" outputs of U2 go high, one-at-a-time, representing each of its 10 counts and is high for only 10% of the total time.
• As shown in the diagram, we select two of the 0-9 outputs of U2, 5 counts apart from each other.  We pick the output that goes high at the same instant that the "glitch" from the receiver's audio arrives.
• Being driven by U2, U1 contains electronic switches that are activated by the two, brief signals from U2 that we have selected to go high when the glitches arrive.  When activated, the appropriate switch inside U1 is closed at a time that coincides with the glitch and this brief signal changes the charges on C2 and C3, the voltage correlating with both the amplitude and polarity of the glitch.
• U3A and U3B buffer the voltages on C2 and C3 and feed it to a zero-centered meter:  The more the charges on C2 and C3 differ from each other, the more the meter swings away from the center.  Since the voltages on C2 and C3 are derived from the glitches that occur, the meter indication tells us not only whether the signal is to the left or right of us, but also something about how far to the left/right it is!

By using a "windowed" detector driven by the relatively brief pulses from U2, we are only looking at our receive audio for 2/10ths of the time (e.g. 20%) and ignoring what is happening during the other 80% of the time and since our meter is connected across these two points it is also only going to react to energy that is consistently "equal and opposite" - as that of the "glitches" depicted in Figure 2.

Because we are looking at only the 20% of the time during which a glitch is coming in, we not only better-reject other audio that might be being transmitted on that signal, but by virtue of some filtering provided by capacitors C2, C3 and resistor R3, we are averaging out the noise and other modulation as well, further improving our effective sensitivity and reducing our susceptibility to effects of noise and modulation on our received signal!

In actual use, one would determine the optimal taps for "X" and "Y" on U2 in the diagram above - either with an oscilloscope, or experimentally by adjusting the taps using a clean tone - no modulation received using an antenna like that described below for the highest meter indication.  For calibration, one would simply set the volume on the receiver to cause full-scale deflection when the antenna was pointing "away" (e.g. 90 degrees rotated from the two elements being broadside to the transmitted signal).  If necessary, you may make R4 variable, placing a 1k resistor in series with a 10k-25k potentiometer.

The antenna:

Up to this point the antenna has been mentioned only in passing.  The simplest antenna - and one that works well for practically any of the simple TDOA systems (of the "left/right" variety) you are likely to find - is depicted below:

Figure 4:
A typical TDOA switch antenna.
The only critical points are that dimensions "L2" and "L3" be equal to each other and cut according
to the lengths calculated using the notes on the drawing above or using the example below.
Click on the image for a larger version.

Note:  The antennas depicted on the WB2HOL pages, linked above, will also work.

While the antenna depicted in Figure 4 looks like a 2-element Yagi, it is not.  What's more, it is important to realize that while you would line up the elements of a Yagi to point it at the signal being sought, this antenna - when "pointed" toward the transmitter - will have its elements oriented broadside to the transmitter.  In other words, if you are facing the transmitter and you are holding the antenna centered in front of you, one of its elements will be to your right and the other will be at the same distance, but on your left and the elements will be vertical.



A few notes about construction:

• The two elements must not be spaced farther than 1/2 wavelength apart at the highest frequency for which you plan to use the antenna.  If they are spaced farther than 1/2 wavelength apart, you'll get nonsensical readings!  Spacing them about 1/4 wavelength apart on 2 meters (144 MHz) results in a fairly compact and manageable antenna.
• Make sure that the two pieces of coax depicted by "L2" are of the same type and length - an electrical 1/2 wavelength apart:  Note that the "velocity factor" of coax will mean that the coax's physical length will be significantly shorter than its electrical length.
• For D1 and D2, use identical diodes.  Preferably, a PIN switching diode will work, but a 1N914 or 1N4148 will work in a pinch with somewhat degraded performance.  Reportedly, 1N4007 diodes (the 1000 volt version in the 1N400x family of diodes) work "well enough" 2 meters for this purpose although their capacitance is a bit on the high side.
• For 2 meters, typical values might be:
• L1 = 16 inches (42cm)
• L2 = 26 inches (66cm) for cable with a solid polyethylene dielectric.
• L3 = 38 inches (97cm) total consisting of two pieces, each of half that length. 

 How the antenna switching works:

If you look at Figure 3 you will see J2, which is connected to the DF antenna and J3, which is connected to the receiver and separating the two is C7, a 47pF capacitor:  C7 is too small to effectively pass our audio-frequency antenna switching signal and is thus able to prevent it from entering the front end of our receiver.

Our switching signal - a square wave - is coupled to the antenna via C6 and this capacitor is large enough that it allows the square wave to pass, but since it is AC coupled, it causes our positive-going square wave from U3D to become bipolar, centered about zero going both positive and negative with respect to ground.  R9 is used not only to limit the level of the square wave being fed to the diodes, but it also isolates the RF signal present at J2/C7 from the rest of the circuit.

The now-bipolar square wave travels down the coax to our antenna along with the RF and when it is positive-going, D1 (in Figure 4) conducts and reverse-biases D2, shutting it off, but when it is negative-going, D2 conducts and D1 is reverse-biased:  It is only when a diode is conducting that it is transparent to RF and in this way, we can alternately select either the left or the right element.

When using the antenna:

• The above antenna only works well for vertically-polarized signals since the antenna must be held with the elements vertical to get left/right indications.
• Remember that you do NOT use this as you would a Yagi.  The tone will disappear when the elements are vertical and the plane of the two elements are broadside to the distant transmitter.  In other words, if you are holding the antenna up to your chest, one element will be near your left arm and the other will be near your right.
• If you "point" the boom at the transmitted signal as if it were a normal Yagi, you will get the loudest tone.
• Because this is FM - and with FM, signal strength doesn't matter once the signal is full-quieting - the loudness of the tone will tell you nothing about the strength of the received signal.  Again, the loudest tone indicates that the antenna is about 90 degrees off the bearing of the transmitter and the tone disappearing tells you that the antenna is perfectly oriented broadside to the transmitter.
• Remember that if the transmitter is behind you, the left-right indications (if the unit has the capability) will become reversed.
• The presence of multipath and reflections can easily confuse a system like this.  Remember to note the trend of the bearings that you are getting rather than relying on a single bearing that might suddenly indicate a wildly different different direction:  If you do get vastly different reading, move to a different location and re-check.  Unless you are very near the transmitter - which probably means that you can disconnect the antenna cable from the radio and still hear the transmitter - a small change in location should not cause a large change in bearing:  If it does, suspect a reflection.

A few comments about some inexpensive imported radios and their suitability for use with these types of circuits:

In recent years there are a number of very inexpensive radios - mostly with Chinese names - that have appeared on the market in the sub-$100 price range - some $50 or below - and the question arises:  Are these suitable of direction-finding?

The quick answer is "possibly not."

Many of these radios use an "all-in-one" receiver chip which has several issues:

• These radios tend to overload very easily in the presence of strong signals.  If one is very close to the transmitter being sought they can do strange things such as experiencing phase shifts.  If one is attempting to use one of these radios with an "Offset Mixer" (a different article...) then it can simply become impossible!
• Many of these radios also have an audio filter that kicks in when the signal is weak and noisy that cannot be disabled.  This low-pass filter - which is apparent when the hiss or audio suddenly sounds somewhat muffled - causes a different audio delay.  While this will likely have little effect with the simplest TDOA circuit where one is simply listening to a tone, it will likely "break" fancier ones that provide left-right indications.

If you have one of these inexpensive radios and can't seem to make the circuit work, try a different radio - preferably one from one of the mainstream amateur radio brands - during your troubleshooting!

In the next part - to be posted some time in the future - we'll talk about how one might implement what we have learned about the circuits, above, in software.

This page stolen from "ka7oei.blogspot.com".

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