An incredibly good article and timely topic, radio direction finding is the ‘other side’ of field communications and is an ever-present threat to teams in the field. Recognizing this, some of the techniques I teach in the RTO Course are designed to mitigate an enemy’s capability through exploiting human weakness. Keypounder provides a foundational piece for developing your own training program and a great primer for what we cover in the RTO Course and upcoming Signals Intelligence course. You are actively training to understand your small unit communications are going somewhere other than where you want them to, right? How about finding and fixing hostiles in your area who don’t have that level of awareness? We can teach you how- and this article is a big step forward. Check out the original on his site. -NC Scout

DF 101

Radio Direction Finding (RDF or simply DF) dates almost to the start of radio; various direction finding equipment pre-dating the use of vacuum tubes existed, and a number of basic DF techniques and systems developed in the early 20th century are still in use. The reason that they are still used is because these techniques, and RDF operations generally, are even more useful than in the early days of radio, when RDF was used primarily for ship navigation. In daily use in the 21st century, RDF provides search and rescue operations the ability to quickly locate crashed airplanes and lost Alzheimer’s patients, among other things. Militaries use RDF to locate enemy radio comm centers, jammers and anything using radio transmissions, and civil government uses RDF to locate jammers and interference plaguing public service communications, while amateur operators use DF to locate sources of interference, either deliberate jamming or incidental interference.

Small loop antennas in various arrays and combinations were the first antennas used; Bellini and Tosi patented the first such system in 1907, using two crossed loops, and a ship-borne modification of this system was used by the Royal Navy and contributed to the sinking of as many as 25% of the U-Boats lost in WW2. Below, left, is a modern version of the BT crossed loop by Alaris Antennas; note the sense antenna on top to resolve the ambiguity of the loops. Alaris is one of a number of manufacturers of commercial DF equipment- link is  http://www.alarisantennas.com.

 Small loop DF is still used today for locating HF transceivers- see photo below of a Russian hand-held small-loop antenna (H/T Joe Moeller’s “Homing In” website.) There are various sizes and types of small loops antennas ranging from small portable versions like the one to the right, up to fixed station DF loops 6 feet in diameter.

The Adcock array, conceived in 1917 by Frank Adcock, a British Army Artillery Lieutenant, and patented in 1919, is still in use today for HF, VHF and UHF direction finding. Adcock’s system can be used with arrays of either vertical monopole /groundplane (U-Adcock, commonly used for MF and HF DF) or vertical dipole (H-Adcock, commonly used for upper HF, VHF and higher frequency) elements.

To the left is a photograph of Alaris Antennas 1 to 30 mHz U-Adcock array; this array has 4 pairs of monopole elements. While the electronics (receiver, etc.) that these arrays feed signals to has changed enormously since 1919, the antenna itself Frank Adcock would recognize immediately.

Here is a picture of what appears to be an H-Adcock array, again from Alaris Antennas; note that there are 2 stacked arrays of vertical dipoles, 4 pairs of vertical dipoles on each array. This Adcock is rated for 80 mhz to 4000 mHz.

Both of the Adcock array types, monopole and dipole, share a number of advantages:

  • The Adcock is extremely broad banded;

  • It is insensitive to wide variations in vertical angle of arriving signals;

  • It is largely unaffected by horizontally polarized signals, unlike loops;

  • It is relatively easy to construct field expedient Adcock antennas with good performance.

As seen above, modern Adcock arrays are usually constructed of multiple pairs of antennas using electronic switching to rapidly connect each pair of antennas and get quick bearings. Arrays can consist of pairs of either monopole or dipole elements fed 180 degrees out of phase with each other.  Common home-brew Adcock arrays consist of one or two pairs of elements.

The point of all this is that DF as a concept is even more useful today, day to day, than it was when these techniques were developed a hundred years ago, and hundred year old techniques are still used today. Looking at an uncertain future, and in the context of a down-grid event or other disruption of normal daily life, DF can be extremely useful, significantly expanding on simple monitoring. Monitoring local communications can tell you a lot about local activity by all sorts of people; DF can tell you where they are, whether and how fast they are moving, and whether they are coming towards your location or moving away. Even if you cannot decipher a particular digital communication, simply knowing that someone in your immediate AO has that capability is worth knowing, and knowing where they are is still more important yet, potentially turning other various bits of information into Intelligence. You do not have to be able to decipher the communication to DF it.

With the above in mind, let’s review some basics on radio propagation.

There are three basic types of radio propagation:

  • Line Of Sight;

  • Groundwave;

  • Skywave.

Line Of Sight (LOS) propagation is mostly what we are dealing with when we are DFing on VHF and higher frequencies, as we are doing when we are trying to track down interference on your local repeater, or when the Civil Air Patrol is looking for a crash beacon from a wrecked aircraft. Remember that LOS does not mean that signals cannot reflect or refract off natural or manmade structures of all sorts. This happens all the time, and can be a significant issue in locating transmitters; it is called “multipath”, a good name because you can receive multiple signal paths from different directions from the same transmitter and transmit antenna. (Multipath is what causes ‘mobile flutter’ or picket-fencing.)

It is also possible, when DFing in the HF spectrum, to get bearings on skywave signals, and on lower HF and MF bands, to DF ground wave as well, but for this post we are going to restrict the discussion to VHF and LOS direction finding; from a practical standpoint, skywave and ground wave for upper HF and VHF direction finding are not very useful because groundwave gets attenuated VERY quickly on VHF and up, and VHF skywave is dependent on atmospheric conditions that are relatively rare. VHF and up are where the local action is; any signal you hear on VHF is likely to be nearby and therefore of interest.  Let’s talk a bit about equipment for VHF and up LOS RDF, especially radios and antennas.

Radio direction finding for a single location, fixed, portable or mobile, requires the following:

  • A receiver for the frequency or frequencies to be listened to;

  • A directional antenna system, based on one or more of the following:

    • signal amplitude variation (Yagi, quads, and Adcock array antennas, for example. )

    • Time distance of arrival (three or more antennas and very precise timekeeping are required. This is how the cell system knows where you are;  each cell call is timestamped very precisely at each tower location, and the difference in arrival time at three or more locations allows the cell system to triangulate your location within a small distance, less than a few hundred yards);

    • Phase variation or interferometry (the Handi-Finder active antenna uses this method);

    • Doppler effect or frequency variation.

  • A skilled operator. (You, right? If not right this moment, soon!) Radio Direction Finding is an art, not a science, and requires training, education, and experience for the operator to be effective.

RDF is different from other kinds of radio operation in a number of ways. One significant difference between DF and other radio operation is the importance of NULLS. In many of the common DF techniques, especially with regard to the manual techniques based on signal amplitude used by many radio amateurs, it is more important to know where the nulls in an antenna are than where the peaks are. Most antenna types in common use for amateur RDF have nulls, and relatively broad peaks, even large multi-element beam antennas.

Another difference is the importance of knowing where a transmitter is not. For radio communication, not being able to hear someone is an undesirable outcome; you want to talk to other people, and most of the time when you cannot hear another station this is a bad thing. In RDF, however, this knowledge is useful information; if someone is jamming a repeater, and you cannot hear them on the input frequency at your location, then you have some idea where the jammer is NOT located, depending on the range of your antenna/receiver combination. When combined with knowledge about the repeater’s planned coverage area, this helps you narrow down the jammer’s location and figure out where to have your DF teams look. Knowing where a jammer isn’t helps when you have limited resources.

( A quick aside: Within the context of dealing with deliberate interference on amateur frequencies under present conditions, if you are motivated to try and help, reach out to the local interference committee. If you don’t know who is on it, or whether there is one, ask around and find out whether the local ham clubs have an interference committee. If the interference is on one or more local repeaters, ask whether the repeater owner (often an amateur club) has organized such a thing. You can also ask the ARRL who the local OO (Official Observer) is- if you do not know, your local ARRL section manager should know, and can pass along your observations.

Keep in mind that the people dealing with deliberate interference are volunteers who are taking on a thankless task, and it is not uncommon for the source of the interference to be an amateur operator known to the local community, sometimes someone known to and friends with members of the local amateur community. This means that interference reports are usually one-way; the person coordinating the hunt for the jammer(s) is not going to advertise his progress or lack thereof, regardless of how trustworthy you think you are, or even how trustworthy HE thinks you are. This kind of thing is best run on a ‘need to know’ basis, and most experienced interference committees are pretty tight-lipped, so don’t be offended if it takes a while to prove your bonafides. And by a while I mean years, not weeks or months. Which points up the need to start working on your DF technique, and building relationships NOW.)

Information you want to get:

Whenever logging interference, whether or not you intend making a report to someone else, include the following information in your log:

  • Date and time of interference;

  • Your location at that time, as exactly as you can provide it;

  • Your equipment (especially antenna type- which might be factory HT to mobile 5/8 on car to fixed site rotary beam)

  • What you heard or did not hear. (example- I heard interference on the local repeater, listened on the input and did/did not hear them on my ________ antenna/radio receiver)

  • Any identifying particulars, i.e. type of interference, whether or not a call sign was used, beeps or tones transmitted, etc. A pocket recorder is very useful.

  • Direction or line of position; this can vary from an approximate location from a momentary peak on a handheld Yagi to very precise bearings from nulls taken over a minute or so.

Even if you do not know to whom to pass these observations along, write down your observations, or log them in your station log and save them for reference. That information may be helpful in future. So, how do we get this information without spending tens of thousands of dollars?

Radios for DF:

The radio must receive on the frequency to be used and needs to have the following characteristics:

  • acceptably portable. Some folks I know would not sneeze at carrying a 15 pound R7000 and a 12AH battery to run it, but I’m not going to do that unless I have no other choice, and there are plenty of small handy-talkies out there with the needed features.

  • Removable antenna or jack for outboard antenna is a MUST; most scanners and HTs have this feature.

  • Signal strength indicator of some sort is desirable. It is possible to retrofit an old Icom 2AT or similar HT with an outboard signal strength meter, but many modern HT radios have at least some sort of signal strength indicator as do many older HT radios; my IC32AT, TH-F6a and VX-7r all do.

  • An actual handheld receiver like the Alinco or the Icom may have an internal attenuator, a distinct plus. (When you get close to the fox, and the signal is overwhelming, tuning off frequency or tuning the third harmonic provides some attenuation, and so does rotating the antenna 90 degrees to get the 20+ db cross polarization loss.) If your radio does not have this feature, you can shop your hamfest flea market for a commercial attenuator, or build one yourself.

  • Having an RF gain control is another plus, as you can reduce the gain and keep the radio from swamping.

A good name brand HT will probably serve the purpose, as will an IC-R6 or an Alinco DJ11E/T, or an older decent portable scanner. Here are some of the radios I use for DF reception:

Left to right:

IC32AT full duplex dual band HT,

Radio Shack Pro-83 scanner,

Kenwood TH-F6A triband HT and receiver,

Yaesu VX-7R full duplex tribander HT

You will also need power cables if using an outboard battery, spare batteries or battery packs, and cables with proper connectors for connecting your radio, attenuator, and antenna together.

Directional antennas- (preferred in Bold)Possible types include but are not limited to:

    • small Yagi-Uda (multi band 144/440 satellite antennas are useful for third harmonic location or locating on 440)

    • small Quad

    • Moxon

    • Adcock

    • TDOA (Time/Distance Of Arrival)/interferometer active antenna (Handi-Finder is one of many types)

    • pseudo-Doppler antenna (generally for vehicle or fixed location use, can be carried)

    • Log-periodics

For purposes of this discussion, we’ll limit this to talking about antennas commonly deployed for portable use, carried by hand. There are two main types I’d suggest you start with, the Adcock and the  Yagi-Uda.

Some RDF hunters, especially those on foot, use the two-element Adcock array; this picture is of my first VHF Adcock. I built it in a couple of hours. The elements are made of 12 gage Copperweld wire, 19” long (resonant on 2 meters) and the tubing and fittings are ½” PVC. The transmission line is high quality RG58. The trickiest part of making this antenna is getting the two sides of the feedline EXACTLY the same length. The next one I build I’ll probably use 300 ohm window line for the cross connecting transmission line.

The technical advantages of the portable Adcock array are:

  • When properly designed and built it has reasonable gain (around 6 dbi give or take, depending on how high you hold it,) with a very broad pattern, so you are >90% likely to hear any signal detectable at your location upon first presentation of the antenna.

  • It has a very sharp and deep null (nulls are around 4 degrees wide for -20db, 88 to 92 degrees from the plane of the elements, and theoretically the null is infinite at exactly 90 degrees) which allows a very precise line.

  • Just like the commercial Adcocks it is usable over a very wide range of frequency, unlike a Yagi-Uda.

A VHF Adcock array with 38” dipoles 14” apart like the one pictured above is usable for 6 meters, 2 meters, 220 and 440. The VHF Adcock is light and handy, and cheap and easy to build. The main disadvantage of the Adcock is that the direction of the signal is ambiguous; it can come from either of two directions 180 degrees apart. It also does not have the gain that a Yagi-Uda or quad can have, but that is not altogether a bad thing. A signal that can be heard on a 5/8 mobile magmount antenna can be easily heard on an Adcock; the Adcock will pull in signals that the mobile 5/8 will not. The ambiguity can be resolved with a “sense” antenna or by phased feedline, but that adds complexity, cost, weight and bulk, and loses the precision of the null.

Most RDF hunters opt for small directional antennas, often 3 or 4 element Yagi-Uda antennas like this one, which is a commercial 4 element 2 meter antenna I picked up at a local hamfest. Note a few things:

  • feed is symmetrical;

  • Balun is also;

  • The feedline is very short.

The tape measure Yagi is very popular for those starting out, and it is very rugged, but I have found that a strong breeze can make the elements flap around, and it is hard to get a good symmetrical null pattern with the tape measure Yagi-Uda.

Other experienced hunters favor quads, which are a bit more compact than a Yagi. The advantage of any of these directional antennas is that it provides an unambiguous direction, and a vertically oriented 4 element Yagi has a more gain than the Adcock, as one would expect. But that is not always a plus; sometimes you can have too much gain! The disadvantage of both Yagi and quad is that the front lobe is fairly broad, and it can be difficult to get a good bearing just using the positive signal lobe.

Both Yagi antennas and quads do have a null, which is generally deepest broadside to the boom, but the depth, location and width of that null can vary depending on element location, and is not as deep or sharp as an Adcock array; best to know ahead of time. EZNEC models are useful to start, but your antenna is almost certainly not going to fit the model precisely, and in DF, precision matters, so best to do some testing and determine performance ahead of need. As mentioned above, symmetrical feed systems result in symmetrical side and back lobe patterns; the gamma match does not.

Also, if you are trying to get a bearing and the signal is coming in off the back of the antenna, (-20 to -30 db) it is possible to miss hearing the signal, which can cause confusion. Lastly, the Yagi-Uda or the Quad are both good for a relatively narrow frequency range. In the context of a foxhunting contest on a predetermined frequency band, (2 meter amateur band is very common,) or when searching for a jammer on a known frequency, that is not a bad thing, but in the context of DFing signals not known ahead of time, as in a down-grid situation, which may be anywhere across a wide spectrum, this can be a very serious disadvantage.

Other antennas:
The Moxon has a good null on a given frequency, is cheap and easy to build, although not as cheap as the Adcock, and has decent directionality although not as good as a quad or Yagi-Uda. It is physically compact like the quad, and relatively rugged, but suffers from being a single band antenna like the Yagi-Uda. I’ve worked with and tested them but don’t currently have one.

The Handi-Finder, seen below, is good for close range location; it has much less gain than the Adcock, but for short range work that doesn’t matter. The location method means that the system is not as subject to swamping as the other systems are. It is also useful across a wide range of frequencies.

HOWEVER, my experience with the Handi-Finder is that it is more subject to common mode than other types, requiring significant attention to attenuating common mode on the feedline to get solid unambiguous results. Note the ferrite cores on the feedline next to the BNC connector to attenuate common mode; there are also three ferrites up inside the handle near where the coax attaches to the circuit board. I’ve got one and am happy I do, but it is not my first choice for general DF work. Also note that the circuit board is exposed to the weather. I have another kit which I intend to build in a weather proof enclosure with removable antenna elements, but right now there are other priorities.

A log-periodic is useful for fixed station DF on a wide range of frequencies; the wide frequency range possible and unambiguous direction make it easy to quickly get approximate lines on new transmitters. As an antenna for walking DF it leaves something to be desired- large, heavy, relatively poor nulls, and not much better gain than the Adcock (around 7 db, depending on design- the more elements and the heavier the antenna, the better the gain.)

Suggested equipment:

When I am planning on doing RDF, I bring both my 4 element Yagi and my Adcock. The Yagi is effective at pulling in and providing direction on distant sources, and the Adcock is useful for precise locations as I close in on the transmitting antenna. Most of the time as I commute to work, I keep an Adcock attached to a dual band (144/440) HT or my Radio Shack scanner in the car; it is relatively small and compact and it will let me DF anything I hear on the 5/8 wave mobile antenna/mobile rig combo installed on the vehicle. If I can hear the signal I want to DF from the car, the signal is within a few miles of my location, most of the time, and I can get a good bearing within 10 seconds at most.

Other things you may need:

  • spare battery for your receiver (Murphy’s Law says that your battery will go flat just as you get close to the fox);

  • A “sniffer” or signal strength meter;

  • Notebook and pencils;

  • A map of the area in which you are operating;

  • Headphones or earphone that fits the receiver you will be using, especially in noisy environments. Nulls are a LOT easier to hear with headphones;

  • A compass or GPS;

  • Outboard signal attenuator;

  • cable(s) to connect to radio and antenna;

  • A clipboard;

  • A small voice recorder for making comments, recording noises or identifiers from the Fox;

  • A bag to put all your stuff in!

Now that we have our equipment squared away, let’s start learning how that equipment gets used.

Starting with basic amplitude methods for VHF and up, the most basic discriminator is whether you can hear a signal or not. If you are attempting to locate malicious interference on a repeater, it is helpful to have a radio that can receive on 2 frequencies at once, ( a lot of modern HT radios can do this, as can most compact handheld receivers) or to have 2 receivers, so that you can monitor both the repeater output, AND the repeater input. (Here is a great use for that cheap Baofeng you first bought when you got your Tech license! Not knocking them, I have one myself.)

There is usually a slight delay for the repeater to receive and re-transmit the incoming signals, and often, if you can hear someone on the repeater input, the path from them to you is shorter than the path from them to the repeater and back to you. Not always true, but often. (Note: repeaters are usually located on prominent elevated locations with good LOS to the surrounding area, and most folks don’t have as good reception as a repeater. That is why we have repeaters!) Any how, if you can hear a transmitted signal on the input, you will usually hear them just slightly ahead of the repeater signal. Sounds sort of like a very quick echo. You ought to know about how far away your radio and antenna system can hear another station, (you HAVE done reception tests with your VHF/UHF radio, right? Right?) so this will give you a radius within which the transmitter you want to find is, or is NOT located. If you hear the repeater signal at the same time as the input, then you know the jammer is slightly FARTHER away than the repeater.

Anyhow, if you can hear the signal on your mobile vertical, then you can hear them on the Adcock, or on a 4 element Yagi.  If you are on or close by an elevated location, then you may be able to pick up a signal that your mobile rig cannot, by using a gain antenna like an Adcock or a Yagi-Uda.  If you can hear them, you can DF them if they keep transmitting.

Here is one scenario of how this might work-

___________________________________________________

Let’s suppose that you are driving along on an errand, with no particular time crunch, and let’s say that you hear interference on the local repeater you monitor regularly. You hear muffled noises and voices, but cannot identify the speaker. Then you hear a clunk, no more noise, but still hear a carrier on the repeater. Now, there are 2 possibilities at this point. Either you can hear the signal on the input, or not. If you know that the interfering signal is making the repeater, because you can hear it being repeated, but you cannot hear it on the input, THAT IS USEFUL information.

Continuing with our example, let’s imagine that you have located the rough area of the interference because you can hear the transmit signal on the repeater on your mobile station. Let’s also imagine that this particular interference is the result of stupidity or cluelessness, not malice- in this instance we’ll assume that some hapless local amateur has has left his mobile radio on, and has his microphone jammed in his car seat cushion and his radio is busily draining his car battery while keeping the local repeater timed out and thoroughly jamming the input. (for those out there who have done this, you are not alone!) So, you know that he is in the area, but where, exactly, is he? You think, “Man, now’s my chance! I’m gonna find this guy!”

Before you do anything else, pull off the road, and note the signal strength on your mobile radio’s S-meter. You then take your handi-talky out of your car and hook it up to your DF antenna. Find the bearing when the signal is strongest, and take another reading on the HT S-meter and compare it the S-meter reading you got on your mobile radio, using the same antenna polarization for the DF antenna as the mobile antenna. Most mobile antennas are vertically polarized, so your DF antenna probably should be vertically oriented, too. Now, rotate the DF antenna so that the DF antenna is horizontally polarized, and take another bearing and S-meter reading. If this reading is stronger, the interfering signal is coming from a horizontally polarized antenna; if it is lower, then it comes from a vertical antenna.

To confirm your bearing, steadily rotate the DF antenna 180 degrees. The signal will drop slowly, then you will hear a sudden null in the signal. For a simple 4 element yagi or quad, the null will be deepest approximately broadside to the antenna, and anywhere from 30 to 40+ db deep, or about 5 to 7 S units deep, and fairly wide; if the signal is weak, you may have to swing the antenna almost in a full circle to hear it again. (It is an excellent idea to test the exact location of your particular antenna’s null in advance of use and to know exactly how deep it is. Another amateur friend with an HT that can transmit on very low power is very helpful here.) If your DF antenna is an Adcock, the null will be much sharper and deeper (the -20 db point is only 4 degrees wide!) so steady movement is essential. I have found it best to hold the antenna steady and rotate my body. Finding the null may help improve the quality of your bearing. It may not, too; multipath bouncing from nearby structures or even wet shrubs may deceive you. In any case, write all of this information down! If you have a map of the area, mark your position, and mark the bearing of the signal you heard.

When taking bearings, BTW, what I do is identify a near-by landmark, like a clearly identifiable tree, shrub or other feature in line with the strongest signal, then take a compass bearing after I move my equipment off to the side so I am not affecting my compass. You can also get a line if you have GPS; walk toward the landmark you have picked out to get your line. (Remember that a compass reads magnetic north, while your GPS usually reads true north, and except for a line running through the central US, they are NOT the same. The difference is called ‘declination’.)

Now, you have one line to the transmitter. Move at about a 90 degree angle to the line you have, and take another signal strength reading and bearing. If the signal was extremely loud, you are probably close, and moving a short distance may give you a good cross bearing; if the signal was weaker, you may have to move farther to get a good cross bearing. Remember, signals can bounce and reflect off all sorts of things, from wet foliage to metal buildings. Getting a second bearing will resolve the ambiguity of the Adcock array, and is required in any event to get some idea of where the transmitter is located. Plot this bearing just as you did the first, along with your location, and move approximately half way to the target location. If driving, obviously this will be easier if you stay on the road network, so some compromises may be in order with respect to location.

You may also find that local topography is limiting what you can hear. Remember, this is Line Of Sight and even though you can hear the signal at one location, terrain, vegetation, buildings, or other obstructions like fences or signs may block the signal when you move to a new location to try to triangulate. Generally, local high points are a good place to get bearings; I have maps of the local area that I use when tracking jammers, and I have some favorite listening points marked in my GPS.

In our example, the signal is continuous. Most deliberate interference is much shorter duration; jammers know that there are consequences to getting caught, so they keep transmissions short, which makes it tougher on the hunter, but there are exceptions, and there are other advanced ways to track an interfering signal…..

Repeat this procedure until you are so close that the signal is so loud that you have trouble getting a null. That is where the attenuator comes in handy; remember that 6 db is about one standard S unit, so if the received signal is S9 plus all the time, try about 40 DB of attenuation; if you still have trouble finding a null, add more attenuation; if your null is now too broad, add less. Other tricks for VHF FM signals are to tune off frequency by 5 or 10 kHz, or to rotate your DF antenna to minimize the signal; if the fox is vertically polarized, then hold your antenna horizontally. You will probably find that over about 60 or 70 db of attenuation, you’ll need a common mode choke on all the feedlines as well as any outboard power cable to get any benefit from the attenuator, as the common mode will be so strong that further attenuation does no good. At VHF, this is easy; a few turns of coax to make a solenoid does the trick, and so does a string of ferrite beads, or a common mode choke using a 43 material toroid and three turns of RG-58. This is especially helpful when dealing with close strong signals.

At this point, if you are using a directional antenna like a Yagi or a quad, you may want to use a technique called “sweep and advance.”  Over about a 4 to 6 second period, swing your antenna from side to side as you move in the apparent direction of the transmitter, going from null on one side to null on the other, 180 degrees or less.  It is not uncommon for the null directions to change, especially if you are in an area with metal buildings or metal roofs. “Sweep and advance” is usually a technique used on foot, although if you have a directional antenna mounted on your vehicle it works there too. (This calls for two people at least in the car, one driving, and one DFing!)

As you close in to the final location, you may find that no amount of attenuation helps. Your receiver is deafened, swamped by the RF coming out of the transmitter. What to do? You can start looking for the antenna; if you are hunting a jammer, look for a tower or other external antenna. In our example, if you are close to a vehicle with a vertical VHF antenna mounted on it, you have probably found the source, and the owner, who is responsible for the unintentional repeater interference, is probably not going to get ugly when you politely ring his doorbell and tell him that his mobile rig is still on.

But you can’t always count on that. Even today, some folks get upset at being DFed and exposed as jammers, so listen to your gut and don’t get any closer than you are comfortable with, and definitely don’t trespass on private property. A fox-box being used in a training foxhunt isn’t going to care about how close you are, but a human jammer knows he is breaking the rules and confrontations with this sort of maggot can get ugly.

Anyhow, you are close, but the signal is so loud you can’t get a precise location of the antenna. For some situations, this is close enough; knowing the location of a transmitter within a few hundred yards in a down-grid situation may be plenty good enough. If you need to get an exact location, a “sniffer,” a simple rectifier using UHF capable diodes and a microammeter, is very useful. MFJ sells one for tracking power line and other VHF noise, but it works fine for close range ‘sniffing’. With the sniffer’s simple dipole antenna, you can use the same techniques that got you close; get a null direction, move off to one side, and get another. Triangulation will get you there.

If you cannot or will not get close, or if you don’t have a sniffer and get completely swamped, what to do? Here is where knowing your antenna’s pattern is very useful. Stay far enough away from the source to be able to get a good null, and take several readings from different directions. This triangulation will get you within a few feet of the antenna. After that, it’s up to the human eyeball Mark 1, Mod 0, to find the antenna.

Herewith, The End of your introduction to Radio Direction Finding; hopefully the beginning of your forging yet another tool in your toolbox.

Afterword-

This post and the books and other information included at the end of this post contain a lot of useful information, but there is NO substitute for actually getting out and trying this stuff for real. Find a local amateur club that is holding foxhunts, and try this out, or try DFing a local repeater or FM radio station if you cannot. An afternoon of hide-and-seek with a couple of ham buddies can be fun too; each person can take a turn at being the fox while the rest of the crew DF him. Have the fox make a 15 to 20 second transmission every minute or so; some gentle teasing and taunting adds to the fun.

If you and your group get seriously interested, make up a couple of Fox-Boxes and start holding your own foxhunts. I use the Byonics Pic-Con with some hamfest rescued Icom HTs in a 50 caliber ammo can as my fox-boxes; they work well and the Pic-Con is relatively easy to setup and use. The remote off and on is a big plus.

Bibliography:

Transmitter Hunting: Radio Direction Finding Simplified Moell 1987

Radio Direction Finding 4th ed Keene 1947

Useful websites:

http://www.homingin.com/ (Joe Moell’s website- EXTREMELY useful)

http://aprsisce.wikidot.com/direction-finding (APRS DF)

http://static1.1.sqspcdn.com/static/f/679473/21303404/1355863220773/seeber-DirectionFinding.pdf?token=5sQ3njSAxNvy0zJ8r0FwNQEpffQ%3D

https://en.wikipedia.org/wiki/Amateur_radio_direction_finding

DF antennas

Moxon 2 meter antennas:

http://www.moxonantennaproject.com/

http://www.qsl.net/dk7zb/Moxon/2-m-Moxon.htm

http://qrznow.com/building-2-meter-vertical-moxon/

http://binnsgenealogy.com/Super%20Duper%20Moxon.pdf

2 meter 4 element Yagis:

http://www.qsl.net/dk7zb/PVC-Yagis/4-Ele-2m.htm (shows EZNEC outputs- note the deep null at 90 degrees!)

http://www.dcarc.club/2016%20N8PR%202%20Meter%204El%20Yagi.pdf (step by step instructions for a simple and inexpensive 2 meter yagi)

http://www.amateurradio.bz/2m_yagi.html

A commercial 4 element yagi- http://www.cushcraftamateur.com/Product.php?productid=A124WB

2 meter quads:

http://www.qsl.net/n1lo/4elquad.pdf

http://www.hamuniverse.com/3el2meterquad.html

Adcock array:

http://www.iw5edi.com/ham-radio/?the-adcock-hf-rdf-antenna,92

Foxbox hardware:

http://www.homingin.com/boxes.html

http://www.wb8wfk.com/equipment.html#tx

https://www.byonics.com/products

Liked it? Take a second to support us on Patreon!
Share this: