The Utah Amateur Radio Club:
Radio Direction-Finding
("RDF")

Radio Direction Finding (RDF) of signals is nearly as old as the art of radio itself.  In the early days, not only was being able to communicate over long distances found to be invaluable, but so was being able to locate the source of such a signal, especially in the event of an emergency.  Although the technology has changed over the many decades, the essential elements of the techniques have not.  Despite advances in technology, it is still largely up to the skill of the RDF equipment operator to make sense out of the information gleaned from their gear.

RDF for fun:

Amateur ("Ham") Radio has had a long history of RDF as well:  It is not surprising that radio enthusiasts would make a sport out of their hobby as well.  This sport most often involves so-called "transmitter hunting" or "fox hunting" in which amateurs compete against each other (individually or in groups) to be the first to locate a hidden transmitter.
 
A DFing excursion:

Throughout much of the month of March, 2002, an odd thing was happening on UARC's 146.62 repeater:  Approximately twice each minute the repeater would key up, having been "kerchunked" by an unknown signal. 

Attempts to hear a signal on the input frequency of the repeater from various locations failed.  What was determined (by placing a very weak, unmodulated carrier on the input frequency and listening to the heterodyne on the output) was that the frequency of the input signal was sweeping from low to high.  This went on for several weeks - seeming to appear and disappear in a manner that we thought might be related to temperature. 

One day the "sweeper" was heard with faint - but distinct - modulation on it corresponding to Bell 103 tones, a clear indication that we were dealing with a telemetry system.  Because of the low "sample" rate (seeming to transmit only every 25-35 seconds or so) we also surmised that it was probably related to a water or sewer system. 

On UARC's Sunday night net the request was made for everyone to listen for the rogue signal on the input frequency of the repeater at their location and one ham reported that yes, he did hear a signal that seemed to correlate with the kerchunking.  We were somewhat skeptical, however, as no-one else in that same general area reported hearing anything - but at least it it was a possible lead. 

A few nights later we (Gordon, K7HFV and Clint, KA7OEI) headed north (well after sunset) toward the QTH of the ham that reported hearing something - listening all-the-while to the input frequency on an all-mode rig (an FT-817.)  We heard nothing until we topped a hill on I-15 near the Roy, Utah exit at which point we heard a fairly weak signal "swish" across the frequency.  Driving past the exit we determined that it was strongest there (although not too strong) and decided to head west into the town of Roy. 

Heading west it seemed to be getting stronger - and finally it was strong enough on an FM receiver that we pulled into a parking lot and fired up the (modified) Dick Smith direction finder.  We quickly got a bearing - to the north - and drove to follow it.  At some point, it was east of us, so we headed in that direction and for a brief instant, it was fairly strong - but then it was weak again.  We decided to backtrack and try an approach from a different direction going north.  It wasn't very strong when the bearing suddenly veered to the east - then the south - indicating that we had passed it. 

"Hmmm..." we thought as we headed east, and before too long, it veered to the south and to the west - never having gotten very strong.  We headed south again and it veered to the north.  At this point, we had more-or-less circumnavigated it - all in a fairly small area - and it had never gotten particularly strong.  This seemed rather confusing, because we were, at this point, about 30 air miles away from the repeater and we were assuming that something that could get into the repeater from that distance ought to be pretty strong - and our signal wasn't. 

We decided to head back to where we'd heard the signal peak up earlier and make some more careful observations.  At this point, the signal appeared to be coming from north of us.  We pulled into a nearby park and decided to study the signal a bit more - and noticed that it was now east.  We also noticed that we were within a couple of blocks of the QTH of the amateur that reported hearing the signal. 

In the dark (after our eyes adjusted) we noticed the outline of two large water tanks - just the sort of thing that one might telemeter this slowly - so we walked toward them (in the dark) with the all-mode radio in hand and sure enough, the signal started to peak up.  When we got to the perimeter fence around the tanks we could hear a distinct "click" on the receiver as the transmitter keyed up, and another click is it unkeyed. 

The next day, a co-worker (John, K7JL) happened to have some business in the general area so we had him take a service monitor (with a built-in spectrum analyzer) and visit the site.  Upon arrival he quickly spotted a small yagi on one of the water tanks and set up his monitoring equipment.  He determined that there were, in fact, three frequencies appearing when the transmitter keyed up.  A quick search of these frequencies on the FCC's online database nailed down the licensee and a contact person.  It was interesting to note that from the location at which we had parked, one of the spurious signals was far stronger than the intended signal. 

Epilogue:

After contacting various people, the radio was pulled and checked out by the water company's radio contractor.  We spoke with the bench technician (who just happened to be a ham) and he'd gone through the radio (a 35 watt radio that was turned down to just 3 watts) and found it to be clean.  The radio was re-installed, and the problem has not reappeared...  yet... 

What happened, then?  Several possibilities come to mind: 

  • There was something wrong with the radio - possibly being slightly out of tune.  Just bouncing it around during transport has "detuned" it so that the spurious signals are either gone, or just not bothering the repeater anymore.
  • There was something wrong with an antenna or power supply connection that was fixed when the radio was reinstalled.  If the antenna's connector was loose, either a mismatch or a lot of RF floating around could have made the transmitter and/or its power supply misbehave.  A loose power supply connection may also have caused the radio to behave erratically under load.
  • Sometimes higher-powered radios start producing spurious signals when their output is "turned down" to a fraction of their designed power level.  It is possible that that may have been the case here.
Whatever the cause, the result (to date) is that it isn't bothering us anymore... 

An abbreviated version of this story, albeit with a few more technical details, may be found on the UARC Repeater Status page.

"Serious" RDF:

There is an aspect of RDF that goes along with the notion of "Amateur Radio Self-Policing."  Amateur radio operators have largely looked out for each other:  If an amateur radio station unknowingly broke the law due to operating practices or due to equipment problems, he/she would often be informed of such an infraction by another amateur radio operator and the problem would be solved quietly.

A matter more serious is the fact that some stations (amateurs and non-amateurs) also break the law, either inadvertently or maliciously:  Occasionally a transmitter (amateur or commercial) will malfunction or be inadvertently keyed up (e.g. sitting on a microphone, etc.) causing interference to amateur (or other) services.  There is also the (hopefully) rare individual that insists on operating in an unlawful manner.

It is at these times that RDF skills are invaluable:  Being able to quickly locate and identify the offending transmitter (and/or operator) is important, especially if such disruptions have the potential of impacting communications that may be critical to life/safety.

RDF for fun and enforcement...

RDF can be great fun.  A lot of amateurs (perhaps even you) get a kick out of using their wits and skills to be the first to locate a hidden transmitter (often called "fox hunts" or "bunny hunts" - depending on your area...)  These activities sharpen skills that may be called on to locate the source of illicit operations.

How do I get practice?

In the Utah area, DFing hasn't caught on to the extent that it has in some other parts of the country:  There are occasional transmitter hunts sponsored by Salt Lake County ARES, or one of the ham clubs (e.g. the Davis County Amateur Radio Club has occasional DFing activities.)  If your club doesn't do as much DF activity as you'd like to see then, by all means, be the one to "take the bull by the horns" and create some activity (in a legal manner, of course!)

One of the people that has been heavily involved with DF activities is Mike Mladejovsky, WA7ARK.  With respect to DFing, Mike's specialty has been locating downed aircraft for the Civil Air Patrol and he has been directly responsible for the Utah team winning the national CAP DF competition on at least one occasion, using equipment of his own design. Needless to say, Mike has applied some of his talent to amateur-related DFing as well.  Follow this link for information on how to build some of Mike's circuits.

What equipment do I need?

It might be instructive to help answer this question by giving a brief breakdown of three of the most common classes of DF devices:

Whatever type of system you have - whether its a humble passive system or a fancy "Doppler" type, it will do you little good unless you practice with it and you learn how to use it.  It takes experience and practice to get a handle on the vaguaries of signal propagation - especially when multipath appears in the picture.  One common misconception is that a "fancier" system can (in part) substitute for skill and experience - but anyone who has experience will tell you otherwise!

How I "do" DFing?

Rather than duplicating effort, we'll defer to links by others who have already put together some fine pages related to DFing. Many of these pages have links to other pages as well:


A few handy links to DF equipment you can build and/or buy:

"Doppler" types of direction-finding systems: Miscellaneous:
If you have immediate interest in this, you may send email - follow this link


Important Notes:


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This page updated on 20061017
 

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