VHF/UHF enthusiasts often install yagis and quads at their home stations. Many take them out on camping trips and use them on public service events. But did you know that some enjoy flying the freeways and beating the back roads with one hand on the steering wheel and the other on a rotating antenna mast?
Perhaps you have seen these hams on weekends, intently driving and turning their beams. What are they doing? They are competing in hidden transmitter hunts.
If you've never experienced one of these mobile radio direction finding (RDF) contests, you have missed some of the greatest excitement a ham can have. While there are several names for it such as "fox-hunting" and "bunny chasing," in southern California this sport is almost always referred to as "T-hunting."
Transmitter hunting seems to be one of the best kept secrets in ham radio, even though dozens of hams here consider themselves to be regular hunters. They range in age from the teens to the eighties. Besides keeping the coordinated two-meter hunt frequency (146.565 MHz FM) hopping, some hunters love to hash over their exploits by the hour on their favorite repeaters.
The idea is simple: One or two hams take a transmitter, antenna, and some sort of distinctive audio source to an carefully selected spot, then make continuous or intermittent transmissions. Usually they remain stationary, though mobile "bunnies" are popular with some groups. Sometimes there are more than one "T" to be found. Surplus ammunition cans are often used as hidden transmitter enclosures. The hunters, as individuals or in teams, do their best to home in on the hidden station(s) with their mobile and portable RDF gear.
"Techies" like the thrill of finding the hidden T with gear they made themselves. They relentlessly work to improve their setups. Mystery lovers and dyed-in-the-wool contesters love the challenge, because very hunt is a fresh start to a new adventure. Your past performances are forgotten. It's just your team and your equipment against today's hider and the other hunters.
At some point, every ham will find knowledge of RDF techniques useful, because it simplifies such chores as finding a neighborhood source of power line interference or TV cable leakage. T-hunters here frequently are called upon to track down sources of "spurs," intermodulation and noise that can plague amateur (and sometimes commercial) repeaters.
RDF plays an important part in Amateur Radio self-policing. In many areas of the country, including southern California, hams are in contact with ARRL and district FCC offices, gathering evidence leading to prosecution in serious cases of malicious interference.
There are several competitive hunt opportunities to choose from every month in Los Angeles, Orange, Riverside and San Diego counties. They are all different in some way, such as time or mileage scoring, day or night start, single or multiple transmitters, intermittent or continuous signal, wide or narrow boundaries. (Or perhaps there are no boundaries at all!)
Most hunts are on two meters with FM signals, but there have been FM hunts on the 50, 223, 440 and 1200 MHz bands. There have even been hunts for Amateur Television transmissions on 434 MHz.
T-hunters have become very sophisticated at finding dastardly hiding places. With the right combination of location and antenna, they make it difficult for hunters to get reliable bearings. Like a ventriloquist, a good hider can make the signal appear to be coming from some other location. With careful planning (and a little luck), the signal's characteristics can cause the hunters to approach the transmitter from the most difficult direction, with impassable roads or other obstructions, even though the T may be easily accessible via other routes. Perhaps the hider will camouflage the setup so well that the hunters won't find the transmitter unless they literally trip over it.
The most challenging of all southern California 2-meter RDF events are the All Day Hunts. Despite their difficulty, many enthusiasts like them best of all. The name is a misnomer, because these marathons often last the entire weekend. The transmitter(s) can be anywhere in the continental USA. The hunt starts in Rancho Palos Verdes. Hiding spots have included locations near Yosemite National Park (California), Las Vegas (Nevada), Yuma (Arizona), and St. George (Utah). The record path distance for a two-meter hidden transmitter signal to be heard at the starting point was set on the St. George hunt, well over 300 miles!
Not every T-hunt is this arduous, of course. Several clubs have sponsored hunts just especially designed for beginners to get things started. In some cases, the hiders make brief transmissions on a repeater, encouraging hunters to come out and find them. After a while, they give clues to narrow the search area. The idea is to give every participant a good first-time experience, including a story-telling session at a restaurant after the hunt.
While some hunters prefer to go it alone, most have more success by teaming up. The driver concentrates on handling the vehicle, while the DFer turns the beam and reads the meters. The DFer also handles maps and plotting, unless there is a third team member for that task.
Yagis are second to quads in popularity. Commercial models work fine, provided that the mast is attached at a good balance point. Occasionally you will see some other kind of gain antenna, such as a "ZL special." Small-diameter loops are rarely used for RDF above 54 MHz because of their bidirectional pattern and low sensitivity.
No matter which gain antenna is used, it is important that to
allow for quickly changing polarization. Hiders can use any wave polarization on
most hunts, so hunters must attempt to determine the correct polarization and
hunt with it. Hunting a horizontal signal with a vertically polarized beam,
for example, causes the direct signal to be attenuated. Reflections and
scattered signals (multipath) from buildings and terrain features are enhanced
relative to the direct signal when the wrong polarization is chosen. There are mechanical and electronic ways to select polarization on VHF beams.
Hunters need sensitive mobile RDF setups for events like the All-Day hunts. They achieve it with their long beams, plus GaAsFET preamps, noise-quieting
meters, and SSB receivers (even when the hider is transmitting FM).
There are two types of dual antenna sets. One type is called a
switched-pattern set and requires a receiver with AM detection. It is used
mostly on the aircraft band. More popular with hams is the phase-front
detector or Time-Difference-of-Arrival (TDOA) set. It is designed to work with
any narrowband FM receiver that covers the frequency of interest. While they
could be used in vehicles, these dual-antenna sets are used mostly for on-foot
RDF, such as closing in at the end of a hunt ("sniffing") or
for wilderness search/rescue work. Be sure to build or buy one with left-right
indicators, or you won't know if the signal is coming from ahead of you or behind you.
Doppler readouts typically feature a ring of at least 16 LEDs, and may also
include a three-digit display in degrees relative to the vehicle. In the
clear, a well-installed doppler has about +/-5 degree bearing accuracy. This accuracy is degraded by multipath, just like it is with the homing RDF, but
"eyeball averaging" while the vehicle is moving helps counteract this
problem.
Although popular in many places, doppler RDF installations have
not caught on among most southern California competitive T-hunters due to their
lower sensitivity compared to beam setups. Vertically polarized
doppler antennas are at an extreme disadvantage if the hider transmits
horizontal polarization, especially if the hider's signal is weak and non-direct, as it often is here.
On the other hand, dopplers are a popular choice of jammer hunters, who are
usually tracking strong vertically polarized signals. They like the rapid
indication update rate and the ability to quickly get bearings on
short-duration signals. Occasionally, you may see RDFers using both a beam
and a doppler set on the same vehicle.
For equipment information, installation ideas, and hunting techniques, read
TRANSMITTER HUNTING---Radio Direction Finding Simplified by KØOV and WB6UZZ,
published by Tab Books (#2701). This book is available at many electronics and
ham radio stores. It is also available by mail from ARRL Bookstore and from the authors.
Have a new ham radio adventure by going out on a hidden transmitter hunt. Be
prepared for some pleasant surprises. Remember, every time you set out on a
hunt, you never know where you'll end up, and you never know what you will
find.
Above article Copyright ©1992 and 2009 by Joseph D. Moell. This article (and only this article on the site) may be reproduced in club newsletters and other non-commercial publications, provided that it is printed in its entirety (either with or without illustrations) and readers are encouraged to visit this Homing In Web site: http://www.homingin.com
If you reprint this article, please send a copy of it via e-mail or by postal mail to: Joe Moell KØOV, PO Box 2508, Fullerton, CA 92837
Surfing suggestions: Read "T-Hunting Then and Now -- From Gooney Birds to GPS" for stories of some classic southern California transmitter hunts from years past.
Back to the Homing In home page
This page updated 22 March 2024
Homing Sets Sniff Well
Another type of RDF instrument, called the homing or dual-antenna RDF, has its
place in the arsenal of the well-equipped hunter. These units have a pair of
vertical antennas, a switching circuit, and a direction sensor with some
sort of left-right indicator, such as a meter or a pair of LEDs. They are easy
to use: When the indicator says LEFT, turn the unit left; when it indicates
RIGHT, turn right. There is a sharply defined crossover at which the unit points
toward the signal source direction. Dopplers Have Their Place
An ideal RDF system would not require constant manual antenna turning. It
would take directional readings hundreds of times per second, and continue to
indicate the bearing after the signal leaves the air. Doppler type RDF sets,
though far from ideal, fulfill all these wishes. The typical four-whip antenna
system can be mounted without drilling holes in the vehicle.How To Learn More
While commercial RDF equipment is available, the majority of southern
California T-hunters prefer to build their own gear. All you need to get
started is a directional antenna, an attenuator to knock down strong nearby
signals, and a receiver with S-meter. You may have it all right now! If so,
it will only take a bit of installation work on the family car to get you
going.