Before you start:
Most beginners start with an HT that has some kind of visual signal strength indication made up of several display
bars (more is better). Make sure your HT has the transmit function disabled in order to not damage an external
attenuator or to interfere with others. If you can’t disable the transmit function then set the HT to the minimum
power level (should be < 300mW) and try to not press the PTT button. Disable the squelch or set it to minimum in
order to hear the weak signals. Also for all techniques except “body block” you will need a directional antenna and
for TDOA you will need a 2 dipole array.
Why is this so difficult:
The difference in signal strengths between just barely hearing a very weak signal and being 5 feet away from a
hidden couple watt transmitter can be over 150dB. Throughout this range of signals the equipment must remain
mostly linear and continue to show which direction the strongest signal is coming from. If this requirement does
not seem difficult to meet, then closely look at the following page where a more detailed discussion about signal
strengths, power levels, and dB is done. Some hunts are held with rules limiting power, polarization, and visibility,
which permits less sophisticated equipment to be used successfully and not frustrate the participants.
Techniques:
The following order is from simplest and least effective to most effective and quickest.
1.) Field strength meter: Only works in the presence of 1 transmitter and only for strong signals. However
when a FS meter is connected to a lightweight yagi antenna and the power of the transmitter matches the
sensitivity of the FS meters you can have a lot of fun. Also due to equipment limitations some people use FS
meters when you get very close to the transmitter and the signal is too strong for their other equipment.
Interference from other 2m transmitters, paging transmitters, and FM radio stations can be problematic.
2.) Body blocking with an HT: This would be my last resort for a technique. You may have some success if you
are lucky enough to have an HT with the S meter on the top of the HT so you can hold it very close to you and
later removing the antenna completely. However your body does not provide nearly the front to back signal
attenuation that a good antenna does.
3.) Body blocking with additional HT shielding: There are several designs for metal tubes you can lower the HT
into and metal vests you can wear but mostly this is effort directed at an already bad idea.
4.) HT with an external passive attenuator: Works very well until the RF leakage into the HT becomes the
dominate signal path.
5.) HT with a TDOA (Time Difference of Arrival) box: These are easy to build and work well for vertically
polarized signals. They work well for extremely strong signals and are not suited for real weak ones. The simplest
designs give you a bearing line but the direction is ambiguous.
6.) HT with an external active (offset) attenuator: These devices are “mixers” and work well for medium
strength signals to strong signals and require the HT to be tuned to the offset frequency. The mixing loss is usually
8 to 15dB so you will want to disconnect it when the signal is weak and tune to the transmit frequency. In the
presence of other very strong signals (TV/FM/Paging) adverse mixing may occur.
7.) HT with a combination external active/passive attenuator: Similar to #6 except that as the signal gets
stronger some passive attenuation is inserted before the mixer causing it to behave better.
8.) Special purpose receiver: Radios designed with gain control in RF and IF stages and shielded work well.
9.) Special purpose receiver requiring no visual feedback: The MK4 sniffer by Brian Ackerly, VK3YNG is a
good example where the signal strength info is conveyed audibly so you can concentrate on paying attention to
the surrounding area.
10.) Doppler: The use of Doppler systems is generally not appropriate for foot hunts and while Doppler systems
have many desirable characteristics (fast response time and frequency agility), they also have many weaknesses
(reduced signal strengths and don’t work well for horizontally polarized transmitting sources).
Note: The following techniques may improve the strong signal capability for techniques 2, 3, & 4 especially and
experimentation will need to be done with your particular HT.
a.) Try tuning off frequency by 5, 10, or 15KHz.
b.) Try tuning to the other side of the 2nd IF frequency (usually +/- 910KHz) to reduce the signal.
c.) Can also tune to the other side of the 1st IF if the radio can tune to this frequency.
d.) Can tune to the 3rd harmonic of the fundamental frequency but the directional antenna may no longer be
properly directional. However you can use a second antenna designed for the 3rd harmonic.
Concepts:
1.) Antenna pattern: Clean pattern more important than gain or SWR.
2.) S-Meter range: Given a clean antenna pattern the S meter range should be about 18 to 20dB from min. to
max. bars.
3.) Signal polarization: Vertical or horizontal and will make a 30dB difference when close. Reflections change
polarization so hunt the correct polarization.
4.) Receiver quieting: Listen for the FM capture for signals below S1.
5.) Location, location, location: Open clear areas are obviously better than gullies and along fence lines.
6.) Reflections: The direction of most reflections changes quickly while the direct bearing does not. Reflections are
not a problem if hunting on 80m.
7.) Knife edge refraction: Top of the hill is not best. Slightly over the top of the hill in the direction of the
transmitter is much better.
8.) Look up and down: When you are very close do not forget to look up and down as the transmitter may not
be at eye level and the antenna pattern will not be correct off the side of the antenna.
9.) Triangulate: This works great when you are very close and still can’t find the transmitter. Allows you to find
transmitter without getting so close as to overload your equipment.
10.) 10dB rule: When you are within 1000 feet of the transmitter the signal strength increases by ~10dB each
time you cut the distance in half.
11.) Practice, practice, practice and know and trust your equipment.
References:
1.) Joe Moell, K0OV is probably the godfather of this activity in the USA and his web site www.homingin.com
contains a huge amount of info relating to all forms or transmitter hunting.
2.) Joe Leggio, WB2HOL was one of the first sources for the construction of the “tape measure” yagi antennas
and his web site http://theleggios.net/wb2hol/projects/rdf/tape_bm.htm gives the dimensions and how to build
them.
3.) Byon Garrabrant, N6BG designs and manufactures several ham radio devices www.byonics.com including one
called a PicCon which turns any HT into a powerful remotely controlled transmitter usable for transmitter hunting.
Having your own “fox” is one of the best ways to improve your abilities.
4.) Brian Ackerly, VK3YNG www.foxhunt.com.au/ manufactures the moderately priced VHF Sniffer MK4 receiver,
which has become the most popular receiver among the top competitors for ARDF.
Transmitter Hunting (known as T-Hunting, Fox Hunting, Radio Direction Finding) is a popular activity among
Amateur Radio operators where a transmitting device is hidden somewhere and "hunted down" using various
equipment and radio direction finding techniques. Those doing the hunting are not required to have an amateur
radio license, as there is no need for them to transmit.
The source(s) of the radio transmissions may be many miles away and the hunt will be done in vehicles or the
sources(s) may be nearby and the hunt can done on foot in a park. Another form of foot hunting is ARDF where
you are given an orienteering map and hunt for 6 transmitters in some primitive area. In most hunts the
transmitter(s) is(are) usually on the air intermittently, and identified either in Morse code or voice.
Any frequency may be used but 2m (144-148 MHz) FM is by far the most common frequency used in the United
States. I have done hunts on other frequencies/modes including 80m CW, 2m AM, 10m CW, 6m FM, & 70cm FM
but different frequencies require different antennas and equipment.
What is Transmitter Hunting?
Page content last updated Mar. 12, 2008
Copyright © 2008 Larry Benko, W0QE
Measurements and analysis showing expected signal strengths for transmitter hunting on on the next page
