Kent KA0LDG discussed Fox Hunting (aka transmitter hunting) during the June club meeting and displayed some of the gear he uses during these activities:
- Directional antenna with an offset attenuator
- Fox hunt transmitter
Plans for home-brew antennas suitable for Fox Hunting are widely available and prebuilt antennas may be purchased from a number of vendors. Those wishing to build their own Fox Hunting antenna may wish to use the design by Joe Leggio WB2HOL described in his Tape Measure Beam Optimized for Radio Direction Finding article.
This antenna evolved during my search for a beam with a really great front-to-back ratio to use in hidden transmitter hunts. This design exhibits a very clean pattern and is perfect for RDF use. It trades a bit of forward gain in exchange for a very deep notch in the pattern toward the rear.1
According to a simulation performed using YagiCAD (4.1) WB2HOL’s antenna has the following characteristics:
|Front-to-Back Ratio||>50 db|
|3 db Beamwidth||E = 67.5 degrees|
|3 db Beamwidth||H = 110 degrees|
WB2H0L’s antenna uses a 1 inch wide steel tape-measure and other easy to obtain materials. It can be easily constructed using common tools.
WB2HOL’s Tape Measure Beam Optimized for Radio Direction Finding article discusses
- The design and various choices the builder can make during construction
- Antenna performance including a plot of a measured sensitivity pattern
Marvin KE6HTS notes a useful addition to the WB2HOL antenna: “A choke balun consisting of 6 - 8 turns of coax around the beam will help reduce the pattern skew significantly. It is not shown on the plans for the WB2HOL Tape Measure Beam, but should still be added to the antenna.”2
ARDF kits available from KE6HTS:
- WB2HOL Tape Measure Beam kit
- K0OV Offset Attenuator
- Complete WB2HOL Tape Measure Beam Kit (includes assembled/tested K0OV offset attenuator w/enclosure)
About Fox Hunting Antennas
The nulls provided by Fox Hunting antennas—unidirectional for Yagi-Uda and bidirectional for Delta-Loops—are the feature that allows hunters to determine the direction to a hidden transmitter.
Dave N1RF indirectly demonstrates the Yagi-Uda null in his Yagi Antenna Teaching Construct Part #3: Directional Gain and Front to Back Ratio article on the Nashua Area Radio Society website.
And there is a brief discussion of Yagi Front-to-Back Ratio in the ARRL General Technical Q & A forum.
Yagi Antenna Gain & Directivity https://www.electronics-notes.com/articles/antennas-propagation/yagi-uda-antenna-aerial/gain-directivity.php
Other Fox Hunting related projects may be found on the WB2HOL Radio Direction Finding Projects page
- Time-Difference-Of-Arrival RDF units
- Fox Timers and Controllers
- Fox Transmitters
- Other RDF Projects
- Designing a Fox-hunt antenna (Rolfje)
- Fox Hunting (Everything Ham Radio)
- Foxhunting (KC9ON)
- Homing In
- KE6HTS ARDF Page
- My Offset Attenuator Project (NT1K)
- Tape-Measure Antenna (ARRL / PDF)
- Transmitter Hunting (Wikipedia)
- Yagi Antenna Gain & Directivity (electronicsnotes)
- Yagi Antenna Teaching Construct Part #3: Directional Gain and Front to Back Ratio (Nashua Area Radio Society)
“Tape Measure Beam Optimized For Radio Direction Finding”, Joe Leggio WB2HOL, Retrieved June 22 2019, http://theleggios.net/wb2hol/projects/rdf/tape_bm.htm. ↩︎
“WB2HOL Tape Measure Beam Kit”, KE6HTS ARDF Page, Retrieved July 6 2019, http://www.west.net/~marvin/wb2hol.htm. ↩︎