Hidden Transmitter Hunts

ARDF means Amateur Radio Direction Finding.  It is also known as "Radio Sport", "T-Hunting", "Fox Hunting", "Bunny Hunting" ... or simply as a hidden transmitter hunt.

 


The next RCT-sponsored hidden transmitter hunt will be held Sunday May 21, 2017, from 10AM until 2PM.  Sign in at the Ft. Nisqually parking lot, Point Defiance Park.  Details are available here.


 

 RESOURCES AND WEB LINKS

Hidden transmitter hunts around the Puget Sound Area, courtesy of Art Jury, KF7GD

Northern California Transmitter Hunting Group

Nice story from the ARRL

USA 2007 ARDF Championship

The "Homing In" website

BUILDING A 2-METER TAPE MEASURE YAGI

For a detailed construction diagrams, see the articles "Adapting a Three Element Tape Measure Beam for Power Line Noise Hunting" (QST, May 2007); and "Tape Measure Beam Optimized for Radio Direction Finding" by Joe Leggio, WB2HOL.

NOTE: Harbor Freight has very inexpensive tape measures for you to hack up. Danger!! The newly cut edge is razor sharp. Use a grinder to round the sharp corners. Some good electrical tape (like Scotch 33) will protect the sharp edge from puncturing your skin.

OFFSET ATTENUATOR

Hidden transmitters are located by watching the signal strenth bars on the display screen of your 2-meter handheld radio. You will need to add a little metal box to your ARDF yagi antenna with a dial you can turn to attenuate the signal level as you approach the transmitter. Without any attentuation between the antenna and the radio, the signal received will be so strong the bars will show full maximum no matter which way you turn! The variable attenuator uses an offset design that requires tuning the radio to receive on a frequency typically 4 MHz above or below the transmitted frequency. For example, if the "bunny" is transmitting on 146.565 MHz, your handheld radio would be tuned to 142.565 MHz or 150.565 MHz. If you're new to bunny hunting, the event organizers will probably have tape measure yagi antennas with offset attenuators that you can borrow for the day.

Instructions for constructing your own variable offset attenuator gizmo can be found on the Homing In website.

Also see Rich KR7W's article describing his design for an active attenuator in the Club Newsletter, April 2008 issue, page 11.

BUILDING A HIDDEN TRANSMITTER

Once you're bitten by the ARDF "bug" you may be thinking about ideas for making a transmitter that you can hide yourself at a hidden transmitter hunt. The basic ingredients are a low power 2-meter FM transmitter operating on battery power, attached to an inconspicuous vertical or dipole antenna, and a programmable transmitter controller. The controller turns the radio on and off at intervals, transmits an audible signal to identify which transmitter is transmitting, and finally, at the end of each short transmission, sends the required station identification callsign.

An international standard calls for up to five hidden transmitters making brief transmissions in rotation. Each transmitter sends audible tones: first the Morse Code letters M (dah-dah) and O (dah-dah-dah), followed by a letter E (dit), I (dit dit), S (dit dit dit), H (dit dit dit dit), or 5 (dit dit dit dit dit). The hunter listens for the number of "dits", one through five, and knows which transmitter's signal is being received.

The PicCon Transmitter Controller can be purchased from Byonics as a kit or fully assembled. The cost for this unit is in the $40 to $70 range.

The ID-O-Matic IV kit available from Hamgadgets is another recommended option. This item is available only as a kit and priced slightly less than the controller unit mentioned above. It's easy to program by connecting to your PC's USB port and making menu choices.

CQ WW FOXHUNTING WEEKEND

The CQ WW Foxhunting Weekend, which is featured in CQ Magaziune and scheduled every year in the month of May, provides a great opportunity to get outside, enjoy some fresh air, and track down those "foxes". In 2011, Club Members participated at Fort Flagler State Park in a transmitter hunt sponsored by Neil Robin, WA7NBF of Port Angeles. In recent years, as the weather warms, local parks have concealed many little bunnies around the lake, in the trees, and (hint, hint) sometimes near the fence by the water tower. Hidden transmitter hunts sponsored by the Club and its Members have been mentioned in:

  • CQ Magazine, May 2012, page 30
  • CQ Magazine, May 2011, page 31
  • CQ Magazine, May 2010, pages 36-37

FOR MORE ARDF INFORMATION, CONTACT

Rich Patrick, KR7W
This email address is being protected from spambots. You need JavaScript enabled to view it.