Spring 2008 Issue

Homing In

Radio Direction Finding for Fun and Public Service

New Gear for 1.25-meter Foxhunting

By Joe Moell, KØOV

Is it in the tree? At a Scout-O-Rama two years ago, this Cub Scout discovered that a pet-tag signal was coming from above. He was searching with a ComSpec PR-50 receiver and
FA-1 Moxon beam. (All photos by Joe Moell, KØOV, unless
otherwise noted)

“I found it!” Few things are more satisfying to me than the surprised exclamations of youngsters as they locate hidden radio transmitters for the first time. This “Aha!” moment is the payoff for their intense concentration and competitive spirit. It’s their realization that ham radio activities can be great fun, and it’s a powerful stimulus for them to learn more about radio waves and propagation.

Wouldn’t it be great if more young people could experience hidden transmitter hunting on ham radio? As I try to promote this idea, one of the most common responses from hams and youth leaders is “We don’t have any equipment.” I reply that tape measure beams1 and offset attenuators2 are easy and educational for young people to build, but I realize that many groups just want an “out of the box” system. Now it is available.

Communications Specialists of Orange, California is offering miniature transmitters and matching radio-direction-finding (RDF) receiver/antenna sets for the 222–225 MHz ham band. ComSpec’s owner is Spence Porter, WA6TPR. His company gained success making subaudible tone encoders and decoders back when hams were first discovering the joys of VHF-FM and repeaters. As transceiver models with built-in CTCSS came out that market diminished, and he turned his efforts to radio tracking equipment for many uses, both licensed and unlicensed.

Two years ago I wrote about ComSpec’s tiny transmitters for 218–220 MHz.3 At the time, these unlicensed emitters were being marketed to pet owners and to flyers of model airplanes and rockets in the USA and elsewhere. Now sold only to foreign customers, this tracking system has 50 channels at 25-kHz intervals from 218.025 through 219.300 MHz. The PT-1 series transmitters put out 28-millisecond pings of unmodulated RF, about 47 per minute. One CR2032 lithium battery, about the size of a quarter, provides a month of continuous operation. As its battery reaches end of life, the transmitter double-pings every 10 seconds as a warning.

For RDF, the mainstays of this ComSpec line are the PR-30 LoCATor and PR-50 R/C-ELT receivers. Both feature a beat-frequency oscillator (BFO) and CW/SSB detector that turn the dits into tone pings. A peak-reading meter makes it easy to distinguish signal-level changes of the pings while turning the supplied FA-1 two-element folded Yagi. This 8" ¥ 20" Moxon rectangle beam is fabricated from circuit-board material and weighs only 101 grams (3.5 ounces).

FCC Giveth and Taketh Away

Unlike the “squegging” oscillators4 in most wildlife tracking transmitters, ComSpec’s little emitters use an elegant temperature-compensated crystal-oscillator (TCXO) design that provides excellent frequency stability and harmonic suppression.

WA6TPR designed the PT-1 to meet FCC Part 15 emission limits for unlicensed operation, when used with a 2.5-inch whip antenna embedded in a pet collar or trailing behind a model plane. RF field strength had to be less than 200 microvolts/meter at 3 meters distance, in accordance with FCC 15.209. ComSpec contracted with an engineering consulting firm to make compliance measurements. Then Spence applied for and received FCC Part 15 certification in April 2004.5

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