The Right Tool For The Job: Toggle Step Attenuator For Amateur Radio Direction Finding (ARDF) / Ham Radio Fox Hunting
A tad bit of spring danced through the air on this mid-March day. It was easy to justify a run up north to find the NEMass Fox Hunters hidden transmitter. From the small dirt parking lot we could hear a few measures of “Mary Had A Little Lamb” bleating out on 146.535MHz through our handheld transceiver. The Lime Quarry Reservation was a small, interesting patch of public land located behind a rest stop on Interstate 495. AA1F, Nellie our dog, and I followed the trails counter-clockwise pausing every few minutes to evaluate the potential direction of the amateur radio “fox”.
Our tools of choice were an Arrow-brand handheld 2M (and 70cm, though the elements are not attached) yagi. I think that this is a piece of radio kit that every ham should either buy or make. I actually made a point of taping a 6 foot run of RG8X down the boom of the yagi, and then threading the coaxial cable through the removable foam handle. This simple step did wonders to keep the set-up reasonably neat and manageable.
The star of the show however was the new-to-me toggle step attenuator. You can read about my history with this component here. In short, each switch attenuates an RF signal by a number of decibels that corresponds to the number written besides the switch. The total number of decibels of attenuation is additive; simply add up the numbers of the switches thrown to get the total reduction in signal strength. This attenuation can range from 0.5dB to 82.5dB, and everything in between. A Yaesu FT-4X, an HT that I am growing to admire more and more (see here for how I am now using it as a mobile radio in my van), was tethered to the attenuator via hair ties. A short RG8X coax jumper connected the transceiver to attenuator. SO-239 to male BNC adapters were fitted on each end of the attenuator, as well as on the yagi. These adapters remind me that I sorely need to make some cables!!
AA1F and a little hint of Nellie’s tail stop for a picture.
Here’s a little proof that we found the fox!
A demonstration of the efficacy of the toggle step attenuator… The signal strength meter (S-meter) of the HT is shown at full strength when the yagi is pointed directly at the fox transmitter from a distance of approximately 12 feet away.
However, when each of the switches of the attenuator is flipped for a total of 82.5dB of attenuation, the signal strength only registers as an S-2. Note, I do not know what the wattage of the transmitter is.
So, what is the strategy with this attenuator? Drop the signal strength until it essentially only registers on the S meter when the yagi is pointed in a single direction, and does not register at all when the yagi is pointed away from that direction. Make a note of the number of decibels of attenuation it took to achieve that diminution in signal…the more attenuation that it took to suppress the signal, the stronger the signal is. Travel toward the direction of the strongest signal taking additional readings along the way to calibrate your travel toward the fox.
In any case, this set-up is undoubtedly one of the most convenient and effective of any of our prior amateur radio direction finding experiences. I am looking forward to flushing out the next fox!
I’m still here.
KM1NDY