Spring 2023 Meetings: Antenna Tuning and Attenuator Building

At SPARC’s April, May, and June meetings, we continued working with the tape measure beam antennas we constructed in February. At the April meeting, we tuned the antennas to make sure they resonated at 146.565 MHz, the local t-hunting frequency. By adjusting the spacing of the steel elements, we made sure each antenna was ready to hear a hidden transmitter.

Checking the antenna on an analyzer
Testing the antenna in Eddie Park

At the May and June meetings, we constructed an attenuator kit to pair with the antennas. Stan, KR6CV arranged a group purchase of kits from KC9ON. The V7 kit provides 4 MHz of signal attenuation. KC9ON explains how that helps:

When you’re closing in on the fox [the hidden transmitter] you may find the signals to be strong enough you can no longer find a peak or null with your antenna. Sometimes the signal is so strong that the RF will leak straight into the radio, connections and other equipment making the antenna useless. The solution is to use an offset attenuator. The circuit consists of a small RF generator, in this case 4 MHz, which will mix with the incoming fox signal (such as 146.52 MHz) and produce new signals at plus and minus the fox signal (142.52 MHz and 150.52 MHz). A potentiometer on the board changes the injection level of the RF generator which in turn attenuates the incoming mixed signal to your radio to a level where tracking can continue.

KC9ON Fox Hunt Attenuator
Product image of the V7 attenuator from KC9ON
Building the V7 kit during the June 2023 meeting

Instructions for building the kit can be found here.

With our antennas and attenuators ready for action, we planned to hold a practice t-hunt at our August member meeting.