Skip to Main Content
This project details the installation of a Yaesu FT-857 and ATAS-120 in a 1997 Toyota Tacoma. After the installation was complete, I used a program called Polar Plot (by G4HFQ) to plot the signal strength around the truck to check the antenna radiation pattern. This program was designed to collect data from a rotor-mounted antenna, but I was curious to see if the reverse would work (i.e. data collected by circling a fixed antenna). I mounted a laptop, FT-817, soundcard interface, 12V battery, and a 15m Iron Horse antenna on a hand truck. I then scribed a 100 foot diameter chalk line in a parking lot and parked the truck mounted ATAS-120 over the circle’s center. With the FT-817 AGC off and the RF gain adjusted to get a usable signal, the audio output was patched into the laptop’s microphone input.The FT-857 was set to transmit at its lowest power of five watts at 21.1 MHz. This proved to be much too strong of a signal at this distance so I substituted an MFJ-259B antenna analyzer as the signal source. The analyzer will output 20 mW into 50 ohms and the resulting signal was more manageable. Tuning the ATAS-120 to 21.1 MHz with the FT-857 and then swapping the antenna coax to the analyzer, I pushed the hand truck around the chalk line until the number of Polar Plot samples was just under 360 for a complete revolution around the truck. I made several runs and compared the results to be sure I was getting an accurate picture. Granted, this test was for one frequency, one elevation (0° take-off angle), and a very short distance from transmitter to receiver, so the results are open to discussion. Nonetheless, the plot does reveal that there are some nulls toward the rear and through the cab, which is probable given the antenna’s mounting location.