Photo of rotor hunter on top of car
'ROTOR HUNTER' is an automated direction finding system developed by VK3ZPF. It consists of two rotating directional antennas, two receivers, a parallel port interface and a lap top computer. The rotating antennas are attached to a motor box which is mounted on the roof of the car. The parallel port interface, the receivers and the lap top computer are mounted inside the car. As the antennas rotate a polar plot is drawn on the computer screen and the direction of the fox is displayed.
I use a five element Yagi for 144 MHz and a four element Yagi for 432 MHz. A diplexer is fitted at the top of the antenna shaft to connect both antennas to one coaxial cable. I use RG213 coaxial cable inside the antenna shaft as it is more durable then RG58. Inside the car is a second diplexer which separates the VHF signal from the UHF signal.

The 432 MHz antenna and receiver are used when we are very close to the transmitter. Nearly all VHF transmitters have a small amount of signal transmitted at three times the carrier frequency. This is refered to as the 'third harmonic' and is usually about 60 dB lower than the main carrier. By having a receiver and antenna on the third harmonic we are able to get closer to the transmitter without needing very large amounts of attenuation.
After being separated, the VHF signal passes through a manual switched attenuator. The attenuator allows the input signal to be reduced so as to not overload the receiver. Future versions of ROTOR HUNTER may have automatic attenuator control as a function of the software.

The parallel port interface is a modified Dick Smith kit while the software is homebrew. At present I use a Kenwood TM742A triband transceiver to receive the VHF and UHF signals. The transceiver has been modified to allow the received signal strength to be monitored outside the radio.
Photo of rotor hunter on top of car

For information about using the signal meter outputs of the Kenwood TM742 click here.

screen display motor box  parallel interface rf coupling software home page vk3zpf@qsl.net

Copyright 1998 - Peter Fraser - all rights reserved.