Corner : The DSE Radio Direc

M ost readers will be broadly familiar with the concept of a radio direction finder (RDF). A basic RDF consists of a receiver and an antenna which can be rotated on its own axis. The direction of the transmitter is found by rotating the antenna for a signal peak or null. You can easily demonstrate the effect for yourself using a portable transistor radio fitted with a ferrite rod antenna. By tuning the radio to a station and rotating the radio about its vertical axis, a null will be found in the signal strength. The ferrite rod antenna will then point in the direction of the station. Of course, this method requires that "fixes" be taken at two or more widely spaced locations in order to find the true location of the transmitter. The exact location of the transmitter is determined by simple triangulation. The classic application of this radio direction finding technique was in World War 11. Many war movies showed how it was possible to track down enemy transmitters using special vans fitted with RDF equipment. Typically, these vans were fitted with a large external loop antenna which could be manually rotated. An operator inside the van listened in on headphones for peaks and dips in the signal strength. Provided the transmitter remained in the one location for long enough, its location could eventually be pinpointed. The Dick Smith Radio Direction Finder is just the ticket for tracking down illegal transmitters and antisocial radio operators. Depending on the antenna system, it can operate on any band from 50 to 500 MHz and will work with FM receivers ranging from pocket scanners to amateur radio and CB transceivers. Physically, the radio direction finder consists of two separate units. One contains the control and display electronics, and the other is a special antenna-switching unit (ASU) which is connected to the control unit via a 4-conductor cable. An electronic "compass" display consisting of 32 LEDs indicates the transmitter bearing. When a signal is received, its relative bearing to the antenna system is indicated by whichever of the 32 LEDs illuminates. In fixed installations, this allows the compass bearing of the signal to be directly indicated to within + 5.6 degrees. When an RDF unit is installed in a car, successive readings allow you to pinpoint the exact location of the transmitter.