Monday, 21 May 2012

Building a dummy load

My first self-build project was a dummy load. I collected various circuit diagrams and reviewed various designs but shown below it the one I went with. The box came from Maplin and the remaining components from Farnell and my junk box.
The load described here is capable of handling up to 10 watts of RF power for a couple of minutes, and is designed for the widely used 50 ohms impedance. It consists of ten parallel connected 560 ohms 1 watt resistors, R1 through R10, a voltage divider, R11-R12, and a rectifier D1-C1. Apart from loading the transmitter output with a minimum of reflected power, the dummy load also provides a direct voltage output to which a voltmeter may be connected to measure the RF power. If the dummy load is used for power levels higher than 10 watts simply use more or higher wattage resistors to give a total of about 50 ohms. For instance, by using twenty 2 watt 1,200 ohms resistors instead of R1-R10 and 150 ohms resistors for R11 and R12, the dummy load is turned into a 40 watt version. The diode may be almost any Schottky type. Types like BAT85 and HSCH1001, for instance, are also suitable. Even a germanium type like the AA119 will work, but then for low powers only.
As illustrated, the ten 560 ohms resistors are soldered in a circle around the centre pin of the BNC socket. Their ground terminals are soldered flush to the inside of the case. Capacitor C1 is a feed through type for which a small hole must be drilled. All resistors should be mounted with the shortest possible lead lengths to keep the reactive component of the dummy load as small as possible. After mounting the parts, the cover is fitted on to the case. Do not drill ventilation holes because that will defeat the purpose of making a non-radiating load. The case may get quite hot when transmitter power is applied for a while but that is no cause for concern. Allow the case to cool between tests.

1 comment:

  1. Thanks for sharing. I like the use of the feed-through cap & Schottky for measuring the output. I think I'm going go to add that element to my dummy load. Keep your iron hot!