Above Image: Large RF dummy load for shortwave transmitter at the Moosbrunn transmitting station of the Austrian Broadcasting Service, Moosbrunn, Austria. It uses a sodium hydroxide solution to absorb the radio energy, which is circulated through the radiator (center) to cool it. It has a capacity of 100 kW. - Source: Wikipedia
At some stage of your amateur radio "career" you will need a Dummy Load. Now what is a dummy load and what is it used for?
A dummy load is a device used to simulate an electrical load, usually for testing purposes. In radio a dummy antenna is connected to the output of a radio transmitter and electrically simulates an antenna, to allow the transmitter to be adjusted and tested without radiating radio waves. A dummy load is connected to the output of an audio power amplifier to electrically simulate a loudspeaker, allowing the amplifier to be tested without producing sound. Load banks are connected to electrical power supplies to simulate the supply's intended electrical load for testing purposes.
In radio this device is also known as a dummy antenna or a radio frequency termination. It is a device, usually a resistor or many resistors, used in place of an antenna to aid in testing a radio transmitter. It is substituted for the antenna while one adjusts the transmitter, so that no radio waves are radiated so that the transmitter does not interfere with other radio transmitters during the adjustments. (As no dummy load is ideal, however, some radiation does occur). If a transmitter is tested without a load attached to its output terminals such as an antenna or a dummy load, the power will be reflected back into the transmitter, often overheating and damaging it. Also, if a transmitter is adjusted without a load, it will operate differently as compared with a load, and the adjustments may be incorrect.
The dummy load ordinarily should be a pure resistance; the amount of resistance should be the same as the impedance of the antenna or transmission line that is used with the transmitter (usually 50 Ω or 75 Ω). The radio energy that is absorbed by the dummy load is converted to heat. A dummy load must be chosen or designed to tolerate the amount of power that can be delivered by the transmitter.
The ideal dummy load provides a standing wave ratio (SWR) of 1:1 at the given impedance.
Veterinarian-grade mineral oil, an inexpensive form of mineral oil, is frequently used by amateur radio operators as coolant in RF dummy loads.
Now that we know what a dummy load is it is time to put one together. The dummy load we are going to construct can tolerate 100 watts of RF Power for one minute. More than enough time to test a transmitter etc. The load uses carbon non-conductive resistors mounted between two PCB's.
The dummy load we going to construct can be used internal or externally. Here I am talking about a project cabinet. Your choice. Mine has been used externally with no cabinet. The dummy load can be made up of 54 x 2k7 2 watt or 48 x 2400 Ohm carbon non-inductive resistors that are sandwiched together, with air spacing of 5 mm, between two identically drilled circuit boards. The resistor ends are mounted flush with the non-clad side of each circuit board. Then the leads are soldered to the foil side and cut. Keep wiring including coax cable as short as possible. This keeps the ground connection short, which eliminates stray reactances at the higher R.F. frequencies. The finished dummy load, shown in Fig. 1 is capable of dissipating at least 100 watts.
I also constructed a 20 watt dummy load using 11 x 560 Ohm 2 watt carbon non-inductive resistors. See images.
Calculation Results:
100 Watt Dummy Load
54 x 2K7 Resistors in Parallel = 50 Ohm
48 x 2K4 Resistors in Parallel = 50 Ohm
20 Watt Dummy Load
11 x 560 Ohm Resistors in Parallel = 50.91 Ohm
