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ANTENTOP- 03- 2003, # 004

LF-Dummy Load




Dummy Load from Uwe, DJ8WX

Here are two dummy loads in use; one is a bulb 110V/DC500W, Z 35 Ohm on 136kHz (bridge measurement) and the other is made of four hollow ceramic resistors 200 Ohm/60W each, makes 200W only (see Picture). but blowing air through the resistors (see attachment) makes a lukewarm 300W dummy on 136kHz (0,4uH only). I tried 800W. A peace of paper above the dummy caught fire within 10 sec.



Dummy Load from Ha- Jo, DJ1ZB

For the time being I am using a dummy load made of ten low inductivity DALE resistors Type NH-50, 499 ohms each, in parallel on a large aluminum cooler (the fins of which could even be immersed into water when necessary), which could be used up to 30 MHz with a coil/capacitor matching arrangement at the input resulting in an VSWR of about 1,2. The matching arrangement has been designed about ten years ago in my qrl by measuring R and X of the parallel resistor arrangement over the frequency range and trying to find a suitable match by employing the old Supercompact software or the ARRL Radio Designer.


DJ8WX Dummy Load



Therefore I guess (I have not tried it yet, no need to do so) for such a small band like 136 kHz it should be possible to build a dummy load using ordinary wirewound resistors (preferably DALE or similars because of their easy mounting on a cooler surface) because their inductance could be cancelled by a suitable capacitance in parallel, or by several distributed capacitors within the parallel resistor arrangement. It should be rather simple to determine the capacitance needed, a VSWR meter designed for LF should do it.


Caution: Nobody should respect such a load to also absorb harmonics of the transmitter frequency, because it is a tuned load. Harmonics may see a short circuit, depending of the Q of the load, and will be reflected. This special behaviour of the tuned load does not matter, of course, if a low-pass filter is added to the tank circuit of  the transmitter. 

73 Ha-Jo, DJ1ZB

Dummy Load from John, G3PAI


About 30 years ago, Ongar Radio station to the north-east of London had a number of nine kilowatt HF ISB transmitters. For dummy loads they used carbon tubes about a foot long and an inch or two in diameter. Resistance was 75 ohms and they were cooled by pumping water through them. I had a box of such resistors, but they went missing in a house move.

John Rabson G3PAI


All of these Dummy Loads were described at LF-Forum:


Page 79





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February 9, 2018 22:05

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