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.
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.
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.
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
Load from John, G3PAI
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
of these Dummy Loads were described at LF-Forum: