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.
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.
73 Ha-Jo, DJ1ZB
"James Moritz" <[email protected]> schrieb:
Whilst on the subject of dummy loads, are Metal Oxide resistors suitable
dummy loads ? because much higher ratings (5 watts) are available for
physical size as the 2 watt carbon film resistors in my home brew dummy
Gary - G4WGT
Dear Gary, LF Group,
The metal oxide types would be fine, as are metal film, etc. In fact,
even some types of wirewound resistors are reasonably non-reactive at
136kHz. Several people have resorted to electric heating elements like
toasters and fan heaters to make satisfactory LF dummy loads. But
obviously, it is a safer bet to stick to known low inductance types like
carbon, metal oxide and metal film unless you have the equipment to
evaluate the actual RF impedance.
The Vishay RCH50 50W film resistors sold by RS components are pretty
good value at about 2-3 pounds each, and work OK up to HF - but
obviously you need a hefty heatsink as well as the resistors.
Cheers, Jim Moritz
73 de M0BMU