For test-puposes I needed a dummyload that could withstand 450W on 136kHz
for some minutes.
Having some 470 Ohm / 25 Watt wirewound resistors in the junkbox I put 9 of
them in parallel and measured 52 Ohm with my DVM. Next I took it to my work
and checked it there on the impedance analyzer :
at 137kHz it was 51.6 + j1.3 Ohms, so more than good enough.
Although 9 x 25 = 225W they withstand 450W for about 2 minutes before it
starts 'smelling'. After several of these 'mistreatements' I checked it
again on the impedance analyzer and it was still 51.6 + j1.3 Ohms, so
overheating the resistors for a shot period does not seem to affect it.
So, if you need a high power 50 Ohm dummyload but don't want to spend too
much money on it : just parralel a bunch of high-power wirewound resistors,
it works fine on 136kHz. For a short period (few minutes) they even
withstand the double power they are rated for.
One final remark : don't put the resistors too close together or the middle
one(s) will become too hot. I kept all resistors about 2.5cm (1 inch) apart.
73, Rik ON7YD
|
