Robin Szemeti wrote:
>
> Hi,
>
> I need to use / beg / borrow / steal a decent dummy load to test this
> amplifier out .. running into its intended load I've had a couple of DC
> flashovers *somewhere* and I'd like to run the amp up into a dummy load, just
> to prove it to myself ...
>
> anyway ... has anyone got, for loan or whatever, a 50 ohm, 50mhz dummy load,
> capable of standing around 20kw CW for at least 5 minutes, water cooling is
> OK, we can do that. Ideally it should be 3 1/8" EIA flange, but I'm
> prepared to get busy on the lathe to make up a fitting if I have to.
>
> Actually, Id even settle for a gigantic 50ohm resistor that I could watercool
> myself, not averse to a bit of lathe work to make a water jacket for it. Are
> normal carbon tubular ceramic resistors OK in a water jacket or do you need
> 'special' ones btw?
>
> I'm UK based ...
I have a 5 kW continuous one sitting in my garage, which would
probably do for 5 minutes, with forced air cooling, but since its not
mine, and I don't know it would, I won't loan it to you.
I recon large carbon resistors would do. I think you can 300 W
continuous rating on them on a heatsink, so I guess 10 of them would
be fine with water.
Another option.
Do you have access to a large drum of coax, just capable of taking the
power? Then a long length of it would become lossy and so dissipate
the heat along its length. If you unwound it, it will do it. Don't
leave it wound up, as it will obviously overheat.
I used to use a 500' real of RG/58 coax as my dummy load on 2m with a
pair of 4CX250Bs. That could melt after several minutes, so was a bit
too thin. But 50 feet would have been fine I am sure - the loss was so
high.
Not great, but it might be an option. Remember the return loss of an
open-circuit (or short circuit) bit of coax will be twice the
attenuation. So if you can get a bit of coax that has 10 dB of loss,
it would give you a return loss of 20 dB, which is more than adequate
for testing an amp into.
Perhaps a long length of RG8 or something like that migth be okay
(perhaps RG8 would melt, but you would know if its getting too hot, as
it would be hotest near the TX, where the most power per metre is
being dissipated).
At leat with coax, there is no need to bother with water cooling
jackets. It just means your dummy load has bit a bit on the long side!
--
Dr. David Kirkby Ph.D CEng MIEE
Author of 'atlc' http://atlc.sourceforge.net/
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