Carbon resistor and dry ice.....(or maybe it would turn into a
superconductor)
73
Baz
end
Barry Kirkwood PhD ZL1DD
Signal Hill Homestay
66 Cory Road
Palm Beach
Waiheke Island 1240
NEW ZEALAND
ph/fax 64-9-372-5161
www.waiheke.co.nz/signal.htm
----- Original Message -----
From: "Dr. David Kirkby" <david.kirkby@onetel.net>
To: <list@redpoint.org.uk>
Cc: <amps@contesting.com>
Sent: Saturday, August 14, 2004 4:01 PM
Subject: Re: [Amps] 20kw Dummy load?
> 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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> Amps@contesting.com
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>
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