[Amps] 20kw Dummy load?

[Alek Petkovic]

The way we Aussies do it is to fill a 44 gallon drum with slightly salted 
water. A couple of hefty electrodes are immersed in the solution and 
connected to the transmitter and amplifier to be tested.

The transmitter is keyed up and we toss in some lobsters and crabs.

At the conclusion of the load testing, we take out the cooked crustaceans 
and enjoy a nice feed of the seafood washed down with a few cold beers.

73, Alek
VK6APK

At 09:13 AM 15/08/2004, Barry Kirkwood wrote:
>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 at onetel.net>
>To: <list at redpoint.org.uk>
>Cc: <amps at 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 at contesting.com
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> >
>
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