Re: [Amps] 20kw Dummy load?

["Dr. David Kirkby" <david.kirkby@onetel.net>]

Robin Szemeti wrote:
> On Saturday 14 August 2004 17:01, Dr. David Kirkby wrote:
>
>
> > 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.
>
>
> hmmm ... just remember a pair of 4cx250s puts out at best 700w on 2m, this is 
> a 20KW device ..


Which is 29 times more. A lot I admit, but not so much that I believe a 
long length of coax could not be used as a large dummy load. Clearly the 
coax would have to be bigger (which is why RG58 would not do as I 
stated), but RG8 might well be practical, although I am not sure. It 
depends on frequency too of course, since loss goes up with frequency. I 
forget what frequency you were working at.

> .  a 3CX15000 ...  think about the dissipation, in a lossy 
> line it will be exponential .... so if 100m had say a 10db loss you'd lose 
> roughly half the power in 30m, that 10kw in 30m ... or 300w per metre ... I'd 
> think its life could be measured in seconds ...  

Yes I know it would be exponential - that the piece nearest the 
transmitter gets the hottest.

> apart from that, RG8 would 
> probably not even stand the voltages from 20Kw into 50R,  

P=V^2/R, so V=Sqrt[P*R]=Sqrt[20000*50]=1000 volt RMS. or 1414 V peak.

That is no problem. Even RG58 could easily handle that voltage - but not 
the power. You can buy various connectors for high voltage DC use that 
use RG58 sized cables for several kV. There is a plug like a BNC 
(thankfully does not mate with a standard BNC), that is rated for 7 kV. 
The voltage rating of the cable is never going to be an issue. The 
connectors would be, but with care you can do that.

> so maybe a couple 
> of dozen kilometres of LDF550 might do the trick?

Sure it would work, but it would also be totally unnecessary - something 
much smaller could be made. f

The physically smallest load you could make would no doubt be made from 
two (or more) pieces of coax, connnected end-to-end. The largest 
diamater coax would be used near the transmitter, where the power 
dissipated per metre is highest. On the end of that, where the power is 
lower, you could use a thiner coax.

> I think I'll continue the search for a load ... I now have a water chiller 
> capable of handling up to 45kw, so a water cooled load is looking like an 
> option :)

Well from initially wanting to borrow a large load for a few days over a 
month ago, you have now got a water chilller, but sitll no load. I may 
well be wrong, but suspect that you could make a suitable load from 
lengths of suitable coax.

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