Re: [Amps] 20kw Dummy load?

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

Robin Szemeti wrote:
> On Thursday 23 September 2004 22:27, Dr. David Kirkby wrote:
>
> > 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. 
>
>
> Not sure ... see, I think the power will be dissipated roughly equally in the 
> outer and the center conductor, so if we go back to our 100m of coax example, 
> the average disspation on the first 30m will be around 300w/m ... or 150w on 
> the inner, 150w  per meter ... on the outer ... and getting that 150w away 
> rfom the outer would be easy enough with water cooling, the inner however is 
> likely to be insulated in a polythene foam core ... its going to fry.
>
>
>  and 
> probably the hamsters too.
>
> I appreciate that coax loads work just fine at lower power levels,  but this 
> is serious cooking power, and its going to have to be either a dipole or a 
> proper load.
You have a point I would admit. Getting the heat out the centre might be 
an issue.

I forget what frequncy this was wanted at, but I don;t thnk it was that 
high. In which case the dielectric losses of the coax will be pretty 
insignificant - it is the the copper losses that will dominate.

20 kW is an RMS current of 20 A RMS in a 50 Ohm system. Anyone fancy 
hooking up a power supply and seeing what happens if 20 A DC is passed 
though the inner conductor of a bit of RG213 for 5 minutes. If that 
survives, then passing the same current though both the inner and outer 
would be a pretty good reprsentation of what would happen with 20 kW of RF.

I was not really thinking of water cooling it - I was rather hoping that 
the air would be sufficient. I know skin effects would come into this 
too, but i think they would be insignificant.

If RG213 will not do, I expect LDF-450 would for the 5 minutes in the 
original spec. Perhaps a bit of LDF-450, with a bit of RG213 on the 
end.might be more sutiable.

I don't have a 20 A PSU handy, but I expect someone will be able to test 
 it.






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