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NNTP-Posting-Date: Wed, 20 Nov 2002 14:19:39 -0600
From: Barry Lennox
Subject: Re: 50 ohm dummy load construction
Date: Thu, 21 Nov 2002 09:20:23 +1300
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On Wed, 20 Nov 2002 09:09:47 +0000, Paul Burridge
>I'd like to make up a dummy load for testing RF amps out with. I'm
>using 30x 2W, 1k5 resistors in parallel. I'll probably go for the old
>oil-filled 'bean can' design. Question is: what sort of internal
>layout would give rise to the best possible useful frequency range?
We did some measurements on this very problem about 20 years ago, and
the lowest reactance was with an outer coaxial shroud ring , I used a
rolled tinplate one about 4 inches in diameter, and the resistors
were soldered radially (like the spokes on a bike wheel) from the
shroud to a center contact bar. File some little nicks into the shroud
ring to make it easy to locate and solder the resistors to. The coax
braid has to be "splayed out" so as to be soldered to the whole 360
degrees of the shroud ring . It gave good results (can't recall the
VSWR) from 30 to 400 MHz.
We also did some fooling about with a solid carbon composition block,
I recall it was material from Morganite (A UK company?) It took a lot
of work to get to 50 Ohms and the end contact pressure had to be kept
constant, but it did look very good up to about 1 GHz.
The resistor types will also be critical for the best results. About
the same time we were using power amps over 225-400 MHz, and had a
spate of catastrophic failures in the output transistors. Turns out it
was a carbon composition resistor of a different style that had got
into the spares pool. They looked the same, and even was much the same
internally, (just a cylinder of carbon comp between the lead-out
wires) but the shape, and hence the distributed C and L to the PCB
were different enough to cause a real problem.
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