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From: "Ian White, G3SEK"
Reply-To: "Ian White, G3SEK"
Subject: Re: "Non-inductive thick film resistors"
Organization: IFW Technical Services
User-Agent: Turnpike/6.02-M (<9qI82xurdTZmwuYn7IpmzNnT4H>)
Date: Sun, 12 Jan 2003 09:00:08 +0000
NNTP-Posting-Date: Sun, 12 Jan 2003 09:54:01 GMT
Paul Burridge wrote:
>I've come across these in the Farnell catalogue. Does anyone know if
>they'd be suitable for use in a 50 ohm dummy load up to say low UHF?
Yes - but you must mount them on a heatsink, so you need to be careful
about the shunt capacitance of a few pF.
I covered this topic extensively in 'In Practice' for June 2002 with a
follow-up in October which said:
John Fielding, ZS5JF, e-mailed with a report of some measurements he
made about five years ago using a vector network analyser (VNA). "Using
a SMA connector with very short leads connected to the sample resistor -
a 100W part made by Craddock - I was amazed to see a perfect 2:1 SWR
right up to 3GHz! Later I obtained some 50 ohm resistors from the same
source and they also behaved perfectly up to 3GHz, the limit of the VNA.
However performance degraded badly when bolted to a heatsink, due to the
capacitive coupling between the flange and the heatsink. As I recall
the SWR was better with the leads connected one way than the other. We
eventually used these resistors as a 50W dummy load for a 30-88MHz
transceiver for 'silent tuning' and they were more than satisfactory.
They are certainly a lot cheaper than the pukka RF load resistors sold
by other firms. When properly heatsinked they will work OK up to about
432MHz at the full rated power." .
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
Editor, 'The VHF/UHF DX Book'
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