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 13:10:32 +0000
NNTP-Posting-Date: Sun, 12 Jan 2003 13:24:47 GMT
John Woodgate wrote:
>I read in sci.electronics.design that Paul Burridge
> wrote (in <9dr02vglshis9qtnrq3cvkaqaj87hnp
>firstname.lastname@example.org>) about '"Non-inductive thick film resistors"', on Sat, 11
>>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?
>Please quote a typical Farnell order code. I looked for them and
>couldn't find them, which is why I posted a question here about sources.
Try 306-6952, 100R 50W (omit the hyphen when searching the Farnell web
The 50W range is better value for money than the 20W.
I asked Farnell for more details of the RF performance, but they don't
have any more useful information than is given in the brief catalogue
>If they do a 50 ohm, it must be intended for use as a termination,
>because 50 ohms is not a preferred value (47, 51, 56).
The catalogue dealers do not stock all preferred values (not even the
full E6 range). If you don't mind the extra capacitance, two 100R units
in parallel will give more dissipation and better spreading of the heat
One way to deal with the stray capacitance is to make it act as the "C2"
of a symmetrical pi-network. In other words, you connect to the
resistors with a small shunt C (equal to the value you're trying to
compensate in the resistors) and a small series L. In practice, the
shunt input C is variable, and is adjusted for lowest VSWR.
This network forms a lowpass filter with a 1:1 impedance transformation,
and is transparent to low frequencies. At higher frequencies, a bit of
modelling will show how the stray reactances can be compensated exactly
at one chosen frequency; the tradeoff is a small increase in VSWR at
lower frequencies, but a steep rise above the compensation point.
I covered this in my November 2002 column, in the context of
compensating the input capacitance of a tetrode operated in 'passive
grid' (with a tube input capacitance of 30-50pF).
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
Editor, 'The VHF/UHF DX Book'