From: JHB@jita.nospam.demon.co.uk (Jim Backus)
Subject: Re: 50 ohm dummy load construction
Date: Wed, 20 Nov 2002 20:18:36 +0000 (UTC)
NNTP-Posting-Date: Wed, 20 Nov 2002 20:18:36 +0000 (UTC)
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On Wed, 20 Nov 2002 18:52:32, email@example.com (Tom Bruhns) wrote:
> Paul Burridge wrote in message news:...
> > Hi,
> > 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?
> > I'd like it to work reasonably well into VHF and UHF if at all
> > possible.
> > T. p
> As important as layout is the type of resistor. Carbon are OK, but
> tend to change value over time. 2-W metal-oxides are actually quite
> good, at least the ones near 50 ohms that I've measured. I've
> measured a return loss that translates to under 1.1:1 SWR at 150MHz
> and as I recall better than 1.5:1 at 450MHz. "YMMV".
> I'd be inclined to go for a different value. For example, if you put
> 8 100-ohm resistors in parallel, and then 4 sets like that in series,
> you end up with 50 ohms. Or you can use a 5*5 or 6*6 set of 50 ohm
> resistors. The reason I'd suggest the lower resistor value is that I
> think you'll get less contribution from parasitic inductance and
> capacitance that way, and a load that's less reactive at high
> frequencies. Best, however, would be to find a good single 50 ohm RF
> load resistor.
No particular experience, but my guess would be that by assembling a
series / parallel arrangement, any benefits of the lower parasitic
inductance of the lower value resistors would be lost by the wire
length to connect them in series.
Jim Backus OS/2 user
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