[Amps] Dummy Load Blues

[Larry Benko]

Manfred,

Just FYI.

Using your numbers and assuming the SWR issues are excess inductance, 
the inductance necessary to turn 50 ohms to a 1.5:1 SWR on 2m is ~23nH.  
If you were to put 8pF from the center pin to ground at the connector of 
the dummy load your 2m SWR would drop to 1.17:1 and the HF SWRs would 
get either slightly better or be virtually unchanged.

73,
Larry, W0QE

On 6/14/2012 9:06 PM, Manfred Mornhinweg wrote:
> I built my very own Cantenna-like dummy load several years ago. The
> resistor is a carbon/ceramic tubular one with silvered ends, about 4cm
> diameter and 10cm long. It's 50 ohm, but I have no idea about its power
> rating. I connected it with silvered copper straps and put it in a metal
> can that came from the supermarket, containing powdered milk from the
> famous brand "Nestle". It's the same size as those gallon paint cans,
> but it has no handle, so that's a downside.
>
> I soldered the lid on to get it oiltight, and added an oil filler nipple
> with cover, that allows air to move in and out as the oil expands. The
> oil is from Shell, specially made for transformers, PCB-free. The
> smallest amount we could buy was a bucket, so several club members
> joined efforts and shared a bucket.
>
> I placed a nicely printed sticker on it, with its name "TarroAntena",
> Spanish for Cantenna.
>
> So far I haven't been able to burn out this load.
>
> The SWR is essentially 1:1 over the HF spectrum, slightly higher on six
> meters, but on 2 meters it's already about 1.5:1. That's because the
> impedance of the strap connections isn't controlled, so the SWR rises at
> the frequencies where they become of noticeable length compared to the
> wavelength. This could be improved by making conical connections. The
> ARRL Handbook has plans for this. But for my use, it's fine as it is,
> because on the rare ocassion when I need a legal limit dummy load for
> VHF, I use a little PI matching circuit to get the SWR down to 1:1.
> Since the Q is low, it's still quite broadband.
>
> Manfred
>
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