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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Visit my hobby homepage!
http://ludens.cl
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