> > > Someone suggested using a CURRENT SOURCE set to 1 AMP
> >> and then measure the Voltage across the antenna joints. Sounds
> > > like a better solution to me.
>
> I'm not an expert on DMMs, but I believe that in resistance-measuring
> mode they _do_ apply a current-source (typically 200 mA) to the
> "load" under test, and measure the voltage across the load. The
> output voltage of the current-source is of course limited, but the
> limit is somewhat above the full-scale-display-equivalent.
>
[Steve Katz] I was just using a Simpson analog meter which provides
a current from a source having 4.5Vdc potential. When measuring 20 Ohms
resistance, the resulting test current is >200 mA. Still, not "Amps," but
significant current.
On a related subject (sort of), zinc is not a great conductor. That
someone measured the resistance of his tower at "zero" (presumably small qty
of milliohms) seems unreasonable to me, although maybe not out of the
question -- there's a lot of plating there, and with three legs in parallel
and all the webbing, maybe there's more surface area than I would have
guessed.
When we used a tower to support an array of switchable half-slopers
(where the tower is half the antenna) for 80m at a contest station we put
together in NJ back in the early 1980's, we noted an enormous difference in
everything (impedance, performance, reliability) when we added a big, fat
copper wire down the middle of the tower to parallel the tower itself,
electrically. Just the tower: Varying feedpoint Z, changing almost with the
wind, and higher Z than we would have liked. With the wire: Steady as a
rock, nothing changes, lower feedpoint Z (close to 50 Ohms). We figured
this was a good use of $5 worth of wire.
-WB2WIK/6
> Many DMM's also have a so-called "Diode" testing mode, in which the
> same thing is done except that the voltage is displayed directly
> (without scaling, or division by the current in amps to get the
> resistance in ohms) so you can simply see the forward voltage drop of
> a diode or a transistor junction.
>
> -Chuck, W1HIS
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