> As to MATCHING the vertical. I'm using, and strongly recommend, the
> technique of using top loading to tune the antenna long (that is, to
> make it enough longer than a quarter wave that it's impedance is 50 +JX
> on the frequency where you want to use it, then add a capacitor of jX
> to tune out the inductance. It isn't practical to use this in some
> QTHs, but when you can, it works VERY well.
For what it's worth, for matching antennas that are less than 50 ohms,
I favor shortening the vertical a tad to show some capacitive
reactance and using a *shunt* inductor to step the feed impedance up
to 50 ohms.
Just to be clear, I'm talking about verticals where adding the extra
length doesn't really materially affect the radiating efficiency, so
I'm not talking going from 6 ohms radiation resistance to 50 ohms
radiation resistance over a 15 ohm ground system, but rather from 25
to 50 over a good ground system.
Advantages to the hairpin approach:
1) No extra length required. In fact, you need to shorten a little.
2) Good capacitors are expensive compared to capacitive reactance from
a shortened antenna.
3) Connected with 2, a shunt inductor for most HF antennas is just a
few feet of #10 or #12 solid ( if you want it to be self supporting)
wire wound into a small coil. Dirt cheap and you already have the
materials.
4) Your antenna ends up being DC and low frequency grounded. At
frequencies comparable to the operating frequency, the inductor is 50,
60 ohms or so, but at low frequencies it's lower. I don't know if
there's any real advantage to this, but you can't get static buildup
on a hairpin matched vertical and I feel better having my 60 foot pole
with a wire on it attached to ground by a #10 wire.
The calculations are a little more complicated, but if you've got your
Cebik.com login there's a section on Beta matching that tells you how
to do it.
There's more than one way to skin a cat but I think this one gets used
less frequently than is justified, given how darn cheap and easy it
is.
73,
Dan
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