> >Wouldn't a shorter than quarter wave result in a feedpoint that exhibits
> >capacitive reactance which would require an inductor in series to cancel
> >the capacitive reactance?"
>
> No, that is one of the neat things about a folded unipole, the sine of the
> reactance is reversed from that of a series fed antenna element. The short
> folded unipole can be viewed as being a section of transmission line which
> is less than a quarter wave, and shorted at the far end. It exhibits
> inductive reactance.
But........ keep in mind it is generally much less efficient for the
same height than a conventional base loaded vertical with inductor
loading unless you use very large conductors for the hairpin and
high current rated capacitors. If you used the same size
conductors in a loading coil of proper form factor, the loading coil
would once again be better!
A shortened hairpin monopole is never as efficient as a reasonably
well constructed top-hat loaded antenna of the same height, mostly
because it has a triangular common-mode current distribution and
a very long loading system carrying high VAR power.
The common mode current (and that is the only current that
causes radiation) is always less than the differential mode current
that contributes to I^2 R losses in the system. A normal loading
system does not have that problem.
One of my unsuccessful early brainstorms was a parallel wire short
hairpin monopole with low loss capacitor loading. It would tune 1.8
to 30 MHz perfectly, but unfortunately it eliminated the coil loss
and replaced it with even more loss in the long copper loop forming
the antenna. The system was over 10 dB better with a hunk of
B&W airdux for loading, because the path through the copper
loading system was shorter and carried much less current!
Most of my days experimenting with folded monopoles were days
spent learning to always look at the big picture of what really goes
on. For multiband and multi-frequency applications where efficiency
doesn't matter, the short folded monopole is just dandy. If
efficiency matters and you only operate a single band, you're better
off with a conventional loading system in a short antenna.
73, Tom W8JI
w8ji@contesting.com
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