AEA Isopole conversion from 2m to 440

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James Chance:
I've got an old AEA 2m Isopole, and am wondering if it can used on 440 by modifying the vertical element length, and changing the skirts spacing. Anyone have a manual for the 440 version showing dimensions?

TIA,

Jim
N3TKD

Dale Hunt:
Well, you could try it if you wanted to, but I don't
think it would work very well.

The Isopole is a dual-5/8 wave design, also known as
an "extended double zepp".  (So are most other 2m
antennas that are about 8' long.)  The input impedance
of such an antenna is capacitive, requiring some sort
of loading inductance at the feedpoint.  That coil is
included in the feedpoint part of the Isopole, and
includes some impedance correction for the use of
the less-than-optimal PL-259/SO-238 connectors.  The
coil induceance and impedance correction will not be
correct for use on 440 MHz.

Even if you could find a combination of settings that
gave a low SWR on 440, the resulting radiation pattern
would probably not be as good as the original was on
2m.  5/8 wave is the maximum length for each section
to optimize gain at the horizon.  If the radiators
are any longer, more of the power goes into lobes at
higher radiation angles that are rarely useful.  So
to get a good pattern you would need to shorten the
top and bottom sections each to 5/8 wavelength.

The top section isn't a problem, but the bottom section
consists of the distance from the feedpoint to the
bottom of the upper cone.  The cone itself is 1/4 wave
on 2m, or about 3/4 wave on 440.  Clearly, even with
the cone moved all the way up to the feedpoint the
lower element will still be too long unless the cone
itself is shortened.  (The lower cone serves only to
decouple RF currents on the mast, and probably could
be left the current length.  It should be positioned
with it's top even with the bottom of the upper cone.)
So basically you have cut the 2m Isopole down to the
size of a 440 Isopole - but you still have the problem
that the internal matching is not correct for this
frequency.

Remember that the Isopole, or any other dual 5/8 wave
antenna (i.e. any 8' tall radiator for 2m) only has 3dB
of gain over a vertical dipole at the same height.
Trying to make it work with elements longer than 5/8
wave will quickly lose most of this gain due to the
radiation at higher angles.  My guess is that any
attempt to convert the Isopole to 440 would not give
any better results than a simple ground plane or J-pole
antenna, either of which would take less work and still
leave the Isopole usable on 2m.

But, of course, you can try it if you want to...

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