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Matching helix antennas



Hello all,

Very interesting discussion going on re the matching of helix antennas.

I've used helix antennas for many years now. My first serious one was
constructed in early 1983 in readiness for Oscar-10's launch. It was a 20
turn 70cm helix made using rg-8 coaxial cable (with the ends shorted) for
the radiating element. The reflector was a bit of over-kill being mesh with
a diameter of 1 metre. It was made according to the conventional wisdom of
the day and it worked very well, although it was heavy and clumsy. I could
regularly access (get a readable return signal from) AO-10 with as little as
100 mW measured uplink power on 70cm. When I found that ... I stopped
trying, hi.

The matching system used was the standard one in use in this country at that
time, namely to progressively alter the "lead" (that's an engineering term
to distinguish it from "pitch") of the first turn of the radiating element.
In other words, the first turn took up the same boom-length as the rest of
the turns but the first quarter-turn was very close to the reflector and the
next quarter-turn a little further away etc. the whole of the first-turn
being bent to blend into a smooth curve so that the important first
quarter-turn was almost parallel with the reflector. By gently bending the
first quarter-turn it was possible to get the vswr down to a very low figure
where no reflected power was measurable.

I used this method successfully on a number of helix antennas, even a
'monster' on 2 metres. 1269 MHz was also a breeze. When it was time to try
2400 MHz I began using the matching method described by James Miller in the
AMSAT-UK publication, Oscar-News. It was the soldered-on shim-brass strip
that some of the recent correspondents have referred to. I found this to
work well on 2400 MHz. As the helix antennas on this band are in the main
used for reception it seems quite aqceptable to simply follow James' design
and leave it at that. With a pre-amp connected (directly) to the helix via
an "N" type adapter, I found it possible to get excellent s/n ratios from
most of the satellites with 20-30 turns of radiating element, using 1/8 inch
refrigeration capilliary tubing and teflon stand-offs.  I used such a helix
with 2.25 turns to illuminate my 1.6m dish for 'mode-s' on Oscar-13 and look
forward to using it again on P3D. I have one in use daily on 1691 MHz for
the geo-stationary weather satellites.

Regards to all,

Bill Magnusson ... vk3jt
Milawa Australia

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