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Helical array (was ... well never mind)



Terry,

Helical array?  Are you thinking of a pair for switching between RHCP
and LHCP?  If so, consider a pair of Yagis, and a coax relay/delay line
to switch between circularity sense.  In my experience, helixes are
cumbersome to balance and will weigh 4 times as much as a Yagi satellite
setup.

The ARRL antenna book has an excellent article on helixes, including a
short piece on matching to 50 ohm coax using brass shim above the feed
point.  I have built a few using this technique.  The more I thought
about it however, the tighter Q of using a coax transformer might serve
the sat op better by being less apt to suffer from desense.  Many of
these newer "do all" rigs suffer somewhat from lack of dynamic range,
and a wide bandwidth antenna, especially on the 3rd harmonic of your 2
meter uplink, is more apt to cause the rig to overload.

Whatever the case, I highly recommend a mast mounted preamplifier for 70
cm, helix -OR- Yagi.  Not only can I hear very weak signals with my KLMs
using an ARR preamp, but the extra "section" before the receiver front
end improves the out of band rejection on my FT-847.

If you're thinking about an array for EME or something, I found a site
for a 4 way power divider made of coax adapters.  It is 3/4 wave on 1296
and 1/4 wave on 432, so ...   Look at:

	http://www.ij.net/packrats/Article_6/w3km_picks.htm#Anchor-1296-27239

Let us know what you decide.  It's great to see the reflector back to
technical discussions.

73, Mike

Terry Bendell wrote:
>  ...  also any thoughts on a 70cm Helical antenna
>  Array in terms of dos and don'ts?
> 
 
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