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Re: Helix feed details



>From: David Rush <ky0dr@earthlink.net>
>Greg:
**snip**
>Then a friend came over with an antenna analyzer that would do up to 3
GHz.  We experimented with the spacing, and found the 
>greatest return loss (best match) with zero washers in place, and the
matching section much farther from the reflector than the G3RUH-
>spec distance.  So I left it there.

Greg and David,

I think one can get too concerned about the placement of helix connections.
 Unless you duplicated G3RUH's design EXACTLY, then the matching will be
different.  Unless you have equipment for measuring return loss [SWR], I
would not get overly concerned.  More important is adding a piece of copper
or brass stripline [1/4 to 1/2 inch wide by ~1 inch long].  This does more
to getting a good impedance.  SWR is more important for transmitters.  I
know someone will jump in with all the theory about proper impedance match
affecting NF, but if you don't not have the equipment to evaluate return
loss and NF, why sweat it!

Here is how I built my helix.  My ground plane is a 3 inch square of 1/8
inch thick aluminum plate.  I use the panel mounted N-connector that has a
square flange with four holes.  I insert the bolts from the helix side and
place the nuts on the connector side, thus minimizing the projection into
the radiating space.  The thickness of the aluminum plate just about
matches the projection of the N-connector "bump", so I can space my strip
line as close as desired.  Mine is about 1/8 inch, but I have no way of
measuring SWR at present, so I can not say this is particularly correct ;-)

Finally regarding reception of AO-40, be sure to compare different feed
results on the same pass.  Squint angle affects your received signal more
than nearly any other factor other than range {maybe more so}.  Tonight my
squint starts at 73 degrees and ends at 113 degrees...impossible to hear at
60,000 km without a preamp [and probably impossible even with a preamp
using my 33 inch dish].  If I used a 60-foot dish it probably squint would
be no problem...yuk, yuk!  

PS:  I was told my db6nt preamp shipped from SSB yesterday, so hope next
week to be able to report how it works with my minimally modified Drake [NF
6 to 8 dB] and a 33 inch offset-feed dish with 5-turn helix.

Ed

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