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Re: Helix for 70cm?



Tony:

I have worked AO-40 twice now with my 6-turn helix for the 70 CM 
uplink.  This antenna has a theoretical gain (untested by your truly) of 
about 11 db.

I would not worry the precision thing too much... do the best you can 
without killing yourself.  If you look at the web reference you quoted, 
there is an end view of one of the described helices.  You will note that 
the turns are far from perfectly aligned with each other.

The axial-mode helix seems to be quite forgiving of small dimensional 
errors, perhaps due to it's broad-banded nature.  Due to some 
miscalculation during the construction of mine, the copper-tube element 
wound up a bit short for the boom-spreader assembly.  I am not sure if I 
measured the tubing too short, or the spreaders were a bit long.  My 
initial solution was to pull the windings a bit tighter, resulting in a 
trochoidal, or rounded triangular, shape, this due to the spreaders being 
located 120 degrees apart.  This has the effect of raising the center 
frequency a bit, but the resulting antenna still seems to have a 1.5:1 SWR 
range of approximately 435 - 446 MHz!

Here is a summary of construction details of my helix, which is based on 
the design detailed in the ARRL Antenna Book, 18th Edition, "Portable Helix 
for 435 MHz", page 19-30:

Boom - two roughly 4-foot sections of tubular fiberglass antenna mast 
(military surplus), purchased at a hamfest for $4 each.  One of these 
serves solely to support a counterweight.

Spreaders - Fiberglass tent poles cut to size, then epoxied to boom.

Reflector - 12 inch aluminum pizza pan mounted in the center of metal 
screen roughly 2 feet square.

Radiator - 1/4 inch thinwall copper tubing.

Matching section - First half turn is of far less pitch than the rest of 
the helix... spacing about 1/8" at the N connector, and about 1/2 inch at 
the end of the first half turn.  Nothing additional was required for an 
adequate match.

There is a (not very good) picture of this antenna online at www.wcars.org 
.  Scroll down to the last of the field day paictures.  Although there are 
two of us standing in front of the antenna, you will get a pretty good idea 
of what it looks like.

Fred W0FMS has an online dimensional calculator for Helix antennas at 
http://vhf.worldsbest.com.au/Helix.htm

A single helix should be plenty to get you on A0-40 via the U band 
uplink!  I would not go to the trouble of building an array of 10-foot 
helices, unless you have a far more demanding application.

See you AO-40!
73,

Paul


At 10:01 PM 8/27/02 -0400, you wrote:
>Group
>
>A close inspection of my old KLM 40CX shows that the driven elements have
>somehow split where they bend to make the loop. Another problem seems to be
>the polarity switch - when switching between RH and LH polarization with
>FO-29's downlink and AO-40's uplink, the difference is very slight about 1
>S-unit at best so something isn't right.
>
>With that said, I was looking for a cheap way to optimize my 70cm AO-40
>uplink capabilities and looked at the Helix design by Mike Cook ( AF9Y )
>http://64.119.173.164/radio40.htm . The gain is impressive and the parts are
>relativily cheap. According to Mike the cost should be around $30. The gain
>on the 10 footer is 15.9dbc.
>
>Having no experience with the helix, it seems that the tough part would be
>shaping the elements to keep within the physical tolerances specified.  I
>was wondering if anyone is building high-gain circular polarized antennas
>like this on the commercial market? If not, it looks like I'll have to build
>my own.
>
>Any suggestions?
>
>Tony - AA2AE
>
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