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Re: Helix for 70cm?



At 10:01 PM 8/27/2002 -0400, AA2AE 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

Tony,

Going from memory Mike's helix was 23 turns.  I built an 18-turn (8-foot)
version of it in 1996 with a couple design differences, I used a circular
aluminum reflector (15-inch pizza pan), rather than the loop reflector.
Subsequent reading has revealed (to me) that the loop works fine.  Mike
built two of these to successfully hear the MGS beacon, so they work good!

I used a wooden closet pole with wood dowel posts to support the turns.  I
wouldn't do it that way again.  Much too time consuming and unnecessary.
The PVC tubing Mike uses is fine and fast and strong.  My closet pole
antenna broke after a year's use (even though painted).

I wouldn't get too worried about precision, as a helix is a forgiving
antenna.  Figure out your spacing and diameter is all there is to it.  I
used No. 10 insulated house wire (solid copper) and it worked fine on
AO-10.  I used a 1/4-wave straight piece of brass 1/2-inch strap to make
the matching at the connector and it matched perfectly without adjustment!

But you can just follow Mike's directions and it will come out fine (437 is
not significantly different from 435-436, so just use his dimensions)!

Ed, AL7EB

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