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Re: 5 1/2 turn helix for offset dish

At 11:41 AM 8/3/2002 -0400, ASTROHAM47@aol.com wrote:
>Can anyone help..who has made a helix for offset dish...I have gotten info 
>off of several web sites...one refers to flattening the 1st 1/2 inch of the 
>helix..and soldering  coax foil sheild to it?? making matching section...I
>not  understand what to do here...I know you dont want the helix to touch
>Mike Miller KA5SMA


I hope others will give you their approach to helix impedance matching.

Most techniques are variations of the method developed by James Miller,
G3RUH, and shown in Ed Krome's (K9EK) "Mode-S the book" available from
Amsat:  http://www.amsat.org

The natural impedance for an axial-mode helix (what we are talking about
here) is approx. 140-ohms, so the trick is to feed it with 50-ohm coax.
G3RUH came up with using a strip line attached to the first quarter turn of
the helix (which happens to be roughly a quarter wavelength).  His was cut
to match the curve of the helix winding and is 6mm wide by 0.2 mm thick
brass by the length of a quarter-turn.  It sits onto the coax connector pin
and spaced 1.2mm above the reflector plate.  The spacing increases to 3mm
at the other end (one quarter turn away).  {from Fig. 2, page-88 "Mode-S
the book", first ed.}

My 5-1/4 turn helix feed (BTW make helix antennas n+1/4 WL long) is
similar, but I used a 1/2-inch by 0.93-inch piece of brass strip bought
from a hobby shop.  I drilled the one end to slide it down onto the
four-hole flange-mount style Female N-connector.  I placed as low as
possible without shorting (maybe 3/32-inch).  The other end is something
like 3/16-inch away from the reflector.  The connector is placed roughly
below the edge of the helix, not in the center of the helix, and the strip
runs straight to the near-end of the helix.  Looking down from the end of
the helix the strip line cuts across the circle of the of the helix on a
slant (this is really hard to adequately describe in words).

Since this is a receiving antenna, and the fact I have no test equipment
for measuring SWR or return-loss, I can only say it works!  If I had a
transmitting source or signal generator I could optimize the match.  But I
think many, if not most, who are making AO-40 helix feeds are in the same
boat and just build-it and use-it!  Fortunately the helix is a forgiving
antenna to build.  It is very wide bandwidth so if you are off a little it
will not matter.  But try to get the dimensions close, space the turns
equally and at the right pitch.  If you use a center support it must not
short-out any turns if metallic (I have a steel bolt running exactly up the
center with a nut and washer to hold the 1-inch PVC tubing form in place.
I found that the solid copper No. 8 wire in the helix is stiff enough to be
self supporting, though.  The weak point is the solder connection at the
connector so I opted for the PVC support.

Anyway, that's how I did it!  There are many other ways...and I'm sure you
will hear a few!


AO-40 mode-S:  33-inch offset helix-fed dish-->MKU-232A2 preamp-->two
Drake-2880 conv.(123 & 435 IF's)-->FT-847
mode-U:  FT-847-->60w amp-->M2-436CP42U/G
mode-L:  FT-847-->DEM 144/1268 (15w)-->no antenna (plan on using my 8-foot
dish with 3-1/4 turn helix feed)
mode-K:  in my dreams: ?dish-->?preamp-->P-com 24G/3456 MHz-->DEM 3456/144

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