[amsat-bb] Helical Antenna for 2-meters

[Edward R Cole]

Dave,

I've built helical beams, before.  Latest was 6-turn feed for an 
offset feed dish to receive AO-40 on 2401 MHz.  But back in 1996 I 
built a 14-turn 437 MHz helical beam to try to hear MGS at 10M km 
from Earth.  Did not finish the 437 converter in time but did try out 
the helical beam on uplink and found it not near as good as my M2 
436CP42UG (not surprisingly).

I used flat brass strip to match 140 natural impedance of a helix to 
50-ohms.  The length was 1/4 wavelength and spacing from the ground 
plane reflector varied from direct connection to the flange-mount 
N-connector to about 3 times further at the connection to the 
starting turn of the helix.   Easy to tune by varying that distance 
for best match.

I think 2m is the lowest frequency I would make a helical beam.  That 
20-MHz monster that MIchael links to - well I think I would rather 
make a x-yagi (which would permit using either RHCP or LHCP).  Most 
amateur Jupiter radio astronomers only use a crossed dipole antenna.

73, Ed - KL7UW

Date: Fri, 24 Feb 2017 12:21:32 +0200
From: Michael Fletcher <oh2aue at kolumbus.fi>
To: amsat-bb at amsat.org
Subject: Re: [amsat-bb] Helical Antenna for 2-meters

 > I just did some calculations from the ARRL Antenna Handbook.
 > I came up with a 6-turn helical with a diameter of 25.826" and
 > a length of 121.65".   My first thought was "whoa, dude, like OMG!"
 > (I'm a high school teacher, ok?)   I rechecked my calculations and
 > came up with same, plus an impedance of 150 ohms.
 >
 > Years ago I used a Fritzel Helical for 2m when I was DA1BB and VP2EHF
 > (during Hurricane Luis in 1994 it ended up someplace out in the
 > Anagada Straights between Anguilla and Sombrero Island).
 >
 > What say, you all?   Advice, comments welcomed
 >
 > 73
 > Dave, N4CVX

Nah Dave,

that's not big, just a scaled down version of e.g. this one (for
~22 MHz, reception of Jovian storms, built by OH8PX, SK):

http://www.rfshamaanit.fi/hf_helix_1.jpg

Your impedance is about right too, but there are several neat ways
of getting this matched to 50 ohms in a fairly broadband manner,
easily googled.

73 Michael OH2AUE


73, Ed - KL7UW
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