[amsat-bb] Re: QFHA for SO-50... polarization issue ???

Art McBride kc6uqh at cox.net
Mon Feb 18 10:32:02 PST 2013

Ing. Pavel,

There are a whole series of QFH antennas. 1/4,turn, 1/4 wavelength through 1
turn 1 Wavelength. 
Gain at the horizon is determined by the ratio of the axial length in
fractions of a wave length. The best performance I have obtained is a 1
wavelength per side, 1 turn, for good gain at the horizon with an axial
length of .65 wavelengths. You need the least gain at the Zenith as the
satellite is closest in range at that point.

The best reference I found is "Resonant Quadrifilar Helix Design" by C.C.
Kilgus, December 1970 Issue, of the Microwave Journal, Pgs 49-54.


-----Original Message-----
From: amsat-bb-bounces at amsat.org [mailto:amsat-bb-bounces at amsat.org] On
Behalf Of Ing. Pavel Milanes Costa
Sent: Saturday, February 16, 2013 8:22 PM
To: amsat-bb at amsat.org
Subject: [amsat-bb] QFHA for SO-50... polarization issue ???

This weekend I tested a Quadrifiliar Helicoidal Antenna for 70cm (LHCP)

I designed it by the online calculations of jcoppens on the internet...

It is a 1/2 turn and 1 lambda, about 5 meters of elevation with ~25 cm 
of RG-58U (a section of it is coiled to form a balum) and then ~15 
meters of A.H. LDF4-50A down to the shack directly to the radio, of 
course BNC to N adapter on the antenna and N to PL259 on the radio 
side... It's at mast top.

Results are good... In passes of over 20 degrees (almost the real 
horizon here) I have a good copy, until... TCA, in which I lost the 
signal completely...

TCA = Time of Closest Approach.

In northbound passes (I've not tested it in southbound) it work as I 
just said...

I have been reading and investigating this strange issue and I think it 
is due to polarization...

I had read that SO-50 is linear polarized on 2m AND 70cm with aprox 45 
degrees each other...

Then Linear to LHCP in the worst case is 3dB loss (half power) but 
consistent over the entire pass... or have SO-50 some 
circular/elliptical component in it's signals that renders useless the 
antenna after TCA? spin maybe?

There is any chapter of the book I missed or other variable(s) that I 
have not taking into account here?

Any comment, link or other kinds of help are welcomed..

73 de CO7WT.

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