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

Ib Christoffersen oz1my at privat.dk
Mon Feb 18 12:15:57 PST 2013

Hi Pavel,
The issue may not be the antenna - but SO-50.
The antennas are linear - but the received signal strength can vary a lot if
you have just RHCP or LHCP.
During passes here in Copenhagen, Denmark I experience large differences in
signal strength which can be counteracted by changing polarity from RHCP to
LHCP during the pass.
With large changes I mean from nothing to 59 signals.
Hope that helps.
73 de OZ1MY/Ib

-----Oprindelig meddelelse-----
Fra: amsat-bb-bounces at amsat.org [mailto:amsat-bb-bounces at amsat.org] På vegne
af Ing. Pavel Milanes Costa
Sendt: 17 February 2013 05:22
Til: amsat-bb at amsat.org
Emne: [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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