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.
73, Art KC6UQH
-----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of Ing. Pavel Milanes Costa Sent: Saturday, February 16, 2013 8:22 PM To: [email protected] 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.