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Re: Cheap yagi info



ctpm@vega.net.dhis.org> writes:

> Can the matching scheme used mess with circular 
> polarization operation?

Claudio,
These antennae will work fine for circular polarization, as the impedance matching scheme has little effect on the field pattern.  For fixed circular polarization, the easiest method is to simply offset the orthagonal elements by 1/4 lambda in the axial direction.  Feeding the two 50 Ohm impedance antennas in quadrature, however, can be done several ways.  The traditional method is to use odd-1/4 wavelength-sections of 75 Ohm coax to a tee.  This is sometimes physically inconvenient and is difficult to tune, especially at 70 cm and higher, since you are dealing with a double-transformation and a recombination.  

Another, simpler method I have developed and used succesfully is to run 50 Ohm coax from each antenna (any length as long as equal) to a tee.  Then, on the common port of the tee, connect a 1/4 wavelength matching transformer of 37.5 Ohm impedance.  This transformer can be constructed easily by using two parallel sections of 75 Ohm cable (e.g., RG-59), each electrically 1/4 wl long, terminated in N-type or PL-259 connectors.  

This method allows each antenna to be independantly tuned for best SWR with the standard 50 Ohm feedpoint, and then the SWR meter can be moved to the common feedline and the transformer can be "tuned" by trimming it for best SWR (of, course, make it slightly long to start).  I used this construction technique on a pair of 70 cm quagi's several years ago that served me well on AO-10 uplink and were good FD antennas (the elements were orthagonal but not concentric as I used separate booms--but the concept is the same as a Yagi).

For switchable circular polarity, simply substitute the coax tee with a coaxial relay and the appropriate phasing/delay line.

Good luck with the project!
73,
Jerry, K5OE

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