[amsat-bb] Questions on physically offset CP yagi feeding
jonny290 at gmail.com
Mon Dec 24 13:59:14 PST 2007
I've decided to rebuild my 2m antenna and have a couple of questions.
I currently have vertically polarized yagis up for 2m and 70cm and they
work fine; however I am becoming more interested in weak signal work and
just want to rebuild my antennas in general, as they need it after a
year. Due to necessity and environment, i can only have a few antennas
up and have thought up a plan to meet all of my needs at a reasonable
price with only one cross yagi in the air.
My thinking is to build a six element wide-band 50 ohm match yagi and
mount the vertically polarized elements 1/4 wavelength
ahead of the horizontal ones, and feed the elements in phase. I have
examined the possibility of masthead CP phasing, but am
concerned about mismatching and like the arguments given for the
physically offset, in-phase fed elements.
My antenna to radio coax run is right around 30 feet. I am thinking of
running two lengths of LMR-240, speced at 3 dB loss/100ft, so loss
should be about 1 dB. If I cut the two feedlines to identical lengths, I
should be able to connect them with a coax tee in the shack and use a
1/4 wave 37 ohm line to bring the system impedance back up from 25 (at
the tee) to 50 ohms. I'd then have an RHCP signal for the birds. Do I
_need_ to cut the feedlines to half-wave multiples, or is it just a good
idea? I plan on tuning the antennas as dead on to 145.8 as I can, so
mismatch will be a minimum at that frequency.
When I don't need RHCP, I can simply use the horizontal feedline for SSB
work and the vertical feedline for FM work, setting the coax tee
nonsense aside. (as a note, the two sets of elements would be tuned
identically, centered for minimum swr on 145.8, but in my experience
operation across the band is no problem).
A good coax relay costs about 60-80 dollars and requires associated
mounting hardware and weatherproofing, this only adds another $15 for 30
more feet of LMR240.
Anybody care to share thoughts on this?
73 and merry christmas
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