[amsat-bb] Questions on physically offset CP yagi feeding

[jonny 290 <jonny290@xxxxxxxxx>]

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
matt kc4ylv

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