I just made a 10 turn helix on 435 MHz and added a 15 db preamp from the local electronics store.
I just copied genesat at S7
I have never seen it so high.
Maybe you might like to try a helix >?
-----Original Message----- From: [email protected] [mailto:[email protected]]On Behalf Of jonny 290 Sent: Tuesday, 25 December 2007 7:59 AM To: [email protected] Subject: [SPAM] [amsat-bb] Questions on physically offset CP yagi feeding Importance: Low
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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