I've decided to rebuild my 2m antenna and have a couple of questions.
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.
Hi Matt, KC4YLV
Mounting the vertical and horizontal elements with an offset of 1/4 wavelength over the boom is good because feeding the driven elements in phase you get almost a one sense CP without adding delay lines. .
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.
There are masthead CP phasing and switching circuits with a minimum of mismatching and in addition the lenght of coax relays is not to be taken into account because the length of relays is compensated for provided all relays are of the same type and the same lenght. As an example using 4 coax relays of the same type antenna mounted you can switch from the shack V-H-RHCP and LHCP as described into the AMSAT-Journal march/april 2007 and may/june 2007
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.
This is correct but you must be able to exactly cut two runs of coax cable 30 feet long and having the same electrical lenght and first of all it is good to use both coax lines coming from the same coil and if you have access to a network analyser you can check if the phase difference between both feed lines is 0° degrees or not.
I'd then have an RHCP signal for the birds.
If the vertical driven element is ahead of the horizontal one to get RHCP the center conductor of the coax cable feeding the vertical driven element must be connected UP and the braid DOWN while looking the elements standing behind the reflector the inner conductor of the coax cable feeding the horizontal driven element must be connected to the RIGHT side and the braid to LEFT side. Since you plan to run two identical coax lines from the shack if you add in the shack a 1/2 electrical wavelenght of coax into the line feeding the vertical driving element then you get LHCP without changing any coax connections on top of the dipoles. The use of folded dipoles with the classic 1/2 wavelength coax balun is recommended.
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.
In theory if you cut the feedlines to half-wave multiples you only have the advantage to exacly measure the impedance of the antenna with an impedance bridge from the shack without to go on the roof and disconnect cables from the dipoles. By the way the VSWR into the line do not change cutting feed lines to half-wave multiples or any other lenght if the impedance of the antenna do not match with the impedance of the coax line.
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.
It is correct
(as a note, the two sets of elements would be tuned identically, centered for minimum swr on 145.8,
It is good
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.
Since your need is satellite and tropo your project sound to be good and simple.
Anybody care to share thoughts on this?
73 and merry christmas matt kc4ylv
73" and merry cristmas from