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

[i8cvs]

----- Original Message -----
From: "jonny 290" <jonny290 at gmail.com>
To: <amsat-bb at amsat.org>
Sent: Monday, December 24, 2007 10:59 PM
Subject: [amsat-bb] Questions on physically offset CP yagi feeding

> 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).
>
Agree

> 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

i8CVS Domenico