[amsat-bb] Linear vs Circular Polarization

Zach Leffke zleffke at vt.edu
Wed Apr 12 23:28:19 UTC 2017


Hi Brady,

My quick three cents:

first penny)  The coax relay at the feedpoint method is feasible and is 
how the M2 Antennas Inc. switchable polarization sense feedpoints work.  
I have no personal experience fabricating these, but we use them in the 
VT Ground station to switch between LHCP/RHCP.  M2 Antennas 
documentation warns of a slight VSWR increase with this technique, but 
so far (couple years now) they have performed great for us.

second penny)  While you might not have to cut phasing lines with the 
1/4 wave offset technique, you will likely need to cut 1/4 wave 
impedance transformation lines.  If your two feedpoints are 50 ohms 
each, then a 1/4 wave transformer of 75 coax will be needed 
(transforming to 100 Ohms) so that when they are placed in parallel (via 
a splitter/combiner) you present 50 ohms to the feedline/radio.  This 
link has a lot of good info on this:

http://www.qsl.net/sv1bsx/antenna-pol/polarization.html


third penny)  There is a great section in the ARRL Antenna Book (I think 
in the Antennas for Space Communications chapter) about using a coax 
relay and a pair of 50 ohm feedpoint yagis to achieve circular 
polarization.  The coax used in that example is both for phasing and 
impedance transformation in one, though requires 100 ohm (or close to 
that) coax (which last time I checked was available at the RF 
connection, who can also fabricate properly phase calibrated cables for 
you).  Also the relay in that example was not at the feedpoint of the 
antenna.  So it does exactly the opposite of what you are trying to do 
(antennas are not offset, and you need phasing/impedance transformation 
lines) but in my experience I found it easier than trying to crack open 
a (potentially expensive) coax relay and integrate it into a feedpoint 
(I left that to the pros at M2 Antennas).


Good Luck!


73s,

Zach, KJ4QLP

Research Associate
Aerospace Systems Lab
Ted & Karyn Hume Center for National Security & Technology
Virginia Polytechnic Institute & State University
Work Phone: 540-231-4174
Cell Phone: 540-808-6305

On 4/12/2017 6:51 PM, Richard Tejera wrote:
> Brady,
>
> Domenici, I8cvs SK had published a pair of articles on making a switching relay between vertical, Horizontal, LHCP & RHCP. he sent me PDF's, which are in my to do someday folder.
>
> Contact me off list (email is good on QRZ), and I will send them to you. They are for 70cm, but certainly can be adapted for 2m easily enough.
>
> Rick Tejera K7TEJ
> Saguaro Astronomy Club
> www.SaguaroAstro.org
> Thunderbird Amateur Radio Club
> www.w7tbc.org
>
> On April 12, 2017, at 14:11, Brady Gordon <ac0xr at lowswr.com> wrote:
>
> Hi Everyone,
>
> I'm working on building a pair of crossed yagis for 2m and 70cm, and I have
> some questions.  I apologize if this has been discussed before, but I
> couldn't find answers online so I was hoping you all could help me out.
>
> First, it seems easiest to me to build an antenna system that can either
> switch between vertical and horizontal linear polarizations, or between
> right hand and left hand circular polarization.  Which would be more
> useful?  And is it worth the extra effort to build an antenna system that
> can switch between all four polarizations?
>
> Also, I've been looking at the different ways of achieving circular
> polarization.  I like the idea of using mechanical 1/4 wave separation
> between the two yagis so I don't have to cut phasing lines.  Would it be
> possible to switch polarizations by putting a relay at the feedpoint of one
> of the yagis and switching which side of the driven element the center
> conductor and shield of the coax goes to?  I've never seen this done
> before, but it seems to me it would work.  Any thoughts?
>
> Thanks and 73,
> Brady AC0XR
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