----- Original Message ----- From: "Greg D." ko6th_greg@hotmail.com To: "Ronald G. Parsons" w5rkn@amsat.org; amsat-bb@amsat.org Sent: Monday, May 19, 2008 9:17 AM Subject: [amsat-bb] Re: AOP-1 Circular Polarization?
Actually, I was thinking about this today... There was an earlier thread which discussed the off-angle performance of a crossed Yagi, and if I recall, the pattern becomes elliptical as you move off the antenna's bore sight. If this is true, then I conclude that it would be better to mount a crossed Yagi in the + orientation, vs X, as that way one of the two antennas will be properly aligned for an off-angle linear station. For satellite work, with accurate keps, this is probably not a significant issue, but for terrestrial work and other situations where the target's position may not be known or tracked, + should be more forgiving than X. At least, that's my theory.
I kind of hope I'm wrong on this... I just got a new 70cm antenna from a swap meet today, and it's already mounted in the X position.
Greg KO6TH
Hi Greg, KO6TH
The interferometer-like effect that you mentioned is referred only to a dual boom antenna mounted configuration no matter if the antennas are mounted in a X or + configuration.
The distance between the two antennas make the phase of the received / transmitted signals to be different when the antennas are moved bore sight from the satellite.
Depending on the squint angle between the off-boom antennas and the satellite the above phase difference can make a RHCP signal to appear LHCP or elliptical or linear.
If both antennas are mounted instead over a single concentric boom this interferometer-like effect cancel out no matter if the antennas are in a X or + configuration.
This effect has been described into "The Satellite Experimenter's Handbook" by Martin Davidoff , K2UBC 2nd Edition pages 7-12 and 7-13
By the way the X configuration is the best if you use a metal-boom while the X or + configuration can be used indifferently if you use an insulated boom like fiberglass or wooden boom as demonstrated with experiments by WA5VJB
http://www.g6lvb.com/fibermetalboom.htm
In addition the X configuration is nice if you want to add V and H linear polarizations using a relay switcher for V-H-RHCP-LHCP as described into the AMSAT Journal March/April 2007 and May/June 2007
73" de
i8CVS Domenico
From: w5rkn@amsat.org To: amsat-bb@amsat.org Date: Sun, 18 May 2008 16:42:02 -0500 Subject: [amsat-bb] Re: AOP-1 Circular Polarization?
Assuming the antennas you reference do produce circular polarization, radiation wise, it doesn't matter whether you mount them in an X or + orientation. The only difference would be the phase of the radiation,
and
you are not concerned with that. There could also be a difference
between
right- or left-circular polarization, but whether that matters depends
on
the polarization the the satellite's antenna. But X or + does not change that.
There might be some mechanical reason for an X or +, such as weather proofing, but not signal wise.
Ron W5RKN
Date: Sun, 18 May 2008 14:28:34 -0400 From: Sebastian Subject: [amsat-bb] AOP-1 Circular Polarization? To: Amsat - BBs Message-ID: Content-Type: text/plain; charset=US-ASCII; format=flowed; delsp=yes
Hello all, I'm getting ready to put back up my old AOP-1 antennas (the old Cushcraft antennas that were popular in the days of AO-10 & 13). I'm wondering whether I should follow the same procedure of cross polarization for the 440 beam? It was recommended at the time to place the antenna in an X pattern instead of the cross pattern. Also, at the time, the 2 meter antenna was suggested to be placed in the standard vertical/horizontal polarization.
Please let me know which would be the best method to use for most of today's birds.
73s de W4AS