----- Original Message ----- From: "Sebastian" w4as@bellsouth.net To: "AMSAT-BB" amsat-bb@AMSAT.Org Sent: Monday, May 19, 2008 4:15 PM Subject: [amsat-bb] Re: AOP-1 Circular Polarization?
Thanks for everyone's replies. It looks like the consensus is to mount the beams in the terrestrial + pattern (vertical/horizontal). I'll be mounting the antennas on a fiberglass pole that served me well years ago (it's been keep safe inside the attic). This is good, because that way I should be able to use those antennas for local repeater work with minimal loss if any, due to using both polarities at the same time.
Hi Sebastian, W4AS
If you mount the beams in the terrestrial + pattern (vertical /horizontal) on a fiberglass pole it is good but to get both polarities at the same time with minimal loss if any is impossible.
Your old Cushcraft AOP-1 satellite antennas consists of a 144-20T crossed yagi for 2 meters and a 416TB crossed yagi for 70 cm and both antennas are mounted with their own elements into the same physical plane (or about)
According to the manual of the 144-20T if you use the phasing harness supplied with the antennas you will get the RHCP or the LHCP and when the delay line is not connected you will get the so called "Axial Radiation" that apparently looking at the manual seems to generate the linear Vertical and Horizontal polarizations both at the same time but this is a misinterpretation.
If you plan to connect the antennas as the manual suggest for the "Axial Radiation" with the purpose to use both V and H polarizations at the same time this is not a good idea and I will demonstrate that if you feed both antennas in phase in to a + configuration as suggested by the manual you will loose 3 dB over the real Vertical or 3 dB over the real Horizontal polarization.
In the Axial Radiation the Vertical and Horizontal fields generated by the dipoles are only the components of the field but the resultant of the field is oriented at 45 degrees or at 135 degrees depending on how you have connected the feed lines to the dipoles.
If we assume that the voltage components over the V and H dipoles are both 1 volt than the voltage of the resultant vector oriented at 45 degrees is the diagonal of a square or 1.41 volt so that over the same radiation resistance the power contained over the resultant vector oriented at 45 degrees is 2 time greater or 3 dB greater than the power radiated by the vertical and by the horizontal dipoles at the same time.
It follows that if you plan to use those antennas for local repeater without to loose 3 dB you must feed only the Vertical antenna and if you plan to use the horizontal antenna for tropo without to loose 3 dB you must feed only the Horizontal antenna. In addition if you run to the shak two coax lines having the same lenght and the same type of coax cable you can add a 1/4 wave delay line and get with easy the V-H-RHCP and LHCP polarizations as the manual suggests
By the way if you mount those antennas in a X configuration over a fiberglass boom it is equally good but in this case the elements oriented at 45 degrees and the elements oriented at 135 degrees when supplied at the same time becames the components of the field and the resultant will be a field horizontally or vertically polarized depending on how you have connected the feed lines to the dipoles.
Again running two feed lines of equal lenght into the shak and using a delay line 1/4 wave electrically long you can switch with easy between RHCP and LHCP but to switch between linear V and H polarizations is possible but it is more complicated.
73" de
i8CVS Domenico
On May 19, 2008, at 4:44 AM, i8cvs wrote:
----- 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
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Sent via AMSAT-BB@amsat.org. Opinions expressed are those of the author. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: http://amsat.org/mailman/listinfo/amsat-bb