[amsat-bb] Re: AOP-1 Circular Polarization?
domenico.i8cvs at tin.it
Mon May 19 18:29:27 PDT 2008
----- Original Message -----
From: "Sebastian" <w4as at bellsouth.net>
To: "AMSAT-BB" <amsat-bb at 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
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
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
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.
> On May 19, 2008, at 4:44 AM, i8cvs wrote:
> > ----- Original Message -----
> > From: "Greg D." <ko6th_greg at hotmail.com>
> > To: "Ronald G. Parsons" <w5rkn at amsat.org>; <amsat-bb at 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 at amsat.org
> >>> To: amsat-bb at 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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