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I: R: Circular patch behaves linear



Hi Tony NX7U,

My answere to you at  nx7u@arrl.net  failed as usually so that i have
to send it via amsat-bb

73" de i8CVS Domenico

----- Original Message -----
From: i8cvs <domenico.i8cvs@tin.it>
To: Scott Townley <nx7u@arrl.net>
Sent: Wednesday, June 25, 2003 3:17 AM
Subject: R: R: [amsat-bb] Circular patch behaves linear


> Hi Tony NX7U,
>
> If a feed is circularly polarized and is looking a reflecting plane of
> metal sheet it acts as a mirror that reflect bach radio waves to the feed.
>
> Since the reflector is plane than the distance from it and the feed can
> be considered almost constant and radio waves reflected by the sheet
> arrives at the feed with almost the same phase but they are all out of
> phase with that transmitted by the feed because the mirror reflects back
> an opposite CP direction.
>
> Since the feed cannot get reflected energy from the sheet this is why the
> VSWR is not to much affected.
>
> If the feed is looking at a dish than the distance from any point of it
> and the feed is different.
>
> Only if the feed is positioned at the focal point of the dish than the
> radio waves reflected by the dish arrives to the feed with the same
> phase but since the dish reverse the CP direction they will be all out
> of phase.
>
> This is why the focal point of the dish is magic.
>
> Coming back to the problem,if the patch alone is adjusted for best VSWR
> looking at free space it see the impedance of 377 ohm
>
> When the feed is looking at the dish and its phase center is positioned at
> the focal point than the VSWR will match with that of the feed alone
> because the feed in the focal point is looking  the same impedance of
> 377 ohm of the free space by reflection from the dish.
>
> Best 73" de i8CVS Domenico
>
> ----- Original Message -----
> From: Scott Townley <nx7u@arrl.net>
> To: i8cvs <domenico.i8cvs@tin.it>
> Sent: Tuesday, June 24, 2003 2:43 PM
> Subject: Re: R: [amsat-bb] Circular patch behaves linear
> >
> >
> >
> > Dom,
> > Interesting.  Although your observation of return losses matching up in
> > free space and at the parabolic focus makes sense, I have to ask, why is
> > the focal point magic?  If a feed is perfectly circular, it should be
> > completely unaffected by any reflecting surface whether or not it is at
> the
> > focus.  If I make a perfectly circular antenna and move it closer to a
> flat
> > sheet of metal, I see no change in VSWR, but the sheet doesn't have a
> focus.
> > ?
> >
> >
> > >Hi Tony AA2TX,
> > >
> > >I have built one patch of K3TZ and four of G3RUH designe all for 2400
MHz
> > >and i have adjusted both for the best compromise of circularity and
> return
> > >loss in my lab with the patch alone looking at  free space out of the
> > >window.
> > >
> > >Without to go in performance comparison i have seen the following:
> > >
> > >Suppose the return loss with the patch alone looking at free space is
25
> dB
> > >and circularity - 1dB
> > >
> > >When i place the patch in to a prime focus dish than the return loss
> changes
> > >greatly as i move the patch up and down  around the focal point but as
> soon
> > >the focal point fall in the exact phase center of the feed than the
> return
> > >loss become again exactly 25 dB as in the lab.
> > >
> > >For the G3RUH patch i found that the phase center fall exactly over the
> > >outer surface of the active element.
> > >
> > >If you measure the return loss before in the lab with the patch alone
> > >looking at free space and if you readjust the patch position at the
dish
> for
> > >the same return loss around the focal point than you have reached the
> exact
> > >position for the phase center of the feed.
> > >
> > >If you get the same return loss with the active element positioned at
the
> > >geometrical focal point it is an indication of a good circularity
> separation
> > >because the feed is LHCP and the dish reflects RHCP energy to the feed.
> > >
> > >If the feed has a good isolation between RHCP to LHCP than the energy
> > >received back by the feed from the dish is very small and return loss
is
> > >small affected.
> > >
> > >For return loss and circularity measurements i have used a rotatable
2400
> > >MHz dipole with balun + a HP8690B Signal Generator + HP11692D Dual
> > >Directional Coupler and HP 8555A Spectrum Analyser
> > >
> > >If any one is interested on pattern diagrams,circularity diagrams and
> pics
> > >of the above test set up please send me a  message and i will replay to
> all
> > >ASAP
> > >
> > >73" de i8CVS Domenico
> > >
> > >
> > >
> > >
> > >
> > >
> > >
> > >
> > >
> > >
> > >----
> > >Sent via amsat-bb@amsat.org. Opinions expressed are those of the
author.
> > >Not an AMSAT member? Join now to support the amateur satellite program!
> > >To unsubscribe, send "unsubscribe amsat-bb" to Majordomo@amsat.org
> >
> > Scott Townley NX7U
> > Gilbert, AZ  DM43di
> > http://members.cox.net/nx7u
> >
>
>
>
>
>
>
>




----
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