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RE: Circular patch behaves linear



Achieving good circularity in any antenna is a *non-trivial* 
exercise...which isn't to say it can't be done, but it's not falling off a 
log either.
NOTE:  I don't think that using a rotatable dipole for a test antenna is 
the best approach to test circularity in most "backyard" labs.  That's 
because unless you get that rotatable dipole way up high (several meters 
for a few tens of meters of antenna separation), the magnitude of the 
reflection coefficient from the earth for horiz vs. vert is different, and 
the field at the CP antenna you're measuring will not be the same.  Better 
to keep the dipole fixed (either horiz or vert), adjust the height of the 
CP antenna until you're in a field peak, and then *rotate the CP antenna 
under test* to see the response variation with rotation.
However, just because I think that, doesn't mean I'm right.  I did however 
work out the reflection coefficient math and for dipole heights of 3m it 
takes ~100m separation to reduce the variation in reflection coefficient 
sufficiently...any comments?




At 00:39 2003-06-24, Achim Vollhardt wrote:
>Hi Patch users,
>just to drop in my two cents:
>
>I built an LHCP and an RHCP patch of the K3TZ style.. exactly to
>mechanical dimensions given by K3TZ with thin metal sheet (0.3mm or 1/80
>inch).
>
>Result: the RHCP patch showed the exactly same return loss plot as
>simulated (double dip with higher frequency dip deeper) with second dip
>at 2440 MHz .. 18dB return loss . At 2400MHz, it still had 13dB ret loss
>with second dip at 2.3 GHz and 8dB ret. loss. so much to SWR.
>
>Circularity: I compared AO-40 beacon strength with LHCP and RHCP patches
>barefoot. Good copy (at leat for TLM, not for SSB, but enough for CW)
>for RHCP, nothing at all for LHCP.. looks rather circular to me.
>
>Conclusion: this K3TZ thing CAN be circular and quite close to the
>simulated performance. I have the impression, that this could have
>something to do with the thickness of the patch.. which is quite thin
>for me.. any comments welcome.
>
>
>Still, I will build up some W0LMD patches.. these look so much more
>mechanically stable to me :-))
>
>Any hints to get them tuned?
>
>DJ9PC suggested to build a signal generator with small dipole and rotate
>it.. turn the tuning screw on the patch until no effect can be seen.
>
>73s Achim, DH2VA
>
>
> >
> > Message: 5
> > Date: Mon, 23 Jun 2003 23:14:37 +0000
> > From: "Frederick M. Spinner" <fspinner@hotmail.com>
> > Subject: RE: [amsat-bb] Circular patch behaves linear
> >
> > Impressive.  Those all appear to resonate a little bit high (ideally the
> > center of the W in the dip should be at 2401 MHz).. but they all look like
> > they'd work fine and be CP.
> >
> > I never got my K3TZ style to show that dip or CP.   But I do know that
> > theoretically that design should work.  I've just never gotten one to do
> > that.
> >
> > Congrats on that.  I probably stand corrected that the G3RUH/W0LMD style is
> > the only one I've ever seen being CP.
> >
> > Also all of the guys who wondered what a CP patch should look like on an
> > analyzer... there you go.
> >
> > Fred W0FMS
>----
>Sent via amsat-bb@amsat.org. Opinions expressed are those of the author.
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Scott Townley NX7U
Gilbert, AZ  DM43di
http://members.cox.net/nx7u

----
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