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RE: Circular patch behaves linear



Achim

I am not sure how the thickness of the patch affects things (probably not 
much) but what I found very critical with the K3TZ patch was its spacing 
from the groundplane.  This had major affects on the circularity (and also 
the VSWR).  After optimisation, the circularity was very good. I found that 
optimisation for circularity rather than VSWR was the better goal.  I will 
try to post info on the web in the next day or so.

Measuring circularity requires a sense antenna with some gain,  or stray 
reflections can really mess up the results.  I use a WA5VJB PCB log 
periodic and had much better results with this compared to a dipole. I 
prefer to leave the patch fixed in position and rotate the test antenna.  I 
have found it OK to sample at 45 degree intervals (across 180 deg).  90 deg 
is too coarse as I have seen the patches produce strongly elliptical 
polarisation at 45 deg to the patch edges, which looks great on a 0 and 90 
deg sample but terrible when you sample at 45 deg intervals!  To minimise 
the effects of moving cables etc getting in the way, I used a rotating 
joint behind the test antenna.  Test equipment was an ANA with the patch on 
port 1 and the sampling antenna on port 2.  Spacing between patch and sense 
antenna was in the order of 15 inches giving a measured loss of about 30dB 
if I remember correctly.


73

Charlie G3WDG




At 09:39 24/06/03 +0200, 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
>


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