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R: Patch

----- Original Message -----
From: Eddy Jespers <ejespers@wanadoo.be>
To: i8cvs <domenico.i8cvs@tin.it>
Cc: Walter Crauwels <Walter.Crauwels@skynet.be>
Sent: Saturday, November 10, 2001 3:42 PM
Subject: Re: Patch

> Hello Domenico,
> I have made some measurements on the patch in a dish. The return loss is
> greatly effected with the patch looks in the reflector. It gets de-tuned
> completely
> when i am not in the focal point.
> I first tried to see the effect, just by experiment. I could see at one
> point the return
> loss was improving, at the exact same frequency as where the patch is
> without the
> reflector. When i moved the patch in and out, the resonance point changed,
> and
> return loss went from 24 dB ( no antenna ) to less than 5 dB, or sometimes
> return
> loss 10 - 15 dB but at a frequency +/- 20 MHz from the normal no dish
> resonance.
> At the best point, where the resonance was the same as no antenna, the
> return loss
> improved to nearly 30 dB !  At that point i measured the distance to the
> reflector,
> i could move from 245 to 255 mm out of the reflector, with minor changes.
> So, best
> at 250 mm.
> I than started to measure my 60 cm dish, to find out the teoretical focal
> distance.
> The dept of the dish is 90 mm, the diameter i used was 600 mm.
> If you calculate the focal lengt from this data, you will see 250 mm !!!
> So, it was 100% correct with what we first measured.
> The f/d of the antenna is 0.41, wich might be to "flat" for this patch
> antenna.
> I will repeat the same measurement in a 120 cm reflector tomorow, i need
> change some mechanical parts.
> All measurements are from the antenna reflector to the PATCH ELEMENT. So,
> your question was where to place the patch, well , the patch element
> goes
> in the focal point, not the patch reflector... .
> My next experiment will be to measure the change of isolation, between the
> 13 cm
> and 23 cm patch. As i told you before, we constructed a dual band S/L
> antenna.
> Together with a few frends, we also constructed the nessesary filters for
> / 13, in
> order to give us an acceptable isolation between TX and RX. You can find
> some
> photo's of our experiments, as well regarding the filters on following
> website:
> http://users.skynet.be/on4bcb
> Here you can see L/S filters, you will find photo's and plots, first
> simulated, and later
> the real measurements on a network analyzer.
> As soon as the L/S prototype is working as we want it to work, we will
> publish the
> photo's as well as all diagram and experiments.
> Best regards,
> Eddy

Hello Eddy,ON7UN

I have built the circular patch for 2400 MHz using copper sheet and an  SMA
connector enlarging its central pin to 4 mm and the rest is the same as per
your drawing.

Here is my test report

Instruments used:

Spectrum analizer HP 141T and 8555A
Sweep generator  HP 8690B with 8699B
Directional coupler HP 11692D
2400 MHz dipole with balun
Parabolic disc 90 cm diameter,13 cm deep
Focal distance= 43,5 cm


The return loss of the patch itself looking out of the window has been
adjusted for a maximum possible value of 25 dB or SWR=1,12

In this conditions the circularity has been measured connecting the patch to
the spectrum analyzer and connecting the dipole to the generator on CW  at
2400 MHz

The distance from the patch and the dipole has been changed from one
wavelength to 20 wavelengts with minor difference in circularity and
rotating the patch  in front of the dipole by 360 degrees the maximum
change in amplitude as seen on the analizer has been around 3 dB

Since the 25 dB of return loss and 3 dB of circularity was satisfactory the
4 mm capacitor screw has been tighted interposing  between the disk
and the nut a berillium copper wavy washer RS cat Nr 289-679

Since the wavy washer is elastic this help in tuning the capacitor screw
without any backlash

Than i have placed the patch in front of the 95 cm disc and,as you
pointed out,i have seen that the return loss is greatly affected by
the disc

Infact,moving back and fort the patch from the deep of the disc the return
loss changes greatly approximately every 3 cm or 1/4 wavelenght
at 2400 MHz

But this situation reduces significantly as soon the element of the patch
approaches the focal point or 43,5 cm in my case

I have accurately trimmed the distance from the element pach around the
focal point and ,same as you, i founded a point with an improvement of
return loss from 25 dB to 28 dB

Now i  have measured again the distance from the element of the patch and
the deep of the dish and,same as you,i found exactly 43 cm or the
geometrical focal distance.

This measurement states,AS YOU POINTED OUT, that the element of the patch
must be placed in the focal point.

In my opinion this measurement help to get easily the maximum return loss
when the patch is mounted in the parabolic disc but in real operation
it is necessary to test for the maximum sun noise in order to verify if the
phase center of the patch match exactly to the point in wich we found
the maximum return loss or not.

I will do this measurement but please you also made your measurements for
comparative report.

The next test will be to sweep the SWR arount 2400 MHz and to measure the
sense of circularity but i have to build first two helices one RHCP and
anoder LHCP

For the moment i am very happy to confirm the results of your previous test
and measurements.

73 de i8CVS Domenico

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