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# R: Circular shaped CP antennas

• Subject: R: [amsat-bb] Circular shaped CP antennas
• From: "i8cvs" <domenico.i8cvs@xxxxxx>
• Date: Mon, 7 Jan 2002 19:02:09 +0100

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----- Original Message -----
From: Frederick M. Spinner <fspinner@hotmail.com>
To: <ccessna@pe.net>
Cc: <amsat-bb@AMSAT.Org>
Sent: Monday, January 07, 2002 6:32 AM
Subject: [amsat-bb] Circular shaped CP antennas

> Clair, (and the group)
>
> OK.. the material Clair sent and some additional reasearch just showed me
> three ways of making a circular (or elliptical) patch do CP.  I think I
have
> this correct now... it is possible to do CP with a circular-shaped patch
> antenna.
>
> #1.  Elliptical.. If one plane of the patch is elliptical so it is a
little
> capacitive (short) and the other is long (inductive) and its fed at 45
> degrees off of the long/short axis, you have CP.  (Similiar to the "nearly
> square" patch I described before)
>
> #2.  Slot in the center.  If you place a slot in the center of the
patch---
> it makes the antenna look capacitive in one plane along the slot --
meaning
> it the patch is a little too large in diameter to start (inductive) what
you
> end up with electically is the same as case #1 when it's fed 45 degrees
off
> of the slot.  (This also would work with square "non-cropped" patches.--
> slot is inline with one set of edges, and feed is on the diagonal)-- this
> led me to:
>
> #3.  Another way of making one "plane" of the circle electrically longer
> than the "other plane"-- place some capacitance along the line that is 45
> degrees away from the feed point-- making the circle electrically an
ellipse
> again.  If its on the same diameter from the center as the feedpoint, and
> the antenna is built to be 50 + j50 Ohms at 2401 MHz, all that would have
to
> be done to make this beast electrically circular is add a capacitor 45
> degrees away (to the right of the feed for LHCP) of -j 50 Ohms!
> ON6UG/G3RUH's design uses this apparently and as such is brilliant!
>
> All they needed to do (and I'm sure they did) is make a linear polaraized
> circular patch of reproducable diameter at a resonant frequency a little
> lower than 2401 MHz (The antenna's diameter is a little longer than
> resonance at 2401 MHz-- equivalent to 50/_45 deg [50+j50] ohms at 2401)--
a
> trim cap is then placed 45 degrees away from the feed on the same
diameter,
> and the antenna is tuned for a 50 + j0 Ohm match at the feedpoint at the
> desired frequency (say 2401.3 MHz) and you have *perfect* circularity.
> (Once the capacitance cancels the inductive reactance under these
> conditions, one electrical "plane" of the patch will be 90 degrees out of
> phase from the other, in the correct orientation sense, and then you will
of
> course have CP)
>
> The cool thing with this is that once the equivalent 50+j0 Ohm frequency
> (overall) diameter for the circular patch is figured out (one that will be
> the same as 50+j50 at say, 2401.300 MHz), one can first make sure that the
> patch is resonant at the correct equivalent frequency (using either a
> network analyzer, or alternatively, a 2400 MHz antenna bridge [sig gen/SWR
> meter]).
> Then, after adding the capacitor 45 degrees from the feedpoint on the same
> diameter tune the cap for min SWR at, say 2401.3 MHz, and at that point
the
> axial ratio and VSWR should be nearly perfect.
>
> The G3RUH/ON6UG patches are made with CNC machines, so all they have to do
> is tune 'em for a good match with the capacitor, and the good axial ratio
> follows.
>
> OK.. well.. I think that filled in the hole in my patch antenna knowledge.
>
> IMHO, The final verdict is that the "cropped" or "truncated"-corner square
> patch is the easiest and most reproducable patch that a
hack-in-a-home-shop
> can build to get CP out of a patch feed.  The circular-patch version with
> the tuning cap 45 degrees out is likely the "best" feed due to mechanical
> reasons above and the ease of adjustments ("tweaks").  Cutting circles is
> harder to do accurately though, and this makes it harder for a "hack" to
> build the feed.
>
> To be used as dish feeds, all should have scalar rings to help not
> over-illuminate a dish, to cut down on sidelobes, and to help the feed's
F/B
> a bit.
>
> 73 de Fred W0FMS
>
>

Hi Fred,

I aegree that in the above mentioned circular patch the feed point to the N
connector pin has been placed  to a radial distance from the center in wich
the impedance is  50 + j50  in order to have 50 /_ 45 degrees at 2401 MHz
but if you make a measurement of the mechanical angle between the N central
pin and the capacitor screw center you will see that the angle is not 45
degrees but 40,5 degrees.

Probably this difference is due to the fact that some velocity factor must
be considered in the space between the inner surface of the active element
and the reflector in order to get 50 /_ -45 degrees in the point in wich the
capacitor is fitted at a mechanical distance of only 40,5 degrees from the N
connector.

I have seen the return loss curve of the above patch with my very well
calibrated HP 8690 B sweep generator using a dual directional coupler
HP 11692 D

Tuning  the capacitor for maximum return loss at 2400 MHz i got 20 to 25 dB
and measuring circularity with a very well balanced dipole for 2400 MHz i
got from 1 to 2 dB of axial ratio

During the return loss measurements and looking at the curve i realized that
using the original size diameter for the above circular patch the return
loss was more than 30 dB at 2450 MHz

My idea was that to increase a litle bit the diameter of the active element
in order to lover the resonance and get 30 dB of return loss at 2400 MHz

Using the metod of cut and try twiking with an autoadesive copper tape
from 3M i finally got a return loss of  30 dB and even better at 2400 MHz
but unfortunately the circularity was completely lost because the axial
ratio increases to more than 10 dB ! !

I hope this information will be usefull  to go more deeply in details about
the many variables involved with a circular patch designe.

73 de i8CVS Domenico

>
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