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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




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