I am trying to design a circularly polarized patch antenna to operate at 915MHz for an RFID reader.  I intend to print the design on a PCB, as that sounds like a reasonable solution, and cheap which is a constraint.  The antenna needs to be circularly polarized, and have a (max-allowed) gain of 9 dBic.

In thread247-288566: 915Mhz Circular Patch RFID antenna
I see that Tony you said you were trying to do this, I feel as if you could be of assistance, but I would appreciate any tips in trying to tackle this problem.

The gain is a bit high. You'll need to use air dielectric to enlarge the element to increase the gain compared to normal patch antennas (usually +5 dBic is typical). So about a 6" square element, etched circuit card. Raised off the ground on plastic posts over a ground plane. Probe location(s) depend on height of circuit card above the ground plane. Your ground plane needs to be larger than you'd probably want so the patch doesn't radiate around the back and reduce your gain. You may need to put it in a metal box to minimize size and add a director + shaped metal in front of the antenna.

Feeding it with a 90 hybrid purchased part makes the antenna design easier than trying to manufacture it without a 90 hybrid.

What is your price and volume target?

 

Hi thanks for the reply, I've been simulating various antenna designs on HFSS, all on FR4 PCBs, and yes i see that the gain is limited.  The bigger problem seems to be that the bandwidth is very small.  I have tried the truncated corners design due to its simplicity and single feed line, but i suspect it may not be the best one -- is there a better one?  I'd really like to stick with the microstrip patch.

I am trying to produce 5-10 to test a system, with less than $500 for the antennas.

I appreciate any further design suggestions.   

I did some modeling: we can get you Circularly polarized 9 dBic Gain at 915 MHz if footprint/ ground plane will be 250 x 250 mm. If you still did not find your solution I offer to construct and send you hand-made prototype for small fee. Tuned properly. It will be suspended plate, truncated patch; probe-fed; terminated with SMA connector. No covers. You can then copy it and build your own.

If you need just one or two Antennas you will be better off buying them from http://www.microcom.us/mt262006trhak.html
Probably the same, slightly larger ground plane.