High Frequency/High Perm CP antenna
I am trying to design a CP patch antenna at 24 GHz. The antenna is to be fed by CPW and is EM coupled into the patch on the other side. The substrate that I am using has a high permitivity value of 12.9. I am using HFSS and would consider myself an experienced user so I am confident the of the values of axial ratio I am getting (which is at best about 10dB)
I have tried chopping the corners, alternating the angle of the feed, alternating the type of feed (cap or inductive), putting slots in the patch but all to no avail. Is there something fundamental I am missing that means I can't generate CP with the parameters set out above.
I think it's related to the Q factor as I am getting v. high values (calculated on the back of a cigarette packet) of Qrad (9million) and negligible values for the rest (lossless substrate, gold metallisation, thin sub of 400um), hence resulting in very low overall Q.
Any thoughts are most welcome.
Thanks
Griogair
You didn?t specify anything about optimization of the substrate thickness.
The height of the patch above the groundplane should be approximately two percent of the width of the patch.
Lower heights result in higher Q, and high currents resulting in higher losses.
Further increase in thickness will reduce the efficiency and the gain of the antenna.
Thanks for the reply.
Unfortunately I don't think I'll ever be able to acheive the gap of 2% patch width between patch and ground. My patch width is approx 1.6mm so that would mean a 32um substrate. I think the lowest I can go is about 100-200um (purely mechanical) so I'll have a bash at that. I've switched the feed's (inset-fed and diagonal) onto the same side as the patch now and they are starting to give me more encouraging results (0.5dB axial ratio) although my return losses are not very good (-6dB and -20dB for the two degenerate modes).
It is still desirable to have the feed on the other side for packaging issues so I will go back to my EM coupled designs. Once again thanks.