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mosizaza
Antenna-Theory.com Newbie

Joined: 17 Dec 2009
Posts: 3

 Posted: Thu Dec 17, 2009 3:19 pm    Post subject: How to creat a Microstrip patch with circular polarization ? Hi every one. i have simulated a simple Microstrip patch antenna at 2.4 GHZ with HFSS and i want to modify it to a GPS's Antenna but i don't know how to make a circular polarization. i give my HFSS design and you can check it out : http://www.4shared.com/file/175175070/9a80fbad/Microstrip_Patch_Antenna.html then if it is possible,help me how to create this kind of polarization for this antenna ? thanks
admin
Site Admin

Joined: 03 Jan 2007
Posts: 197

 Posted: Fri Dec 18, 2009 5:31 am    Post subject: I'm not downloading your files, but I can offer some tips. First, it is pretty difficult to make a patch antenna circularly polarized. When it is done, the antenna is only circularly polarized over a narrow bandwidth, and also only over a small angular range (the axial ratio might be <3:1 for theta=30 degrees or less, with theta=0 degrees normal to the patch). That said, some methods: a) have a primary feed, and another delayed feed that feeds an adjacent side of the patch, but has an additional 90 degree phase delay due to the extra length of transmission line b) for a single feed, you can cut the corners at a diagonal c) add a centralized diagonal patch cut out, typically rectangular For b) and c), the design is very tricky, you have to basically adjust parameters to get it right. Check out balanis' Antenna theory book for descriptions of all of these methods (you will still have to optimize).
nurul
Antenna Theory Regular

Joined: 26 Apr 2011
Posts: 23

Posted: Mon May 09, 2011 11:18 am    Post subject:

hello admin,

 admin wrote: the axial ratio might be <3:1 for theta=30 degrees or less, with theta=0 degrees normal to the patch

what does this means?

does axial ratio to be monitored is around the angle of 0-30 deg ?

thanks
admin
Site Admin

Joined: 03 Jan 2007
Posts: 197

 Posted: Mon May 09, 2011 9:16 pm    Post subject: "the axial ratio might be <3:1 for theta=30 degrees or less, with theta=0 degrees normal to the patch " What this means: An antenna is considered circularly polarized if the axial ratio between two orthogonal polarizations is low (i.e. <3:1 or whatever you want to call it) and the time-phase delta between the two polarizations are 90 degrees. Now, this won't ever be perfectly circularly polarized for a real antenna. In the case of microstrip antennas designed to be circularly polarized, it is important to note that the fields are more or less circularly polarized only over a certain beamwidth. If the maximum radiation direction is theta=0, they might be "reasonably circularly polarized" over a beamwidth of say theta=0 to theta = 30 degrees. At theta=90 degrees, for instance, the fields won't have the requirements met (i.e. the axial ratio might be high or the time delay between the polarizations might not be close to 90 degrees). Hence, I'm trying to say that even though an antenna is classified as "circularly polarized", this tends to mean that it is CP in the direction of peak radiation, and as you look away from that direction the quality of the circular polarization dies off. In general, you want to monitor the polarization of your antenna as a function of position if you are trying to control the polarization is several specific directions. As a real world example: GPS signals from satellites are Right Hand Circularly Polarized. Mobile phone antennas for GPS would benefit then from being RHCP antennas. However, because the volume is constrained, they aren't designed to be CP. Even if they were able to get CP for a particular direction, it wouldn't be CP in all directions. And since the phone is held in any number of positions, and you don't know which direction the satellite(s) signal(s) are coming from, there is no point in trying to get CP in a GPS antenna on a mobile phone. Conversely, the satellites do use RHCP antennas, because the signals they send are pointed directly at the earth, and so they need RHCP fields only from a small beamwidth (i.e. they don't care if the radiated fields away from the earth towards the sun or wherever are not circularly polarized). Hope this long winded explanation clear this up!
nurul
Antenna Theory Regular

Joined: 26 Apr 2011
Posts: 23

 Posted: Tue May 10, 2011 7:51 am    Post subject: hello admin. cool. it's clearer now one more thing, kudos for antenna-theory website and this forum. It's very2 usefull especially for me who need 'layman' cum interactive explainations about antenna.
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