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guess_me87
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Joined: 26 Mar 2009
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 Posted: Wed Apr 29, 2009 2:45 pm    Post subject: Multiband Microstrip Patch Antenna Hi... I am student of undergraduate in Electrical Engineering... I hve designed a rectangular single patch microstrip antenna which is resonating on multiple bands... Can some body just convence me how single patch could be multiband?? Kindly if you provide me theoratical explantion i would be thankful..

Joined: 03 Jan 2007
Posts: 197

 Posted: Wed Apr 29, 2009 4:52 pm    Post subject: Multiband Microstrip Antennas A patch antenna can be multiband if: 1) there is a parasitic patch antenna next to it, designed to resonate at a different frequency. Typically the directly fed patch will be smaller and resonate at the higher frequency. A closely spaced patch will be next to it. At the lower frequency, the energy is more easily transferred to the adjacent patch, and the adjacent patch increases the "effective size" of the overall patch antenna, which lowers the resonant frequency. 2) there is a slot of some shape cut into the patch. This will introduce different modes or paths that the current can travel along on top of the patch, which may induce different resonances. Often a U-shaped slot carved into a square patch can significantly increase its bandwidth as well. 3) A regular square patch that radiates at f0 will also radiate at 3*f0, because this is 1.5 wavelengths. These are just some thoughts.... If you go to IEEE Explore and search for multiband patch antennas, there will be dozens of papers on this. I would recommend looking for conference papers (which are usually much shorter and will focus on one thing) instead of journal articles (which will go into depth on a bunch of topics that will make it seem more difficult than it is)
guess_me87
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Joined: 26 Mar 2009
Posts: 4

 Posted: Fri May 01, 2009 7:43 pm    Post subject: Still confused..... Sir... First of all thank you for your rply.. I want to ask that i hve designed lot of regular rectangular shape antenna but all are resonating on single frequency but in one antenna i hve changed its dimensions slightly and also feeding point location.. Now it is giving multiband charterstics... So what would be arguments of this???? Its simple regular rectangular patch antenna.....

Joined: 03 Jan 2007
Posts: 197

 Posted: Fri May 01, 2009 8:19 pm    Post subject: Characteristics Without seeing the layout I can't be sure. But, I would guess that you moved the feed to be off-centered. If the feed is still centered, then moving the feed away from the edge only changes the input impedance. If you moved the feed away from the center of the patch, it would likely still resonate at the same center frequency as before. However, you may introduce an asymmetry that would allow a resonance at another frequency. As long as you end up with fringing fields at some frequency that are not canceled out on one side of the patch antenna relative to the other side, you will get some radiation. I suspect there is some mode or current resonance in there, caused by the asymmetric feed that allows radiation.
guess_me87
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Joined: 26 Mar 2009
Posts: 4

 Posted: Fri May 01, 2009 10:00 pm    Post subject: Some more..... Well Sir kindly explain the term asymmetry interms of antenna theory and its effects??. According to my information impedence change will just effect on return loss(S11 parameter) not on oprating frequency.... I am totally agree with you that changing feeding point location will just change its impedence.. Then question is how this impedence change effects the oprating frequency and becomes cause of genrating multiple oprating frequencies?? I hve feed an antenna at near corner point... It is resonating at 9 bands of which THREE are GSM and ONE WiMax band. Thanks... Looking for your positive response...

Joined: 03 Jan 2007
Posts: 197

 Posted: Sun May 03, 2009 11:51 pm    Post subject: Impedance and Assymetry You are correct about impedance - if this is the only thing that is changed then it will only effect return loss or S11. The impedance change does not introduce the broadband effects. As to why an offset feed may produce multi-band operation.... I'll try to offer my thoughts by drawing a [crappy] picture. If the patch is fed in the center (with width W and length L): <---W---------> ...................... _ |||||||||||||||||. |||||||||||||||||.L |||||||||||||||||. ........||||.........- ........||||......... ........||||......... ........||||......... Then both sides of the patch (on either side of the feed) will have the same current distribution, and there will be one dominant mode. Here, the dominant frequency is determined by the length L of the patch. Suppose now we feed the patch antenna at the corner (same dimensions L and W): ||||||||||||||||| ||||||||||||||||| ||||||||||||||||| |||| |||| |||| |||| As far as the patch antenna and current propagation is concerned, it doesn't know if W is the length or if L is the length here. So you'll end up with two modes propagating: one in which the patch resonates as if the length is L and a mode resonating as if the length is W. In addition, you may end up with other current modes/resonances that occur (by diagonal propagation for instance). For radiation we need the current to line up such that the corresponding out-of-phase voltage produces fringing fields that don't cancel each other out at opposite edges, and ideally the fringing fields should reinforce each other. For each wavelength in which you observe good radiation, try to think about how the current/voltage might be distributed at that frequency such that radiation occurs. This might take some thought, but will give a better understanding of the radiation mechanism. I hope this helps in the understanding....
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