Why my microstrip antenna resonates at twice the frequency?

[scorpio07]

tm10 mode excitation

hi friends,
i am doing a project o microstrip antennas. i have designed a simple rectangular patch and centre fed it. the probem is it resonates at twice the frequency i have designed for.

any ideas

 

[kprokopi]

microstrip antenna l= lambda/2 why

hi,

Can you give us a more detailed description of your antenna design?
Have you tested your antenna with some numerical method (i.e. FDTD) or any microwave CAD software? Maybe you measure a higher mode.

kprokopi

 

[gymissyou]

Re: Microstrip antenna

i am also designing microstrip antenna ,i want we can help each other.

 

[kleinesmurf69]

Re: Microstrip antenna

hello,

microstrip antennas are usually not center fed. feeding in the center of the patch may lead to the excitation of higher order modes instead of the fundamental mode. the fundamental mode for a rectangular patch is the TM10 mode. by feeding at the center you are probably exciting the TM20 mode. the TM10 mode has an impedance minimum at the center (Z = 0 theoritically). the TM20 mode has a maximum at center. the resonance frequency of the TM20 mode is twice the resonance frequency of the TM10 mode.
i hope this helps.

best regards.

 

[scorpio07]

Re: Microstrip antenna

sorry for not getting back,
i did not use any software for designing it. ii used the results from the method of moments and some formulas from T.Cedwards and BahlBhartia to design it i have two patches one with a semi rigid cable(1) and the other with flexible coaxial. the patchesa re designed to work at 2.4GHz. the first one did get excited at 2.38 G but the results from the anechoic chamber showed an assymmetric lobe. and i had some mech problem with way in which the probe was fed. so i made another one with a hole oin the groundplane with the dimension of the dielectric of the coax
this one worked at the second harmonic.

kleinesmurf69 can u shed somemore light into TM10 mode coz i am not good in it

scorpio07

 

[yingyang]

Re: Microstrip antenna

Please check your physical antenna in terms of wavelength and the feed.

 

[kleinesmurf69]

Re: Microstrip antenna

what do you mean with an asymetrical lobe? if you have 2 main lobes and a radiation minimum at broadside you are definitely exciting TM20 mode.
TM10 --> L = lambda/2
TM20 --> L = lambda
A rectangular patch has 2 dimensions. The modes TMnm correspond to the variations of the fields beneath the patch in x and y directions
Any good book about microstrip antennas will explain this in detail.

Best regards.

 

[blueant]

Re: Microstrip antenna

what do you mean with an asymetrical lobe? if you have 2 main lobes and a radiation minimum at broadside you are definitely exciting TM20 mode.
TM10 --> L = lambda/2
TM20 --> L = lambda
A rectangular fix has 2 dimensions. The modes TMnm correspond to the variations of the fields beneath the fix in x and y directions
Any good book about microstrip antennas will explain this in detail.

Best regards.

Can you tell me what boos are useful for microstrip antennas and where can find these books ?

 

[kleinesmurf69]

Re: Microstrip antenna

my primary resource for microstrip antennas is:
Microstrip antenna design handbook, R. Garg et al., Artech House, 2001
you can buy it from hxxp://www.amazon.com

best regards

 

[George137]

Re: Microstrip antenna

You can read the book microstrip antennas by David Pozar to get full illustration of this.

 

[ssue]

Microstrip antenna

book-Microstrip antennas by Bhal and Bhartia is good for beginning design