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dipole antenna close to metal ground plane

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shahid78

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i am using CST to model dipole antenna near metal ground plane. the dipole is parallel to the ground plane.
when i simulate this dipole antenna for 2.4Ghz centre frequence and place it very close to ground plane 3mm to be exact i get badly matched dipole shown by s parameter however when i check the farfield pattern at 2.4 ghz , the gain is around 8dbi and its giving smooth and wide main lobe with maximum in θ=0 degree.

can anyone explain this why this is happening because in my view if antenna's s parameter is not good it should not radiate and its gain should be very low.
any help would be highly appreciated.

thanks
 

hello,

first, I think placing your dipole very near to the ground in this way will reduce its radiation resistance tremendously (by image theory, this is as if you have a parallel dipole with opposite current very near to it leading to small total radiation or radiation cancellation).

concerning your question about the gain, the gain is the directivity multiplied by the radiation efficiency, and since you are using a metallic (I think you made it PEC or copper) resonant antenna then your radiation efficiency should be very high (near to 100%) so the gain is high. Some simulators include the matching into the gain (i.e. multiply the gain by 1-|S11|^2 but I don't think this is correct.

you may try to match the dipole using lumped capacitors or so, but this usually comes at the expense of narrower bandwidth.

best regards,
Adel
 

in order to improve ur s parameter try to use ebg like mushroom ,it is supressed the surface current at ground plane which has aclear coupling on the s parameter,u can search here about ebg structures
regards
 

so it means we should have 2 gain definitions:

1) gain = directiviy* radiation efficiency
2) gain=directiviy* total efficiency

so cst is showing high gain because it is using radiation efficiency which is very high for copper based dipole
 

Check whether the directivity and gain are more or less the same. This must be the case. In other words, mismatched antenna does not mean that it cannot have directed radiation patterns.
 

Return loss is not the only candidate for efficiency. However i am doubtful about the efficency calculation of CST most of the time. however i will reccomend u to increse the deembeding in CST and see how it works
 

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