dipole antenna parallel to metal/pec ground plane

04 Jul 2007 18:08 dipole antenna close to metal ground plane


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i am using CST to model dipole antenna which is lying parallel to metal ground plane. i want to study the effect on antenna characteristic by varying the height of dipole over ground plane.
when i simulated 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 poor s11 parameter. however when i check the farfield pattern at 2.4 ghz , the directivity is around 8dbi which is the same when i place it far above ground and its giving radiation pattern 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
s11 parameter is not good it should not radiate efficiently as it is not properly matced and its directivity should be very low.
any help would be highly appreciated.

thanks

Kraus (Antennas) has a discussion of what happens when you place a conductive sheet in the vicinity of a radiator. You might start there. I am not surprised that the simulator tells you that it does not work well 3 mm from the surface. that is what I would expect.

Try starting with your spacing around a quarter of a wavelength and vary it around that distance. I would anticipate gains on the order of 5 dBi but not much more. There will be significant changes in the pattern as you change the distance so if you are not seeing that there is something wrong with your simulation.

Impedance does not have anything to do with directivity or radiation pattern.
You can add impedance matching networks to improve S11.
It appears that you put infinite ground plane, but when you build a real
antenna the ground plane size will be limited.
S. H.

The directivity of an antenna is based on only the sturcture of the antenna. It doesn't include the loss due to the antenna mismatch. If you check the gain of the antenna, you should find that the gain of the antenna changes with the input impedance mismatch.

When a plane is too close to a dipole as in your
antenna, this is shorted out. This means the power
that is getting into the antenna is very small.
Although the power is small the directivity is based
on that small amount of energy, this is based on the
power effectively delivered to the antenna.