ground plane dipole directivity

Hi,everyone

I am using HFSS to simulate dipole antenna near metal ground plane. the dipole is parallel to the metal ground plane.

when i simulate this dipole antenna for 2.4Ghz centre frequence and place it very close to ground plane 1mm 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 9dbi and its giving smooth and wide main lobe with maximum in Theta=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

Kuanfu

Hi,

placing a dipole too close to a ground plane means that its matching will be severely affected (because of the cancellation between the direct field and the reflected field) as you said. But the gain will be higher. This is correct because the gain is calculated from the accepted power of the antenna not from the incident power and thus is independent on the matching. You can check HFSS help file on the gain to see the formula.

regards,
Adel

An antenna can have a good gain or a good directivity,
because these calculations are based on the accepted
energy no including the reflected as Adel point it out.

I once did a parametric study using HFSS changing the
distance to the ground plane, and I could verify that
the antenna efectively gets shorted out as the ground
is close. Do it yourself and you will see the change on
the insertion loss as the ground is closer.

The gain of the combination of groundplane(reflector) has a maximum at 0.25 wavelenght spacing. As you decrease the separation a variety fo things go wrong as your simulation shows.

You are probably asking for directivity in your post processing rather than gain. Gain includes antenna efficiency and mismatch loss and is always less than directivity. Marketing guys like to use directivity. The rubber meets the road with gain.

Hello Azulykit,

The inclusion of the mismatch loss in the antenna gain is mainly dependent on the Friis transmission equation you are using. The most common one is:
Pr = Pt *Gt * Gr * (lmda/(4pi R))^2 * (1-|S11|^2) * (1-|S22|^2) * Polarization_loss_factor.
and as you see it separates the matching from the gain.
On the other hand, some people use the simpler equation
Pr = Pt *Gt * Gr * (lmda/(4pi R))^2
and in this case, the matching has to be included in the gain. I agree with you that the second equation makes the antenna gain much more meaningful (since it includes the power loss due to dielectrics & conductors AND the power loss due to the mismatch) but the first equation is the most common (you will find it in most antenna theory books).

The antenna directivity is defined as the gain/radiation_efficiency and again in this case the radiation efficiency can include the mismatch (again, much more meaningful) but the most common definition is that he efficiency is only related to conductor and dielectric losses.

Regards,
Adel