Effect of metal plate on dipole antenna

[brayn64]

Hi, everyone

I try to study the effect of placing a horizontal dipole antenna above a metal surface with a height lower than 0.01*wavelength. In theory it is said that this leads to two effects:

The first is that the radiation efficiency (without taking into account the mismatching of the antenna) will be very low due to the wave reflected by the metal surface that is phase shifted by 180° relative to the direct wave, therefore it will be a cancellation or fainting of the the far-field.
The second is the mismatch of the antenna due to the mutual impedance between the dipole and the surface of the metal.

But, when I do the simulation with CST and HFSS (antenna + metal surface), I find that the radiation efficiency is equal to 1. On the other hand, the total efficiency is very low. This means that the first effect does not exist. And that only the mismatch of the antenna which affect the radiation of dipole when it is near the metal. Because the mismatch prevents transmitting the total power present at the port, therefore the total efficiency of the antenna is low.

Is that the reflected wave does not cancel the direct wave when the height (distance between the dipole and the metal surface) is less than 0.01*wavelength?

thanks

 

[vfone]

I don't think a dipole antenna will radiate at all when is at 0.01*lamda from a metal surface. Is almost like the entire antenna is shortened to the metal ground.