Weird radiation pattern from monopole antenna

[Ioanniss]

Hi all,

I am trying to get the 3d radiation pattern of a monopole antenna with a rectangle ground plane using HFSS. I expected a radiation pattern like the picture , but I have a pattern exact the same as a dipole antenna.
I dont know why is that, but I thought this happened because I use a ground plane made of vacuum(2 dimenions). I tried to set the PEC material using a 3 dimensional cube for ground plane instead of a 2 dimension rectangle, but the results are still the same.
If you can imagine any explanation I would be grateful!
The parameters I set in ADK are:

frequency: 435 MHz
antenna lenght:17 cm
radius: 0.3445 cm
Feed Gap: 0.3445 cm
Ground plane width: 10cm

Thanks in advance. Any opinion would be very helpful.

Regards

 

[biff44]

well I assure you, the radiation pattern will have a dip at around 0 degrees elevation +/- a few degrees, UNLESS the ground plane is way too small

 

[Ioanniss]

Hi biff44,
thanks for your response,
I was expected something like what you describe, but instead I get this pattern. It seems wrong to me.
I tried the antenna with different ground plane dimensions. I tried 2cm x 2cm, 10cm x 10cm, 49cm x 49cm, 100cm x 100cm and 500cm x 500cm. The radiation patterns are the following, respectively.


Do you think ground plane is small?

 

[jiripolivka]

An antenna above the ground plane typically is a quarter-wave and the GP is "infinite". If you vary GP size like shown, the structure becomes a non-symmetrical dipole with the pattern shaped as shown.
Try other radiator thickness, also its distance from GP at feed point can be significant.

 

[Ioanniss]

Thanks for your replies,

Although I still cant take the appropriate patterns.
Do you believe that is possible to take the ground plane as part of the radiator?
Also is there any difference in the results if I use lumped port or wave port? I dont understand the meaning of wave port so I use lumped port.

Thanks again guys! Your help is very important to me!

 

[biff44]

An antenna above the ground plane typically is a quarter-wave and the GP is "infinite". If you vary GP size like shown, the structure becomes a non-symmetrical dipole with the pattern shaped as shown.
Try other radiator thickness, also its distance from GP at feed point can be significant.

Maybe, but I have done actual testing of monopoles over small ground planes, and then do indeed have a radiation null as you look slightly underneath the ground plane elevation. For instance, 5.8 GHz monopole on top of a 2" ground plane (circular) exhibits this effect strongly. As I recall, there was a phase reversal too!