Effect of Ground Size on Verticle monopole

What is the effect of ground on vertical monopole. Does it improve the radiation pattern or how the performance of antenna especially UWB is dependent on the size of Ground plane.

From Modern antenna design by Thomas milligan, it says that ground plane will severely effect F/B ratio but according to my simulation on CDM by varying ground size. I realized that it might effect at lower frequency but over all effect is negligible sometimes acutually (100x100 to 100x20mm ground size) small ground it much better at some frequency.

Any ideas

if you are talking about UWB monopoles, you are right, the F/B ratio is going to be affected mainly at low frequencies but at higher frequencies the ground size would be already electrically large and will act like an infinite ground.

however, you should not only rely on simulations in this regard, since you sometimes need a large enough ground plane size to prevent unbalanced currents that will affect the measured results of your antenna. Unfortunately, it is quite tiresome to model unbalanced currents in most simulators.

best regards,
Adel

Thanks for your prompt reply. Would u like to briefly explain the unbalanced current in antenna.

The dimension a ground plane under a monopole has significant effects on the pattern and match.

Refer to 3-23 in Jasik, "Antenna Engineering Handbook" . Take a look at figure 3-34 in particular.

The basic idea is that the "antenna" is not just the monopole, it is also includes the structures in the immediate vicinity. This idea is important when one tries to reduce the antenna size.

Hi
Azulykit
you make a valid point but for very specific case when we know about surround like for base station antenna. What i got from simulation results. Azimuth radiaiton pattern has hardly effect but at some frequencies i can see the effect of reduced ground.

My point is that the size and shape of a groundplane can have a significant effect on the pattern and impedance characteristics of a monopole. This is particularly evident when the ground plane is not symmetric and smaller than around 4 wavelengths in diameter. The same is true if it is not flat, but some other three dimensional shape. A simulator is a good way to evaluate a specific situation. My example was for a particular case, but the message is that the groundplane has significant effects in other situations as well. The example I cited is one of many. Be casreful with your site and ground planes or you may be surprised with unanticipated performance corruptions.