Effect of ground plane size of mono-pole antenna radiation patterns

[Engineer4ever]

Hi,

I am designing a printed strip mono-pole antenna and I simulated it of HFSS for different ground plate sizes and extracted the radiation patterns in the three co-ordinates, but I don't quite understand how the size of the ground plate affect the three radiation patterns I have.

Thanks in advance,

 

[WimRFP]

Assuming you have a fully planar antenna (that is both radiator and ground plane are in the same plane).

The current that goes into your monopole radiator is drawn out of the ground plane. In other words, there is also current in the ground plane. Depending on geometry and size of groundplane, the electrical current in the ground plane can generate significant far field radiation. This far field interferes (constructive or destructive) with the field generated by the monopole, hence causing distortion of the pattern.

The effect can be severe with relative small ground planes that resonate in a half wave mode. If so, you may see a change in input impedance of the monopole when touching the ground plane.

When simulating surface current density, the current in the ground plane may not be visible. Reason is that the groundplane is mostly wider then the monopole. Therefore a low current density in the ground plane can represent a high current (current = current_density*width).

 

[Engineer4ever]

I got it. Thanks

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But, I have another question: How does the size of the ground plane affect the resonant frequency? in the simulation of HFSS, I assigned the solution frequency=2 GHz, and as I increase the dimensions of the ground plate, the antenna resonated at about 5.5 GHz, why is that?

 

[WimRFP]

Best is to post a image of structure (I don't have HFSS). In fact the RF source is in between two pieces of metal. In a dipole case, each piece of metal has same size. In your case, one side is called "monopole" and the other side is called "ground plane". I see the ground plane more as a counterpoise.

The impedance as seen by your souce is the sum of the monopole impedance and ground plane impedance (roughly spoken). When the monopole is in quarter wave resonance (low-Z) and the ground plane is more or less in half wave resonance (high-Z), the impedance as seen by the source is dominated by the ground plane impedance as the ground plane impedance is the highest of the two. If so, the ground plane provides most of the radiation.

This is not just a theoretical issue, but can be seen in practical antennas, especially in case of ground planes with large length/width ratio.

 

[Engineer4ever]

Just to make sure I got this right, do you mean that as ground plane increases the total impedance increases so impedance mismatch takes place resulting in a shift in the resonant frequency?

 

[WimRFP]

The impedance may increase or decrease, it depends on whether it behaves more like a quarter wave or half wave low Q resonator. So when it was in sort of half wave resonating mode (a mode with relative high impedance), the impedance will reduce when you increase or decrease the size somewhat.


Large ground planes (length and width >> 0.25 lambda), show less variation in impedance then smaller ones. You will notice this from simulation. If the size is less then 0.25 lambda, you can be sure impedance increases (capacitive) when you further decrease the size.

 

[Engineer4ever]

Alright. Thanks a lot