Why thick substrate results in broad impedance bandwidth?

Hi everybody,

For most printed antennas, thick substrates usually results in broad impedance bandwidth. Can anyone kindly advise the reason or mechanism for this?

Thanks in advance.

Really? I think thick substrate for higher power capacity. That is important for high power transmitted.

With the increase in height, the fringing fields from the edges (which makes printed antenna to radiate) increase, but also they are reflected, refracted and scattered in a different manner, which affect the bandwidth.

Yes, but how do the scattered and refracted field affect the impedance? Can you provide a more detailed explaination?

In addition to what Vfone mention, surface waves increases.

Looks like a duplicate thread to me.. See the below one. Discussed this before.

https://www.edaboard.com/thread147511.html

So.......?

---------- Post added at 08:24 ---------- Previous post was at 08:24 ----------

Hi cnm, thanks. I will check it.

Consider a half-wavelength microstrip patch antenna at resonance on an electrically thin substrate (thickness a small fraction of a wavelength).

The radiated power is dependent on the voltage at the end of the patch and the patch width and is independent of the substrate thickness. If we hold the patch width and voltage constant and decrease the substrate thickness we increase the stored reactive power, hence increase the Q and decrease the bandwidth. To understand the basic relationship between the radiated power and the voltage/patch width, look up the 2 slot model of the patch.

I couldn't find a simple paper explaining it, but this would be a start:

Microstrip antenna technology
Carver, K.; Mink, J.;
Antennas and Propagation, IEEE Transactions on
Volume: 29 , Issue: 1
Digital Object Identifier: 10.1109/TAP.1981.1142523
Publication Year: 1981 , Page(s): 2 - 24

Or in other words just going to the very basic formulas of impedance Z, and Q factor:

Q = 2*PI*f_res*(Energy Stored / Power Loss)
Q (or bandwidth) is changed not only by the stored energy, but also by the power loss due to refraction and scattering.

Impedance Z=U/I is changed when varying the edge currents and voltages due to material thickness.

That's what I did - showed that if you kept the radiated power constant then the stored reactive power increased as you decrease the substrate thickness.

What is not clear from your analysis is why the ratio of the radiated power to the reactive power drops as you decrease the material thickness, you need some way of quantifying the radiated power and this is where I used the two slot model for the patch. Think of a circuit model for the patch - typically a length of transmission line with shunt conductances 'g' at each end representing the radiation. You need to be able to estimate 'g' if you are going to calculate the Q, and this is where the two slot model is useful.

I'm not sure what you mean by the power loss due to refraction and scattering, if it is a reasonable antenna then the power loss should be dominated by radiation.

Thanks Fred. It really helps. That's exactly what I need.

---------- Post added at 02:16 ---------- Previous post was at 02:15 ----------

Hi Vfone. Thanks again for your inforamtion.