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# understanding the physics of coupling in a aparture coupled patch antenna

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#### yefj

Hello, i have read the following article and i have a basic missunderstanding regarding imagining the physics of coupling in such case.
i know from coupler that as two lines here are closer then we have stronger coupling.
so in this case we need to tranfer the maximal amount of energy from the line to the patch.

in the article they dont specify
1.the role of the substrate height
2.the role of substrate permitivity
3.the role of the width and legth of the slot for the coupling.
they only say ,we get good coupling for measures of such and such without getting into the reasons of why?

Why the coupling is increasing or decreasing from each case?

#### vfone

I think the paper explain very well the coupling phenomenon.

Since patch antenna was invented in 1972 by Howell, few things are well known about it:
- The bandwidth of the patch antenna increases with height of the substrate.
- Decreasing the substrate dielectric constant the bandwidth of the patch increases (e.g. bandwidth is about 10% with air dielectric 1, and about 5% with FR4 dielectric 4.4).
- The bandwidth is directly proportional to ratio of width W and length L. Usually highest bandwidth happen when W=1.5*L.
- Using a stacked multilayer parasitic patch, bandwidth can be increased up to 20%.
- Aperture coupling provides about 10% bandwidth.
- Aperture coupled stacked patches could provide bandwidth up to 50%.

#### yefj

Hello Vfone, igot even better article of POZAR regarding this.
The problem is that i am loking for the physical explanation of why these laws gives us better coupling?

for example why thinner feed line gives better coupling
why the feed substrate nned to be with higher dielectric constant the patch dielectric constant?
Thanks.

#### vfone

For the same length, a thinner strip line have higher inductance than a thicker one.
In addition of the coupling due to the fringing fields across the gap between coupled lines, there is also a mutual coupling of the equivalent inductance of the strip lines (see the equivalent schematic of the coupled lines).
Thinner coupled strip lines (with higher inductance) will have better (mutual) coupling.

I think they recommend higher dielectric constant for the feeding substrate to get lower insertion loss in the feeding strip line (reducing radiation losses).
The patch antenna needs in opposite (lower dielectric constant) to get better radiation.

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#### yefj

Hello Vfone,from slot theory i know that the feed need to be quurter wave long from the slot to get constructive interference.
but in fact this is not the case in real simulation.
is there a way by looking at the fields that we can see the we dont have yet constractive interference?

Also ,Is there a way by looking at the field of the aparture fed patch antenna that i can see with my own eye that
for example
1. i need to decrease the substrate of the patch
2.i need to increase the length of the slot
3.i need to move the location of the slot.
is there a way by looking at the field so i could get to these conclutions ?

If you want i could show you the fields from my simulation so you can tell me how to interpret them for the purposes above.

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