Microstrip patch: Reflection coeff. and grating lobes

[KRAIG]

position grating lobes

Hi

I am in the process of designing a thinned planar microstrip antenna array.
I have a problem with reflection coefficient i dont know what parameters is depended on,what should i change in order to minimize it in appr. -30dB for every source(patch).The source is a coaxial feed for every patch with linear polarization.

Also how can i find the maximum distance that the patches should be so as not to have any grating lobes in the far field diagram.
Any clues about those two parameters would be of great help to me

 

[juppydu]

grating lobes position

hello,

for patch input matching, as rule of thumb, you can try to move the input port (caxial) toward the patch edge if you have an input impedance less tahn 50 Ohm, or toward the patch centre if you have an input impedance more than 50 Ohm.

for the seconfd question: grating lobes are mainly related to the array periodicity. If you hae a regular lattice the grating lobe positions are well known. Anyway you have alsto take into acount for the maximum scan angle before you select the minimum inter-element spacing.

Bye.

 

[KRAIG]

hello,

for patch input matching, as rule of thumb, you can try to move the input port (caxial) toward the patch edge if you have an input impedance less tahn 50 Ohm, or toward the patch centre if you have an input impedance more than 50 Ohm.

Bye.

What if my input impedance is exactly 50ohm?
Also i thought that when moving towards the center you loose the linear polarization and you get circular/elliptical.

 

[juppydu]

but if you already have 50 Ohm wht is the problem ? do you have problems at array input port ?

if the patch is sqaure or rectangular and you move the probes toward the inner part the cross-polar rises.

Anyway, what kind of patch you have ? how many inputs ports per patch, 1 or 2 ?
linear or circualr polarization ?

bye

 

[etoile]

The situation is really interesting as if it is already having 50 ohm, then i agree with the guy who responded u .. and so far reflection coefficient relate dto matching that u have laready.. could u explain the shape of patch , and ur specifications u wanna have ...

 

[KRAIG]

I have a square patch with one port (coaxial feed) and i want to minimize the return loss.I use this equation:

X = (Zin-50.)/(Zin+50.)
r = 20*LOG10(ABS(X))

to calculate S11.
Now, i have a plannar antenna of this kind 10x10 patches and try to find optimum configuration for linear polarization.

 

[juppydu]

Hello,

Ok, you are calculating the return loss by that formulas, then ???

You second statement is meaningless ! what does it mean "optimum" for linear polarization ? (optimum for what ???, peak gain, coverage gain, beam shape, etc...)

I think you are missing somthing.

Bye.

 

[KRAIG]

well, my optimization goes both to reflection coeff. and peak gain-shape but my original question was if there are any "rules" about the position of the source so as not to go blindly about it.

 

[juppydu]

Okay

we can tray to do the first step "How does the Input impendence vary ?"

I hope it can be useful for you

"it has been found that a coaxial-probe fed-patch antenna's input impedance exhibits behavior that follows the trigonometric function:

cos2[π(y0/L)]

where:

L = the length of the patch and

y0 = the position of the feed from the edge along the direction of the patch length L.

On the other hand, it has been found experimentally4 that on low-dielectric-constant materials, the input impedance of an inset-fed probe antenna exhibits fourth-order behavior following the function:

cos4[π(y0/L)]"

is this good for you ?

 

[KRAIG]

Yeah its good.
Thanks for your help!