Micostrip antenna- square or rectangle?

[chanzg]

Hi All,

I am designing a 2.4GHz microstrip antenna. Becasue of the fringing field the patch is designed to be rectangular in shape. However i realised that to have a circular polarised antenna made from truncated corners the patch needs to be in a square shape. However, this would affect the radiation and gain of the antenna.

Any one could explain or give any solution to thsi?

 

[Ankesh Garg]

Hi,
You can get circular polarization without going for truncated option by exciting two orthogonal modes of a rectangular patch antenna. Use a 90 degree phase shift for other using a branch line coupler and then excite the two modes (one along width and other along length). Hope this helps.

-Ankesh

 

[chanzg]

anyone knows what is the typical gain for a microstrip antenna?

i try simulating a 38 x 29.4mm without truncated corners. The gain is 5.27dB

a 29.4 x 29.4mm with truncated corners antenna only has about 2dB... is it that square patch has a lower gain?

 

[volker_muehlhaus]

Hello!

Here is a theoretical estimate for a lossless patch antenna:
https://en.wikipedia.org/wiki/Patch_antenna#Patch_Antenna_Gain

I would expect ~5dB if you include metall loss and dielectric loss for common microwave substrates.

What substrate are you using?

Best regards
Volker

 

[chanzg]

i got 5db gain when i simulate the 38.4 x 29.4 antenna patch (FR4)using ADS.

But i want to have a circular polarized antenna so i made it square 29.4 x 29.4 with truncated corners, the gain drop to 2 dB. Varying the length on the square patch does not improve the gain much.

 

[volker_muehlhaus]

I think that something might be wrong with your substrate definition in ADS.

FR4 is pretty bad for patch antennas, because of the high substrate loss. This reduces the gain, compared to low loss substrates.

I did a quick test with Sonnet EM, using FR4 with 1.5mm height, epsr=4.4 and tand=0.017. This substrate loss is what I measured for my own design work on FR4 in this frequency range. FR4 has much variation between different manufacturers, so you might find other values for FR4 from other manufacturers.

With this, the gain (E_total) is 1.6dBi for the 28.5mm*38mm patch and 1.2dBi for the square path with 28.5mm *28.5mm. This are normal linear polarized patches with pobe feed.

You can see that the patch width changes the gain only a little bit, not by 3dB. Maybe you have a problem with the substrate definition and/or with the defintion of the gain that you look at (polarization).

Added after 1 hours 32 minutes:

circular polarized antenna so i made it square 29.4 x 29.4 with truncated corners

I tried to build something similar, and with my substrate properties given above, I end up at 29mm length. The gain is around 1dBi with left hand polarization. This value is not much different from the linear polarization square patch, where I had 1.2dBi.

 

[chanzg]

Ya, there is some error in the loss tangent of the substrate.

Thanks a lot, that really helps.

FR4 is too lossy resulting in the low gain.

 

[apurvmittal]

@changz

Can u specify the dimensions of your antenna when you get 5dB gain with fr4 as the substrate?

 

[sagar441]

hello.
i have made an aperture coupled microstrip antenna using square patch for 2.28 Ghz.
now i want to change it to 2.45 Ghz.
someone told me to use optimetrics.
but i dont know how to.
plz help.

 

[ferdows]

hi
pls upload u file

 

[redcrystalline]

i have this problem too because frequency transition cause changing in axial ratio!!!

 

[medaziz]

Dear sagar441

can you give us the figure of the antenna to undestand the problem exctly