height of the substrate and antenna

hey guys....i am done with my design of microstrip patch antenna to resonante it almost over 1.5 GHz....but i need to widen its bandwidth to about 300MHz but i cant do so....i tried but my results are destroyed then.....i am uploading two of my results here one is at 1.475GHz and the other is at 1.52GHz.....so please help me out here.....thanx

vary size of the patch..

i have already done so...but my results get different and then just cant regain them precisely and also not much of the bandwidth changes....

The Bandwidth of patch antenna increases with height of the substrate, and/or decreasing the substrate Dielectric Constant Er.

Maybe you can also check/improve the Input VSWR.

but vfone i cant change the dielectric constant as it is the only standard material available in market....and also tried with increasing height...what happens is actually frequency shifts higher and nothing more.....would be better if you people look at my design

I would expect that when you change the dielectric height, you will have to slightly resize your patch to keep the same resonant frequency. If the resonant frequency shifted up when you increased the height, you will probably need to increase the dimensions of the patch slightly (or at least the length). Try doing that and see if the bandwidth increases. However, 300 MHz seems like a lot ask at 1.5 GHz. Also, when you increase the dielectric height you will have to make tradeoffs with things like increased feed loss, cross-polarization, etc.

If you have the space, have you tried adding some parasitics to the edge of the patch. Also, there has been research done on adding a U-shaped (or possibly similar shapes) slot to the center of the patch (increases cross-pol though).

Check this attached document.
This doc is from chapter3 of Compact and Broad Band Microstrip Antennas by Kin-Lu-Wong.

Regards,
Balaguru.

Hi arsi15,

I took a look at your final.hfss design. There are some problems with the design I guess:

1. Air boundary: it's dimension should be at least quarter wavelength at all sides. For your case, you need to increase the z size to at least 50 mm (1.5 GHz design). Moreover, you didn't define the radiation boundary. define all faces of the airbox as radiation boundary except the bottom one. define it perfect electric.

2. Waveport: dimension of the waveport isn't correct. as far as I remember, width of the port would be around 5 times the microstrip line and height would be around 4 times the height of the substrate. Please take a look at the HFSS tutorial.

The substrate you used is one of the best for patch antenna design and very well known. Therefore, it is better to use this substrate for your design.

I would like to ask you one more question: How did you find the position to feed the patch? generally, you should feed at the position where the patch has 50 ohm impedance (Zo). Did you consider that? If not, please do so and as an example please take a look at the book of Banalnis (chapter 14).

REgards,
Shameem

one more question; why do we choose airbox λ/4 far away from every side , i thought i should only care radiation direction , but if it doesnt make any radiation to the sides , is that important?

You are right! airbox should be λ/4 far away to the radiation direction.

thanx shameem....
but i read in a paper that the air boundry is to be from lemda/8 to lemda/12 from the patch and i have considered lemda/8.....and also that the air isnt necessary to be at all sides ,it could also only be on the top....i will provide you with the paper in which there is an example of it.....and also i have used air as a material so no need for assigning boundry to it i guessed from my search...and about the waveport i am not sure i will go through the tutorial and check again and also i didnt checked for the 50 ohm patch feed i will consider it now...i just placed it in the centre.....thank you so much shameem ...i shal now alter it again and let you know....thanx.....

Airbox should be λ/4 far away to the direction of propagation. So for patch antenna, it should be at the top. and I don't think λ/12 or even λ/8 distance give you correct results.

you definitely define the air boundary as radiation boundary to emulate the far-field wave propagation criterion.

but shameem when i tried givind radiation boundy to the air box....my boundries overlapped.....

It's easy to fix this overlap problem. Just remember PEC and radiation are all boundaries, so the gnd should be set as PEC while the other five surfaces of the air box should be set as radiation.

just increase the height of substrate and u can increase the bw of antenna

Another way to increase the bandwidth is by introduction of slot in the patch.
I havent done it myself but I have been told by my teacher.