Patch antenna design
Patch antenna design
(OP)
Hi guys,
I was recently given a project to design a broadband patch antenna with a center frequency of 900MHz. Here are the specs that I used for my desing..
1. Center Frequency Fc=900Mhz
2. Er (Dielectric Constant)=2.2 (PCB Board)
3. Substrate Height = 1.5mm
Calculated dimensions were w=110mm & l=97mm.
Did some simulation using Ensemble Design
After fabrication, I plugged it into a network analyser and was just surprised that the only place where I got a decent return loss (-12dB)was at 1500MHz! I've looked through everything and I can't seem to figure this one out. Any thoughts would be highly appreciated.
Meant to use parasitic elements to broadband it.
I was recently given a project to design a broadband patch antenna with a center frequency of 900MHz. Here are the specs that I used for my desing..
1. Center Frequency Fc=900Mhz
2. Er (Dielectric Constant)=2.2 (PCB Board)
3. Substrate Height = 1.5mm
Calculated dimensions were w=110mm & l=97mm.
Did some simulation using Ensemble Design
After fabrication, I plugged it into a network analyser and was just surprised that the only place where I got a decent return loss (-12dB)was at 1500MHz! I've looked through everything and I can't seem to figure this one out. Any thoughts would be highly appreciated.
Meant to use parasitic elements to broadband it.





RE: Patch antenna design
Is this a standard half wave resonant patch antenna?
Your coax. probe from underneath? It should be located just off the center line, probably 5% out from the center towards the edge. Did you use a connector on the bottom? and cut away some copper on the base so the center conductor doesn't short to the bottom ground plane?
What size ground plane did you put underneath it?
kch
PS: What's your bandwidth? did know know that 2x thickness of substrate = 2x the bandwidth of the antenna (thicker substrate needs the coax. closer to the outside edge of the patch too).
RE: Patch antenna design
The ground plane dimensions are 300x300mm. The substrate thickness was about 2mm.
Was aiming for a 80% bandwidth, but with the aid of the parasitic elements.
Thanks
Bmnet
RE: Patch antenna design
Three bandwidth examples of mine;
1) at 3.5 times your (0.2 inches) and 8 Ghz frequency (11x frequency) we achieved 20% bandwidth for a thick patch which required the probe to be attached on the edge of the antenna.
2) at 1.33x your thickness for GPS which is 1.575 Ghz, we achieved only 30 Mhz bandwidth (1.9%)
3) 433 Mhz quarter wave patch (2"x4" size) using 60 mil FR4 had 20 Mhz bandwidth (2:1 VSWR point).
Antenna bandwidth depends on the volume of your antenna per the laws of physics (except for Marketing, Sales and Management which break the laws of physics daily), i.e. LengthxWidthxHeight sets the bandwidth of the antenna, and there is no getting around that unless you own negative dielectric constant maybe.
kchiggins
www.toyon.com
RE: Patch antenna design
http://www.emtalk.com/tut_1.htm
and the measured versus hfss results are really good:
http://www.emtalk.com/forums/viewtopic.php?t=7
Enjoy.
RE: Patch antenna design
As for the simulation Savedadogs, it did agree with theoretical design calculations I did initially.
Currently am suspecting the coax connector I've used is 75ohm.
Will check and keep you posted. Unfortunately I don't have another simulation software..
RE: Patch antenna design
I don't think the 75 ohm impedance would be the reason for no resonance at 900 Mhz. Make sure your dielectric is correct.
If all else fails, build one extra long with your probe in the middle, then start hand trimming each edge for match and frequency.
kch
RE: Patch antenna design
RE: Patch antenna design
A point of interest when trimming, I made a 433 Mhz patch that had fingers on the end that were used for frequency trimming. I found that I didn't have to remove the fingers of copper to achieve a frequency shift, only making a cut in them was necessary. Each finger was 4 Mhz of tuning, and the difference between just a cut and removing the entire copper finger was maybe 0.1 Mhz. So if you trim all the way across the patch to remove metal and you get really close to your frequency, you can just cut a corner to raise the frequency slightly instead of removing a sliver of copper all the way across the patch.
The really thick patch may have some trimming challenges, so good luck if you try that one.
kch
www.toyon.com
RE: Patch antenna design
It brings up another question though: Did I manage to change the dielectric constant of the material or the 15holes (x 1.3mm) have got no effect?
Will still try the trimming just for future reference.
Thanks heaps Higgler.
Bmnet.
RE: Patch antenna design
If things don't make sense, often just starting from scratch can shed some light on the subject. Hence, build another one and it might work fine.
I just calculated 97 mm gives 1.54 Ghz half wave frequency in air, maybe just a coincidence. Your rtn loss should be at least -10 dB at resonance. Suggest making your patch length x width about a 2:1 ratio just for safety sake if you retry. More rectangular patches also have better crosspole purity which is often desired.
kch
www.toyon.com
RE: Patch antenna design
Thanks guys for all the help.
Bmnet
RE: Patch antenna design
If you want it twice as short, go to the exact center of the patch, cut it in half, (just the top part, leave the ground full size) and short the top of the patch to the bottom of the patch all the way across it with metal tape or solder metal across (you can also just drill holes and put screws from top to bottom, use 5 or more evenly spaced). The frequency will stay approx. the same, you're size is halved, but your bandwidth is cut in half and the patterns become slightly asymmetrical (higher gain towards the open end) but now you'll have a quarter wave shorted patch antenna.
Many receivers are sensitive to static blowing them out. Having your center conductor of the antenna shorted prevents any static from getting into your receiver.
kchiggins
www.toyon.com