rectangular patch antenna 2

[91happy]

Hello,
Im doing a design: a patch antenna which works in 1420mhz.It will be put at the focus point of a dish(D=2m)From the rim(angle=65degree)to the center(0degree),there should be a fall of 10dB.I calculated the dimention was:width=106mm,length=86mm.Choose transmission line to feed it.I dont know how to design the feed line so that i can get 50ohms match.The substrate is air.
Please give me some idea!Thanks a lot!

 

[Higgler]

Looks like you calculated half wavelength in air, 106 mm, which you called the "width". INormally the "length" definition sets the frequency. Your actual patch will be a little shorter than 106 mm due to fringing fields, so plan on trimming it slightly shorter to set the frequency at 1420.
You need to add a coaxial connector to the back of the antenna. Coax. center pin connects to the patch, Coax. ground goes to your patch ground plane. The impedance (50 ohms) is achieved by locating the connector along the center line of the patch, somewhere between the middle and the end of the patch. Hence draw a 53mm long line from the dead center of your patch to the center of the edge along your 106 mm dimension. Position your connector along this line, place it close to the middle for a thin patch (0.1" patch thickness), close to the outer edge for a thick patch (1.0" patch thickness). You could calculate the location or find it by cut and try by making a slot in the patch and in the ground plane whereby you can slide the connector along the slot to find the correct impedance point. You of course have to make it connect to the patch and ground plane, but that can be done with solder or silver paint. Then make a new one after finding the 50 ohm point. It takes about 4-8 hours to tune one up if you have all the materials and an analyzer.

What is this used for?
kch

 

[91happy]

Higgler,thank you so much!this antenna is used for a radio telescope to receive 1420mhz radio wave.I've tried a simulation in ADS with 106*96mm patch and 18*16mm for the feeding line ,the center working frequency is perfectly 1420mhz and the impedence matching good(in the origin of the smith chart).But i find the effeciency is 100%!I dont know what is wrong.And what else should i think about??
Sorry Im totally new person in this field.

 

[Higgler]

I don't think ADS is used for detailed antenna analysis as far as I know. Does ADS show you fields within the antenna?

I'm guessing ADS only approximates results based on antenna size and doesn't do any real detailed analysis of it.

If the patch is exactly 1/2 wavelength and it gives you exactly that frequency within even 1 Mhz, then the analysis program is using simple lookup formulas. Antenna pattern calculations will be approximately correct. It's pretty simple to build, so give it a try. Your ground plane behind the patch will change the antenna pattern and gain of the patch, so be aware of that. Large ground plane, more patch gain, but added blockage. For full analysis of your reflector, you need HFSS from Ansoft and a powerful computer and lots of experience (2+ years running HFSS).

kchiggins

http://www.toyon.com

 

[savedadogs]

If you want to learn how to simulate in HFSS, this site has some good tutorials and a forum for questions:

http://www.emtalk.com

HFSS is not hard to learn but you need some experience to use it efficiently.

 

[91happy]

Thank you for all of your suggestions!
I have to di it as a side feed(a transmission line)feed antenna.Higgler How do you usually design the feeding line?I calculated the width for a 50ohms transmission line ,it is 88mm in width!!

 

[Higgler]

My normal is a coax. antenna from underneath, but a microstrip connector from the side is ok too.

You have to get the 50 ohm line to end at the correct spot on the patch for good VSWR match. Find some photo's of the layout for a microstrip fed patch. I guess it could be easier to tune since you can cut away side channels to make the feed progress further into the patch for better match. Photo's will show you. I don't have any link handy, but I'm sure you can find some.

kch

 

[91happy]

Thank you again!
If i want to add a box and have a UV material cover,is it essential to simulate it?Or is its affection much?

 

[Higgler]

I assume the box is on the bottom of the patch and the UV cover is atop the patch. The UV cover if it's attached directly to the patch (say using glue) will change its frequency and that's not good. I suggest adding a foam spacer (1/4 inch thick or more), you can glue the foam onto the patch circuit card perimeter and it won't change the antenna frequency, then a UV cover atop the foam. If rain water hit's the surface, the patch won't shift frequency if you have that foam spacer. It will shift frequency without the spacer. I'd add more of a rain shield atop the patch, thin plastic.

good luck,
kch

 

[91happy]

Thanks Higgler ,
How about it with a totally metal box?I have another LNA that will be put together with the patch antenna in the box,any suggestion about how to place them ?
Yan

 

[bmnet]

Hey, I think the metal box will block all your signals, or make your antenna receive from a certain direction only...
As for the LNA, would be better placing it behind the ground plane of the antenna...

 

[Higgler]

I'd make the LNA have it's own metal box. Have an rf connector pair sticking out of this metal LNA box, one to connect the Patch directly to and the other one for your output.
LNA's are static sensitive, so be careful touching your patch, it might blow the LNA, especially since you're using a plastic case for environmental protection. I like "grounded center conductor" antennas when they are connected to sensitive LNA type hardware.

I'd suggest, Metal box with LNA, then patch connected directly to metal box RF connector, then put both in water tight plastic box. at 1420 Mhz, you might get some antenna pattern effect from the plastic if the plastic is thick, keep the front and sides of the plastic box less than 0.1 inches (0.01 Lambda) thick if possible.

kch