Dual Band Antenna Matching

[rvargo]

Hi,

We are looking at using a dual band chip antenna for a mobile device. When the antenna is put into our device, it is slightly mismatched at both bands (2.4/5GHz). So I need a matching network.

However, if I design a network to match one band, it has a negative impact on the other band. Are there any techniques to match in both bands?

Any advice is appreciated.

 

[Higgler]

How did you mount it to the groundplane?
Many chip antennas are very groundplane size dependent (i.e. length mainly in the E plane = primary polarization).

You could measure VSWR and change the length of the groundplanes (add metal tape to your groundplane, or solder wide copper braid to groundplane and cut it to length) as you look at VSWR, that may be all that's needed. Or if it helps at one frequency, then you only need to match the other freq.

kch

 

[rvargo]

Thanks for the response.

The chip manufacturer has a recommended layout. This includes size of ground plane and distance to ground plane. Unfortunately, we are size constrained, and I was not able to meet their requirements for ground plane size (I did meet the requirement for distance to ground plane). I was hoping that I could make up for this by using a matching network, but this is proving very difficult.

 

[Higgler]

There is the law of physics that puts a limit on the VSWR if the size is small. i.e. small antenna, bad VSWR, low gain.
At 2.4 ghz, wavelength is 11.8"/2.4 = 4.92", your ground plane should be in the 3-5 inch size.
You don't actually need a full ground plane width, you could use some metal stubs at each end of the ground plane and bend them top and bottom to form an S shape. These can be on your circuit card too.

What size circuit board do you have? What is the antenna part?

You should possibly add the metal to extend your groundplane and demonstrate to management that you've hit a law of physics limit so they don't get upset with you for not being able to break laws of physics.

kch