Antenna trace to Chip Antenna for Bluetooth module

[docmur]

Hey

I'm a little confused about how to calculate the trace length / width for for an a module that is using a chip antenna.  I know the antenna trace has to physically match as close to 50 Ohms as possible, but what I can't figure out is if I need to calculate the length of the trace to the 1/2 wave, 1/4 wave or etc..

I picked a generic chip antenna for my Bluetooth module and nothing in the datasheet talked about being careful about the length, so do you guys know what is the best method to use for designing this trace?  I used the auto router to give me the width of the trace that would match 50 ohm, which it said was about 20.5 mils, so that is how wide I've made the the trace, I been careful to make to that no cooper is left under the antenna or around the antenna, in accordance with the datasheet.

I've posted a picture of the actual layout of the chip / module, maybe it would help.

Thanks

Docmur

[RF-Tech]

Hi Docmur,

I'm not an expert antenna guy, but I have done a few - we recently evaluated a number of them, and I can tell you that most if not all were not impressive.  I did find an excellent website that will answer all if not most of your questions and has good examples as well:

http://colinkarpfinger.com/blog/2010/the-dropouts-guide-to-antenna-design/

Looking at your design, you don't have to worry about the length of the feed-line, as long as its 50 ohms - and you usually don't route with 45 degree angles, you will route 90 degree turns and miter the corners.  If you want to stay with the chip antenna I would suggest that you move it as close to the via as possible, shortening the trace length as much as you can.

But if you want to reduce your component count, you can eliminate the chip antenna by printing an F antenna, making it a little longer than the calculated 1/4 wave length (so you can shorten with an x-acto knife), and tune to the proper frequency - which will mean having access to a network analyzer, which may not be easy.

The calculated 1/4 wave length for 2.485Ghz is approximately 1.13", ( using the formula: 234/F(Mhz)*12 = 1/4 Wave antenna length in inches ).

-S

[free_electron]

Your antenna cant sit underneath the module ! It needs to be off side , have no plane under it , but does need a plane to the side of it.

Ti has an appnote for their cc3xxx based bluetooth chips on antenna design. Those work very well

[ajawamnet]

I will say that you want to watch any bends in the trace.

After setting up the proper impedance up to the end of the plane, limit the amount of bends as much as possible. In the new AD15, there's an option to do RF mitres.

http://techdocs.altium.com/display/ADOH/Chamfered+Routing

There's a great site for working with RF mitre/miters-
http://www.microwaves101.com/encyclopedias/mitered-bends
http://www.microwaves101.com/encyclopedias/microwave-rules-of-thumb