915/868 ISM band PCB antenna

[Gami]

Hi,

I am working to implement a small size 915/868 MHz PCB antenna. I studied some earlier forum threads like thread247-75816.
What is the starting point for this? I read that an inverted F will be a good configuration.
Is it possible to design this without the aid of EM simulation tool?
If not, what is a reasonably priced simulation tool to perform antenna modeling and simulation for 915 and 868 MHz frequency? How about NEC from Nittany scientific? Is it good enough? The antenna should work for both frequencies.

 

[Comcokid]

I've not use any NEC software in an attempt to analyze any PCB or planar antenna. From what I've read, NEC is geared towards a an antenna that can be modled as 1-D lines.

Flat antennas, or those with 2-D members need something more powerful like Ansoft Designer. I attempted analysis of a F antenna using the free student version of this, but it was too limited, and wouldn't allow for the analysis.

With a lack of tools, I designed several F antennas using just a network analyzer and making sure the S11 was well matched. I prototyped a F for the analog cellular band (824 to 894 MHz) that was semi-circular due to a need to fit a particular enclosure, and it work well. However, the issue with this approach is you're not sure what your antenna pattern is, unless you use a two-port network analyzer and try to take data points at different angles in a open-space area with no reflections.

If I could empirically design a F for the analog cellular band, you should be able to do one that has both the North American and European ISM band. There are also methods by which two resonant elements are put on a F to handle different bands, but the interaction of the two elements will make it really impossible to guess-ti-mate what the end pattern is.

 

[biff44]

I agree, you can do a fairly quick job of it by just taking an existing design from some paper and scaling it. I would caution you about things I have seen that complicate this. Usually the PCB is near something else, like a plastic or metal housing, or external objects in close proximity. This will detune the antenna, so you have to try to tune up/design the antenna while it is in its final resting place!

I have had virtually NO luck using the chip or coil type antennas that you can buy at digikey,etc. Every time I get one that seems to be in my band, and I actually measure it, I get a horrendous vswr.

Any antenna on a pcb board will suffer from weird radiation patterns as the board is rotated. Sometimes a simple resonant loop works well, as it tends to radiate pretty well in a 360 degree arc. At lower frequencies I like to use a loop with a small tuning chip capacitor in series--that way I can trim out the board/housing effects without a respin.

 

[Higgler]

What size is your ground plane on the pcb?
I can send you dimensions for a standard notch antenna, not specifically a PIFA if you have enough room.

kch
khiggler
at
yahoo
dot
com

 

[Gami]

Thanks Higgler/Comcokid/biff44
Thanks all for valuable input. Looks like the good starting point is to take an existing design and scale it. The antenna i am looking for does not necessarily have to be PCB antenna, it could be board mount but contained within chassis which is 4 x 2 x 0.5 inches.
No off-the-shelf supplier seem to have dual band antenna for 915 as well as 868 MHz bands. Do you guys know any?

The ground plan size is about 3 x 1.8 in.
Higgler, i will email

Thanks