Antenna Question

[docmur]

Hey Guys

So I have a new product, that will use 916 MHz RF (CC1101) and Bluetooth, due to some requirements.  I found two patch antenna's from Linx:

https://linxtechnologies.com/en/products/antennas/usp

That are exactly what I need.   So my question is, what kind of considerations do I have to make, to use two antenna on the same board and get reasonably good signal strength out of both of them?  I'm not a professional PCB designer, so I have limited hands on time with doing RF design.

Thanks
Docmur

[geoffp]

The spec sheet say that you need a 1/4 wave counterpoise to work with the patch antenna. An ideal counterpoise for an omnidirectional radiation pattern would be a copper disc with a radius=1/4 the wavelength of the frequency. But you can compromise and use a spoke of traces (or maybe just one for a somewhat directional response).

The wavelength is calculated using the formula lamda = c/f   Where lambda is the wavelength in meters, c is the speed of light of the medium in meters/second, and f is the frequency in Hz. In free space, c = 300x10^6 m/sec. For pcb material, the speed will be reduced by a factor depending on the material and thickness.

Since you need to work on two bands, 916MHz and 2.5GHz your should have 1/4 wavelength length traces  centered on both 916MHz and 2.4Mhz bands. The 1/4 wavelength traces act as 1/4 wavelength transformers. For the design frequency each trace will transform the high impedance of it's end (which is connected to nothing) to a low impedance at it's beginning (which is connected to the 'ground' of the antenna).  You can have different counterpoise for different frequencies connected in parallel without them affecing each other too much. A discone antenna works on this principle.

[Gribo]

A ground plane (This antenna is grounded) can be used instead of a counterpoise trace. Make sure it has the required length (~8cm for the 915MHz antenna). Also, try to separate the 2.4GHz and 915MHz antennas as much as possible, you don't want them to interfere with each other.

[geoffp]

If you have the space, a ground plane for each antenna is preferable to counterpoise trace(s). My idea of using counterpoise traces of both wavelengths in a star configuration  with the patch antennas located at the center is a way of making a multi-band antenna. Not necessarily the most ideal configuration.

Instead of using the Linx patch antenna product.  Have you considered designing the whole antenna as PCB traces?  See this Hackaday article: http://hackaday.com/2014/02/28/pcb-antenna-reference-designs/ 

As for how the two antennas affect each other  in close proximity, that is a difficult question to answer. It may be possible to simulate with software antenna modeling software such as NEC,  A practical approach is to create a test board with multiple antenna mounting configurations, try each one out and pick the one which works the best.