Bluetooth antenna orientation.

Hi Gautam,

     Yes, the antenna placement has an impact on performance. For guidelines regarding this topic, please refer to the PAN13xx Datasheet.

     http://processors.wiki.ti.com/index.php/CC256x_Modules#Panasonic_Modules

Regards,

Miguel

Hello,

This is an area that interests me as well. I've been looking at chip antenna radiation patterns, which are also indicative of sensitivity. In particular, I'm interested in transmit/receive performance within a 3D space.

TI has some information on antenna performance that you may find useful in extrapolating the effects of orientation.

See: http://www.ti.com/lit/an/snoa519a/snoa519a.pdf for an example.

Another [non-TI] example can be found at www.pulseelectronics.com/download/2964/w3008_w3008cpdf

Best wishes to you.

Thanks Tim. I had gone through that earlier. I understood it to some level but not much. The radiation pattern shows that it covers most areas but I am not sure whether I have to consider polarization as there is no notes on it. Am I missing something? The device I am designing is a  device stuck to a body whose orientation can be anything.

Just to simplify things, if you have two antennas of a particular type - in this case chip antennas mounted on circuit boards - and these antennas are physically oriented the same way (i.e. their tops point "up" and each of their other corresponding faces are similarly oriented) then their polarizations are matched. For antennas on mobile devices, polarization is useful as a measurement reference, but since the devices orientation is uncontrolled then it is useful (to me, at least) to consider best & worst cases. This is one reason for looking at the radiation patterns for different antenna geometries - to see if they have any potentially troublesome "dark sides."

Your use case sounds something like a biometric telemetry application in which one or more devices attach to the body and collect data describing things like temperature and heart rate and then transmit this data to one or more other devices (likely also in close proximity to the same body). In such a case you may have some control over the sensor antenna orientation, but the receiving device may be in any orientation. If you don't have an omnidirectional radiation pattern then you may discover that, under some conditions, you don't have a reliable signal at either or both sides of the "conversation."

This may not be a problem with Class II devices, which can transmit signals well beyond the range of a physical Personal Area Network (10 meters is beyond my own reach). With such a transmit power level you may not have any real problems, even when antennas are not optimally oriented.

Another factor in your favor is that people tend to be around things that reflect RF signals. Even if you have a master and a slave located on opposite sides of a very large person who, for some reason, absorbs abnormally high amounts of 2.4GHz signals, nearby objects may reflect enough energy to allow the system to work fine anyway.

Ultimately, you can do some empirical testing out in a field (or in a wooden boat on a lake...or something) and see what happens when you move the radios around with respect to each other. I suspect that you are okay if you are using an off-the-shelf Bluetooth module.

Lastly, if you are developing a proprietary system (and not creating "dongles" that communicate with a 3rd party device, such as a smart phone), then you might consider using a Body Area Network rather than a radiated energy system. I don't know what your application is or what bandwidth you might need, but body area networks can be a very reliable and low energy means of shipping data among points on the human body.

Best wishes.

Thanks for taking the timeout to write a detailed reply. 

I have some doubts below:

Tim Bergin said:

Just to simplify things, if you have two antennas of a particular type - in this case chip antennas mounted on circuit boards - and these antennas are physically oriented the same way (i.e. their tops point "up" and each of their other corresponding faces are similarly oriented) then their polarizations are matched. For antennas on mobile devices, polarization is useful as a measurement reference, but since the devices orientation is uncontrolled then it is useful (to me, at least) to consider best & worst cases. This is one reason for looking at the radiation patterns for different antenna geometries - to see if they have any potentially troublesome "dark sides."

Your use case sounds something like a biometric telemetry application in which one or more devices attach to the body and collect data describing things like temperature and heart rate and then transmit this data to one or more other devices (likely also in close proximity to the same body). In such a case you may have some control over the sensor antenna orientation, but the receiving device may be in any orientation.

It is exactly a biometric telemetry application. I didn't get as to how to how I would have control over the antenna. The data collection (or receiving)  master might be a mobile device or a station mounted beside his bed. The patient might be oriented anyways either in sleeping horizontal position or slanted or vertical. In fact even both the sending and the receiving device can be in any orientation.

Tim Bergin said:

If you don't have an omnidirectional radiation pattern then you may discover that, under some conditions, you don't have a reliable signal at either or both sides of the "conversation."

If I have an omnidirectional radiation pattern would I be getting a good signal in all orientations? For eg for wifi I thought its usually vertically polarized or is there somethingabout the chip antenna's I am missing where polarization does not matter?

Tim Bergin said:

Ultimately, you can do some empirical testing out in a field (or in a wooden boat on a lake...or something) and see what happens when you move the radios around with respect to each other. I suspect that you are okay if you are using an off-the-shelf Bluetooth module.

Yup. I should do this. When you say a field or a lake it means that there shouldn't be any reflections(eg: off the walls) right?

Correct me if I am wrong but in the TI application note at Page 5 and Page 6 the radiation looks somewhat spherical as the elevation cut and the azimuth cut does not have any NULL beam right (which is what I am worried about)? The elevation cut looks particularly nice with a slight bump between 120 and 180. In the pilkor chip antenna link are they re-orienting the antenna in each of the 3 radiation patterns?

NULL beam, as in transmit power / receive sensitivity dropping to zero at some angle? No, it doesn't get that bad, but the pattern doesn't become spherical, either. I've never seen a single antenna pattern that is spherical, particularly after the antenna and PA are bundled into a package. I have a need for a relatively spherical pattern within a large volume and will likely need to use more than one BT radio & antenna (I'm looking closely at this).

The elevation and azimuth graphs also don't tell the full story - I prefer the 3D renderings (such as http://www.antennawizards.com/images/M2M_pattern.PNG ), which are created through modeling and/or painstaking measurements. Note that the colorful image is for a planar inverted F antenna, which are sometimes used for Bluetooth.

The Pilkor example is just one more step above the elevation-azimuth plots in that it provides more information, though it is not as complete a visualization as is a 3D plot.

I don't think you'll have a functional dead zone (meaning that you can't ship data reliably) if you are using a Class II to communicate within a purely personal space (literally, a PAN with all devices within arm's reach). However, if you have a life safety application and need to be worried about black swans then it would be a good idea to bring in a skilled RF engineer. In such a case it's worth the money to have an expert looking over your shoulder.

For fun, do a Google image search on phrases like "bluetooth chip antenna radiation pattern" and see how things vary.