For GSM, the PTCRB defines very specific standards for radio characteristics, such as radiated spurious emissions, antenna radiation efficiency, etc. These standards apply equally to the radiation characteristics of M2M devices as to standard consumer handsets. Obviously performance against these standards are significantly influenced by antenna design, but they are also affected by the properties of the radio module and the overall device itself. That his why PTCRB certification is based on testing of the entire device, not of any individual component such as the antenna.
There are a number of independent laboratories that are experienced in testing M2M devices to comply with the industry standard on emissions.
I think OTA remains the best and most relevant way, in general, to test all cellular devices, regardless of whether you require USA network certification or not. The reason I say that is because customers who target it seem to be more successful in the global market, no matter what their design topology and device size. I believe it is because the more successful devices simply display better transmit and receive sensitivity on the networks. Using peak antenna gain can be misleading as it is the best possible measurement in one particular direction, at one particular angle in one particular set up. As regards antenna measurements the issue is when devices are turned off it is only a snapshot of performance at one stage of the wireless device design process. The way to properly design cellular device antennas is to start with this method but to also optimize at the end of the design process when your device is on and is in transmit mode. Antenna efficiency and TRP have a very close relationship as antenna efficiency is a measure of how much power conducted to the antenna is actually transferred into radio waves (over the air). If you have a good impedance match between an efficient antenna and a certified module (with the device in active mode) you have good TRP. Generally TIS follows unless you have RSE issues and that is another day’s discussion as you know! For devices that require a directional pattern it has to be solely up to the network whether or not, based on the business case, they allow that product onto their network. From our experience all that metal on one side means you are destroying antenna efficiency and hence TRP on the other side! Means you require a custom antenna (to allow for the presence of the metal) and a step by step design process with the all relevant parties, including the operator, to show that you have considered your design to have that directional pattern optimized, based on your own individual application. Love to hear more on this one, Dermot
With the endless array of applications and form factors for M2M devices, it's very common that the device under test really shouldn't or can't be tested in the standard way.
If you plan a M2M device with external cellular antenna, then you will find a supplier for common antennas easy. If you plan a small device with integrated antenna, then you will run in trouble easy as soon the size of the PCB is smaller than 40 mm x 100mm. Some designs accept 40mm x 90mm. I am working on a book how to design a M2M device. Chapter 1 is talking about embedded antennas. By comparing more than 10 "out of the shelf antennas" I have seen that all reference PCBs have a size of close to 40 mm x 100 mm ground plane. They all have an antenna efficiency of 50% to 70%. Some customized PIFAs are listed with up to 90%. BTW, just measure the size of your mobile phone and the phone of your friend and you will see that the size is close the same.
The two main questions at hand are how should these devices be tested, and what parameters should be used to quantify their performance (i.e. other than TRP & TIS)?
TRP is related to the antenna efficiency. If the antenna you select has from beginning to less efficiency, then you have no chance to pass. The matching circuit on first PCB should hold the components mentioned at the reference PCB. The final tuning of the matching circuit makes sense only if you have the final plastic enclosure. An enclosure made by rapid prototyping has maybe another Epsilon R. The Epsilon R of the plastic will interfere with your antenna.
The TIS of your device can be damaged by display or microcontroller easy. By selecting a microcontroller with internal Flash-ROM, RAM and oscillator you minimize the risk of "digital noise" on your PCB. As lower the power consumption of your uC is as lower the chance on noise. See also:
Best will be to try to get the Wireless modules, the antennas, the micro controllers, the power supply, battery and charger from one hand. If you like, I can help with resources for developing a prototype up to final product. Just drop an email to harald.naumann at gsm-modem.de
Thanks for the posts. I wanted to wait awhile to see the different responses to my posts before replying.
You each made very good points about the challenges of certain designs, testing methodologies, etc. Anyone searching for details on antenna design should certainly heed your warnings and adhere to the guidelines suggested. However, my intention was to focus on a couple of scenarios that are very common: devices where either the device itself or the installed environment has a large enough ground plane where the lower azimuth pattern is completely nullified; and devices that are installed against the body (wrist, ankle, waist).
For the first case, antenna efficiency measurements seem to be a better way to evaluate the antenna performance. Alternatively, if the ground plane is known, I would think a modification to the CTIA OTA scan angles would also be an effective way to determine performance.
For the latter case, I guess the approach that falls in line with the current methods would be a full body phantom. Although, I still don't quite see the value in the current head phantom testing, especially when you throw in SAR requirements. If the only field use case is for the device to be worn on the body, does it still make sense to evaluate the antenna performance for the full 360 degrees? Doesn't the device see higher signals from reflections in the antennas upper azimuth, away from the body? If so, shouldn't the requirements and measurements be for the effective antenna pattern?
I look forward to your input and guidance.