One of the many challenges involved in designing an IoT product is off-the-shelf antenna implementation. There are many off-the-shelf antennas that are easily available from antenna companies. Off-the-shelf antennas such as GPS, Wi-Fi, Bluetooth and cellular antennas, are often easy to source and readily available. However, the efficiency range will be from 40% – 60% for ceramic chip antenna and 50%-70% for PCB antenna. Its like I could almost hear voices saying those numbers are good enough for their application. Stop right there! You are still not done and there is a trick to this. Some of the challenges we face with such implementations are:
1) Smaller chip antennas are an attractive solution for many ISM band technologies however, one thing we fail to notice, is that the efficiency of the smaller chip antenna is typically lower, around 40% – 60%. This lower efficiency will affect the field performance of the product. I regarded this efficiency as low because with custom designed antenna with Innowave, you can easily achieve 80% – 90% of efficiency.
2) Moreover above mentioned efficiency of the off-the-shelf antennas are only achievable if we allocate a gap between the chip or PCB antennas and other components in the PCB. Usually, this gap will need to be PCB ground cleared. In typical cases, designers will always struggle to allocate such a huge space so the efficiency will further be degraded.
3) Off-the-shelf antennas will usually have poor second harmonic, noise and spurs rejection. This is because these off-the-shelf antennas are only designed to perform at the band or frequency of interest. Due to this, products incorporating off-the-shelf antennas are prone to fail at certification especially FCC-Radiated Spurious Emissions (RSE) and PTCRB-RSE tests.
4) There are no smaller antennas available in many applications such as cellular, NB-IoT, satellite, RF energy harvesting (915MHz) and many more. Bulky off-the-shelf antennas will increase the product size and might make it unattractive and inconvenient to the customers. Moreover, bulky antennas are prone to interference such as user touch or body loading, placed against a wall and other environmental obstacles which will significantly reduce the antenna or product performance.
5) There are surface mount and UFL cable off-the-shelf antennas available. Antennas with cable will have some losses which will reduce the antenna performance.
6) In many cases, off-the-shelf antennas need retuning due to impedance miss-match between the antenna and the output of the radio. The reference matching circuit given by the antenna manufacturer will not work because designers will be using different PCB stack ups, PCB materials etc. This antenna re-tuning will incur additional costs because antenna manufacturers will usually charge for the service.
7) All off-the-shelf antennas requires a certain ground plane length and this length of the ground plane will be ridiculously large for a wearables or smaller wireless devices. If you are not able to allocate enough ground plane as per the datasheet, your efficiency will be reduced further.
I have given an example of off-the-shelf GPS antenna tuning here. This GPS antenna is tuned by our team member. The measured data of the input impedance, return loss and the efficiency is given below. The GPS antenna below is tuned with a 120mm length ground plane.
Figure 1: impedance measured after GPS antenna tuning
Figure 1 shows the impedance data on a smith chart. The GPS frequency, 1.575GHz is matched to 47.1 -j1.4 ohm input impedance.
Figure 2: Return loss after GPS antenna tuning.
Figure 2 shows the return loss of the GPS antenna. Return loss at GPS frequency, 1.575GHz is -29.9dB which is excellent.
Figure 3: GPS antenna efficiency measured on an anechoic chamber
Figure 3 shows 47% of efficiency at 1.575GHz. This data is measured on an anechoic chamber at Cetecom Milpitas.
This data proves that even an off-the-shelf antenna requires antenna tuning using matching components. Ceramic antenna efficiency is generally lower, and it gets reduced even further if we don’t allocate enough ground plane length or enough ground clearance between the antenna and the other components.