Off-the-shelf antennas are usually big in size for low frequency band technologies and prone to external interference. We have also given an example of a Wi-Fi antenna tuning done by our engineering team, taking external interference such as human touch and human body loading into account in the previous article. Off-the-shelf antenna are often designed for ideal condition without human body loading. Due to this, we shouldn’t get too excited about efficiency and gain numbers in the datasheet. Often, what you see is not what you get if your product is a body worn, wearable or handheld device.
Some off-the-shelf antennas are connected to the radio output/input through a RF coaxial cable with a UFL connector (UFL cable). Theoretically, the 50 ohm UFL cable should be lossless but in the real world, a lossless system is non existent. Due to this, the 50 ohm UFL cable will cause additional losses to the system which will subsequently reduce the antenna gain and efficiency.
Moreover, the off-the-shelf antennas are tested in an ideal environment without any circuits nearby. They also have adequate ground clearance between the adjacent ground plane. In the real world, IoT designers struggle to allocate space for ground clearance and also space between other circuits nearby due to smaller size requirement. If this statement applies to you, then the datasheet efficiency and gain numbers are meaningless im sure your antenna will be doing much lesser than what is claimed on the datasheet.
Furthermore, off-the-shelf antennas need input impedance tuning using a matching network. Even though the matching circuit is given on some antenna datasheets, IoT designers usually won’t be able to use the recommended matching circuit on the datasheet. This is because the matching network will only work if we follow the exact PCB design guidelines given on the supplier datasheet. Engineers often end up minimizing the cost of the product by simplifying the PCB, i.e. using thinner layer stack, using FR-4 instead of Rogers material and so on.
Some off-the-shelf antennas might not have recommended matching circuits in the datasheet. However, matching circuit is required if we are unable to follow any one of the design and implementation guidelines mentioned in the supplier datasheet. Even if we follow all the guidelines mentioned; which is a rare occurrence, it is safer to implement a matching circuit to tune the antenna input impedance to 50 ohm.
Over the years, we have used many off-the-shelf chip antennas in our embedded system designs and in all instances, we requireimpedance matching network to match the antenna input impedance to 50 ohm. This is done to get the optimum performance out of the antenna or the radio. Figure 1 shows the antenna block diagram which illustrates the implementation of the matching circuit for antennas with 50 ohm UFL cable.
Figure 1: Block diagram of the antenna with 50 ohm UFL cable and antenna matching circuit.
I have given an example of an off-the-shelf antenna below and the requirements as given on the datasheet for it to work at its best. Figure 2 below shows Chip Antenna X. This antenna needs to be mounted at the centre of a 50mm x 100mm Rogers PCB as given below for optimum performance. This means, this single antenna requires your total PCB size to be 50mm x 100mm.
Figure 2: Mounting guideline for Chip Antenna X
It is recommended that a 5mm gap between the antenna and surrounding parts should be given for the antenna to work. No conductor, including the copper of the PCB and components are allowed to be adjacent to the antenna top side, bottom side and board side. Figure 3 below, shows the ground clearance recommendation for chip antenna X. All the layers should have a 6mm x 6mm ground clearance for optimum performance of the antenna.
Figure 3: Ground clearance recommendation for the Chip Antenna X.
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