Antenna Matching within an Enclosure Part II: Techniques & Guidelines
Antenna selection and placement can be a difficult task, and the challenges of implementing the antenna design are not over once it’s placed on the board. As previously discussed in Antenna Matching Within an Enclosure Part I: Theory and Principle, the enclosure affects the antenna match. The process of matching an antenna can be a very complicated process. Not only is an in-depth knowledge of RF Design principles and components needed, but proper technique and understanding of the antenna properties is required. Any error introduced into the measurement while matching the antenna will ultimately reduce the effectiveness of the antenna and its performance. For this reason, as much care should be taken in the setup of your measurements as the actual matching process itself.
The most important factors in antenna tuning are proper connection to the PCB, calibration and port extension of the network analyzer, and knowledge of the antenna to ground plane relationship. These three factors must be taken into account when attempting to tune an antenna for an enclosure, and will be covered in this paper. Tuning an antenna is not as easy as simply connecting it to a network analyzer and chasing the impedance around the smith chart. Without doing the little steps right, it is easy to introduce error into the measurement and improperly match the antenna.
Before matching can begin, a proper connection must be established to the PCB. There are usually two methods for connecting to the PCB to match an antenna. The first is using a U.FL connector. This involves finding an area big enough to place the connector down and an area that allows for good grounding. It is often difficult to find enough room near the antenna trace for this method; additionally care must be taken to make sure you don’t introduce additional parasitic effects. The U.FL is typically only effective for tuning if the PCB was designed with specific component pads for placement of the U.FL. The second, more common method is to strip a thin coaxial cable and solder it directly to the trace line. This requires less space on the PCB and is more flexible than placing a U.FL on the board.
When soldering a coaxial cable to the PCB, the inner conductor must be soldered to the beginning of the trace and the outer shield well soldered to ground. Occasionally, it is not possible to solder to the beginning of a trace. This will give an inaccurate measurement of the antenna’s impedance. The RF signal propagates down both ends of the trace at the same time. The signal will reach the end of the stub, reflect back, and interfere at the solder point to give false impedance readings. To eliminate this problem, the trace behind the solder point has been cut. The RF signal will now propagate down the cable and only to the antenna.