This section discusses antenna issues, including connecting the antenna, self- interference and co-existence with other wireless technologies.
4.5.1 Connecting the Antenna The antenna is one critical component of the radio system or, more precisely, the radio system front-end. The front-end is comprised of the antenna, any components (like filters, amplifiers, switches) between the antenna and the RF chain of the radio, and the connections between the various components. It is critical that the various connections are made with controlled impedance structures. These structures can be coaxial cable, controlled impedance PCB traces like microstrip or stripline, spring-loaded pogo pins or spring fingers, flex circuits with controlled impedance lines, and so forth. Attention must be paid to the various interfaces between these structures and the antenna and circuit terminals. Circuit components and antennas that are well-matched when measured in isolation can suffer from impedance-matching issues when used in combination with poorly designed interconnects. For example, spring fingers have become common in cellular handsets (to connect to the antenna mounted in the housing) and work quite well. However, during the design process, both the impedance and noise-pickup potential of the fingers must be taken into account. Another good example of the front-end interconnect issue is the use of a filter which utilizes antenna element as one termination. Very often, antennas in actual devices may have VSWRs of 3:1 (possibly higher), especially at the band edges. Most reflective-type filters rely on good impedance matching to provide the specified filtering and a VSWR of 3:1 is typically not adequate to properly terminate the filter component. Some tuning, external to the antenna and filter, may be required to insure that the filter is operating correctly. An improperly terminated filter can introduce substantial insertion loss in addition to a distorted filter response. Cables and PCB traces will introduce losses (in both transmit and receive) and must be carefully considered. Cables and traces create opportunities for noise to enter the system at the especially vulnerable point of the receiver input. Care must be taken to route all interconnects away from or around noise sources.
Antenna Fundamentals – Technical Brief