7 Ways to Quickly Judge the Quality of Circuit Board Design (Part 4)

4-Antenna feeder

If your design includes a radio transmitter, receiver or transceiver (transmitter and receiver combined), it must have an antenna. For best performance, the feed line between the radio frequency (RF) pins on the RF chip should match the impedance of the feed line connected to it. In turn, the feeder must match the impedance of the antenna. In order to maximize the power transfer between the antenna and the radio chip, this impedance matching is necessary. Any mismatch will result in a reduction in the actual transmission power, thereby reducing the operating range.

The feeder is just a PCB trace with a controlled impedance that matches the antenna impedance (usually 50Ω). If the output impedance of the transmitter does not match the impedance of the feeder, a matching network composed of inductors and capacitors is usually used. In order to achieve controlled impedance, the feeder is a PCB trace whose calculated width extends on the ground plane. The width of the trace depends on the thickness of the copper trace, the thickness of the PCB substrate and the dielectric constant.

There are many online tools available to calculate the exact width required for a given copper thickness and substrate material, and it is best to confirm this in the actual PCB. My favorite is the free software called AppCad that can be downloaded from Broadcom. If the antenna is a PCB antenna, it should be located on one side of the PCB without any ground plane.

Any other traces should be removed and any large components should be kept away. The screen-printed markings around the antenna are usually good, but copper markings (such as the PCB number or company name) can detune the antenna.