A failure to meet FCC regulations or PTCRB radiated-spurious-emissions- conformance requirements due to a level of harmonic energy generated by the device is often blamed on the antenna. In truth, the antenna can often influence the level of the harmonics but does not generate these signals. An antenna almost never has gain at the harmonics of the intended band of operation. Provided the radio has acceptable harmonic performance, excessive harmonic generation is typically an interaction between the impedance of the antenna and the impedance of the final stage of the transmitter.
The antenna is designed to have acceptable impedance in the bands of use. Outside of these bands, the impedance is usually uncontrolled (and often unknown). Unusual or unanticipated impedance conditions can sometimes raise the harmonic levels of a Tx power amplifier by 10 or even 20 dB. As with other failures, the out-of-band impedance of the antenna is difficult to change at the regulatory or conformance test phase. Often, filters between the antenna and transmitter are added as a last resort, but these components have their own impedance issues and can often reduce one harmonic while raising the level of another.
Therefore, as early in the project as possible, the antenna and transmitter should be tested together with harmonic generation in mind. It is very difficult to design or purchase an antenna that has a specific impedance at each potential transmitter harmonic. Approaching the issue in this way will not yield good results. Instead, the power levels of the harmonics should be measured as soon as it is practical to do so, when mitigation steps can be readily taken to reduce individual failures.
6.5 Device Pre-Testing
Most radio-module vendors offer some limited pre-testing of the radio and antenna as a system in their own laboratories. While these facilities can vary widely in accuracy and repeatability, they should be used if available. Very often, a radio vendor will be familiar with the issues that their particular module may have when
Antenna Fundamentals – Technical Brief