COMMENT Partial list of RF interferers
Radio frequency interference (RFI) RF Venue’s senior applications engineer, Don Boomer, looks at the causes of RFI and how to achieve a reliable performance WHEN CONSIDERING THE operation of wireless microphones and IEMs, radio frequency interference (RFI) is any natural or man-made electromagnetic energy that creates unwanted interactions between radio equipment, whether it be intentional or unintentional. Causes of RFI interference include external, naturally occurring sources like thunderstorms and static electricity, as well as man-made sources created through out-of-band transmissions, radiation from network antennas and cabling, and the proximity of adjacent radios operating on overlapping frequencies. Every one of these things is going into your antennas and down into your receiver, making its job much more difficult. In order to help handle RFI, most industrialised nations have set up user hierarchies that dictate who has the “right of way” to operate radio equipment on a particular frequency. In the US, unlicensed UHF wireless microphones have the least authority of all the devices permitted to operate there. They must accept any interference they receive from other unlicensed users and licensed users, like television stations. Because of this limitation, users of wireless mics and IEMs have a limited but powerful set of tools at their disposal. The goal of these tools is to improve the dynamic
The signal from the transmitter must reach the receiver at least 20dB stronger than the residual RF noise floor range of your transmitters to your local noise floor. It is this dynamic range that is the principal gating factor of reliable on-the-air performance. Many of these tools work by improving the signal-to-noise ratio to mitigate interference. Understanding the relationship between noise floor and signal is crucial to correct wireless systems operation. The noise floor is the average level of the RFI in a given location that creates a “floor” of RF energy. If a microphone signal arrives at an amplitude that is too near that noise floor, the receiver will have difficulty distinguishing signal from noise, no matter how expensive the gear. For this reason, one (perhaps even the most important) goal in wireless audio is to ensure the highest possible signal-to-noise ratio between noise
floor and transmitter signal. The goal is to maintain at least a 20dB dynamic range, that is the signal from the transmitter must reach the receiver at least 20dB stronger than the residual RF noise floor at your venue. This factor is much more important than actual signal voltage, although there are limits below which a receiver cannot distinguish your intended signal.
Rejecting the effects of RFI Practices to improve dynamic range (also called carrier-to-noise ratio or CNR) include using directional antennas as opposed to simple omni whips that are included when you purchase wireless mic systems. Since a directional antenna is only picking up signals from the direction
it is aimed and not from behind it (think cardioid microphone pattern), it will typically be picking up only one-third of the surrounding RF interference. Your antennas are going to pick up whatever RFI there is in your venue, but getting the antennas closer to the transmitters results in a stronger intentional signal without a rise in the noise floor level. There will be less loss in the coax than through the air. Reducing the number of antennas will also result in a stronger CNR since antennas actually interfere with other antennas until those antennas are about three wavelengths apart (about 3m for 500MHz operation). So the best practice is to use a single A/B pair and an antenna distro to couple that pair to all of your receivers. Using the newer 100% shielded (foil plus braid) coax will ensure less signal is lost over distance and prevent ingress from outside signals (such as other cables or other equipment in your rack) and therefore better CNR. And, finally, the use of bandpass filters can block RF signals that are out of your band of interest and keep them out of your receiver entirely, making its job much easier. www.rfvenue.com
6 WORSHIP AVL January–February 2024
WAVL Pg6-7 Comment RF.indd 6
27/11/2023 12:03