Using the RF Venue Diversity Fin to Eliminate Wireless Microphone Interference

by Alex Milne

"Multi-path interference is the most common cause of wireless drop-outs, and if you can get rid of multi-path, you can get rid of a good number of headaches. A new antenna from Boston based RF Venue, the Diversity Fin, can eliminate multi-path drop-outs by using a novel technique called polarization diversity, all in a single unit—doing away with bulky twin paddle antennas.

But before you understand why the Diversity Fin works so well, you have to understand multi-path. When a microphone transmitter sends out a radio wave, it spreads through a room, like ripples on a pond. As the wave encounters flat surfaces, like walls and ceilings, it reflects and continues forward at different angles. Since there are multiple surfaces in every room, there are multiple reflections and hence multiple paths—some longer and some shorter—that a wave takes before reaching the wireless receiver.

Usually, the receiver is able to process two or more signals arriving at slightly different times without difficulty. But if the signals overlap in such a way that they cancel each other out (creating a “null”) you get a drop in volume or complete drop-out. Sometimes, the shape of the room can cause a multi-path null to perpetually hover over a receiver. Other times, when the speaker walks past a certain spot on the platform, a dead spot will develop and you’ll hear a quick drop-out.

Most dropouts are not caused by insufficient transmitter power, because even at the low power used in wireless microphones, as long as there is line of sight and the mic is within a few hundred feet, the receiver will be able to pick up the signal. Most drop-outs are caused by persistent or transitory nulls developing over the receiver.

Diversity receivers filter out multi-path interference by using two antennas instead of one. Most wireless receivers that have two antennas are diversity receivers. Since a multi-path null occurs only in specific and relatively small locations, it is less likely that a null will exist over both antennas. This is called “spatial diversity.” Diversity receivers are given two variables—antenna one and antenna two—and then special circuitry “votes” to decide which signal from which antenna is better. But spatial diversity does not work 100% of the time. The null can cover both antennas, that two nulls can be over each antenna, or the dead spot can move around as the microphone moves across the stage or platform.

The Diversity Fin exploits a byproduct of multi-path propagation: as waves bounce around inside a room, they also change their orientation, or polarization. Polarization is the type of shape of movement that the wave takes as it travels. So, when multiple waves arrive at a transmitter, they may be out of phase, but they also vary in polarization.

The Diversity Fin uses a cross-polarized design that discriminates between waves of different polarizations, instead of waves that are in different locations. With a Diversity Fin attached, the receiver circuitry gets to vote between differences in polarization, not between two antennas in two different locations. The result is zero multi-path interference in a single antenna device. As an added benefit, audio quality is significantly improved since the number of times the receiver votes between antenna A and B is reduced.

The Diversity Fin comes is available in a compact folding design with a tactical fabric cover, for live sound, as well as bothmatte black and white unmarked designs for installed sound in churches, theaters, and auditoriums. "