Wireless in-ear monitors (IEMs) have been a boon for live performance. They offer mobility and better sound for performers, lower stage volume, and less bleed into mics. The RF link that makes this possible, though, has also been a weak spot.
The problem is that historically, IEM systems have mostly been sold with simple whip or paddle antennas that work well almost all of the time. Yes, almost. Few things will throw a performer off his or her game more than an IEM dropout — a loss of audio, a noise hit, etc. Even 99 percent reliability isn’t good enough; it means a two-hour show would be marred by over seven seconds of dropouts, which would be seriously bad.
The culprit is a phenomenon we call polarization cross-fade, and it tends to be unique to wireless audio because we’re trying to maintain an RF connection between an antenna that is stationary and another that can be moving around in any position or orientation.
Polarization refers to a particular property of a radio wave. Radio waves are a portion of the electromagnetic spectrum, and they comprise two parts: an electrical field and a magnetic field. The electrical field is produced by an alternating voltage between two points in space and the magnetic field is produced by the current running back and forth between those points. This conversion of electricity into a radio wave occurs in an antenna.
On the receiving end, the radio waves pass over an antenna, and their electrical and magnetic fields produce a corresponding signal in the form of electricity. The receiver demodulates this signal to obtain whatever audio was put into the transmitter.
The polarization of an antenna is determined by a straight line between those two voltage points. That’s the plane of the electrical field. If the antenna is a whip or dipole, then those points are at the ends of the antenna elements, and there’s only one plane of the electrical field. We call this a singular-polarized antenna. If the transmitting antenna is singularly polarized and so is the receiving antenna, the radio signal will couple between them most effectively when they are parallel with their broadsides facing each other (see Figure 1). But if the antennas are at right angles to each other, the receiving antenna will not catch the electrical and magnetic fields effectively. We call this phenomenon polarization cross-fade.
Now remember, with an IEM system, one of these antennas is a small whip on a receiver that a person is wearing. We can’t control that one. It could be in any position and oriented in any direction. This poses a problem, because the transmitting antenna can’t change its position to keep itself aligned to the moving receiving antenna.
But what if we could make a transmitting antenna that is not restricted to a single polarization? Then, maybe it wouldn’t matter how the IEM receiver is oriented. That’s the idea behind using circularly polarized antennas for transmitting IEM signals.
RF Venue offers three models of circularly polarized antennas: The CP Stage, the CP Architectural, and the CP Beam. The CP Stage and Architectural models are spiral panel antennas and the CP Beam is a tapered helical antenna. All three are a type of traveling wave antenna, meaning that instead of there being only two points defining the electrical and magnetic fields, the RF signal is sent along a somewhat circular path in the antenna. The result is that at any point along that circle, the voltage at whatever point on the RF signal is at that point, has the mirror image voltage on the other side of the antenna. So now we have RF voltages distributed all the way around in 360 degrees producing electrical fields. And offset 90 degrees from any given voltage, we have current flowing in the same direction on each side of the circle to produce the magnetic field. RF signal polarizations in infinite angles all the way through 360 degrees, all the time.
Figure 2 shows a head-on view of a simple helical antenna. At every spot around the circular path of the antenna element, there is a voltage and a mirror image voltage on the opposite side.
And there is current, driven by the voltage. And interestingly, we find that it’s the same magnitude and direction on opposite sides of the circle. So for any angle we choose to look at, we have a complete radio wave being produced.
Mind blown, seriously.
Even though I’m an RF geek, it took me a very long time to get my head around circular polarization. I attribute this to the terrible ways in which circular polarization is depicted, even in textbooks. Invariably, they showed transmitted signals spiraling off of transmitting antennas in corkscrew patterns. That didn’t make sense. Radio signals travel through space in straight lines; they can’t curve around in a spiral. The problem was the bad descriptions and bad illustrations. Radio waves don’t corkscrew, despite how the poorly thought-out texts describe them. I had to figure out the actual physics and think in terms of waves in order to start understanding circular polarization.
What circularly polarized antennas produce are not corkscrew waves, but continuous radio waves polarized in all angles. But there is a phase difference between any two angles, and that phase difference is equal to their angular difference. For example, if you compared the signal in the vertical angle of polarization with the signal in the exact horizontal angle, you’d find that there’s a 90-degree difference in phase between them (see Figure 3). That’s the only way this works, but fortunately, we don’t need to worry about the phase. Just know that it works.
All that really matters is that these circularly polarized antennas, because they produce RF that is not restricted to a single polarization as with a paddle or whip antenna, will never cause a polarization cross-fade dropout. The result is happier performers and a less stressed-out monitor mixer. Circular polarization wins every time.