Working with wireless gear can be one of the most frustrating things we do in the world of production. Personally, I prefer to go wired whenever possible, since it's more reliable and usually sounds better. Wireless is often necessary, though, so we need to be armed with knowledge. I'll attempt to give an overview to common RF (radio frequency, aka "wireless") concerns, and while we don't have space to get into much detail here, hopefully this will point you in the right direction for continuing RF enlightenment.
Let's start with a simple setup as an example and build from there. Imagine we've got one wireless microphone (a handheld) and a receiver with only one antenna (although you would usually have two with modern wireless mics). In this example, no antenna distribution or special antennas will be used; we have only a “rubber ducky” antenna (the same kind usually supplied with wireless gear). We'll forget about frequency choices for now and assume we've got a clear channel.
As simple as this setup is, there are still a few things that could go wrong, even assuming the frequency stays clear. The most obvious is that we get too far away from the receiver, and the signal isn't strong enough any more for the receiver to detect. As much as we tend to assume weak signals are the cause of our wireless woes, however, this is probably one of the least common problems.
It's actually pretty common to overload the receiver. That may be counterintuitive, as it may seem like you'd want the strongest possible signal hitting the receiver, but that's not necessarily true. Just as we can overload a microphone preamp, we can overload a wireless receiver, and it happens all the time. Normally, this happens with directional antennas and RF amplifiers, close proximity to the antennas, and choosing the high power option on the transmitters (or some combination of the above). We'll cover some of those in a moment, so don't worry if some of that was unfamiliar.
Another problem arises when the transmitted signal reaches the receiver antenna more than once: initially from the transmitter directly, and also as reflections bouncing off of building steel. A reflection will have a later arrival time and can combine with the direct signal to cause a signal cancellation. If that sounds familiar to you, it's the same idea as comb filtering in audio. This problem in the RF world is known as multipath propagation, and it is a leading cause of dropouts. Have you ever wondered why most receivers have two antennas? RF reflections that might cause a problem in one antenna will have to travel a different distance to the other antenna, and that is enough to not cause a cancellation in that antenna (this is called “diversity”). Keep in mind that as the transmitter moves around, the relative difference in arrival time changes between the direct signal and the reflections. That's why you may have noticed a certain spot in your room where there always seems to be a dropout.
Okay, now we know a few basic potential problems that can cause a loss of RF signal: we can be too far from the receiver; we can overload the receiver; and we can have multipath propagation. Let's make this more complicated.
It only takes a modest amount of wireless channels in use before you have a few thousand frequencies to worry about.
Let's assume, now, that you have several wireless systems in use. At this point you have two general choices with your antennas: you can either keep using the supplied rubber ducky antennas, and have yourself a mess of criss-crossed antennas in your rack; or you can opt for antenna distribution so that all of the receivers share the same pair of antennas.
Having multiple receivers with antennas in close proximity in a rack can actually interfere with each other in two ways: the nearby antennas can unexpectedly alter each others' directivity; and the internal oscillators that many receivers use as part of the tuning process can interfere with each other. This is where antenna distribution comes in.
An antenna distribution system takes two antennas and replicates the signal for many receivers. This allows you several advantages: you can use the most appropriate type of antenna for your application and only buy two of the them; you can put the antennas where you need them (and outside of the less-than-ideal location in the racks); and you won't have the problems mentioned above where antennas interfere with each other.
That leads us to the main types of antennas. The most common one you see is the “rubber ducky” included with wireless gear. It's essentially omnidirectional, which means it picks up equally from all directions. I said “essentially” because it doesn't pick up well from the direction in which the tip is pointed, which is why you want them pointing up or sideways, rather than pointed at the transmitter. You also want the pair of antennas to make a 90 degree angle, which helps with signal polarization issues that we don't have the space to cover here.
Directional antennas, on the other hand, give you two advantages: the rejection of RF transmissions (potential interference sources) from one or more directions; and gain. Gain, in directional antennas, is an automatic outgrowth of being directional and requires no additional electronics. This is nice because directionals can pick up weaker signals (they can be farther away). Common directional antennas in live sound include the “fin” or “paddle” antenna (technically a “log-periodic dipole array”), and the less well known, but up an coming, helical design.
In most cases it is better to use the low-power transmitter setting.
Gain can be a good thing when you've got distance to cover. But, it can be a bad thing, too, as we mentioned earlier. Consider that some directional antennas also have active gain (amplifiers) built in to make the signal even stronger (in addition to the "free" passive gain already built in to directional antennas), and you can start to see where we sometimes go wrong in setting up our wireless systems. Many RF problems are caused by too much net gain (in transmit power, amplifiers, directional antennas, and being too close to the receiver antennas) causing receiver overload. Also, keep in mind that RF amplifiers raise the level of any received interference sources, too, so they often cause more problems than they solve.
The solution is to consider how far the antennas are from the receivers (the coaxial cable connecting them loses signal, so the longer the cable the more loss you have to compensate for) and how far the antennas are from the transmitters. If your antennas and receivers are both near the side of the stage or altar area, with relatively short coax cables, you probably don't need RF amplification (and maybe not even directional antennas). In fact, you might need a pad. Pads reduce the signal strength, and while that might seem like a counterintuitive solution to RF problems, sometimes that's exactly what you need to make the receivers happy with the signal strength. Keep in mind that interference sources will be reduced, too, with a pad.
As if all that isn't enough to consider, we also have to actually pick good frequencies to use. If we're just using one system, it's basically as simple as looking for a clean channel on the receiver and then using it. However, things get hairy when multiple systems are in use.
The problem is that transmitters will interfere with each other in a process called intermodulation distortion (IMD, or “intermod”). This interference creates additional RF products in the air that weren't there before, and those RF products may end up landing on other frequencies we're trying to use. So, RF coordination involves considering not just that every system needs a clean frequency, but that the intermod products between each system and each other system can't land on frequencies already in use. It only takes a modest amount of wireless channels in use before you have a few thousand frequencies to worry about.
How do we deal with intermod? There are three keys: 1) Good RF coordination. Some of the major wireless product manufacturers offer free software you can download, and there are also commercial products such as the Intermodulation Analysis System software from Professional Wireless. 2) Keep transmitters physically separated as much as practical. Intermod gets worse as they get closer together, since it's a function of the transmitters causing distortion products in each other. 3) Use low-power transmission if your products give you the option. Sometimes you really do need to use the high-power setting, but unless you're absolutely sure you need it, low power is often better. It allows more channels to operate happily together and is less likely to overload the receiver if you have lots of RF gain involved in your system. Plus, it leads to better battery life, and that's a nice side effect.
Even though we have really just scratched the surface, I hope this provides a good starting point for you. The major manufacturers have some good technical papers available on their website, as well as free RF coordination software; I highly encourage you to read and experiment. RF can be a bit mysterious, but with a little education and experimentation, it can be a lot more reliable. Happy wireless!
