The phrase, “It’s never too late to do the right thing,” applies to so many situations, including wireless audio, particularly in-ear-monitors (IEMs). How many times have you seen a bunch of IEM transmitters in use, each with a whip antenna connected to it? Yeah, that’s not the right thing to do. So let’s do the right thing.
“Antenna farms” — unorganized collections of multiple antennas that could be replaced by one transmitting antenna or a diversity pair of receiving antennas — are about as detrimental to performance for IEMs as they are for wireless microphones. It may be tempting to simply use the whip antennas included with the transmitters and give it no more thought. But having a bunch of random antennas close together causes a number of problems that can interfere with the reliability of your IEM system. And don’t underestimate the severity of IEM issues. Even though they’re less obvious to the audience than wireless mic dropouts are, remember that performers might not always be singing or speaking, but they are always listening. Breakups and dropouts in the IEM system can be extraordinarily distracting and upsetting to them.
An antenna farm of IEM transmitting antennas creates a lot of interference among them, with a rise in the local RF noise floor, alteration of pickup patterns, and generally weak performance. Also, the whip antennas themselves are omnidirectional, so they’re not focusing their energy where it’s most needed. IEM transmitters typically put out about 30 to 60 mW of RF signal, so you end up with a lot of total power going out in unwanted directions and very little going toward a useful direction. These problems are compounded when the antennas are attached directly to the rear panels of transmitters installed in an equipment rack! Imagine enclosing your antennas within a rack enclosure. That would certainly crush their effectiveness.
The key to making IEMs work properly is to use one antenna, preferably a directional one that you can aim at the performers. And also preferably, a circularly polarized antenna. Use a good combining system to sum together all of the IEM transmitter signals into a composite signal sent to the antenna.
Let’s focus on combiners for now. A combiner actively sums signals from multiple IEM transmitters into one output, so you can use a single antenna instead of a separate one for each transmitter. The inputs are isolated from one another, so signals from one input will not backfeed into any others. The combiner also permits you to use a better-suited antenna. For example, if you have six IEM transmitters, you eliminate having six omnidirectional whip antennas and instead have one directional one that you direct toward the performers.
A combiner is sort of a simple mixer for RF signals. The Combine6 HDR model shown in Figure 1 has six inputs and one output. Thus, you can send RF signals from as many as six IEM transmitters into one antenna (Figure 2). An LED on the front panel indicates power, and an LED on each channel glows green when there is a good RF signal on the input or red if the signal level is excessively high, over 60 milliwatts.
An interesting feature on the rear panel (Figure 3) of the Combine6 HDR is the 2:1 passive combiner (or “link”). If you have more than six IEM channels, you can put this Combine6 HDR’s antenna output into one of the Link inputs and the output of another combiner into the other Link input. An internal RF transformer will sum the two sets of signals and put them out at the Link output to feed the antenna (Figure 4). This is the proper way to jointly use a pair of combiners; unlike distros, you mustn’t cascade IEM combiners, because of their higher RF levels. If the two combiners are both Combine6 HDR, their half-rack physical form will allow you to accommodate as many as twelve IEM channels in one rack space.
The HDR designation in the model name stands for High Dynamic Range. The device’s circuitry was purposely designed to have a very low noise floor and high linear headroom. This is to avoid the overdrive/nonlinearity situations that would cause intermodulation, or IM. This is especially important for IEMs because the combiner is handling relatively high RF levels (compared to the receive-antenna levels that distros handle, for example), and subpar circuitry would easily overdrive and distort the composite RF signals, causing IM products that raise the noise floor and possibly clash with the channel frequencies in use. Intermodulation artifacts in IEMs might not be heard by the audience, but they will certainly disrupt the performer’s mood on stage.
RF Venue’s older combiner models are the Combine4 (up to four IEM channels) and the Combine8 (up to eight). They both have full-rack chassis and provide DC power ports on the rear for powering 12-volt IEM transmitters.
While combiners can be designed to reduce the risk of IM, it’s also important for the operator to manage RF signal levels and prevent IM. While the RF Venue combiner models can handle up to about 60 mW on each input, it is usually a good practice to operate the receivers at a lower level — maybe 30 to 40 mW — as a further cushion against nonlinearity, especially if you’re using a large number of channels.
When the Combine4 and Combine8 came onto the market, IEM transmitters powered by 12 volts DC were dominant. But in recent years, an increasing number of transmitter models, particularly from Shure, are meant to run on 15 volts. Check the requirements of your IEM equipment. Transmitters made for 15-volt power might not operate properly on the 12 volt supplies of the Combine4 and Combine8. The Combine6 HDR, in its half-rack size, did not have the real estate available for power outputs, so you will need external power for your IEM transmitters regardless of whether they run in 12 or 15 volts.
IEM channel count can be tricky, but here’s a little-known strategy for possibly reducing the number of IEM transmitters and channels you use: keep in mind that you can transmit any IEM channel to more than one receiver. You do not have to assign transmitters and receivers one-to-one. If you’re mixing monitors from FOH and there are, say, eight aux outs, you will not need more than eight IEM transmitters for your system because you can’t produce more than eight monitor mixes. If some of the performers can be on the same monitor mix, then use just one transmitter for them, and use whatever number of receivers you need.
A so-called “paddle” antenna, which is actually a log-periodic directional array and typically manufactured as a double-sided printed circuit board, is a popular choice for IEM transmission, but you can do better.
Did you notice the funny-looking antenna in Figures 2 and 4? That is the RF Venue CP Stage antenna. It is a circularly polarized panel antenna that mounts onto mic stand threaded hardware. It is ideal for IEM transmission because it produces circularly polarized radio waves. Unlike a vertical paddle or whip antenna, which will only emit vertically polarized radio waves, a circularly polarized signal is directional and has RF energy continuously in all angles of polarization, all the time. So the angle or orientation of the IEM receiver’s little whip antenna will not affect reception and will stay strong.
The wide pattern of the CP Stage makes it possible to set up at the side of the stage (maybe over near monitor world) and still cover all of the performers wherever on the stage they may be. The enclosure of the CP Stage is constructed from ABS plastic, and it is IP 44 rated, so you can use it for outdoor events, festivals, and other shows when rain might occur.
RF Venue’s other circularly polarized antennas are also ideal for IEM transmission. The CP Beam is a tapered helical antenna that has a tighter directional focus than the CP Stage, but from that it will have a significantly longer usable range. The CP Beam antenna has a synthetic fabric enclosure that permits the antenna to be squashed down for storage or transport and defines the antenna’s shape when it is in use, but it is not weatherproof.
The CP Architectural antenna is very similar to the CP Stage in its functionality, but it is meant to be mounted on a wall or ceiling. It has a square cover with an offset low pyramid shape, so that it looks more like a piece of décor than an antenna. And it’s paintable (as long as the paint is non-metallic)
