As the airwaves become more and more crowded due to increased cellular activity, the plethora of new devices operating in the Wi-Fi band, and RF generated by video walls or stage lighting in large productions, it’s become more and more difficult to operate wireless mics, IEMs and instrument systems without encountering problems. Here are some ways you can avoid issues when using your wireless gear.
Don’t Hide the Antenna
Most rackmount wireless units have antenna mounts on the rear panel. This is great for transport but not so great for reception because the rack itself and the chassis of the wireless units can block transmission. Getting the antenna out of the rack improves RF performance and that’s why a lot of manufacturers of wireless gear offer kits for mounting antenna on the front of the rack. Speaking of antennas: use the antenna recommended by the manufacturer (label them if necessary), always extend telescoping antenna to maximum length, and never cut an antenna.
Getting the antenna up front is a great start but may not help you if the wireless rack is on the floor next to the monitor engineer and the stage is five feet high. That’s where remote antennas help maintain — or at least increase — the chances for “line of sight” between transmitter and receiver. If you’re using multiple IEM systems, it’s a good idea to use a combiner and one shared antenna (such as the Sennheiser ADP UHF shown in Fig. 1) placed on stage — but beware of connecting the monitor engineer’s IEM system to the combiner because it may put the antenna out of line-of-sight for the ME’s receiver (which is probably sitting right next to them).
Have A Donut
Pay attention to the deployment of the antennas, which typically radiate omnidirectionally in a plane perpendicular to the orientation of the antenna. Yikes. Imagine you’re playing ring toss with a donut and you toss the donut so that it lands with the hole around the antenna. The donut represents the radiation pattern of the transmission. Maximum efficiency is achieved when the transmitter antenna and the receiver antenna are arranged in the same direction (usually parallel to each other). Transmission is the least effective when the transmitter and receiver antennas are at 90 degrees to each other, and the RF loss could be as high as 20 dB. This can be a challenge with handheld mics and performers who move around a lot, which is why a diversity system can help improve performance — you can orient the two antennas in different planes.
Make your talent aware of the fact that if they cover the antenna of a handheld wireless mic with their hand, transmission will be compromised. In general, keep antennas as far away from the skin as possible, and that’s a good reason belt packs should be worn on the belt (duh) or guitar strap as opposed to being placed in someone’s pocket. That’s not always possible — especially in productions where a pack has to be hidden under clothing — so do your best to sneak the antenna out of the clothing and keep It away from the performer’s body.
Less Equals More
Common sense would dictate that — for transmitters with adjustable output power — the highest output would yield the best signal. Not so, especially in productions where there are multiple wireless systems. You may be familiar with the term intermodulation distortion, which refers to the way multiple RF systems behave when operated simultaneously in the same locale. Specifically, when signals from two or more systems are combined, new signals are generated at different frequencies in the same band, creating interference. As transmitter power is increased, intermodulation distortion increases, introducing new signals that pollute your RF environment.
Of course, the temptation is to run the transmitters at the highest possible power, but you must resist temptation! Try each system during sound check (one at a time) with the transmitter set to the lowest power setting and walk the performance area to the maximum distance you’ll need. If you have sufficient coverage, then leave the transmitter power at that setting. If you have problems receiving a clean signal, then raise the transmitter power up to the next highest setting and perform the test again. The newest RF systems can run transmitters as low as 1 mW (with options typically at 10 or 20 mW) while older systems may offer 10, 20 and 50 mW settings. An added bonus is that battery life is extended at lower power settings.
Filter That Mouth
Given all the RF crud that’s floating about these days, it’s a wonder that RF mics, instruments and IEM systems work at all. You can reduce the amount of RF interference “seen” by your receiver by using RF bandpass filters. An RF filter is a gadget (usually passive) that patches inline between a remote antenna and the receiver, reducing out-of-band signals that can overload the receiver. RF filters are “tuned” to specific bands so you’ll need to get a filter that covers all or most of the same band as your wireless system. If you’re running a diversity system, you’ll need two. The good news is that they’re really easy to use; the bad news is that they aren’t cheap. RF Venue offers a range of filters in various bands (Fig. 2). Expect to spend somewhere in the vicinity of $300 to $400 per filter.
Watch Out for Wiffie
There are a lot of wireless systems on the market operating in the 2.4 GHz band. The good news is that the 2.4 GHz band is license-free across the globe. The bad news is that the 2.4 GHz band is license-free across the globe. It’s like the wild west of wireless. There are an awful lot of devices operating this band including Wi-Fi routers, Bluetooth devices, baby monitors, cordless phones, garage door openers, etc., all posing a potential threat to your wireless mic or instrument system. What’s worse is that you won’t see the interference coming until it’s too late. Yes, that wireless guitar system might work great at sound check when the building is empty, but what happens when the room fills up with a few hundred people who have BT ear pods connected to their phones? I can’t predict it and neither can you, and that’s why 2.4 GHz wireless mics and instrument systems are probably best for applications where there aren’t a lot of wireless channels in use, and the RF environment is relatively uncrowded.
You May Need Help, Really
Wireless manufacturers have gone to great lengths to provide users with tools that make their systems easy to use. Take advantage of them. Just about every manufacturer of wireless gear has management software that’s available free of charge. Capabilities of the software typically include the ability to create a network, automatically recognize devices from the same manufacturer on the network, name devices; calculate, coordinate and deploy frequencies without need for an RF analyzer; and monitor performance data such as RF strength, audio signal strength and battery life, such as Shure’s Wireless Workbench in Fig. 3.
Rack Attack
In permanent rack installations (such as theater), leaving a blank rack space between receivers can reduce the possibility of RF emissions from one receiver reaching an adjacent receiver and causing interference. If you want to do this in a mobile rack, fill the empty space with plenty of foam so that the chassis of the receivers won’t bounce around during transport when you hit the road.
Steve “Woody” La Cerra is the tour manager and front of house engineer for Blue Öyster Cult.
