Wireless microphones have become a necessary tool in today's sound reinforcement environment, whether for stages, tours or churches. They give performers and designers freedom from cables, but they also bring challenging technical issues. And while the technology is better than ever and much easier to use than in the past, the basics for how to get the most out of these systems remains relatively the same. For one thing, physics is physics, regardless of whether the wireless systems are digital or analog, and whether your system cost $500 or $5000 per channel. The ever-growing challenges of spectrum loss and crowding has even veteran wireless mic gurus concerned. Despite these challenges now and in the future, there are steps we can take to ensure reliable performance day in and day out.
The main concept to learn here is that by getting the basics right at every stage in the system, no single problem will bring the system down. However, if the system is poorly purchased, poorly designed and poorly implemented, then even the smallest hiccup might just have your minster go off the air right at the worst possible moment.
So, what are the basics? First, let's assume you have a quality microphone system that cost a minimum of $500 per channel. Because if you don't, you are opening yourself up to problems, especially as the available spectrum shrinks and what's left becomes very crowded. What's the technical reason for this? Filtering. Low cost units have very basic designs with loose tolerances, and this can spell disaster when the chips are down. The reputable manufacturers will include important technical specifications in their data sheets so you can see this. It may be explained as a minimum spacing between channels, for instance. And in low cost systems, this is likely to be 1 MHz or more, while the best systems can have channels spaced less than 300 kHz apart. This not only points to how many channels you can get into a given amount of spectrum, it also tells you the likelihood of interference from other signals.
Next, we must remember that a wireless microphone is an audio system unto itself. Start with proper gain structure by setting your transmitters appropriately for the source. This means that there is no one gain setting on the transmitter that will give you good results for all users. Mainly, we are interested in getting the best signal-to-noise ratio by increasing gain at the front end until we "bump" the limiters at the highest peaks. Following that, we should be careful to set our receiver audio output levels appropriately for the next device in the chain, usually a mixer. If we can run our outputs at line level and go into a true line level input at the console, we can get about 3 dB better signal to noise vs. going mic level.
Our receiver antennas must be purchased correctly and placed correctly. Many users think it is necessary to use directional antennas, when in fact they may be wasting money on these items. If you have a short distance between the transmitters and your antennas, and you have a short antenna cable run, you probably do not need directional antennas. However, they can be useful for blocking unwanted signals, such as from DTV towers in your area.
Another common mistake is for the receiver antennas is to be placed too closely together. For best diversity results, give them several feet apart, even 10 to 14 ft. is good. One on either side of the house mixing console is a typical successful setup. Place them high enough so that they have direct line of sight to the transmitters, as to avoid the signals having to go through bodies before being picked up.
Don't forget that antenna cables have loss, and it can add up. Typical RG58 cable loses 12 dB per 100 ft at 400 MHz, so this gives you some idea. The shorter the cable, and the lower loss the cable, the better. Related to this, the only reason to use RF amplifiers, either separately or built into the antennas, is if you have long cable runs. A common mistake is to use amplified antennas with short, low-loss cable. The result is worse range than just putting a couple of whip antennas on the back of the receivers. Receivers are designed for a specific range of RF signal strength and too much will definitely reduce performance.
Choosing frequencies for your various transmitters should not be taken lightly, either. The random approach, or using pre-set groups, does not work well in today's crowded RF world. Instead, I recommend using software for this purpose. Most manufacturers make a package, and there are some 3rd party software packages available as well. Any of these are only as good as your willingness to learn how to best use them. Experience counts, and the only way is to get in there and put it to the test. The main issue these software systems manage is to mathematically calculate which frequencies will interact with each other, then choose ones that don't. The more channels you add to your system, the more complicated these calculations become. So, choose a software package, learn it inside and out, and practice using it as often as possible to gain experience.
So I've mentioned spectrum crowding and this has become a very serious issue in the past 10 years particularly, with the advent of DTV and the spectrum auctions back in the early 2000s. The FCC is auctioning off more spectrum, and this time the band between 600-700 MHz is to be affected. What can we do about this? First, don't panic. Instead, come up with a careful plan. Come up with a list of all the wireless microphones, IEM systems, intercom systems or anything else with an antenna on it, and include what frequency ranges they operate on. From there, sort the units that operate anywhere between 600 and 800 MHz. Any units between 698 and 806 MHz are already illegal, and you need to stop using them immediately. Any units between 600 and 698 MHz are at risk, and will likely be unusable after about 2018 or 2019. These, you should plan to phase out and replace with units under 600 MHz.
What are you looking for in new systems? I mentioned high-quality filtering earlier. Look for wide tuning ranges, but not too wide. Look for quality construction, and reputable companies that will stand behind their products. Look for companies that educate their customers and potential customers, rather than just trying to sell a magic bullet. These systems are tools, and when designed, built, installed and configured correctly, will give years of reliable operation.
