Wireless microphones are essential to the modern production values we’re all accustomed too. And while wireless systems have freed performers from the shackles of cables, they’ve also presented an entirely different set of challenges to the engineer. Still, it doesn’t have to be a guessing game. Most dropouts or interference issues are the result of very common and avoidable mistakes. Here are 5 fundamental faux pas to avoid.
1. Blocking the Signal
Rule number one is always maintain line-of-sight between the transmitter and receiver antennas. It is good practice to avoid large objects such as metal and walls. Most importantly, however, it is best to avoid large numbers of people. The human body is largely composed of water, which absorbs RF energy like there’s no tomorrow. As an example of just how effective our bodies are at absorbing RF, try cupping your hands around the antenna on a handheld transmitter. In doing so, its effective output can be reduced by more than 50 percent. For the best results, place the receiver antennas in the same room as the transmitters and elevate them above the audience or other obstructions.
Finally, always avoid coiling or folding a flexible antenna. It might sound obvious, but you’d be surprised how often this issue comes up. The message here is clear – anything that obstructs the line of sight between your transmitter and receiver is a threat to the reliability of your wireless signal.
2. Incorrect Antenna Type or Placement
You can go a long way toward improving your wireless signal by properly understanding which antenna type to use and how to place it correctly. Mistakes in antenna selection, placement, or cabling can cause short range, dead spots, or low signal strength leading to dropouts. The good news is, modern diversity receivers offer a much more reliable performance than single-antenna types, but the right antennas must still be placed correctly to maximise performance and reliability. The following are some best practice examples you should follow:
First things first, try to keep antennas close to transmitters and in line-of-sight as previously mentioned.
Secondly, to ensure good diversity performance, always space the antennas apart by at least one-quarter of a wavelength (5 inches at 600 MHz). Increasing the separation between diversity antennas to one wavelength (20 inches at 600 MHz) will improve diversity performance further still. Beyond one wavelength, however, extra distance between the antennas will not significantly improve diversity performance but may improve coverage of a large venue. Also, for the best pickup of moving targets, receiver antennas should be angled apart in a wide ‘V’ configuration.
If the receiver is located away from the performance area, 1Ž2-wave antennas or directional antennas should be remotely mounted – ideally above the audience for a clear line-of-sight. Directional antennas will significantly improve reception by picking up more signal from its forward direction and less from other angles.
Note: Short 1Ž4-wave antennas should never be remotely mounted because they use the receiver chassis as a ground plane.
If you are connecting your antenna to the receiver using a length of coaxial cable, you might need an antenna amplifier to overcome signal loss in the cable over distance. Just how much signal loss will occur depends on the length and cable type; simply follow the manufacturer’s recommendations to ensure that the total net loss does not exceed 5 dB.
Finally, antennas can be frequency band-specific. Make sure you check the frequency lists first before trying to use an antenna from another system.
3. Poorly Coordinated Frequencies
A suitably coordinated set of wireless frequencies achieves two imperatives: 1) They avoid local active TV channels, and 2) they are mutually compatible.
As TV transmitters can operate at power levels around one million watts – much higher than a typical wireless system at 50mW – use of local TV channel frequencies should be avoided.
Up to 50 or 60 miles is typically considered local, but can vary from location to location. Fortunately, indoor setups are less at risk than outdoor setups due to the buildings attenuation of TV signals. If the building is particularly substantial, you might get away with ignoring TV stations as close as 30-40 miles away. Still, since the locations of television stations are well known, it is best to set your system up using the safest wireless system frequencies available in your area.
Once the local TV channels have been taken into account, it is necessary to ensure a mutually compatible set of frequencies using one of two methods. The simple approach is to use the group and channel frequencies programmed into your wireless systems. This method ensures compatibility for small setups of similar equipment.
For more complex wireless setups – such as using wireless microphones and in-ear monitors together – frequency coordination using a computer program such as Shure’s Wireless Workbench can help to ensure compatibility.
Remember, though, there is no such thing as ‘set and forget’. One set of frequencies does not fit all, and if you’re touring, you will need to coordinate for each city you visit.
In fact, even if your audio system is static, the RF landscape can change unexpectedly. Television stations might remain relatively constant, but if there are other wireless systems in the frequency band – such as additional systems at your venue or interference from down the road – your wireless frequencies may need to be adjusted accordingly. You cannot guarantee that your sound check setup will work when the show begins. Therefore, the lesson here is simple – always reconfigure and monitor your frequency coordination. Check, check, and check again!
4. Poor Battery Management
Cheap batteries have a greater impact on performance than you might think. But despite this, some engineers continue to cut costs at the first point in their signal chain – not a sensible move. High quality alkaline or lithium single-use batteries have the most stable output voltage throughout the batteries life. Voltage level is important, because when supplied with a low voltage, transmitters can exhibit audible distortion or even signal dropouts. Consumer rechargeable batteries can seem like the ideal solution, but many provide about 20 percent less voltage than a traditional disposable battery — even when fully charged.
The best way to combat battery problems is to use a rechargeable system designed specifically for wireless microphones. Rechargeable battery systems found in many new Shure wireless systems feature medical-grade lithium ion rechargeable batteries, and intelligent information about your systems battery life and health. When managed effectively, rechargeable batteries are a great way to save money and landfills, while still delivering a reliable performance.
If your system will only support traditional batteries, you can help improve the performance by carefully comparing the transmitter’s voltage requirements with the battery’s output voltage over time to make sure that the battery will last through a full performance.
5. Not Setting Your Gain Properly
Last but not least is the vital adjustment of input gain. On the one hand, gain set too high can lead to distortion. While on the other hand, gain set too low can result in poor signal-to-noise ratio.
What tends to get overlooked, is the fact that your wireless system likely has a gain control on the transmitter itself. Much like the gain control on your mixer, its purpose is to set the input sensitivity low enough to prevent clipping, but high enough to preserve a good signal to noise ratio.
The best way to achieve a balanced result is to set the gain control so that the loudest input peaks barely light the overload indicator. If the peak indicator flashes constantly, you have a problem. Simply reduce the transmitter input gain until it only flashes very occasionally on the loudest signals.
Next to consider is the output level control featured on many receivers. This control only affects the receiver output and has no effect on improper gain adjustment in the transmitter. In other words, if you have a weak or even distorted signal coming from the transmitter, it cannot be fixed by changing the receiver output. Most engineers will recommend leaving this control at maximum for the best possible dynamic range; providing, of course that the mixer input can accommodate this level. If not, dial it back until the gain hits the red only occasionally as described above.
Become a Wireless Expert
RF spectrum can seem complex. And, with the recent changes coming thick and fast, it can all seem like a lot to take in. If so, you’re not alone. Keep up to date with the latest changes and get your FREE guide to wireless frequencies in the UK by visiting LosingYourVoice.co.uk