Wireless microphones are an essential part of modern shows with a high production value. Although this has liberated the artists from the limitations that cables imposed on them, for wireless technicians wireless systems are a completely new challenge. Rest assured, you are not the only one with these problems. Most failures or signal interruptions are caused by understandable mistakes that are easy to prevent. Here is an overview of five basic mistakes that you can easily avoid.
1. Something blocks the signal
The main rule is always to create a free line of sight between the antennae of the transmitter and the receiver. Make sure there are no major obstacles that break the line of sight, such as metal objects or walls. However, the most important thing is to avoid large crowds. The human body consists largely of water, which absorbs radiofrequency energy like a true sponge. Do you want to know how well the body absorbs the radio spectrum? Then fold your hands around the antenna of a hand transmitter! This reduces the effective signal strength by sometimes more than 50 percent.
For best results, place the receiving antennas in the same room as the transmitters and higher than the public and other obstacles.
Finally, make sure that flexible antennas are nicely straight and are not folded or rolled up. That sounds logical, but you do not want to know how often this causes problems. So the message is - anything that blocks the line of sight between your transmitter and receiver is a threat to the reliability of the wireless signal.
2.Inverted antenna type or wrong placement
You can greatly improve your wireless signal by using the right type of antenna and by correctly positioning the antennas. If you use the wrong antenna (s) or cables, or place them incorrectly, their range will be limited and you will receive lower signal strengths, which may cause failure. The good news is that modern diversity receivers work far more reliably than receivers with only one antenna. However, you still need to place the right antennae in the right place to ensure maximum performance and reliability. Some practical guidelines for this are:
First, keep the distance between the antennas and the transmitters as small as possible and create a free line of sight.
Secondly: Optimize the diversity effect by spacing the antennas at least a quarter of a wavelength (that's 12.5 cm by 600 MHz). Increase the distance between the diversity antennas to a whole wavelength (that is 50 cm at 600 MHz) to further improve the diversity effect. More than one wavelength distance between the antennas will not improve the diversity effect, but it can provide better coverage at large locations. Also place the antennas in a wide "V" shape to optimally track "moving" transmitters.
If the distance between the receiver and the stage is too great, connect them with a cable, 1/2-lambda antennas or directional antennas - and preferably position them higher than the audience to create a free line of sight. Head antennas significantly improve reception because they pick up more signal from the front and less from other directions.
Note: Never connect the short 1/4-lambda antennas to the receiver via a cable, because such antennas only work properly if they are placed on a ground plane, such as the receiver housing.
If you connect antennas to the receiver via coaxial cables, you may need to use an antenna amplifier to prevent signal loss over longer distances. The size of the signal loss depends on the length of the cable and on the type of cable; follow the recommendations of the manufacturer and ensure that the total net loss does not exceed 5 dB.
Finally: Some antennas are only intended for a specific frequency range. Always check the frequency specifications before using an antenna from another system.
3. Poorly coordinated frequency use
A well-chosen set of wireless frequencies distinguishes two important points: 1) They avoid local television channels, and 2) they fit well together.
Television channels have transmission capacities of up to one million watts - which is much higher than the 50 mW of the average wireless system - so do not use the frequencies that television channels use.
Even television channels at distances up to 100 kilometers can still cause interference, but that differs from place to place. Fortunately, indoor locations are much less susceptible to breakdowns than outdoor locations, because buildings shield most TV signals. In the case of large and heavy buildings, even television channels are not a problem at less than 50 kilometers away. Nevertheless, television channels remain notorious sources of interference, and it is smart to set your system to the safest wireless frequencies available in your area.
Once you have taken into account the television channels, choose a set of frequencies that fit well together. The easiest way is to use the existing frequencies for groups and channels already pre-programmed in your wireless systems. This method works best in small setups where you use systems that are compatible with each other.
In more complex setups - where for example wireless microphones and in-ear monitor systems are used interchangeably - there are computer programs that provide frequency coordination and coordination, such as Shure's Wireless Workbench.
Remember, however, that there is no such thing as 'turn on and ready'. One and the same set of frequencies can not be used equally well, and if you are on tour, you have to check the settings in each city again.
Even with permanent systems, RF spectrum use can change unexpectedly. Now TV channels will not change that fast, but other wireless systems of the same frequency band - at your own location or a bit further away - can also force you to tune your channel usage accordingly. There is no guarantee that the settings you used during the sound check will still work when the show starts. The lesson is simple - keep an eye on your frequency usage and adjust it if necessary. Keep checking, checking and checking again!
4. Bad battery management
Cheap batteries ruin more performances than you might think. Nevertheless, some sound engineers continue to cut down on this first link in the signal chain - not smart! High-quality disposable batteries (alkaline or lithium) provide the most stable output voltage over their entire lifetime. The voltage level is important because transmitters do not work properly when the power supply voltage is too low and they cause audible faults or even fail completely. Ordinary rechargeable batteries seem an ideal solution, but the voltage that such rechargeable batteries provide is usually about 20 percent lower than that of disposable batteries - even if they are fully charged.
The best way to avoid battery problems is to use a charging system that has been specially developed for wireless microphones. Most of Shure's new wireless systems use rechargeable lithium-ion batteries - which are also used for medical applications - in combination with intelligent charging stations that inform you about the status and condition of the batteries. Effective use of rechargeable batteries is a good way to save money and to spare the environment, without sacrificing reliability.
If your system only supports regular batteries, check how long they supply the voltage that the transmitter requires, so that you know that the batteries last a long time and deliver optimum performance.
5. The wrong gain factor
Finally, the entrance reinforcement also plays a crucial role. A too high gain factor causes distortion. A too low gain factor produces a moderate signal-to-noise ratio.
What is often overlooked is that the wireless transmitter itself can also be set to a specific 'gain'. Just as with the 'gain' you set on the mixer, here too the intention is to set the 'gain' low enough to prevent overdrive, but high enough to achieve a good signal-to-noise ratio.
For the best result, set the gain so that the overload indicator lights up very briefly for the loudest sound peaks. It is not intended that this peak indicator will flash continuously. Decrease the gain set on the transmitter until the indicator flashes incidentally only at the loudest sounds.
Then adjust the output level of the receiver. This arrangement influences only the output of the receiver, and does not affect the 'gain' of the transmitter. In other words, if the transmitter delivers a weak or just overdrive signal, this can not be compensated with the output level of the receiver. Most sound engineers open this controller completely to maximize the dynamic range; provided that the entrance of the mixing console can handle that level at least. If this is not the case, turn the control back until the signal only incidentally appears in red, as described above.
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