Until recently good wireless microphone systems were very expensive & the cheap ones performed poorly. However, the technology has become a lot cheaper. Wireless microphone systems can either be VHF (very high frequency,) UHF (ultra high frequency,) or digital. (With VHF being the cheapest & digital the most expensive.)
You can run about 3 VHF radio mics together, but for theatre work this will often not be enough. UHF systems allow you to use far more radio mics than this & are less likely to suffer from interference. (That is the reciever picking up anything other than the desired mic signal.) UHF arials are also short & this is important when you consider that the transmitter with its antenna will have to be hidden on an actor; either on a belt under their costume or in a pouch secured to the costume somewhere by the costume person.
The transmitter will require power of some sort & this is normally provided by batteries. Depending on the model these may be disposable or rechargeablel, the important point is:
IT IS YOUR RESPONSIBILITY TO ENSURE THAT RADIO MICS' BATTERIES
Obviously budgets come into play, but where money allows, it is good practice to use new batteries for each performance. Discarding the old ones at the end. There is nothing you can do if a radio mic's battery dies during a big musical number; you are not going to be able to go on stage & change the battery & you may well have spoilt the paying audiences enjoyment of the production.
ON/OFF SWITCHES & MUTE SWITCHES
Depending on the model being used there may be on/off & mute switches on the transmitter.
IT IS YOUR RESPONSIBILITY TO ENSURE THE MIC/TRANSMITTER IS TURNED ON &
Do not rely on the actors, stage managers or assistant stage managers to do this as it is not their responsibility.
Normally in theatre you will be working with lavalier mics. Very often for television work you will see these clipped onto the presenter's lapel. This is less sucessful in theatres because if an actor turns their head they will no longer be speaking into the microphone. Also on the lapel, the microphone is quite a distance from the sound source (ie the mouth.) In order to minimise feedback you want the mic to be positioned close to the sound source.
Usually it is a good idea to either hide the mic in the actor's hair/wig or place the mic on the actors cheek & use tape & make-up to conceal it. You can get flesh coloured mics for this. For a good description of positioning & concealing mics on actors visit:
USING WIRELESS MICROPHONE SYSTEMS
This web site provides a very good & detailed description of using wireless systems. I would recommend that you read it thoroughly.
Each transmitter has to transmit on its own frequency. This means that if you have a show that requires 8 radio mics, each of these mics will have to be transmitting on its own frequency, different to all the others. Likewise, the receivers have to be tuned in - like a radio - so that each receiver is only receiving on one transmitter's frequency.
Some systems provide frequencies already set by the manufacturer, others allow you to change the frequency. The receivers will have antennae (or a means of attaching them.) It is recommended that you use a "diversity" system. Inside buildings radio waves bounce off walls, floors & ceilings creating occasional deadspots where the signal from the transmitter does not reach the receiver. A diversity receiver has two antennae so that the chances are if one antenna encounters a deadspot the other antenna will pick up the signal successfully. On the picture on the previous page you can see there are two antennae on the receiver & the model is called "true diversity."
It is best to position the antennae as closely as possible to the transmitters & as far away from any possible sources of interference. This unfortunately is usually a compromise. The best place for the antennae is right in front of the stage, but of course this will ruin the audiences' view. Therefore the antennae will normally have to be positioned to the side of the stage, you can then use cable to connect the antennae to the receivers. (Very often two antennae can be shared between several receivers.)
As a performer moves further & further away from the receiver the signal will eventually no longer be received & will drop out. Usually radio mics have a "squelch" control which you can set so that the receiver mutes before the signal disappears & you are left with amplified static.
Once the system is on, tuned & functioning, you should be able to see information provided on the receivers screen if it has one. Usually you can see which of the arials are picking up the transmitter's signal & sometimes the strength of the signal is also displayed. You should also be able to see the level of the audio signal that is being sent from the receiver to the mixing desk. You may also get a visual feedback if the transmitter is not on or is muted. All of these help to prevent avoidable errors when operating radio mics.
There will be some further information on setting up wireless microphone systems in the chapter that deals with the technical rehearsal.