Rough Guide to Radio Microphones

Using Radios

Radio mics transmit the audio signal from a transmitter to a receiver, which is connected to a mixer or plugged directly into the camera. There are three types of transmitter: A pocket transmitter with a small tie microphone (often called a lavelier mic. in the USA), a hand-held transmitter (like a normal microphone with a bult-in transmitter), or a plug-on transmitter (this converts a normal microphone into a radiomic. and are mostly used for converting a long-handled reporter mic. into a radiomic., or for making a wireless fish-pole). They allow the user to move around freely, un-tethered by a cable. They are particularly useful for presenters (especially if they are ‘out and about’).

The working range for a radio mic. can be up to one or two hundred metres in line-of-sight, though it is best to keep to 50 metres or less for reliability. Receivers can be "diversity" or "non-diversity", a diversity receiver uses two antennas and the antenna with the strongest signal is the one used and the receiver switches antennas silently to the strongest signal (nb: the G3 pocket receiver uses the output cable as the second antenna). Cheaper systems tend to have less selectivity on the receiver, and fewer can be used in the same location. Modern radio systems use DC/DC converters which give full power to the transmitter until the battery dies; this can happen very quickly, so do not try and use batteries from the previous day. Older or cheaper systems do not use a DC/DC converter and the transmission power will drop quite quickly, it will then level off for a while before slowly dying. Again, a good reason not to use batteries too long.

The transmission frequency must be carefully chosen as interference with another transmitter could occur. Each manufacturer will have a recommended frequency plan of frequencies that will work together and this should be adhered to. Always switch on the receiver first at a new location or scan to find out if anyone else is transmitting. Do not switch on a transmitter before checking as you could cause interference to others. At large events pre-planning of frequencies is essential. The radio mic frequency band is shared by various different users, TV and video radio equipment being only some. You may be in range of someone else using the same frequencies as you, so continual monitoring is necessary as this may happen at any time. If it does you can change the frequency (most systems will be switchable). Be aware that the human body absorbs - if the antenna of a pocket transmitter touches the body you can attenuate the signal by as much as 70dB, vastly reducing the range. Keeping the antenna off the body by as little as 1cm will get most of this back. Consider mounting the transmitter with the antenna pointing down as this can often help.

As well as setting levels on the camera/mixer in the normal way, radio mics have their own levels to set. The Transmitter will have a sensitivity control to maximise transmission level without distortion. The microphone has its own noise floor so the sensitivity of the transmitter must be set to give a good output but must never peak. The Receiver pack will normally have an output level control, many can output either microphone level or line level; wherever possible it is always best to output line level and go into a line input.

Tip: When setting the radio mic levels, remember that the sum of these levels add some noise to the channel and it is important to set your levels correctly and evenly before starting the job. Try to make sure the transmitter audio input level is high, but not too high, leaving a bit of headroom is preferable to clipping and distortion and the receiver output level should be adjusted to as high as possible without distorting the input of the receiving equipment. Line level is better than mic. level - if you do have to use a microphone input make sure that no phantom or plug-in power is being sent into the receiver, or you have a receiver with blocking capacitors that will prevent this power doing any damage.

You are effectively setting up to 2 extra levels (depending on which system you use) so the potential for getting it wrong is increased. This is the reason that they should really only be used if you need the mobility or cable-free operation. With sit down interviews a wired solution is preferable. Reading the instruction manual is essential. Do some web research for the make model you have. A beginner can get good results but it is risky. A good sound recordist is a massive benefit and will minimise the risk.

Makes of Radio Mic

There are many good makes of radio mic available. Manufacturers include: Sennheiser, Audio Limited, Micron, Lectrosonics, Zaxcom and Audio Technica. Best of the budget range has to be the Sennheiser evolution G3 series (ew112-p G3 - around £500 - or the ew 100 ENG G3 which is the same kit but with an added plug-on transmitter). Next up is the Micron Explorer 100 Series 16CH - SDR116 which is a true diversity system, on sale at some places for under £1000. Second hand ones can be considered but read the next section and make sure you are not getting one which will not be usable for you after 2012.

Licences and Changes in the Air

You MUST make sure you are aware what frequencies are available in your country and find out if they require a licence. It is very dangerous to take radio microphones across borders without first checking the legality as you could find them impounded by Customs - in the same way, it can be dangerous to purchase radio systems from another country as you could easily end up with something that is illegal in your own country.

In Europe there is a small frequency band - 863MHz to 865MHz (in Ch.70) that is available for use licence-free in every ETSI signatory country. This frequency band will not change and will continue into the future. In practice up to about four frequencies can be used intermodulation-free. Cheap systems only two or three, though Sennheiser can get six frequencies in G3 systems.

All other frequencies require a licence in the UK. There are two types of licence available: "co-odinated" (fixed site) or "shared" (mobile). In the past the shared frequencies were 14 fixed frequencies in TV channel 69, though in practice only four to six were usable in cheap systems, though better systems could get eight or twelve working together. This frequency band will become illegal during 2012, but is guaranteed up until the end of the London Olympics. The replacement frequency band is channel 38 which is currently used by radio astronomy and some radar stations. These will cease at the end of 2011 and the full Ch.38 will be available for radio microphones. Up until the end of 2011, channel 38 can be used, but with exclusion zones (the largest is centred on Jodrell Bank radio telescope). A licensed user must first look up the availability at his location and if it is within an exclusion zone, then frequencies in Ch.39 or Ch.40 will be allowed (the look-up table will advise). From January 2012 onwards, Ch.38 will be fully available for shared frequency use and Ch.39 and 40 for licensed co-ordinated (fixed site) use only.

After the shut-down in 2012 Ch.69 will be licensed for a Europe-wide wireless broadband system (including many other TV channels) and will be totally unusable and illegal for radio microphones.

You may find many second-hand systems for sale cheaply - be aware that if these are old Ch.69 systems they will only be of use if they are under 10mW and can be switched to the de-regulated 863MHz band.

·         Full details on UK licenses are on the JFMG website.

·         Their is also lost of information on the change-over on the BEIRG website.

·         The website of the Institute of Broadcast Sound is also useful.