Radio microphones
Technical information ...

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Radio frequencies: The frequencies made available depend on the type of business the user is involved in (e.g. Theatre, Broadcast, Film, Entertainment, etc.). Many of the frequencies authorised by the D.T.I. interfere with each other, so where more than one radio microphone is to be used in a location, we take every precaution to provide a combination of frequencies that work correctly together. However, we cannot guarantee that there will not be interference from, or to, other radio equipment in the area (taxis, walkie talkies, broadcasting, commercial communications systems, other radio microphones in the vicinity). If interference is encountered, (usually evident as a varying whistling noise called 'birdies'), we may be able to offer you an alternative system. Most frequencies are made available on a 'secondary' basis, which means that if the radio microphone interferes with any other licensed radio user, then the radio microphone user must cease transmission. Fortunately, this is an exceptionally rare occurrence.


Batteries: Systems are supplied with batteries in a usable condition. It is the user's responsibility to keep a check on battery condition, and provide replacements as required. Note that Micron systems with only one battery must only be replaced with Alkaline PP3 batteries, (e.g. Duracell, Gold Seal, etc.). Micron transmitters send an inaudible signal to their receivers when their battery level becomes low. This is apparent as a flashing of red lights (at 1 Hz), to alert the operator to the problem. Not all PP3 batteries are the same size! When replacing batteries in Micron hand-held mics, take care to ensure that the battery is making a permanent contact with the two terminals (a small piece of cardboard may be wedged behind small batteries to push them against the terminals). Use your rehearsal time to calculate convenient breaks for battery changes, making allowances for extended performances and aiming to change batteries within half the quoted life if possible.


Switching On & Off: Micron Hand-held & Sennheiser pocket transmitters have On/Off switches. Micron pocket transmitters are only switched 'On' by the process of connecting the microphone. To avoid producing unwanted noise in the audio system, check that the volume from the receiver is not 'turned up' before the transmitter is switched 'On', and conversely, 'turn down' the receiver's volume before switching 'Off' the transmitter.

When a receiver is On while its transmitter is Off, there is often a loud sound of white noise heard. This is quite normal but can be damaging to ears and loudspeaker systems, so care must be taken to ensure that the volume from a system that is not being used is 'turned down'. In certain cases, this white noise may even be heard faintly in adjacent receiver modules in multiple receiver racks, and can even 'break through' from one channel of a mixer to another!

When using more than one system together, be warned that by switching On or Off a transmitter, a brief but loud noise may be produced in any other receiver operating at that moment. (If costume changes necessitate switching transmitters Off, and if the dressing room is near enough the receiver antenna for these switch-on noises to be heard, try removing the transmitter antenna before switching off, and replacing it after switching on).
Diversity Receivers: These receivers actually consist of two completely independent receivers tuned to the same frequency. Each receiver will be connected to separate antennae. There is an automatic system which blends the sound from both of these receivers to take advantage of whichever one is being received with the stronger of the two signals at any moment in time.

This feature of diversity receivers is necessary because the transmitters produce radio signals which reflect off walls, etc., and cancel out the original signal at certain points in the room. (In reflective steel clad studios, these cancellations appear at œ wavelength intervals). As the transmitters move, so these 'dead spots' move, and when one passes an antenna, the signal is momentarily lost, often producing a 'pop' or a 'fizzst' noise. Even outdoors, the signal can disappear completely in some spots. By using a diversity receiver, it will generally be true that one or other antenna will receive a strong signal. In cases of interference, the Micron diversity receivers also have monitoring facilities which, with the transmitter switched off, can assist in quantifying the strength, frequency and direction of the interference.


Receiver Antenna Positions: The best position for the antenna will be right next to the transmitter.

The received signal strength falls off rapidly as the distance between receiver and transmitter is increased. In general, good positions will be high, away from large metal surfaces, and in line of sight of the transmitter, particularly avoiding any metal objects in between transmitter and receiver antennae. As it is clearly impossible to see radio signals, some experimentation can be helpful in avoiding 'dead spots'. The receiver's meters are vital in finding positions of consistently strong signal strength.

In the attempt to place the receiver antenna close to the transmitter, the use of long antenna leads should be avoided, as there is approx. 10dB loss of a 180MHz signal in 100m of low loss co-ax cable. It is better to use long audio cables from the receiver, than long radio cables to it.

Where a diversity system is being used, the two antennae may either be used in identical manners (similar positions but spaced a few meters apart and with different orientations), or be used in complementary positions, (e.g. one directional antenna at the back of an auditorium pointing at a stage, and one omni-directional under the stage).

Directional antennae (Yagi) are available on request, and while giving a few dB s of additional signal strength in the direction they are aimed at, they provide many dB s of attenuation of unwanted signals from the sides and from behind the forward field of pick-up. This property greatly helps in reducing interference, and is particularly useful where long transmission ranges must be used. Beware that the wavelength of some VHF systems may render these antennae too unwieldy for some applications.

During your rehearsal do remember to monitor the receivers' signal strength meters to locate any dead spots, and to move the receiver antenna as required. This will be as important to achieving repeatable results as will attention to the audio quality.

MICRON POCKET TRANSMITTERS

Micron pocket transmitters (pics, left) are available with a variety of microphones and other inputs. The most popular are the Sony ECM 50 & ECM 55 series of clip-on lapel mics, the Sennheiser miniature MKE-2 which can be taped to the head or body, and input leads for other microphones such as Shure SM10 head-worn mic (with microphone boom in front of the mouth) and Sennheiser MKH 416 gun mic for remote outdoor work.



These transmitters are switched ON whenever the microphone is connected. The microphone should therefore be unplugged at the end of a session. The antenna connects on the top of the transmitter, at the opposite side from the microphone, and should be kept reasonably straight and allowed to hang freely, away from the wearer's body and from the transmitter's case. The battery compartment can be opened by pushing then twisting the end of a small slotted screwdriver or Yale style key in the opening on the bottom of the pack. The unit uses one or two PP3 batteries. If only one battery, then it must be alkaline. Each alkaline PP3 can provide up to 8 hours operating life, the 'low battery' warning signal being transmitted when the battery falls to 6.5 volts - at which time the battery should be changed within about 15 minutes.


The controls on the top of a pocket transmitter allow the level to be set by a small screwdriver potentiometer and 2 LEDs. The LEDs will only operate while the "SET" button is pressed, in order to conserve battery power.
The "SET" and "TONE" controls can be locked in their ON positions by pushing them in and then slightly sideways - don't forget to release them when you have finished adjusting the transmitter level!
There is a volume adjustment on the top of the unit near the microphone socket. To help in adjusting the volume, press the small white SET LEVEL button, and lock it in by pressing it sideways. Adjust the level so that typical speech illuminates the -10 LED, but not the 0 LED. Do not forget to release the button when this has been done.

To confirm that there is a good radio link between transmitter & receiver, there is a small white TONE button on the top. This can be operated by pressing the button in. This button can also be locked in the On position, and will be the cause of a continuous 1k Hz tone at the receiver, at 0dBm (PPM 4).


MICRON HAND-HELD TRANSMITTERS

The aerial screws onto the threaded stud on the base of the unit. These are switched On/Off by way of a three position switch on the base. There is also a three position volume switch on the base with unusual markings. The 'loud' position makes the system quieter, and is made for loud voices. The 'quiet' position is similarly for quiet voices. There is a 'mute' position to the On/Off switch, which allows the user to silence the microphone, without turning off the radio signal which would produce the undesirable noises mentioned under On & Off, above.

The battery compartment is opened by unscrewing the large slotted screwhead on the side of the body. Batteries must be alkaline PP3s, and battery life is approximately 8 hours maximum, but allowances should be made for much less than this. The same 'battery low' signal is transmitted to the LED display on Micron receivers as with portable receivers, at which time the battery should be changed within a few minutes.

For outdoor use where wind noise may be a problem, or even in certain indoor situations where unwanted low frequency noise is present, a bass roll-off switch with three positions is provided. This is in the battery compartment next to the terminals, and can be adjusted with a small screwdriver or match, etc. The battery terminals bend a little each time a battery is inserted. Gently pull them back towards the battery if they appear to be making contact without much pressure.


SENNHEISER POCKET TRANSMITTERS

(See pic, right).

The battery compartment is opened by holding the transmitter in the palm of one hand with the identity label uppermost, and with the thumb and forefinger of the other hand, pulling the two black ribbed finger-holds down, towards the bottom of the transmitter, and pulling the hinged cover downwards.
The transmitter takes 3 x AAA size batteries, 1.5v. Care should be taken to check that the springs which hold the negative battery terminals in place are not pushed against the body of the transmitter which could "short out" one or more batteries!

The controls for these transmitters are on the top panel inbetween the microphone and antenna sockets. These sockets are of the same type and size, and are distinguished by the symbols marked next to them (see pic, below - antenna symbol on the left, microphone on the right). They have an On/Off switch located next to the antenna socket, a screwdriver volume control in the centre and a limiter In / Out switch next to the microphone socket.


To assist in setting a working level on the transmitter, the receiver's LED display can be switched to show 'deviation' which is effectively the received audio signal level. This display will allow you to optimise the transmitted audio level (within the available dynamic range).


MICRON DIVERSITY RECEIVERS

There are On/Off switches on the front and back panels of these units.

The audio connections offer a microphone level output from the male XLR on the rear, (-53dBV, balanced & floating), and a line level signal from a jack socket, also on the rear panel ( 0dBm, unbalanced) which will accept a P.O. jack, or two or three-pole A gauge jack. As there is no adjustment over this level, be sure to use the correct socket for your associated equipment's input. Always use the diversity mode of operation, unless there is some particular interference product appearing from one antenna that can be silenced by switching exclusively to the other antenna. On the separate Micron diversity receivers (MDR 3 or MDR 530), the audio is monitored using the small 'A-diversity-B' switch, which selects which receiver is monitored on the headphone or loudspeaker. The speaker switch and volume control allow the operator to monitor the microphone and the reception conditions of each antenna's tuner, independently of the subsequent sound system.

When the transmitter battery is low but just usable, the receiver's red LED columns will pulsate once a second, from left to right. If the receiver is also being powered by batteries (12v), a low receiver battery warning appears on the top two green LEDs, which will flash. To power the receiver from a D.C. source, use the 240 degree 5-pin DIN socket, Pin 2 negative, Pin 4 positive, input voltage range 10œ - 15 volts. Typical current per channel is 150mA.
Never use these receivers with poor or no earth on the mains supply, the interference suppressor can pass electrical noise voltages to the chassis, which will can be dangerous if touched and not properly earthed through the mains. When using racked receivers, note that the numbers against each of the audio output sockets on the rear panel indicate the physical position of the corresponding receiver module in that rack, they do not indicate the channel number of the operating frequency. The channel numbers appear on the transmitters and the front of the receiver modules.

MICRON PORTABLE RECEIVERS

There is a multi-function switch on the front of these receivers, whose function has slightly changed over the years, and has been internally linked to provide the required operation, in later models. The functions are explained on the panel on the top of the receiver, but the differences only apply to the centre 'off' position. On some models, this switches off the receiver completely, saving battery life, and avoiding 'white noise' when the transmitter is 'off', but it also causing an audible thump when switched 'on' & 'off'. On other models, the centre 'off' position only switches off the LED display, saving battery life at the receiver; in these units, there is no 'Off' switch. The other two positions select the function of the LED display-: The 'up' position displays transmitter information (signal strength, tuning, low battery warning - 6.5 volts or less); The 'down' position displays receiver information (battery condition).

The antenna input is by way of a BNC connector on the back, which in portable applications will accept direct connection to a half dipole.
The audio output is from a 3-pin locking Preh (DIN) connector, balanced across pins 1 & 3, with screen and centre-tap on pin 2. Adaptor leads to XLR 3pin male are normally provided, with the signal balanced across XLR pins 2 & 3, and with screen and centre tap on pin 1. Audio output is also available on a TRS 'B' gauge jack socket, via a screwdriver adjustable level control on the side of the unit (this will drive high impedance headphones at an adequate level for most purposes) note that the audio appears across the tip and ring, with the sleeve not connected.
The unit may be powered through the 5 pin Preh (240 degree, DIN) locking connector in a variety of ways: 12 volts DC across pins 4(pos) & 2(neg), or 18-25 volts DC across pins 5(pos) & 1(neg). This is compatible with Micron MMS1 mains power supply units, Micron battery packs (containing 8 x AA size 1.5 volt batteries which fit into the receiver carrying case), the Micron power output socket on some receiver rack units, and 12 volt lead-acid batteries with the appropriate leads. The current requirement of the MR510 receiver with LED illuminated is approx. 55mA, (and with LEDs off, 45mA.).



SENNHEISER DIVERSITY RECEIVERS

The mains On/Off switch is on the front panel. The mains socket does not have an earth pin, the ground being taken from the audio cable screen. Usual 3 pin IEC mains sockets will fit the two pin socket correctly. The audio output is available on an XLR male socket, (balanced & floating at low impedance) on the back panel, next to a screwdriver level control. This socket is capable of producing a line-level output ( 0dBm ), if required.
The input sockets for the RF antenna on the rear panel are not straightforward, and require some care. The two (diversity) receivers within each unit are identified as A and B, and it is vital for proper diversity operation that separate antenna inputs are provided to each of these. There is only one socket for the 'B' receiver, but two for the 'A' receiver, one being an input, and the other an output. This arrangement allows two diversity receivers to be housed in a single 19" rack with only two antennae whose signal are split as follows:-
One antenna is connected to each receiver's 'A' input, then two short BNC link leads connect each receiver's 'A' output to the other receiver's 'B' input.

Most antennae are completely passive devices, but the Sennheiser directional UHF antenna, which looks like a flat black plate, is an active device, which needs to draw power from the receiver. That power is available on the 'A' and 'B' inputs to each receiver, and the wiring method described above will operate perfectly.
The front panel LED meter can be switched to indicate either the radio signal strength, "RF", or the audio signal level, "DEVIATION". The front panel "SQUELCH" control allows you to adjust radio signal strength level at which the output becomes muted. This is also a screwdriver adjustment, where the lower the number on the control indicates the lower the RF level at which the mute will operate. The final front panel control is the headphone monitoring socket and level control.



R.F. DISTRIBUTION AMPLIFIERS

These allow the signal from one antenna to be routed to up to four receiver inputs. These amplifiers must be powered from a Micron 12v power supply (or other 12v outlet). There are no controls and no indication of the presence of power. Remember that for useful diversity operation, each channel of a receiver must receive signals from a different antenna, so where r.f. distribution amplifiers are being used, there must be one for each antenna.
For example, in a system of four diversity receivers, one antenna would be connected to an R.F. distribution amplifier which in turn would be connected to the four 'A' inputs of each receiver, and a second antenna would be connected to another distribution amplifier, in turn connected to the four 'B' inputs of each receiver.

ANTENNA TYPES
There is a variety of types of aerial, each suited to different applications:-

V.H.F. helical half dipole. This is the most compact, and is generally supplied with a right angled BNC socket for direct connection onto the rear of a portable receiver; this is usually the antenna used for mobile work. It is limited by being situated only where the receiver happens to be placed, though they can and do often give excellent results even in fixed installations. Experimentation will confirm whether an adequate signal is being received or not.

V.H.F. helical dipole. This is the most frequently used receiver antenna, consisting of a central connector with BNC and crocodile clip, and two threaded sockets for attaching the two halves of the dipole. (The threaded studs that connect the dipoles to central connector can un-screw and disappear - sawing the head off 2BA bolt will make a perfect replacement). These can be attached to walls, etc. using the crocodile clips, and in diversity applications can be set at differing orientations, at opposite sides of the performance area. Where possible, avoid attaching them to earthed or other metal objects such as water pipes, but if only earthed fixings are available, they may offer better reception than an unearthed position twice as far away or out of sight. For portable work, the dipole can be clipped to the shoulder strap of the recorder, and probably give better reception than a half dipole under the receiver (at leg height).

V.H.F. Yagi. These 3 element antennae are superb in applications where their pick-up from the front gives useful directional properties. This can help eliminate much of unwanted r.f. signals from the sides and rear, if the transmitter(s) are always going to be within the same few degrees of sight from the front of the Yagi. However, they are large, unwieldy to transport, unsightly to place in auditoria, and impossible to use with mobile receivers. For diversity operation, two antennae must be used, and although they need not both be Yagi, the other should still have a reasonable chance of receiving a strong a signal in order to be useful.

An example application of Yagi antennae is at a sports event with a number of transmitters in the stadium, and a Yagi on a mast on top of the sound recording vehicle outside the grounds, connected to a rack of several receivers. In this case the one Yagi antenna is pointing at the area where all the wanted transmissions shall be coming from, and rejecting all the other r.f. that may be 'hitting' the high mast position from all other directions. Another application is a water-sports event where the distance between the land-based commentator, and the P.A. system may be quite great, and where the waterborne commentator may be even farther away, but always in the same direction so that a Yagi may be aimed at both commentators.

U.H.F. Yagi. These have all the benefits of directional antennae mentioned above, but are more compact than V.H.F. yagi. They consist of about six short metal elements on a central rod, of about half a meter in length, which can be clamped to a mast or bar. The r.f. terminal is BNC. For diversity operation, two antennae must be used, and should both be pointing towards the area where the transmitters will be used, though each from a different direction. U.H.F. signals are severely attenuated by objects in the line of sight between the transmitter and receiver, so a directional antenna can be vital in achieving a usable signal strength.

U.H.F. Ground-plane. This is a compact arrangement of three 'legs' radiating outwards and slightly downwards from a central connector, with a fourth 'leg' pointing upwards. This is omni-directional, but can achieve good reception if mounted above most of the 'earthy' objects in the area. It is attached by a 3/8" thread on the hub, which suits most microphone stands and fittings, and can also be used with special 'G-clamps' available with a 3/8" microphone thread.
The r.f. connector is a UHF threaded socket, supplied with an adaptor to BNC.

U.H.F. directional plate. This large plate should be mounted vertically, with its arrow pointing towards the area of strongest reception. It offers the same directional advantages as the V.H.F. Yagi mentioned above, but is small enough to be used in any auditorium. This is ideal even for use at the side of a stage in theatre applications, where the rejected signals from the sides and rear may just be the signal reflected off the walls, but which may contribute to cancellations if a non-directional antenna is used. This antenna is attached by a 3/8" thread, which should suit most microphone stands and fittings. It should be mounted with the 3/8" thread socket facing downwards. The r.f. connection is BNC.

All antennae (transmitters and receivers) have a length that is selected to suit the frequency of operation, and in some systems, there may be a variety of frequencies in use, requiring a variety of lengths of antenna. In the case of helical dipoles, it is not possible to tell how long the arms of the antenna are by measuring them, however there is often a coloured band on two of the frequency bands: red for 185 - 205MHz, and green for 205 -225MHz. No coloured band is probably the 168 - 186 band. Note that a tolerance of plus or minus 5% is quite acceptable, and 10% should still provide reasonable results if necessary. (Better with one antenna in a good position even if its length is 15% 'wrong', than an antenna position that is not in line-of-sight). The U.H.F. ground-plane has adjustable 'legs' which are marked in MHz and whose length must be tuned to the operating frequency.

The Aluminium Yagi antenna is of fixed length, but can be given a wider tolerance of, say, plus or minus 10%. The straight transmitter antenna will be either a quarter or a half the wavelength, and for the technical, wavelength (in meters) = 300 / frequency (in MHz). So for example, for a frequency of 177 MHz, the wavelength will be 1.69 meters, therefore the length of a quarter-wave antenna will be 1.69 / 4 = 422.5 mm. This should be good for all frequencies in the range 168 - 186 MHz. In emergency, a low frequency's long antenna can be cut to the required length of a higher frequency's shorter antenna.



TROUBLESHOOTING AUDIO PROBLEMS

Feedback. This is the enemy of many productions using tie-clip microphones with a P.A. system. Careful speaker placement, microphone placement, equalisation and skilful use of the faders are the principal techniques; frequency shifters, supplementary 'fill' speaker arrays, phase cancelling speakers, etc., etc. are available in difficult situations. In all these cases, it is helpful to ignore the radio part of the link, while trying to obtain a satisfactory sound balance, and consider the microphones as wired omni-directional mics. In the case of continual howl from a Micron pocket transmitter, check that the 1kHz test tone button has not been locked 'in'. Also check that a line level signal from the receiver is not being connected to a mic input on the mixer.

'Birdies'. (Changing whistling noises). This is a result of using a combination of radio frequencies that produce intermodulation frequencies that are picked up in one of the receivers. It may occur when three or more transmitters are used together which do cannot, in fact, operate together for this reason, or it may be the result of some external radio source that is creating the intermodulation frequency along with one of the radio transmitters. This is not really an audio problem, but a radio problem, and although it may be cured by revising the relative positions of the transmitters, or of the receivers, or experimenting with the antennae's positions. It may become necessary to exchange one or more systems for a system operating on another frequency.

Audio level changes. When this happens gradually, it is generally the distance between the microphone and speaker's mouth that is changing. When this happens with sudden and sharp jumps in level, then it is probably the result of the Complementary Noise Reduction System in the receiver loosing track of the incoming signal for a moment, and therefore failing to correctly follow the corresponding gain adjustments in the transmitter. In this case look for faulty antennae and antenna connections, antenna cable or poor receiver antenna positioning relative to the transmitter antenna. Also check for good received signal strength at the receiver. Check for loose audio connections, which can 'trick' the noise reduction system in the same way. There may be a weak microphone connection to the transmitter.

Distortion. With loud voices, particularly singers, or tie-clip mics fitted to instruments, it is likely that the signal level will distort or clip at some point in the system. All the transmitters have a level control or switch to reduce the volume transmitted.

No audio. Check batteries and all connections from the microphone through to the mixer, using the meters to monitor received signal strength, and headphones to monitor the audio in the receivers, where possible. By systematically following the signal from microphone through receiver to the mixer, it should be possible to identify where the sound path is broken - usually a switch has to be turned to the correct position or a cable has to be plugged into the correct socket, somewhere.

One microphone's signal is heard on another's receiver. Check that the transmitter whose signal is being 'invaded' is switched 'On' and with a good antenna connection and good received signal strength. (Some receivers will try to lock on to another signal if their own transmitter is not 'On'). Try moving the transmitters apart, to more than half a wavelength of each other. If the transmitters are too close together, some curious intermodulation products can be created from the combination of the two signals with the other frequencies present within the transmitters. If two transmitters must be used close together, there may be another pair that works better close together in your studio or stage. Check that the combination of frequencies (and types of transmitter) is capable of operating simultaneously.

Loud white noise heard instead of audio. Presumably there is no transmitter operating on that frequency, but the background RF noise level is enough to 'lift' the automatic mute (or 'squelch') control. Check transmitter is On, with good batteries and antenna.

Crackling and/or intermittent audio. Check transmitter microphone and antenna connections. Try substitute mic & antenna. If noise occurs when transmitter passes through certain places or angles, try to improve the receiver antenna positions relative to the receiver positions (the closer they are to each other the better). Check for normal audio causes, such as mains borne interference, bad connections, bad grounding, etc.

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