GB Audio. system design, system, design, mic, microphone, radio mic, radiomic, radiomike, radio mike, mike, wireless, radio, cordless, vhf, uhf, fm, rf, regulated, unregulated, deregulated, de-regulated, legal, illegal, not legal, licence, license, diversity, aerial, antenna, antennae, freq, frequency, frequencies, hz, compander, transmit, transmitter, receive, receiver, hand, hand held, handheld, hand-held, held, lapel, tie, clip, head, head worn, head-worn, mpt1350, mpt, 1350, mpt1345, 1345, 1311, en300, en, 300, 220, sound, audio, hire, rent, rental, sales, installation, service, repair, dealer, theatre, conference, concert, licence, license, mhz, hertz, UK GB Audio. Scotland's sound system design company - hire, sale, design and installation service - Dr. Know's radio microphone help file 2016  Go Back one page
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Dr. Know is our resident Technical guru, responsible for the award-winning Reference Library, Technical Info, Data Sheets and the Audio Clinic. Visit the Wireless Mic Clinic or write to

Please note, we have heard of unscrupulous individuals dumping their unwanted wireless kits on an unsuspecting public via eBay. There is no law against selling or owning channel 69 equipment but it is now illegal to use and possibly unusable due to interference from new services which have takeb over channel 69. Don't just buy the first kit that looks like a bargain because you may get ripped off. Remember - Caveat Emptor - buyer beware!

In 2012, Ofcom re-allocated some of the radio mic spectrum. If you currently use radio mics or in-ear monitors you must check out this summary:

General notes

Wireless microphones are best considered as a system thus: The microphone capsule picks up the sound in the normal way. Audio Technica microphones are known for reliability, and high quality sound. Then there is a battery powered radio transmitter pack (or built in to a handheld mic) which converts the audio into a radio signal, which transmits out in all directions. A receiver unit with antennas picks up the radio and converts it back into audio, which then feeds into a mixer channel, where it is controlled just like a normal microphone.

Wireless gives a great freedom of movement but is not without it's problems. The number of connections and processors added into the audio chain don't help reliability, let alone the unreliable and unpredictable things that can happen to low level radio transmission. Our rule is: if you are going to be performing in a fixed position or can manage to cope with a cabled mic then use a cabled mic. They virtually never fail. If you need unrestricted movement, then consider using a wireless system but please be aware of the potential pitfalls and try to minimise the risks by understanding how the system works and following good practices.     top

Frequently asked Questions

What is the difference between Single channel, Diversity and True Diversity?
Single channel receivers have one antenna and one receiver circuit. Diversity receivers have two antennae and a circuit which selects the strongest signal. True Diversity receivers have two totally independent RF receiver modules and a circuit which selects the best channel for both signal strength and signal-to-noise ratio.

How many transmitters can I use with a single receiver?
Only One. Two transmitters operating on the same frequency will interfere with each other. It is possible to have both a handheld and guitar transmitter on the same frequency, and use them with the same receiver, but not simultaneously.

How far from the receiver can I place my antenna?
As long as you use 50 Ohm fully screened cable up to six metres, but the closer the better. It is possible, by employing antenna amplifiers to run cables of up to ninety metres.

Can I mix systems from different manufacturers?
Theoretically yes, but careful attention must be paid to specifications, because different systems have different technical parameters for equalisation and compression. Sometimes there are even compatibility issues between different models from the same manufacturer. It would be prudent not to assume cross compatibility between different manufacturers, unless tried and tested first.        top

Is there a performance difference between UHF and VHF?
No; there is little audible performance difference but, with the high demand for a greater number of usable, interference free frequencies, the UHF band offers more opportunities.

What is an LN Compander?
A means of preparing a signal with a wide frequency range for transmission in a compressed form and expanding the signal back to it's original state in the receiver, thereby improving the signal-to-noise ratio.

What is FM Modulation?
FM is the abbreviation for Frequency Modulation, which is the method of transmitting information by modulating a carrier frequency on a radio transmitter. The frequency of the carrier wave is varied in accordance with the amplitude of the input signal, while the amplitude of the carrier remains unchanged.

What is the difference between Squelch and Mute?
Nothing; the output of a receiver is muted by a squelch circuit when the received signal falls too low for reception. Some units have preset mute levels, others have manual settings. It should be set so that the receiver mutes when the transmitter is switched off, or goes out of range, before the signal is swamped with noise or interference. To achieve the optimum setting, place the transmitter at a distance so that the receiver shows the carrier at about half strength. It may help to remove the antenna from the transmitter.
Set the squelch control to maximum (ie muted) and then back it off until noise starts to appear.
Then trim it back a tiny amount until it just mutes. If you are using multiple systems, you will also need to check and adjust each squelch with all the other transmitters switched on to make sure they don't interfere with each other.

Your Questions: real problems solved. Dr. Know is our resident Technical guru, responsible for the Reference Library, Technical Info, Data Sheets and the Audio Clinic. Visit the Audio Clinic by clicking here or write to

Common Problems

Fragile connections between the very thin cable of lavalier mics and beltpacks are very vulnerable and liable to problems caused by pulling, twisting and straining when worn under costumes or during vigorous acting or aerobics.

Unwanted noises can be introduced in a number of ways: Bangs are commonly caused by intermittent battery contact, due to dirty or loose terminals. Clicks and scratchy noises are usually associated with fractured transmission antennae or poor earth continuity in the case of lavalier mics, due to fractured cable, dirty connectors or loose terminals.

Distorted sound is usually caused by the beltpack being overloaded by the microphone. Most beltpacks have a gain control to match the output of the microphone to the input sensitivity of the beltpack. For speech, a higher gain will be required but for singing or shouting, this will need to be attenuated for best results.

Dull sound can be caused by several things: the mic capsule poorly placed or trapped under clothing, the tiny mesh grille on top of the mic caked up with makeup or sweat. An unmatched transmitter and receiver can also cause this effect.

Drop out happens when wireless signals reflect off certain surfaces and therefore cancel out the direct signal on arrival at the antennae. Antennae should be positioned at least 1 metre from walls, floors or any other flat surface. Careful positioning of antennae to check for possible areas of drop out should be conducted before a performance. The closeness to antennae does not automatically guarantee operation.         top

Poor RF signal strength may be caused by the transmission aerial being tangled or having dirty or broken contacts to the beltpack. Please don't just stuff it into a pocket. Try to dress the aerial in a straight line and preferably not on highly mobile places eg arms. Don't let the antenna contact bare skin as this has the effect of detuning the antenna - grounding RF through the skin - known as body absorption. Watch out for actors sweating; a sweaty shirt can act as a good conductor straight to the skin. One possible solution to this problem is to attach the transmitter to the belt with the antenna pointing downwards (giving clear instructions to the actor to take care when sitting down to avoid stretching or breaking the antenna). Incidentally, unavoidable Body Absorption is the reason why beltpack transmitters are allowed to have a more powerful output (typically 50mW in UK) than handheld devices (max 10mW in UK). Also check for low batteries. Only use alkaline batteries. Never use rechargeables as they only produce 80% of the normal voltage when fully charged and only last a very short time making them unreliable.

Very poor RF signal strength can be caused by metal getting in the way. In the theatre this is often a problem; steel girders, lighting bars, trussing, metal framed stage flats/platforms/risers, safety cages, suits of armour, even tin foil can reduce your signal strength. Remember that metalwork in the vicinity will act as an antenna and suck the transmitted signal away to ground. Sheet metal or bars or grille in between the transmitter and receiver will act as a Faraday Cage and stop radio from passing through.

Breakthrough can occur with cheap kits or when a large number of wireless systems are used together. Each system has it's own frequency but the actual bandwidth of the transmission is typically 30kHz i.e. 15kHz on either side of the centre frequency. Cheap units have a wider bandwidth which can spill and be picked up by other receivers on close frequencies, just like the breakthrough interference which occurs when two FM radio station frequencies are too close to each other. High quality transmitters have a tighter defined sideband and are therefore less likely to interfere with each other.          top

Harmonic interference can also be picked up on other channels by receivers with poor input sections. Low level harmonics are generated and transmitted along with the original signal. These harmonics or ghost channels, although very weak, can sometimes interfere with other channels. Again, high quality receivers are better able to reject unwanted signals, whilst remaining locked on to the true signal.                          

Nervousness and anxiety have been cited as possible factors affecting radio mic performance. The reaction of electrical circuits to personal electromagnetic energy fields is a difficult phenomenon to understand, let alone measure scientifically in a performance scenario. However, I am certainly not alone in having witnessed situations where a wireless kit will work perfectly on Actor A but will drop out or give lower RF signal strength on Actor B without any obvious reason. Or Actor C who, after a perfect rehearsal, drops out or loses strength during the show. Indeed I have even heard anecdotal evidence of a certain actor for whom three (tested) industry standard lavalier mic capsules literally stopped working, dead. Changing to a different brand capsule made the same wireless system work, faultlessly. These cases are rare and the key is to address the proven scientific points (above) as far as possible. However, if every step has been taken to optimise performance then you may have to consider other methods of miking up a problem case, such as gun mics, plates or wired mics because there's nothing worse than a mic cutting in and out during a performance.      top

Your Questions: real problems solved. Dr. Know is our resident Technical guru, responsible for the Reference Library, Technical Info, Data Sheets and the Audio Clinic. Visit the Audio Clinic by clicking here or write to

UK frequency information - Updated 2016

This summary users guide should help you understand the main choices and limitations of radio microphone use in the UK. We have a range of systems available for hire in most of the categories below, with licence cover if required. We have hand held mics, lapel (tie tack) mics, headworn mics, instrument mics and line inputs.
Some other VHF and UHF frequencies are available for temporary use or for specific geographic locations. You are welcome to call us for further information or visit the new OFCOM website: They administer licences and site allocations for Programme Making and Special Events (PMSE). Reserved news and broadcast frequencies are not shown here. To the best of our knowledge, this information is accurate at the present time. Please contact us to discuss the latest position if you are planning to purchase new wireless systems. No liability is accepted for any errors or inaccuracies or changes which may occur or the consequences thereof.

VHF Deregulated Frequencies.
Anyone can use these frequencies for MPT1345/1311 type approved equipment.
VHF Fixed Site Frequencies.
MPT1350 type approved equipment may only be used at the licensed site.
VHF Shared Frequencies.
MPT1350 type approved equipment may be licensed for use anywhere in the UK.
Still available for use
Free of charge
£168 for a block of thirteen frequencies
or £28 each per year or £8.50 each for 48 hrs
£85 for a block of fifteen frequencies per year
 (£10 cheaper online)

173.800 MHz
174.100 MHz
174.500 MHz
  174.800 MHz *
175.000 MHz



* OK on it's own but may interfere with other units.

176.400 MHz
177.000 MHz

192.100 MHz
192.300 MHz
192.600 MHz

200.100 MHz
200.800 MHz
201.700 MHz
207.700 MHz
207.900 MHz
208.100 MHz
209.400 MHz
210.000 MHz

  175.250 MHz *
175.525 MHz
  176.600 MHz *

191.900 MHz
192.800 MHz
193.000 MHz
199.700 MHz
200.300 MHz
200.600 MHz
208.300 MHz
208.600 MHz
209.000 MHz
209.200 MHz
209.600 MHz
209.800 MHz


UHF Deregulated Frequencies Ch70.
Anyone can use these frequencies for EN300 220 type approved equipment anywhere in UK.
UHF Shared Frequencies Ch38.
MPT1350 type approved equipment may be licensed for touring use in most locations in the UK from 2010. Check availability by postcode.
UHF Shared Frequencies Ch33 - 37 & Ch39 - 40.
MPT1350 type approved equipment may be licensed for use in certain locations in the UK from 2010. Check availability by postcode.
Still available for use after 2012
Free of charge
£85 for the whole block shown below per year
or £155 for two years (£10 cheaper online)
Licence £28 per unit per year
Block band
863.100 MHz
864.900 MHz

863.100 MHz
863.700 MHz
864.100 MHz
864.900 MHz

Block band
606.500 MHz

613.500 MHz
Contact us or OFCOM for frequency allocations in your area.

Licences can also be obtained for fixed site use only on channels 21-30 and 41-60. Check local availability with OFCOM.

April 2015 - Following an EU Implementing Decision on harmonised technical conditions of radio spectrum use by wireless audio programme making and special events equipment in the Union (2014/641/EU), new 823 to 832 MHz and 1785 to 1805 MHz bands are to be made available in UK for wireless audio PMSE equipment. 1785 to 1805 MHz bands will not be available in Northern Ireland.

Audio-Technica range availability.     Sennheiser free frequency planning software.             

European wireless microphone manufacturers' UHF frequency range options:

Range A = 516 - 558 MHz (includes frequencies in TV channels 26 to 31)

Range G = 566 - 608 MHz (includes frequencies in TV channels 33 to 37 and a bit of 38)

Range GB = 606 - 648 MHz (includes frequencies in TV channels 38 to 42)

Range B = 626 - 668 MHz (includes frequencies in TV channels 40 to 45)

Range C = 734 - 776 MHz (includes frequencies in TV channels 54 to 59)

Range D = 780 - 822 MHz (includes frequencies in TV channels 59 to 64)

Range E = 823 - 865 MHz (includes frequencies in TV channels 65 to 70)


TV channel / frequency chart (2016)
21 22 23 24 25 26 27 28 29 30
470-478 478-486 486-494 494-502 502-510 510-518 518-526 526-534 534-542 542-550
31 32 33 34 35 36 37 38 39 40
550-558 558-566 566-574 574-582 582-590 590-598 598-606 606-614 614-622 622-630
41 42 43 44 45 46 47 48 49 50
630-638 638-646 646-654 654-662 662-670 670-678 678-686 686-694 694-702 702-710
51 52 53 54 55 56 57 58 59 60
710-718 718-726 726-734 734-742 742-750 750-758 758-766 766-774 774-782 782-790
61 62 63 64 65 66 67 68 69 70
790-798 798-806 806-814 814-822 822-830 830-838 838-846 846-854 854-861 863-865

Channels 21 - 37, 39 - 60

 - Programme Making & Special Events (PMSE) is interleaved with Digital TV transmissions throughout the UK.
Channel 38  - Used for PMSE on frequencies  606.500MHz - 613.500MHz. - no change. Licencable for mobile radio mics in most of UK
Channels 61 - 69  - Cleared spectrum - no access allowed. However, see EU Implementing Decision (2014/641/EU) which will give access to 823 - 832 for PMSE
Channel 70  - De-regulated free access for wireless microphones, IEMs - no change

Your queries
(selected email correspondence)

Dr Know's Answers
I am a sound recordist and I record sound from small to medium film productions. I am planning to buy a Sennheisser G3 EW112 but first I would like to know if this product:

is this compatible with the range  GB (Ch38 2012 compliant) , also  in order to use the radio system in the UK do I need a licence from: ???.

Also I want to know if I can use this microphone in other European countries for example: Germany or Spain - I mean if I buy a GB CH38 System can I use it outside of the UK for example other countries in Europe ?.

I know that each system is tuned to a channel when you buy it, will that mean that if I buy it with CH38 I won't be able to use legal frequencies in other parts of Europe?

I know that  Band E channel 70 is still free to use in Europe, but I heard that the range is quite tiny and is over-used making it difficult finding an available frequency, so I am not sure is a good idea to buy a Band E system.

Please let me know if you can help me, Thanks! Luis (10/08/2013)

Dr. Know's reply:  Yes the Sennheiser EW100 system operates on either channel 70 (free) or channel 38 which requires a licence from JFMG (2016: now OFCOM). It is not possible to get a system which can operate on both channel 38 and 70 because the frequencies are too far apart. We do not know if channel 38 is legal for use in other countries. You will need to contact the broadcast regulator in each country to find out.

I hope this is helpful to you.

(2016 update): JFMG has now been replaced by OFCOM for licensing wireless systems.


"Hi There, I am looking at purchasing a Shure PGX 24 but it is American – yes an e.bay special!! I can see that 2 of the American frequencies in this country are assigned to Digital TV Channels 35 & 36. The other one is L5: 644.000 to 662.000 MHz. Which at the mo is awarded to PMSE licence until 2018. So - will the mic work if I buy it??? Any advice please?? Thanks T" (31/03/2011)
Dr. Know's reply: Thank you for your recent email enquiry. You have two issues here:
1. Will they actually work? They are on UK channels 42 and 45 so will be amongst high power Digital TV transmissions. In theory there is vacant space between the digital channels which may be usable for agile wireless kits (ie tuneable or selectable frequencies) but I do not have any experience of trying this yet as kit on these frequencies is not common. It sounds like these two units are fixed frequency units, in which case I would be very surprised if they don’t pick up DTV interference. The chances of them just falling in usable spaces by luck is a bit of a long shot.
2. Licensing. You will need to talk to JFMG (2016: now OFCOM) to see if its possible to get a license. Licenses may be site-dependent or site-limited. JFMG (2016: now OFCOM) should be able to advise you on both issues. Good luck.
"I'm looking to purchase a set of five radio mic's for the brass section of our band and have become enmessed in this whole question of radio frequency changes.
We play all over the country (location seems to be an issue).
Can you explain what's going on and give me any advice as to how to move forward please.
O. C. Oct 2010


Dr. Know's reply: Channel 70 is currently usable UK wide without licence and free of charge and will continue to be available without licence and free of charge after digital TV switchover in 2012. Normally a maximum of four mics can be used simultaneously on Channel 70. However, Sennheiser have just introduced a version of the Evolution G3 Series which, uniquely, could run up to a maximum of six kits simultaneously on Channel 70 by using tighter bandwidths. Please note that four kits are guaranteed to work together but there is no guarantee that six kits will always work in all locations. 

Channel 69 is currently usable UK wide with a licence at a cost of £85 for the whole Channel 69 block per year and will not be available after digital TV switchover in 2012. Normally a maximum of twelve mics can be used simultaneously on Channel 69. 

The replacement for Channel 69 is Channel 38 which is just beginning to become available in many parts of UK but NOT UK wide. Channel 38 will require a licence at a cost of £85 per year for the whole Channel 38 block and will be usable after digital TV switchover in 2012. Theoretically, a maximum of twelve mics can be used simultaneously on Channel 38.

Some additional frequencies in Channels 39, 40, 41 and 42 will be made available in parts of the UK where Channel 38 is not available but this will need to be negotiated with JFMG (2016: now OFCOM) who manage the radio spectrum for Programme Makers & Special Events (PMSE) users and administer the licences.

"Hello there Doctor Know! Tyron here from South Africa, I’ve been searching the internet for a few hours now looking for some answers to a radio/UHF mic problem I’m being faced with.

I recently bought my first set of wireless microphones (I bought 2 sets in fact). I’ve had them for about 3 months now. They are manufactured by Audix, I’m sure you are familiar with the increasingly popular Audix brand of microphones.

Since purchased, I have been hiring them to a theatre production running into its 8th week now. Last week I got a call from the producer of the show telling me that the one RECIEVER has just gone into “MUTE” mode.

Of course, I went through all the necessary quick troubleshooting procedures a good few times on the TRANSMITTER, and nothing worked (changing frequencies, batteries, resetting etc.) the receivers just showed MUTE. At the same time, the RECIEVERS weren’t getting any RF signal at all – which would explain why the receivers go into MUTE mode.

In your experienced opinion – what are the common flaws in transmitters that cause them to fail? I haven’t been present on the set of the production everyday, so I am not sure where the mics have been stored etc. however, I don’t believe they were purposefully badly treated, but they could have been – I mean – it is 8 weeks.

How do the transmitters transmit? What components are inside the transmitters? Which of these would cause the RF signal to fail?

Sorry for all the questions, the local repair guys here haven’t been very helpful with regards to info – and I found your super informative site and thought “here’s someone who can help me out finally!”

I am keen to know more about wireless tech & construction as I wanna invest in some more in the future (and decent ones at that!). So knowing what makes them “stop working” would be of great help to me!

Thank you so much for your time Dr Know!!"
Dr. Know's reply: Check our handy radio mic advice and FAQs at for lots of useful hints and tips about wireless mics.

Assuming that the receiver is working OK (check squelch/mute sensitivity, receiving antennas) and the transmitter battery is OK (the transmitter power light comes on?), the most likely reason for your system not transmitting would be a broken transmitter antenna. The floppy wire type tend to break at the point where the wire exits the beltpack (hard-wired type) or connector (plug-in type). Confirm by gently pulling the antenna wire to see if it stretches. If it does stretch, that will confirm that the internal conductor has broken and the antenna needs to be replaced. Alternatively, if it is a connector type antenna, you can swap the antenna with the second system to confirm the fault.

If the battery and antenna are both OK, then you probably have an electronic fault in the transmitter pack which will require specialist service or possibly a manufacturer service. Generally wireless transmitters are not user-serviceable.




"Our church uses some of your equipment... Recently the VHF wireless Belt Pack quit working....(Beyer Dynamic TSP - VHS Wireless Belt Pack ---  serial # 13903). We're trying to determine if it is the "unit" itself  problem or the connecting "wire" from the belt pack to the microphone. Do you have any suggestions about how to correct and/or fix ?
Thanks, B.K."
Dr. Know's reply: If the transmitter is working properly (red LED indicator on), the receiver should display an indicator to show the presence of RF. If you wave the transmitter around or rotate its orientation you should see the receiver indicators flip between the two receivers. So, if your transmitter is talking to the receiver and there is still no audio, then it is most likely that the mic cable is broken. There is always the remote possibility that the mic capsule is dead, but thankfully this is much more rare. Re-attaching the four pin Lemo connector is a specialist job. If you send us the microphone and transmitter we can do this for you.
"Dear Dr Know, On EBay I am currently watching an AKG  HT40 Mic Fq - 863.100 MHz but the receiver is a SR40 Receiver - 858.200 MHz. Will they ever talk to one another? Is it possible to change frequencies of mics or receivers without a thorugh understanding of audio technology?  It also looks as though the receiver falls into the licensed bracket?
We already have two older Shure SM58s in our duo and mine is beginning to get a bit shabby (battery terminals and antenna are loosening) would I have to replace Mic and receiver or can I get another 2nd hand mic and would it have to match the 173.800 frequency of the receiver? Yours with apologies for my naivity I'm somewhat of a beginner with microphones etc.. We're an opera duo and hope we're using the right kit! D.C."
Dr. Know's reply: Briefly, no they won’t work together. The HT40 is on channel 70 so it is legal and free to use. The SR40 is on channel 69 so can only be used with a licence and will be unusable and illegal after 2012. Some of the high quality (more expensive) wireless systems are designed to be factory re-tunable but these budget systems are not re-tunable. In theory you could use any other transmitter on 173.800MHz and it will talk to your existing receiver but, if it is a different model/make, it may not sound great if the compander circuits are not compatible. You may get lucky and find a matching one which works OK. The older VHF systems on 173.800 – 175.000 MHz are still legal and free to use in UK (June 2010).
"Dear Dr. Know

We are an amateur theatre group and own two Sennheiser EM 1031-V VHF True diversity radio mics. These have six adjustable frequencies, four of which fall into the deregulated band and are free, but two fall into the regulated band. Unless we expand and purchase more we won't be using the two regulated frequencies. Do we need a license? Would be interested in your opinions.                       A C"

Dr. Know's reply: No, you do not need a license in the situation you describe. Frequencies 173.800, 174.100, 174.500 and 175.000 mHz are free to use anywhere in UK, subject to the equipment conforming to type approval MPT1345 or 1311. Your Sennheiser 1031V system does conform to this type. Your 176.400 and 177.000 mHz channels may be used at any fixed site venue which holds a license for those frequencies i.e. it is the site which requires the approval and not the equipment. You cannot get a license to use them at different locations. In order to add more channels in the future, you will have to use high band VHF or UHF units (see above for frequency choices and equipment specifications). These will require a license (except the newly de-regulated UHF band) but you would be able to use them almost anywhere in UK. Meantime, it is perfectly legal and OK to own this equipment so long as you do not transmit on 176.400 or 177.000 mHz
"Question: As a general rule, do these type of radio microphones use FM or AM? I also (regretfully) bought a very cheap (made in China) radio mic for less than £20. I think it works on freq just above the FM band. The receiver is very noisy; in fact too noisy to use - so I've effectively wasted £20!!! Another question:- What is the significant difference between these "cheap" mics and the more expensive ones? Is it a question of poor IF design or PLL detector, companding circuitry, etc - or just plain all round cheapness to cut costs?    J.B." Dr. Know's reply: The answer is FM. Please check above for legal UK frequencies. Only those shown may be used in UK. Other frequencies are allocated (and licensed) to broadcasters, military, police, taxis, etc.
Unfortunately, I agree that you have probably wasted your £20
You wouldn't expect a £20 alarm clock to sound very good so why would a radio mic? The ancient rule of "You get what you pay for" applies here.
Consider the mic capsule - bear in mind that a decent wired hand held or tie mic costs £100+ 
There are so many factors which affect the radio section eg: transmission signal strength, frequency stability, compression. The receiver must also have precise sensitivity, frequency stability and compression. Broadcast radio mics can cost up to £3000 - this reflects the quality and reliability of the radio link.
"Dear Dr., Thanks for a very informative and helpful section on radio microphones. I have suffered almost all of the problems you describe and tend to avoid using them whenever possible, having been let down badly during performances, by sudden outbursts of loud hissing noises, cutting off signal etc., even after flawless rehearsals, with the same equipment, people and positions. I would like to ask you for a bit of clarification on one point which you did mention. Our kit consists of two hand held Audio Technica mics, 174.5 and 175.0. We also have two Audio Technica lapel mics, 174.1 and 175.0. They all have their own antenna/receivers. Does a "system" as you describe, mean the transmitter/receiver pair or just one frequency? What I am trying to find out is if I can use the pair of 175.0's at the same time.?

Another question I have is about the positioning of of the receivers, which I have normally crowded round my mixing desk. Do they interfere with each other?
Thanks for a very helpful site.  J. M."

Dr. Know's reply: We refer to one system or one kit as a complete mic/transmitter/receiver/antenna on the same frequency.
You can only use one kit on each frequency ie you can only use one kit on 175.00MHz.
Which model number are the AT kits?
We have an AT reference chart which shows which frequencies will work together without interfering.
We always try to persuade users to only use radio mics when no other alternative is viable, because they introduce an extra degree of unreliability to the overall system.
A wire is much more reliable and predictable.

If you have an antenna distribution amplifier, you can stack all the receivers together and use only one pair of aerials.
Otherwise, if you use the individual antennae on each receiver, it is better to place them at least one metre apart from each other (and from anything else). ie ideally 1 metre off the floor, 1 metre away from any wall or solid object.
Keep receivers well away from anything metal or electrical - such as dimmers, amps, heating, refrigeration, even your mixer/FOH position.
If you put the receivers onstage, make sure they are at least 1 metre away from any actor wearing a mic (avoid walkways).
If you have diversity receivers (ie with 2 antennae each) set the antennae at about 90 degrees to each other.

"Dear Dr Know. Help! I'm very worried about these rumours about my radio mics being made illegal. What will become of our school's end of term show if we can't use radio mics? J.M." Dr. Know's reply: Don't panic. OFCOM are still considering how to allocate the current radio mic bands and the rest of the freed up radio spectrum following the UK's switch over to digital TV which starts in October 2007. We've put together a separate information page explaining more about the Digital Dividend Review (DDR) and its potential impact on all UK radio mic users - and what you can do about it.
"Hello, I just discovered your site and I think it is great. Finally a place where questions get an answer. I have to teach some people how to work with wireless systems and I do not seem to be able to declare in simple words the meaning off squelch and why it is needed. Please help me Thanks J. G. Belgium" Dr. Know's reply: The output of a receiver is muted by a squelch circuit when the received signal falls too low for reception. Some units have preset mute levels, others have manual settings. It should be set so that the receiver mutes when the transmitter is switched off, or goes out of range, before the signal is swamped with noise or interference. Full instructions for setting up the optimum squelch setting.
"I have just purchased brand new channel 38 microphones and purchased a whole set of frequencies for use on our permanent site. GREAT! PROGRESS! I have plugged a t/c and h/h into the mixer in one of our theatres, eq’d and both are sounding fine and no interference etc etc. however when I turn off the h/h transmitter I start to get a load of interference/crackling type noise coming through the h/h channel, when I turn down the fader it disappears and when I move it back up it is still there. If I turn the h/h transmitter back on the noise goes away. Why is this? Is there some interference coming through the channel from the t/c? is the channel on the desk a bit faulty (it is an old desk)? I know it cannot be the frequency as I have purposely put the frequencies apart as I knew they would be working in the same environment, the frequencies are as follows: 614.150 & 617.550.

Not an ideal situation as I need to be able to turn the h/h off, have the fader up and not get noise coming through the p.a. please note this channel on the mixer has been working fine."
Dr. Know's reply: The issue is that the receiver needs to Mute when there is no transmission signal. There should be a Squelch or Mute sensitivity adjustment on the receiver. Adjust this up very gently until the noise stops. Do not adjust it further up as it will reduce operational sensitivity even when the transmitter is switched back on. Full instructions for setting up the optimum squelch setting.

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Related sites: 

JFMG UK radio mic licensing and radio frequency information.  Check availability by postcode

BEIRG UK radio mic users campaign group

OFCOM UK telecommunications regulator

Audio-Technica's list of products which can be modified for ch38

Third Party links:   5.8 GHz Wireless Headphones