Wireless & RF Systems

Introduction

Wireless is the only part of an audio system that fails in front of the audience. A bad EQ decision is a matter of taste. A dropout in the middle of a keynote is a hole in the show that everyone in the room notices, and it is remembered long after anything else you did.

It is also the part of the job most often handled by hope. Turn it on, it works in the shop, assume it will work in the venue. Then the venue turns out to contain a television transmitter, forty delegates with phones, an LED wall, and another production company's radio rack.

Why this book has frequencies in it

Wireless is not really an audio discipline. It is radio, and radio is arithmetic. Which frequencies are legal, how far apart channels must sit, what two transmitters do to each other when they mix, how much signal survives a length of coaxial cable, how long an antenna needs to be. None of it is guesswork, and all of it is learnable.

So this book gives you the actual figures. Not so you can memorise them, but so that the reasoning becomes visible and you can apply it to whatever equipment and whatever country you find yourself in.

How to Use This Book

Each chapter follows the same shape, and the shape matters more than it might look. You read the ideas, then you see them play out in a real situation, then you get the procedure, then you are shown what goes wrong and how to recognise it early. Reading only the parts that look new is the fastest way to arrive on a job holding half of something.

1. Read the chapter body first. This is the understanding layer, and it is written to be read straight through.
2. Read On the job. The same material as a real situation on a real show, which is where it stops being abstract.
3. Work through How to do it. These are procedures you can follow on a job this week.
4. Study What goes wrong. Experienced technicians are not people who never make mistakes; they are people who recognise mistakes early.
5. Do the exercise before reading the answer key. Recognition feels like learning and is not.
6. Take the checklist with you. Photograph it on your phone. Use it until you no longer need it.

Keep a notebook

One habit is worth starting today, before the first chapter: keep a small notebook and write the technical details in it as you meet them. Wireless runs on records — which frequency is on which channel, which pack is on which performer, what coordination gave you in this building last time, which batteries went in and when. A radio problem is almost always solved by knowing what you set, and memory is not a record.

Write it by hand rather than photographing this page. Copying a number puts it on paper; rewriting it in your own words is what puts it in your head, and deciding how to word it is most of the understanding.

Then use it twice. Read a few pages on the way to a job, which takes five minutes and turns things you half-know into things you know. And carry it on the job, because the number you cannot remember is always the one somebody is waiting for.

A note on the words

RF vocabulary is inconsistent and you should expect that rather than be caught by it. A channel means a numbered slot on a receiver to one person and a television broadcast band to another, and the difference matters when somebody tells you which channels are clear. Squelch is a threshold on some receivers and a whole mute circuit on others. Coordination means the software, the calculation and the piece of paper that comes out of it, depending on who is speaking.

None of that is worth arguing about. What matters is that you can say what a setting does, because a person who can describe the function will be understood on any system, and a person who only knows one brand's menu will not.

A note on safety

The hazard that belongs to this part of the job is hearing, and it is somebody else's. An in-ear mix sits inside a performer's head with nothing between it and them, so limiting the output is a safety matter rather than a preference: a radio dropout or a patching mistake can arrive at full level with no warning and nowhere for it to go.

Antennas also go up, and racks carry mains power. This book gives you awareness, not authorisation. If you are ever unsure whether something is yours to touch, the answer is to ask.

Where this book sits

This book assumes you have read Audio Basics, or that you already know what is in it.

Experience counts instead. If you have experience on audio crews where you already know what mic level is and what a stage box does, start here.

If none of that is true yet, read Audio Basics first.

What pairs with this

• Front-of-House Mixing — the desk end of the same signal

How Wireless Audio Works

A radio microphone is a small transmitter carrying an audio signal on a carrier wave, and a receiver tuned to that wave recovering it. Everything that goes wrong with wireless is explained by one of three things: the wave does not arrive, something else is on the same wave, or two waves have combined to make a third.

Wireless replaces a cable with a radio link. Understanding that link explains everything that can go right or wrong.

RF and the wireless link

Wireless audio sends sound over radio instead of down a cable. In a wireless microphone, a transmitter (in the handheld or a bodypack) converts the audio into a radio frequency (RF) signal and broadcasts it; a receiver picks up that RF signal through an antenna and converts it back into audio for the console. The audio still flows source to destination as always, but one link in the chain is now an invisible radio signal traveling through the air — and that invisible link is where the special knowledge of this course lives.

RF behaves differently from a cable. A cable reliably carries signal from end to end; a radio signal weakens with distance, can be blocked by bodies and walls, and shares the airwaves with countless other RF sources that can interfere. This is why wireless is less inherently reliable than wired and demands real expertise to make dependable. The whole craft of RF is managing this invisible, shared, fragile link so it performs like a cable.

Frequencies and the RF spectrum

Radio signals occupy frequencies, measured in megahertz (MHz) or gigahertz (GHz), and wireless audio operates in particular frequency bands set aside (and regulated) for it. Every wireless device must be tuned to a specific frequency, and the airwaves are a shared, finite space — many devices (your wireless gear, other productions, broadcast TV, Wi-Fi, phones) all compete for room in the spectrum. Crucially, regulations govern which frequencies you may use, and these vary by country and change over time, so using legal frequencies is part of the job.

Because the spectrum is shared and finite, you cannot just turn wireless gear on and hope. Two devices on the same or overlapping frequencies interfere and both fail. This is the root reason for frequency coordination, which is its own chapter: deliberately assigning each device a clear frequency so they coexist. Understanding that RF is a crowded, regulated, shared space is what makes the rest of wireless make sense.

Wireless audio replaces a cable with a radio link: a transmitter converts audio to an RF signal, a receiver picks it up via an antenna and converts it back. Unlike a cable, RF weakens with distance, is blocked by bodies/walls, and shares crowded, regulated airwaves measured in MHz/GHz. Every device needs a specific legal frequency, and because the spectrum is shared and finite, devices must be coordinated so they don't interfere.

What actually travels, and what it costs

A wireless microphone is a radio station with a very small audience. The transmitter takes the audio, uses it to modify a carrier wave at the frequency you set, and radiates that from an antenna. The receiver listens on the same frequency, extracts the audio again, and hands it to the desk at line level. That is the whole chain, and every fault you will ever chase is somewhere in it.

What matters on a job is what the link costs you compared with a cable. It costs a battery, which is a countdown. It costs a frequency, which somebody has to choose and which somebody else may already be using. It costs a small delay — a few milliseconds on a digital system, and audible if you also have the same source on a cable. And it costs certainty: a cable either works or does not, while a radio link degrades, and degrades differently depending on where the performer is standing.

Analog and digital systems trade that certainty differently. An analog link at the edge of its range gets noisy — hiss creeping up under the voice — which is unpleasant but gives you warning. A digital link holds perfect quality right up to the point it stops, and then it is silent. Neither is better; they fail on different schedules, and knowing which one you have tells you what "it is starting to go" sounds like.

Companding, and why a wireless mic sounds like one

Analog radio cannot carry the full dynamic range of a voice, so it cheats: the transmitter compresses the signal on the way in and the receiver expands it on the way out. Compressing and expanding, shortened, is companding, and it is the reason a wireless microphone has a characteristic sound that a cabled one does not.

Done well it is nearly invisible. Done at the edge of the system's range, or with the transmitter gain set wrong, it becomes audible as pumping — the noise floor rising and falling behind the voice, most obvious in the gaps between words.

This is also why transmitter gain is not a volume control. It sets where the voice sits inside the range the companding expects. Too low and you expand the noise along with the voice; too high and the transmitter clips before the desk ever sees a level to work with, which no amount of gain at the console repairs.

Digital systems do not compand, which is most of why they sound closer to a cable. They spend the same problem elsewhere, on latency and on the cliff at the edge of coverage.

The spectrum you are allowed to use

Wireless microphones are secondary users of spectrum that belongs primarily to broadcast television. That single fact explains most of the rules: you may use the gaps, you must not interfere with the primary user, and the gaps change from city to city.

Licensing, using the US as the example

• Unlicensed operation in the UHF TV bands is generally limited to 50 mW of transmitter power.
• A Part 74 licence is available to eligible users and permits higher power, up to 250 mW in the UHF TV bands.
• Operating on an active local TV channel is not permitted, and in practice will not work anyway.
• A DTV channel occupies 6 MHz in the United States and 8 MHz in much of Europe. Stay out of occupied channels entirely.

Wavelength, and why it decides everything physical

Antenna length, antenna spacing, and how well a signal gets around obstacles are all consequences of wavelength. One formula gives you all of them.

What this tells you in practice

• Wavelength in metres equals 300 divided by the frequency in MHz.
• Higher frequencies mean shorter antennas, which is convenient, but also worse penetration through walls and bodies.
• A 2.4 GHz system is far more affected by a human body standing in the path than a 500 MHz system is.
• Diversity antennas should be separated by at least a quarter wavelength so they experience different reflections.

On the job

A conference in a hotel that you have worked in twice before. Last time you used the same eight frequencies and everything was fine, so you load the same file.

Twenty minutes into the opening session, channel three starts to crackle. By the afternoon it is unusable and channel six has joined it.

Nothing about your equipment has changed. What changed is the world outside the hotel. A local television station has come on air on a channel that was dark last year, and the ballroom's glass frontage does nothing to stop it. Your frequencies were legal and clear in March, and are neither in October.

This is the single most common wireless failure and it has one cause: trusting a saved file instead of scanning. Spectrum is a local and time-varying condition, like weather. You would not use last year's weather forecast, and a frequency plan has exactly the same shelf life.

How to do it

How to establish what spectrum is actually available in a venue.

1. Arrive with all your own transmitters switched OFF. You cannot scan for other people's signals while making your own.
2. Scan the full tuning range of your equipment using the receiver's built-in scan or a dedicated scanner.
3. Scan from the position where the receiving antennas will actually be, not from the loading dock.
4. Scan at the time of day the show runs. Television transmissions and other users vary through the day.
5. Identify the occupied blocks, then choose your frequencies in the clear gaps between them.
6. Ask the venue and any other production companies on site what they are using, and agree who owns which range.
7. Save the resulting plan with the date and the venue, and treat it as a record rather than a template.
8. Re-scan on multi-day events, because conditions change overnight.

What goes wrong

Practice it

Work these out before checking.

Wireless Mics & In-Ear Monitors

Microphones receive and in-ear monitors transmit, and putting both in the same rack without thinking about that is how a well-specified system becomes an unreliable one. This chapter is about the equipment itself and the settings that decide whether it behaves.

The two everyday wireless systems send audio in opposite directions — and both must be managed.

Wireless microphones

A wireless microphone system frees a performer from a cable. It comes as a handheld (transmitter built into the mic) or a bodypack (a transmitter worn on the body, connected to a lavalier or headset mic) sending to a receiver at the rack. The audio quality and reliability depend on a good RF link and correct gain on the transmitter, plus all the mic fundamentals from Audio Basics. Wireless mics are essential for presenters, performers, and anyone who must move, which is why they are ubiquitous and why their reliability matters so much.

Wireless mics introduce things to manage that wired mics do not: the transmitter has its own battery and gain, must be tuned to a coordinated frequency, and depends on staying in range of the antennas. Each wireless channel is a small system to set up and watch. The more wireless mics in use, the more RF coordination is needed, which is why large shows with dozens of wireless channels require serious RF expertise.

In-ear monitors and other wireless

Wireless also goes the other direction: in-ear monitor (IEM) systems send a performer's monitor mix to a bodypack receiver and earpieces they wear, so they hear themselves without stage wedges. Here the transmitter is at the rack and the receiver is on the performer — the reverse of a wireless mic. IEMs are RF systems too and must be coordinated alongside the mics, all sharing the same crowded spectrum. They also tie back to the monitor mixing from Audio Basics and Live Sound: IEMs are how those monitor mixes reach performers wirelessly.

Other wireless systems live in the same world: wireless intercom for crew comms, and IFB (interruptible foldback) for sending cues and program audio to presenters' earpieces. All of these are RF links competing for spectrum and needing coordination. The key insight is that on a modern show there may be many wireless systems — mics, IEMs, comms, IFB — all transmitting at once, and they must all be coordinated together as one RF environment.

Wireless mics free performers from cables (handheld or bodypack transmitter to a receiver), each a small system with its own battery, gain, frequency, and range to manage. In-ear monitors run the opposite way — transmitter at the rack, receiver on the performer — delivering monitor mixes wirelessly. Comms and IFB are wireless too. On a modern show many RF systems transmit at once and must all be coordinated together as one environment.

Batteries, and the only failure with a clock on it

Every other wireless fault is a maybe. A battery is a certainty with a time on it, which makes it the one failure you can actually plan away.

Use fresh alkaline or lithium rechargeable, and know which, because they behave differently as they discharge. An alkaline cell's voltage falls steadily, so a meter reading is a useful estimate of time left. Many lithium packs hold voltage almost flat and then drop, so the same meter reads healthy until it does not. A pack that shows four bars for two hours and then dies in ten minutes is not faulty; it is chemistry.

The rule that survives contact with real shows is to start every performance on a known quantity and never on yesterday's remainder. Log what went in and when. On a long day, change at a scheduled point rather than at a warning, because the warning arrives during the thing you cannot interrupt.

And carry more than you think. A pack is the cheapest item in the rack and the only one whose failure has a countdown running from the moment you switch it on.

In-ears, and the responsibility that comes with them

An in-ear monitor mix goes directly into somebody's head with nothing between it and them. There is no room to absorb a mistake and no distance to lose level over, which changes what a patching error costs.

A dropout on a wedge is a gap. A dropout on in-ears can be silence followed by full level returning at once, and a performer wearing them has no way to protect themselves. This is why limiting the output is a safety matter rather than a preference, and why the limiter is set before anybody puts them in rather than after somebody complains.

The second thing in-ears change is isolation. A performer wearing them properly cannot hear the room, which means they cannot hear the audience either. Some want ambience microphones fed into the mix for exactly that reason, and a performer who feels cut off will pull one ear out — which unbalances what they hear and usually ends with them asking for more of everything.

Treat the in-ear mix as a performance instrument rather than a utility. It is the only thing the person on stage can hear.

Transmitter settings that matter

Most wireless problems that are not spectrum problems are gain problems. A transmitter has its own gain stage, and it is the first place the signal can be ruined.

In-ear monitors are different animals

• An in-ear system transmits from the rack and receives on the performer, which is the opposite of a microphone system.
• Keep in-ear transmit antennas physically separated from microphone receive antennas, ideally several metres apart.
• In-ear transmitters are a strong local signal and will overload a nearby microphone receiver's front end.
• Coordinate in-ear frequencies together with microphone frequencies. They all live in the same spectrum and all generate intermodulation.
• Limit the output of an in-ear mix. A dropout at full level goes straight into someone's ear canal.

RF signal levels at the receiver

Receivers display incoming signal strength, usually as a bar meter and sometimes in dBm. Knowing what good looks like turns a vague worry into a measurement.

On the job

A theatre production with twelve radio microphones on performers and four in-ear packs for the band. The rack is built neatly, everything in one case, antennas on the front, and it fails intermittently from the first technical rehearsal.

The build looks tidy and is wrong. The in-ear transmit antennas are within thirty centimetres of the microphone receive antennas. Four transmitters radiating a strong local signal are sitting directly in front of twelve receivers trying to hear a 50 mW transmitter forty metres away across a stage. The receivers are not deaf, they are deafened.

The fix is physical, not electronic. Move the in-ear transmit antennas away from the receive antennas, ideally to a different position entirely, several metres apart and preferably with something between them. Get the receive antennas out of the rack and up in the air with line of sight to the stage. Nothing about the frequency plan changes and the problem disappears.

Wireless is mostly geometry.

How to do it

Preparing a wireless rack so that it behaves on site.

1. Coordinate microphones and in-ear systems together as one frequency plan, never separately.
2. Physically separate in-ear transmit antennas from microphone receive antennas by several metres.
3. Fit fresh batteries to every transmitter and pack a labelled spare set.
4. Set transmitter gain with the performer at their actual performance level, not their speaking level.
5. Set squelch a few decibels above the measured noise floor, not at maximum.
6. Label every transmitter and its matching receiver clearly, and label the performer it belongs to.
7. Walk the whole performance area with each system while watching the RF meter, including offstage.
8. Note the weakest position found, and fix it by moving antennas rather than by raising power.

What goes wrong

Practice it

Answer before checking.

Frequency Coordination

Coordination is the part people skip, and it is the part that decides whether twelve channels work together or interfere with each other. The hard truth is that transmitters do not merely coexist; they combine, and the products of that combination land on frequencies nobody chose.

The single most important wireless skill: giving every device a clear frequency so none interfere.

Why coordination is essential

Frequency coordination is the process of assigning every wireless device a frequency where it will not clash with the others or with outside RF. It is the heart of reliable wireless, because the most common cause of wireless failure is two signals fighting over the same frequency space. With a handful of channels it is manageable; with dozens, it becomes a careful technical exercise — and skipping it is the classic reason a show's wireless mysteriously falls apart.

Coordination starts by understanding your RF environment: scanning to see what frequencies are already in use (by TV stations, other productions, anything nearby) and therefore which are clear. You then assign your devices to clear frequencies. Software tools help calculate compatible sets of frequencies, especially as channel counts grow. The principle is simple even when the execution is complex: find the clear space, and put each device in its own piece of it.

Interference and intermodulation

Two kinds of clashes cause trouble. Direct interference is another signal on (or too near) your frequency — your gear, a neighboring production, or a TV broadcast. The other, subtler one is intermodulation (intermod): when multiple transmitters operate together, their frequencies can interact to create new interfering signals at other frequencies, even if each device's own frequency was clear. This is why you cannot just pick clear frequencies one at a time — you must choose a compatible set that does not produce harmful intermod, which is exactly what coordination software calculates.

The practical upshot: coordinate all your wireless together, test it all running at once, and re-check on show day because the RF environment changes (other productions arrive, TV signals differ by location). A set of frequencies that worked at one venue may not at another. Treating coordination as a deliberate, tested, location-specific step — not a one-time guess — is what separates dependable wireless from the kind that fails at the worst moment.

Frequency coordination — assigning every device a clear, compatible frequency — is the heart of reliable wireless, because the top cause of failure is signals fighting for the same space. Scan the RF environment to find clear frequencies, then assign each device its own. Beware intermodulation: multiple transmitters can create new interfering signals, so you need a compatible set, not just individually clear frequencies. Coordinate together, test all on, and re-check on show day.

How coordination is actually done

Coordination is the process of choosing a set of frequencies that will all work at the same time in one building. It is arithmetic, it is done before the day, and the reason it is not optional is that the failures it prevents are invisible until every transmitter is live.

Start with what is already there. The spectrum belongs to broadcast television first, and which channels are occupied changes from city to city. A scan of the room with a receiver or a dedicated scanner tells you what is present TODAY, in THIS building, which is the only thing that matters — a frequency that was clear last month is not evidence.

Then add what you are bringing. Every transmitter in the rig, including in-ears, including comms, including anything the video department has. A coordination that covers audio and ignores the wireless camera link is a coordination that fails when the camera switches on.

Then let software do the intermodulation arithmetic. Third-order products from every pair, fifth-order from every pair, checked against every channel you intend to use. Two channels is arithmetic you can do on paper. Ten channels is forty-five pairs and ninety third-order products, and by twenty channels it is hopeless by hand — which is why coordination software exists and why "we spread them out evenly" is not coordination.

Write the result down and use it. A coordination that lives in somebody's head is not a plan, and the number one cause of a rig that worked yesterday failing today is a pack that got tuned to something convenient by a person who did not know.

And re-run it when the rig changes. Adding one transmitter adds its products against every existing one, and the new channel is rarely the one that fails.

Spacing, and why even spacing is wrong

The instinct when choosing frequencies is to spread them evenly across the available band. It is the worst possible arrangement, and understanding why is the fastest way to understand coordination.

Evenly spaced transmitters produce intermodulation products that land exactly on other evenly spaced transmitters. If f1 and f2 are 400 kHz apart, then 2f1 minus f2 sits 400 kHz the other side of f1 — precisely where you put the next one. Every pair reinforces the same grid, so a neat plan generates the maximum possible number of collisions.

What works instead is deliberate irregularity: frequencies chosen so that no product from any pair lands on any channel in use. The resulting list looks arbitrary and is anything but.

A few working figures. Keep transmitters at least a few hundred kilohertz apart as a starting point, and keep more space between channels in the same physical area than between channels at opposite ends of the venue. Keep in-ear transmitters in a different part of the spectrum from microphone transmitters where you can, because in-ear transmitters are always on and always at full power, which makes them the loudest neighbours in the room.

And leave a guard band around anything that must not fail. The keynote speaker's microphone is not the channel to squeeze into the last available gap.

Intermodulation, the thing that catches people out

When two transmitters operate near each other, their signals mix in the non-linear parts of receivers and amplifiers, producing new signals at predictable frequencies. These products can land squarely on a channel you are using, and they appear only when both original transmitters are on, which is why the fault seems to come from nowhere.

A worked example

• Transmitter A is on 520.000 MHz. Transmitter B is on 521.000 MHz.
• 2A − B is 1040.000 minus 521.000, which equals 519.000 MHz.
• 2B − A is 1042.000 minus 520.000, which equals 522.000 MHz.
• So two transmitters 1 MHz apart have just contaminated 519.000 and 522.000 MHz.
• Put a third microphone on 519.000 and it will suffer, and only when both A and B are live.

Practical spacing and how to coordinate

You do not calculate intermodulation manually on a show day. You use the tools, and you understand enough to recognise when the tool is being ignored.

The order to do it in

• Scan the venue with your transmitters off, and import the scan into the coordination software.
• Enter every transmitter you will use, including in-ear systems and any other department's radios you know about.
• Let the software calculate a compatible set rather than choosing frequencies yourself.
• Deploy the calculated set, then verify by walking the space with everything switched on together.
• If you cannot use software, use a single manufacturer preset group and do not mix groups.

On the job

A multi-day festival. Your eight channels are coordinated, tested, and perfect on day one. On day two, a second production company arrives for a side stage with their own twelve channels, coordinated equally carefully.

By the afternoon both systems are suffering, and both engineers are certain the other is at fault. Neither is wrong and neither is right. Each plan is internally clean. Together they are twenty transmitters generating intermodulation products across each other's channels, and no amount of individual competence prevents that.

The fix is organisational rather than technical, and it must happen before anyone rigs. Someone has to own the spectrum for the whole site, divide it into ranges, and allocate a range to each production. Then each company coordinates inside its own allocation, and the two plans cannot collide.

On any site with more than one production company, ask on the first phone call who is coordinating RF. If the answer is nobody, volunteer, because the alternative is a day like the one above.

How to do it

Coordinating a multi-channel system properly.

1. Establish who owns spectrum for the whole site before anyone rigs, and get a range allocated to you.
2. Scan the venue with all your transmitters off, from the intended antenna position, at show time.
3. Import the scan into your manufacturer's coordination software.
4. Enter every transmitter in your system, microphones and in-ear packs alike.
5. Add any known third-party systems and the local television allocations.
6. Generate a compatible frequency set and deploy it to every transmitter and receiver.
7. Switch everything on together and verify, walking the whole performance area.
8. Save the plan with venue, date and time, and re-verify each subsequent show day.

What goes wrong

Practice it

Do the arithmetic before checking.

Antennas, Range & Deployment

Once the frequencies are right, everything else is geometry and cable. More range is almost never bought with more power; it is bought with better antenna placement and shorter, better cable. This is the chapter that turns a marginal system into a reliable one.

Good coordination still fails with bad antennas. Physical deployment is half of reliable RF.

Antennas and range

Antennas are how RF signals leave and reach your gear, and good antenna practice is essential for reliable range and coverage. Receivers need their antennas positioned with a clear line of sight to where the transmitters will be, away from obstructions and metal that block or reflect RF. For larger or more demanding setups, antennas are often remote (mounted out near the performance area and cabled back to the receivers) and may be directional (focused toward the stage) to improve reception where it is needed.

Most systems use diversity reception — two antennas per receiver — so that if one antenna's signal momentarily drops (as bodies and movement cause), the other maintains the link, greatly reducing dropouts. Understanding that range is limited and that obstacles, distance, and antenna placement all affect it explains why a transmitter that works fine near the rack can drop out across a large stage. Good antenna deployment is what extends and stabilizes that fragile radio link across the whole performance space.

Batteries and deployment

Wireless transmitters run on batteries, and battery management is a constant, critical discipline because a dying battery causes dropouts and dead mics at the worst moments. The professional practice is to start every show with fresh or fully charged batteries, monitor battery levels (good systems report them to the receiver), and have spares ready. Many productions use rechargeable systems and a strict charging routine. A flat battery is an entirely preventable failure, and preventing it is basic wireless professionalism.

Deploying wireless well ties it all together: coordinate the frequencies, place the antennas for coverage, fit transmitters to performers correctly (secure, with the mic placed well and gain set right), label everything so the right pack goes to the right person, and test every channel in the actual space before the show. Wireless rewards preparation more than almost any AV discipline, because once the show starts, an invisible failure is very hard to fix live. Set it up right, and it disappears into reliability.

Antennas must have clear line of sight to transmitters, away from obstructions and metal; remote and directional antennas extend coverage, and diversity reception (two antennas) reduces dropouts from movement and bodies. Range is limited by distance and obstacles, so placement matters. Batteries are critical — start fresh, monitor levels, carry spares — a flat battery is preventable. Deploy by coordinating, placing antennas, fitting transmitters, labeling, and testing every channel in the space.

Diversity, and what it actually solves

Almost every professional receiver has two antennas, and the reason is not range. It is reflections.

Radio arriving at a receiver has usually taken more than one path — straight from the transmitter, and bounced off a wall, a truss, a lighting bar, a person. Those copies arrive at slightly different times and can cancel each other, producing a null: a spot in the room where the signal almost disappears even though the transmitter is close and working. Move thirty centimetres and it is fine again.

Diversity solves it by listening twice. Two antennas in different places will not both sit in the same null, because the null is a position in space. The receiver picks whichever is healthier, moment to moment, and the performer walks through the room without knowing any of it happened.

Which is why spacing matters more than height for the second antenna. Two antennas bolted to the same rack are one antenna with extra steps — they occupy the same point in space, so they share the same nulls. A quarter of a wavelength is the minimum separation and more is better; at 600 MHz that is about twelve and a half centimetres, which is closer than most people assume and further than most racks provide.

Angle them apart as well. A pair splayed rather than parallel picks up differently polarised reflections, which is another way of not sharing a fault.

And remember what diversity does NOT fix. It does nothing about a dead battery, nothing about intermodulation, and nothing about a transmitter behind a body. It is one specific answer to one specific problem, and reaching for it as a general cure is how people spend money without changing anything.

Antenna placement

A receiving antenna needs to see the transmitter. Almost every range problem is a line-of-sight problem wearing a disguise.

Cable loss, and why long runs fail

Coaxial cable attenuates signal, and the loss increases with frequency and with length. A long run of the wrong cable can throw away more signal than your antenna placement gained.

How to think about it

• Keep coaxial runs as short as physically possible. Move the receivers towards the antennas rather than the antennas towards the receivers.
• For anything beyond about 8 metres, use low-loss cable rather than whatever is in the case.
• An inline amplifier compensates for measured cable loss. It does not add range, and it amplifies interference along with signal.
• Use an antenna distribution system to feed multiple receivers from one pair of antennas rather than filling a rack with individual whips.

On the job

An awards show in a large hall. The radio rack lives in a back-of-house room forty metres from the stage, because that is where the power and the desk space are. Antennas are the little whips on the back of each receiver, inside the rack, inside the room, behind a metal door.

Every receiver is being asked to hear a 50 mW transmitter forty metres away through a wall and a metal door. It will not work, and no frequency plan fixes it.

The answer is to move the antennas, not the rack. A pair of proper antennas on stands at the edge of the stage area, above head height, with line of sight to the performers, feeding back to the rack through low-loss coaxial cable and an antenna distribution unit. If the run is long, an inline amplifier trimmed to compensate for the measured cable loss.

Now the receivers are effectively standing at the stage while the rack stays where the power is. This one change turns more failing wireless systems into reliable ones than any other single intervention.

How to do it

Deploying antennas properly.

1. Decide where the antennas need to be based on the performance area, not on where the rack is convenient.
2. Place a diversity pair with clear line of sight, above head height, separated by at least a quarter wavelength.
3. Angle the two antennas about 45 degrees each side of vertical.
4. Keep them at least 3 metres from any transmitter and as far as practical from metal, LED walls, and motors.
5. Run low-loss coaxial cable back to an antenna distribution unit feeding all receivers.
6. Measure or calculate the cable loss and add inline amplification only to compensate for it.
7. Walk the whole performance area with a transmitter, watching the RF meter for weak spots.
8. Fix the weakest spot by moving antennas. Only when placement is optimal should you consider more power.

What goes wrong

Practice it

Answer before checking.

Troubleshooting Wireless

Wireless faults have a small number of causes and a large number of appearances, which is why they feel mysterious. Learn to sort the symptom into one of four buckets and the mystery largely disappears.

When wireless fails it's invisible and urgent. A methodical approach finds the cause fast.

Diagnosing dropouts

A dropout is a momentary or sustained loss of the wireless signal, and it has a few usual causes: interference (another signal on your frequency, or intermod), range or blocking (the transmitter too far away or obstructed by bodies, walls, or metal), and battery (weak power causing an unstable link). Diagnosing wireless uses the same systematic method from troubleshooting: check the simple things first (is the battery good, is the transmitter on and in range, is it on the right frequency), then isolate the cause. Many dropouts are simply a low battery or a transmitter that wandered out of antenna range.

Because wireless is invisible, the receivers' indicators are your eyes: most show RF signal strength, audio level, and battery, and reading these tells you whether you have an RF problem, an audio problem, or a power problem. A strong RF signal with no audio points one way; a weak, fluctuating RF signal points to range or interference. Using the receiver's information turns an invisible problem into a readable one, which is the key to fixing wireless quickly.

Prevention and the RF mindset

The deepest truth of wireless is that the best troubleshooting is prevention. A show that is properly coordinated, with well-placed antennas, fresh batteries, correctly fitted transmitters, and every channel tested in the venue, simply has very few live failures. The expertise of an RF professional is largely front-loaded: the calm of a show with flawless wireless is bought by careful preparation beforehand. When you do face a live problem, having set everything up methodically means you know your system and can find the fault fast.

Carry forward the RF mindset: respect that the radio link is invisible, shared, regulated, and fragile, and therefore demands deliberate coordination, good physical deployment, disciplined battery management, and testing. Wireless rewards the prepared and punishes the careless. Master that discipline and you become the person trusted with the wireless on important shows — one of the most valued and specialized skills in live audio.

A dropout comes from interference, range/blocking, or a weak battery; diagnose it with the systematic method — check battery, power, range, and frequency first, then isolate — using the receiver's RF, audio, and battery indicators as your eyes into the invisible link. But the best troubleshooting is prevention: proper coordination, antenna placement, fresh batteries, and testing every channel in the venue mean very few live failures. Respect the invisible, shared, fragile RF link and prepare deliberately.

Glossary

Practical sign-off

Wireless is learned by deploying it. The learner scans and coordinates several wireless channels, places antennas, fits and tests transmitters with fresh batteries, and troubleshoots a deliberate dropout. A mentor signs off when the learner can deploy and run wireless reliably.

• Scanned and coordinated several wireless channels (legal frequencies)
• Placed antennas for reliable coverage
• Fitted, labeled, and tested transmitters with fresh batteries
• Diagnosed a dropout using the receiver's indicators

A method, rather than a guess

Wireless faults are the ones people troubleshoot by swapping parts, and swapping parts is the slowest possible method. The chain is short enough to work along deliberately, and the order is chosen so each step rules out everything before it.

Start at the transmitter, because it is where the cheap failures live: battery, power switch, mute switch, gain. Complete silence on one channel is almost always here, and it takes fifteen seconds to eliminate.

Then the link itself. Noise or hiss with audio present is a gain problem — transmitter gain first, then squelch, which the book puts at five to ten decibels above the noise floor. Squelch set too low lets noise through; set too high it mutes a signal you could have used.

Then coverage. Dropouts that move with the performer are a geometry problem: line of sight, antenna height, a reflection null in a particular spot. Walk it with somebody watching the meters and the fault will show you where it lives.

Then the rest of the rig. Dropouts only when everything is live are intermodulation, and the fix is coordination rather than anything you can adjust on one channel. Dropouts near the rack are the opposite problem — a transmitter too close to a receive antenna, overloading its front end.

Five steps, and each one eliminates a class of cause rather than a single component. That is the difference between a method and a guess.

Sorting the symptom

Almost every wireless fault is one of four things. Identifying which bucket you are in takes seconds and eliminates most of the search space.

The order of checks, cheapest first

• Battery, power switch, mute switch. These cost five seconds and are the most common cause.
• Is the receiver showing RF signal? That single indicator splits the problem into radio or audio.
• If RF is present but no audio, the fault is after the radio link: audio output, cable, console channel.
• If RF is absent, the fault is the transmitter, the frequency, or the path between them.
• Only then start moving antennas or re-coordinating.

On the job

Live broadcast. Presenter's lapel microphone starts crackling thirty seconds before a segment. You have one minute.

You do not have time to diagnose, and diagnosing is the wrong instinct anyway. Look at the receiver. Is there RF? If the RF meter is solid, the radio link is fine and the problem is a connection, the lapel capsule, or the cable to the pack, so the fastest fix is to swap to the spare pack you prepared. If the RF meter is jumping, the link itself is failing and swapping the pack will not help, so you go to the handheld on the desk or the presenter's backup.

Either way you are restoring sound inside thirty seconds, and you are doing it because you decided which of two situations you were in by glancing at a single meter.

After the segment, and only after it, you find out why. That is the discipline: a show is not the place to be curious.

How to do it

Diagnosing a wireless fault when there is time to do it properly.

1. Check the transmitter first: battery level, power switch, mute switch, and that it is on the expected frequency.
2. Look at the receiver's RF meter. Present or absent splits the problem in half immediately.
3. If RF is present but audio is absent, work forward from the receiver output: cable, input, console channel, routing.
4. If RF is absent or unstable, check that transmitter and receiver are on the same frequency, then check line of sight.
5. Walk the performance area watching the RF meter, and map where the signal weakens.
6. If the fault appears only with the full system live, suspect intermodulation and re-coordinate with everything entered.
7. If the fault appears only near the rack, suspect front-end overload and increase separation.
8. Change one variable at a time, and record what you changed and what happened.

What goes wrong

Practice it

Answer before checking.

Quick Reference

Every number here is worked through somewhere in the book. They are gathered on one page because a coordination problem at doors is not the moment to go looking for them.

The numbers you will actually reach for

• Wavelength in metres equals 300 divided by frequency in MHz. At 600 MHz that is half a metre.
• Keep at least 3 metres, or 10 feet, between a transmitter and a receiving antenna.
• Keep diversity antennas at least a quarter wavelength apart, angled about 45 degrees each side of vertical.
• Third-order intermodulation products land at twice one frequency minus another.
• Typical minimum channel spacing is 250 to 400 kHz, but follow your manufacturer's figure.
• Scan at the venue, at the time of day the show happens, with the venue's own systems running.
• Use low-loss coaxial cable for any run beyond about 8 metres.

About Reboot AV

Reboot AV exists because the audiovisual industry has a map problem. The work is there and the ceiling is high, but almost nobody entering it can see the route from where they are to where they want to be.

What the gear actually looks like

Every device named in this book, in one place. Recognition is the whole point of Chapter 4 — use this page to attach a picture to each name before you meet it on a dock at six in the morning.

Audio

Dynamic microphone

Condenser microphone

Wireless handheld

Radio rack

Mixing console

Stage box / snake

Power amplifier

Main loudspeaker

Subwoofer

Stage monitor wedge

In-ear monitors

DI box

Video

Camera

Video switcher

Scaler / processor

Projector

LED wall panel

Confidence monitor

Lighting & control

Conventional fixture

Moving-head fixture

Lighting console

Dimmer

Show controller

Media server

Networking & infrastructure

Network switch

Patch bay

Equipment rack

Power distribution

Cable tester

A properly coiled cable

Connectors, close up

XLR

TRS / TS ¼-inch

SDI on BNC

HDMI

Cat5e / Cat6 (RJ45)

Fiber optic

DMX (5-pin XLR)

AC power — not signal