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Illuminating The Dark Art: A Practical Step-By-Step Guide To Success With Wireless/RF

When approached systematically, managing RF for wireless microphone and IEM systems can actually be quite enjoyable in a nerdy kind of way!

On larger, complex shows and festivals, there’s usually a dedicated RF tech setting up and managing wireless systems, but on many tours it falls to the monitor engineer. While RF often gets referred to as a “dark art,” in truth it’s a science which, approached systematically, can actually be quite enjoyable in a nerdy kind of way!

While I’m by no means a mega RF boffin, in my 20-plus years of touring I’ve developed a practical step-by-step approach that has served me well and which I hope will help you too.

Getting Started

Use RF only where necessary. Some band members/performers need to be completely mobile and so wireless in-ear monitor (IEM) and/or microphone systems are a must. But consider who needs them – some musicians such as drummers and keyboard players probably won’t need to move, so they can be outfitted with a hardwired IEM pack. Check with them before assuming – there might be some acoustic part of the show where they move downstage and actually do need to be mobile after all – but don’t use RF unless it’s necessary.

Check what’s required where you’re going. Some countries require the purchase of licences for the specific frequencies you want to use on the dates you’ll be there; some have frequency bands which are banned from use; others have large unusable areas of the spectrum which are blocked out for TV and phones; and in some places it’s an unregulated Wild West. Check with the local promoter about the rules where you’re going, and/or this information is usually easily accessible online – forewarned is forearmed, and choose frequency bands and book licenses accordingly.

Make sure you have the right tools for the job. Just because a transmitter and receiver from different manufacturers are in the same frequency range and appear to work together, it doesn’t mean they’re compatible.

Compansion (compression > expansion) is the process by which a signal is compressed before transmission, and then re-expanded in the receiver. It’s important for the compansion circuitry in a system to be compatible with its “other half” for optimum performance and signal-to-noise ratio, so make sure transmitters and receivers are designed to be used together.

The broader the range of frequencies that systems can be tuned to, the more options once you hit the road, and in these days of governments selling off the airwaves to digital TV and mobile phone providers, you do need options. Bear in mind that an antenna combiner is a crucial part of the chain – if it dies, you lose everything going into it. It’s not common but it does happen, and it’s a potential show stopper, so consider carrying a spare that sits directly above or below the active unit in the rack that can be quickly swapped in if needed.

Get the right personal tools. I highly recommend investing in a handheld scanner if you tour and use RF regularly. In some places you will switch on your receivers to see a hot mess of RF coming from who knows where, and without a visual of what’s going on in the spectrum you’re flying blind. I’ve had my little scanner in my fly-kit for years and it’s saved me on more occasions than I can count.

Lay the groundwork. Systems have a number of variables such as gain/sensitivity, squelch, pilot tone, EQ boosts, limiter, mono/stereo and so on. Set all of these variables the same, and if you plan to use the sync function for fast tuning of mics and IEM packs, ensure that you’ve chosen exactly what parameters you want to include in the sync.

Squelch is a muting mechanism that silences the audio output of a receiver should an erroneous signal cut across it. This is a good thing – that signal can be a lot louder than the desired one (i.e., the IEM mix) and can give the listener a nasty blast of noise.

Set the squelch low enough to allow the desired signal through but high enough to keep out the uninvited. I find that 7 to 11 dB is a good all-rounder – if you set it too high, the desired signal will also be muted more easily when your artist moves further away.

Pilot tone is a “secret code” embedded into a radio signal, which takes the form of a tone inaudible to the human ear. The wireless receiver must ‘hear’ the pilot tone in the transmitter signal before allowing it to pass through the squelch controls. In this way the pilot tone prevents the receiver from passing unwanted audio and noise. (Be aware that if you’re transmitting in mono, a pilot tone will not function and you need to switch it off or the system won’t pass audio).

Antenna Selection & Placement

Choose the right antennas. Do your homework and know the polar coverage of your chosen antennas, so you can keep that in mind when you position them. Use directional paddles instead of “twigs” for wireless mics, and if they’re active, it’s generally best to set them on the lower gain. (Higher gain means they pick up over a greater area, but they pick everything up, not just the frequencies you want, and 3dB is ample for most stage applications.)

It’s been my experience that helical or “bubble” antennas with IEM systems offer superior performance to a paddle due to the higher gain and reduced dropouts characteristic of its construction.

Minimize connections. Every connector in the path of an antenna cable results in some RF signal loss. Avoid extending RF cables, and use adaptors and panels only when absolutely necessary. RF signals always prefer traveling through air rather than cable, so keep cable lengths to a functional minimum – never use a 30-foot cable if a 10-foot cable will reach. If you need more than 30 feet, reassess the positioning of your racks to see if you can get them closer

Maintain direct line-of-sight between transmitters and receivers. An antenna that’s tucked around a corner and can’t ‘see’ the stage won’t do its job well, and keeping artist IEM bodypack antennas on the outside of their clothing is good practice where possible. You may have to negotiate with the wardrobe department if it’s a costume-heavy show, but it’s very normal for them to make a little fabric pouch for the pack to sit in.

Use the right cables. It’s easy to mistake a BNC cable that’s intended for the back of a desk (i.e., MADI) for an RF cable, as they have the same connectors – but they have different characteristic impedance and must be kept separate. RF uses 50-ohm cable, digital data uses 75-ohm. It’s also worth using a specific low-loss cable such as RG-213 with N-type connectors for IEMs – they’re thicker than standard cables and BNC connectors, and lose a smaller amount of RF signal – especially useful in circumstances where you have no choice but to run longer cables.

Get high. Height is our friend when it comes to antenna placement, so take stands up to their fullest extension. Diversity receiver paddles for radio mics can be close to each other – a minimum of 1/2 wavelength is good practice – the wavelength for a 700 MHz signal is about 40 centimeters (15 inches), so a wide T-bar on a single stand is fine. Keep some distance between receiver paddles and your IEM transmitter antenna though – I usually put my IEM antenna up high near the downstage edge of my desk and the receiver paddles at the upstage side.

Tuning Up

When I arrive at a gig but my gear is still on the truck, I use this time to have a look at the frequency ranges of my equipment on my handheld scanner to see what’s in store. Then once I’ve got everything set up and switched on, I’m ready to tune my frequencies. Here’s the rest of my process:

– If we’ve booked frequencies through a licensing body or the promoter has given me a list, I’ll use those; if I have free rein, then I can choose frequencies through comprehensive RF coordination software such as Sennheiser Wireless Systems Manager or Shure Wireless Workbench – or make use of the banks of compatible frequencies within the system by using the scan function of the receivers and seeing which bank reads the most clear frequencies.

– I then set the receivers and transmitters accordingly, and then switch all of the transmitters off and look at the receivers. The pre set-up can has already provided me with a pretty good idea what to expect, but if everything is quiet (i.e., receiving no RF at all) then I know I’m off to a good start – there’s no local interference in the frequencies I’m already tuned to.

– I switch on all of the transmitters back on and spread them out (both handheld mics and bodypacks) along the top of an adjacent flight case. I make sure the mic “tails” (where the transmit antennas are located) are pointing away from each other – if they’re all in a pile it can create proximity intermodulation that will totally cloud the next step.

– Starting with the IEM systems, I switch off the first transmitter and see if all RF signal disappears from the associated pack. If it does, happy days, it’s clear; I switch the transmitter back on and do the second one, and repeat the process for every transmit/receive pair.

If any receiver is not completely “quiet” (i.e., interference and intermodulation free when its own transmitter is switched off and everything else is switched on), I set it to one side and come back to it at the end – if I have to retune anything then I’ll repeat this process until everything is quiet.

– Next I do the same with the mic systems, switching off each transmitter in turn and making the sure the receiver goes quiet. This is the most time-consuming process of the day, but the time spent here reaps dividends later – it’s vital to not only be using clear frequencies, but that all of those frequencies play nicely together.

(Some frequencies interfere with one another even though they are miles apart in the spectrum, in a phenomenon called intermodulation; and the more frequencies you use, the greater the potential for this to happen).

– Once all of this is set, I send pink noise to the left then right of each bodypack in turn (shows up any patch boo-boos and lets me know that there are no hums on one side or the other) and line check the mics. Then it’s time to…

– Walk the room. I test out the end user’s experience before I hand them their RF equipment

– I walk the performance space with their pack (not a PFL pack on engineer mode – that won’t tell me if there’s anything wrong with their hardware) and talk to myself in the mic the whole time. This way I’ll experience any problems for myself and have time to fix them before the band walks on stage.

Common Downfalls

Be aware of the effect that LED screens have on RF. They transmit low-level interference, so you may need to play around with optimum antenna placement. If there’s just a single backdrop screen it shouldn’t be too bad, but if it’s an entire stage made from LED screen, as on one tour I worked, it may be time to enlist the help of an RF expert.

Be aware that RF hates metal. Wireless gear does not play well with metal hardware, so keep packs off of metal belts, costume parts, and the like, and also make sure antennas aren’t resting on metal walls or truss – ideally they’re located at least 3 feet away from them. It’s all to do with an interesting phenomenon called the Faraday effect.

Plan for the worst-case scenario. I always have a couple of spare bodypacks on dedicated frequencies ready to go, and I also carry several extra hardwired packs. A couple of them are always patched into the system, ready to go if needed. It’s imperfect if the performer needs to move around, but in an emergency it will get her/him through the gig.

Another option when facing an unworkable RF nightmare is to go mono with the IEM systems – again, deeply imperfect, but a mono signal is much less susceptible to interference than stereo as well as throwing further, so if you’re truly stuck it can be a “the show must go on” option.

Although this is a simple guide that is adequate for most monitor applications, RF is a highly complex and fascinating subject, and it’s always great to increase your knowledge. There’s a wealth of additional resources online at sites like ProSoundWeb, I also highly recommend attending the training offered by manufacturers like Sennheiser and Shure.

As with everything about touring, be practical and systematic in your approach to wireless and you’re well on the way to having an enjoyable and trouble-free gig!

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