Wireless Spectrum Management: Part 2

In part one of Wireless Spectrum Management, we were guided through the process of setting up and implementing a spectrum management plan by Steve Caldwell, technical manager and radio frequency (RF) engineer at international high-flyers Norwest Productions since shortly after the discovery of sound.

In recognition of the certainty that no spectrum plan will survive a production unscathed, this second instalment is about making it through the event with your system (and hopefully also your sanity) intact.

WEAPONS OF CHOICE

Once all your radio devices are up and running with no interactions, it’s time to start monitoring your entire environment for those unexpected signals that can ruin your day. Steve’s weapons of choice for broadband scanning are Winradio’s G33WSM a software-defined receiver designed specifically as a spectrum monitor for sound engineering (see the sidebar), and its newer sibling the G39DDC Excelsior which has more sophisticated demodulating capabilities and an impressive sweep rate of 1GHz per second.

“I’ve had a G33WSM for almost 10 years now and actually started out with one of the prototypes. It’s been absolutely fantastic,” enthuses Steve. “I use a normal spectrum analyser and VNA (vector network analyser) for more generic tasks but this is great. It’s a fairly small box that you attach to your computer. It’s a broadband scanner that’ll scan the entire broadcast span and display a standard graph of what the spectrum looks like. More importantly, it’s capable of placing software screen markers on each of your known transmitter frequencies or entire frequency ranges, such as known TV channels to avoid.

“It’s got a maximum and minimum hold system so you can actually dial in your particular band of frequencies to scan and the maximum hold will show you all peaks that have occurred since the scan commenced, while the minimum hold will eliminate any short-duration signals. You can let the scanner run continuously, go about doing your other duties, then come back and look over the log and see straight away if there’s anything there that shouldn’t be. It’s all graphical so you can grab the mouse and scan up and down the chart very quickly.

“One of the beautiful things about this piece of equipment is that it’s a software-defined receiver, so not only will it give you an RSSI (received signal strength indication) map of the band, it will actually let you tune in and listen to any of the RF peaks that you see using your computer’s audio system. So you can listen to your radio mics and your IEMs and what have you, which is helpful, but if a rogue frequency comes up you can actually listen to it. This is one of the most effective ways to trace a rogue frequency.

SEARCH FOR STRAYS

“You can spend hours trying to locate a transmitter by direction finding but in most venues, the somewhat vague nature of directional antennas along with multi-path reflections will defeat you every time. You’re much better off putting a set of headphones on and listening to what’s being transmitted on that rogue frequency. It’s easy to hear if it’s comms or an ENG crew but the great thing about it is that if it’s nearby, you are most likely to hear your own PA system in that signal, and using the speed of sound through air, you can determine how far away the transmitter is from your PA. If you’re hearing comms it’s likely to be coming from somewhere backstage or back of house, while an ENG crew will usually be found front of house or just outside the venue. The audio you hear over that transmitter will give you the most information about its location, without a doubt.

“Be very sure that what you’re hearing is the fundamental of the rogue frequency and not an intermod product [see part one for more about intermods – Ed] from either your own radio mics or that of another authorised user. It’s very easy to confuse the audio from an intermod of an authorised radio mic with that from the fundamental of an unauthorised one. If you feel there is a possibility of the rogue signal being a product of two or more of your own transmitters, find an opportune time and switch off your transmitters one by one to see if the rogue signal disappears. Start with IEM transmitters as they usually have the highest power output and also often mix in their own combiner systems to produce intermods. Although it’s often not possible to account for all intermod products, most of the lower order ones should be accounted for in the initial solution calculation and will be avoided.