In today’s ever-more-crowded RF environment, wireless system users need every advantage they can get to make sure their show comes off without a hit.
While wireless frequency coordination is not a new thing, I find that many in our industry are unaware of it. This article will explain what it is and what it does, and take you through some scenarios to show how you can benefit from it.
What’s the core issue? Wireless systems, specifically wireless transmitters, interact with each other. In much the same way that musical notes will combine to create overtones and undertones, wireless transmitters will combine to produce (and occupy) additional frequencies.
In its simplest form, wireless frequency coordination is a methord for calculating these additional frequencies so that they may be avoided.
While I won’t go into all of the math involved, the basic story is that two wireless transmitters will produce, below and above each frequency, new frequencies that are the same spacing as the two frequencies are apart. These new frequencies are called intermodulation (or just intermod) products.
Frequency 1 = 501.000 MHz, Frequency 2 = 502.000 MHz, Spacing = 1.0 MHz
Therefore, the intermod products will occur at 500.000 MHz and 503.000 MHz, which means that if you’re looking for two more channels, you can’t use 500.000 MHz and 503.000 MHz. Just to make life more interesting, this same thing occurs between every new and existing frequency.
So, if you bring on another system at, say, 505.000 MHz, say good-bye to: 497.000, 498.000, 499.000, 508.000 and 509.000 MHz. It adds up, and it adds up quickly. (You may have also gathered by now that it’s not a good idea to use even spacing when selecting more than two frequencies).
But these intermod products are only potential problems, because the transmitters have to be able to interact with each other to produce them. This usually means that they have to be in close proximity to each other. Does this get you off the hook? No.
Following are some scenarios of how this can bite you in real life, but for now, keep in mind that transmitters that are putting out a steady signal, like In-ear transmitters or Intercom base stations, can interact with wireless mics, guitar packs, etc. that are in the same area.
Also understand that if signals are being combined, like in an IEM system, and there are intermod products from poorly selected frequencies, you’ll be broadcasting the intermod products as well as your in-ear mixes.