Wireless microphone frequencies and what the spectrum auctions took

By Joey Calkins · Section Sound

A radio microphone is a small transmitter, and transmitters need somewhere to sit. For thirty years that somewhere was the ultra high frequency television band, which was quiet, propagated well indoors and had gaps between broadcast channels wide enough to hide in. Two rounds of spectrum policy have since taken most of it away, and the practical consequence for a production is not inconvenience but arithmetic: fewer megahertz means fewer microphones that can work at once.

Jurisdiction matters throughout this article. What follows describes the United States and the Federal Communications Commission. Allocations elsewhere differ, sometimes sharply, and equipment legal in one country is frequently illegal in another.

Two auctions, two evictions

The first came with the transition to digital television. The band above 698 MHz, known as the 700 MHz band, was reallocated to mobile services, and wireless microphones were required to stop using it in June 2010. A great deal of equipment became unusable at a stroke, and some of it is still circulating second hand.

The second was the broadcast incentive auction of 2016 and 2017, which cleared a further block. Microphone users were required to vacate the reallocated 600 MHz service band by 13 July 2020. What is left for ordinary use runs from roughly 470 to 608 MHz, with narrow slivers above it under restrictions.

These dates and boundaries come from the regulator's own rulemakings, which is the strongest kind of source available here: the body that set the rule, publishing the rule. Nothing in a manufacturer's brochure overrides it.

Why losing a third of the band costs more than a third of the channels

Frequencies cannot simply be packed shoulder to shoulder. When two transmitters operate near each other their signals mix in the front end of a receiver and produce intermodulation products at predictable arithmetic combinations of the originals. The third order products, sitting at twice one frequency minus another, land close enough to the originals to matter, and a system placed on one of them will hear interference that exists nowhere on the air.

Coordination therefore means choosing a set of frequencies where no member of the set falls on a product generated by any other pair. As the available width shrinks, the number of mutually compatible slots falls away faster than the width does. Halving the band does not halve the channel count; it does considerably worse.

The scan is the coordination

Because the band is shared and the occupants change, a frequency plan is a measurement rather than a document. A receiver sweeping the range in the room, with the house lights, dimmers, video walls and LED fixtures running, produces a different picture from the same sweep taken in an empty hall at nine in the morning. Switching power supplies in modern lighting are a common source of broadband noise across the lower UHF, and they are usually off when the quiet scan is taken.

The sweep also has to be repeated. A touring production arriving in a new town enters a different broadcast landscape entirely, and a run long enough to cross a broadcaster's own schedule change can lose a channel mid-week.

Sharing what is left

The remaining UHF is not exclusive. Television broadcasters have priority, and white space devices are permitted to use vacant channels in the same range. A frequency that was clear during the technical rehearsal is not guaranteed clear on the second Saturday of the run, which is why coordination is a scan performed in the room at the time rather than a list written once and filed.

Licensing separates two classes of user in the United States. Part 74 provides licensed operation for eligible users, and eligibility was widened in 2017 to include professional users routinely working with large numbers of microphones. Everyone else operates unlicensed under Part 15, which means no protection from interference and no recourse when it appears. A community theatre with eight channels is almost always in the second category.

The alternatives, and what each costs

Manufacturers moved into other bands as UHF contracted, and each move buys something and spends something.

The 900 MHz industrial band and 2.4 GHz are crowded with everything from telemetry to the audience's own devices, and 2.4 GHz in particular competes with every wireless network in the building. The 1.9 GHz digital cordless band is comparatively orderly and tightly regulated, which limits both interference and range. Portions of 6 GHz opened to unlicensed use in 2020 offer width, and pay for it in propagation: the higher the frequency, the less forgiving it is about walls, bodies and scenery.

Wired capsules remain the option nobody advertises. A float, a boom or a hardwired headset costs nothing in spectrum, cannot be jammed, and does not need a battery tracked through a two-act show. Where staging permits it, one wired channel is one fewer coordination problem.

What we cannot verify

Channel count claims in product literature depend entirely on assumptions about band width, filtering and permitted intermodulation performance, and those assumptions are rarely stated. They are published by the companies selling the transmitters. We do not reproduce them.

Regulation moves. The allocations described here reflect the rulemakings named above, and anyone specifying equipment should check the regulator's current position for their own country rather than rely on this or any other article. Second hand equipment sold as working may be legal to own and illegal to operate.