What Spectrum is Right for You?

Shure UK Pro Audio Group  Manager, Tuomo Tolonen walks us through the most common bands of  spectrum used for wireless microphones to help you determine what best  suits your application.

For radio microphones to work reliably, we need  access to clean spectrum - this much is well known and  documented. What's less well known, is which bands of spectrum are best  suited to each given application. Unfortunately, we can't just use any  portion of spectrum; and while in theory there's a lot of spectrum  available, the parts useful for the reliable operation of wireless  microphone or in-ear monitor systems are limited.

Why?

Different parts of spectrum have different performance  characteristics, of which, propagation is arguably the most important to  understand.

What do we mean by propagation?

In very simple terms, RF propagation refers to how well a given  frequency can travel over distance and penetrate through surfaces such  as walls or other obstacles. Strong propagation characteristics are  critical to obtaining a solid, stable signal across a wide operating  area in applications such as live performance.

Here's how it works:

One of the shared characteristics between radio frequencies and sound  is wavelength. Just like sound waves, lower frequency radio waves have  larger wavelengths while higher frequencies are smaller in wavelength.  To fully understand this, we must also understand the wave formula (C = L  x F), which we can use to calculate the wavelength of any given  frequency. Yes, we know, it's maths, but bear with us....

In the case of radio waves, we are, of course, dealing with the speed  of light, which in this case forms our constant, or the letter C in our  equation. (The speed of light is 300, 000, 000 metres per second, for  your reference). To calculate the wavelength (L), we would take our  frequency and divide it by the speed of light. See the example using  600MHz below:

300, 000, 000 ÷ 600, 000, 000 = .5 metres

In other words, one cycle of 600MHz is half a metre long, which makes  for a sizeable enough wavelength to transmit over a healthy operating  range while easily penetrating many surfaces and walls. In contrast,  if we take a higher frequency, such as 10GHz, then the wavelength would  be so short it wouldn't even be able to travel through lighter surfaces  such as curtains or thin walls. In professional applications, this level  of performance is simply too prohibitive.

Understanding UHF & VHF

Over the last 50 years, the vast majority of wireless microphones and  in-ear monitor systems have operated in the VHF and UHF bands of  spectrum. VHF stands for Very High Frequency while UHF stands for Ultra  High Frequency.

Looking at the chart below we can see how each frequency varies in  wavelength based on the wave equation we just covered. Some of the  previous systems operating in the UK occupied the VHF space (around  300MHz and below). As you can see, these systems had a nice, long  wavelength allowing for exceptional operating range and propagation;  what was missing, however, was enough quantity of spectrum to cater  for larger events. As a result, the bulk of our industry today make  wireless systems that operate in the UHF bands (from 470MHz and up).

In the UHF bands, which start at 470MHz and go up to the mid 800MHz,  we still retain excellent wavelength – only this time, there's a much  greater quantity of space from which to operate. Historically, though,  users of wireless microphones have always shared this space with  terrestrial television, which requires some coordination to avoid  interference. The operation of TV channels will vary from  place-to-place, with occasional gaps known as 'white spaces'. It's  important to remember that we operate as a secondary user – next to TV –  meaning you must programme your wireless systems into these clear white  space gaps. This arrangement makes for an operating environment where  interference is predictable; we understand where the other users are,  and we can safely coordinate a large number of channels for large  professional events.

1.8 GHz, 2.4 GHz, and DECT

While the UHF bands are the preferred operating space for  professional live events, not all applications require such a high level  of performance for vast channels of wireless systems. If your channel  counts are smaller, there are other bands available. Here's a brief  explanation of each option:

The 1.8 band (1786-1800 MHz) was recently announced in the UK as  available for wireless microphone operation. The wavelengths at these  frequencies are smaller than our ideal UHF bands, however, for many  smaller applications they remain more than adequate.

2.4 GHz is commonly used for WI-FI, making for a challenging  operating environment shared with lots of unlicensed devices.  Smart 2.4GHz systems combat the challenge of WI-FI interference with  intelligent frequency scanning and interference avoidance; Shure's GLX-D  system is a perfect example. Perhaps the greatest advantage of 2.4GHz  is that it's globally licence free, and you don't need to worry about  whether or not your system will work when touring abroad.

Finally, we have the DECT (Digital Enhanced Cordless  Telecommunications) bands. Most systems occupying this space are  designed for installed applications, again where intelligent frequency  scanning and allocation is required. The Shure Microflex Wireless system  is a great example of DECT applied to corporate conference room  applications.

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