All About Wireless: Waves and Spectrum
As governments around the world gradually sell off portions of the spectrum available for wireless microphone and IEM use, it is becoming increasingly important for audio engineers to have a thorough understanding of wireless microphone and IEM system design and configuration. For this reason, Shure has partnered with Entertainment Technology Asia to present a 12-part series covering RF Engineering theory and best practice. The intention is to provide readers with the knowledge required for the successful configuration and operation of their wireless systems.
The series will cover topics such as RF wave propagation; antenna types and positioning guidelines; signal distribution techniques; transmission line theory; sources of noise and noise mitigation techniques; mixing for IEM systems; analog vs. digital wireless systems; spectrum management and frequency coordination. In this first issue, we will begin by looking at what Electromagnetic waves and the Electromagnetic spectrum actually are, and why we need to modulate signals for wireless transmission.
An electromagnetic wave consists of two oscillating fields, an electric field and a magnetic field. There are two key characteristics that define the Electromagnetic wave. Firstly, there is no phase shift between the electric and magnetic field waveforms. Secondly, the polarization of the electric and magnetic fields, and the direction of propagation, are all orthogonal, or at 90° angles to each other. Additionally, electromagnetic waves do not require a medium through which to travel. Unlike acoustic waves, electromagnetic waves propagate most efficiently through the vacuum of space.
The Electromagnetic spectrum is simply the frequency range over which Electromagnetic waves are propagated. One interesting point when considering the Electromagnetic spectrum is that different frequency ranges often accommodate different types of signals, often with very different purposes. AM radio, for example, is typically broadcast from 550kHz – 1640kHz, while FM radio usually operates on frequencies from 88MHz – 108MHz. Visible light is simply electromagnetic radiation from 429THz – 750THz. The human eye is sensitive to electromagnetic radiation in this frequency range, which allows us to perceive light and color. Humans are also sensitive to the range of frequencies just below visible light in the Infrared range, which we perceive as heat.
The most commonly used frequency ranges for wireless microphone and IEM operation are the UHF band from 450MHz to 698MHz, the DECT band at 1.9GHz and the 2.4GHz band, though there may be region-specific restrictions on which parts of these ranges can be used legally. There may be additional slices of VHF or UHF spectrum available for use, such as 902MHz to 928MHz for example, but the legality of operating on these frequencies is, again, region-specific.
Operation in each of these ranges presents its own challenges. The 450MHz to 698MHz range is generally preferred for high channel count systems, or systems requiring more than about 30m operating range. The challenge here is that this range is also typically used for land mobile radio, public safety and television broadcast, the transmit power of which typically far exceeds that of wireless microphone and IEM systems. Wireless microphone and IEM systems must therefore be coordinated around these existing services, which can be challenging in locations where available spectrum is limited.
Recently, the 2.4 GHz band has attracted some interest for wireless microphone use. Potential advantages of this band include the use of smaller antennas, the lack of other high-power transmitters that may cause interference, unlicensed availability across most parts of the world, and the lack of government imposed bandwidth and modulation restrictions. Unfortunately, these same features have already attracted a considerable volume of traffic from Bluetooth and Wi-Fi devices, in addition to ISM (Industrial, Scientific, and Medical) users. At face value, the 2.4GHz spectrum sounds like an excellent alternative to the 450MHz to 698MHz range; and in certain applications, it is. However, in most countries the 450MHz to 698MHz range represents the largest contiguous block spectrum, so although special 2.4GHz modulation schemes may mitigate the risk of interference to some extent, the 450MHz to 698MHz range remains the most suitable for high channel count systems.
Now we have a basic understanding of the frequency ranges commonly used for wireless microphone and IEM operation, the first question most audio engineers ask is why – why do we need to convert an audio signal into a high frequency RF signal to transmit it wirelessly? There are two equally important reasons. Firstly, transmitting audio signals at their native frequencies, that is from 20Hz to 20kHz, is essentially impossible because the antennas required for efficient propagation would be miles in length. Secondly, even if the antenna size wasn't a limiting factor, radio frequencies transmitted at 20Hz to 20kHz would be destroyed by interference. Use of a high frequency carrier for wireless transmission allows for smaller antenna design and provides the opportunity interference avoidance.
What is Modulation?
'Modulation' is the term given to the process of superimposing baseband information onto a high frequency signal for the purpose of efficient wireless transmission. In an IEM system, for example, the transmitter takes an audio input signal, modulates this onto a high frequency carrier, then via the antenna, radiates the signal as an Electromagnetic wave. The opposite process occurs at the belt-pack receiver; the high frequency Electromagnetic wave is detected by the receive antenna and is demodulated to recover the original audio signal. There are a multitude of modulation schemes available, both analog and digital. While we won't examine the details of any particular modulation scheme as part of this tutorial series, we will look broadly at the pros and cons of each.
Next month we will dig deeper into Electromagnetic waves, defining the terms 'wavelength' and 'polarization'. We will also learn about propagation losses and look at the effects various obstacles can have on wave propagation.
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