How external antennas improve signal-to-noise ratio

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Live  |  03/03/2016  |  By Alex Milne  |  Add Comment

When it comes to wireless audio signal quality, nothing is more important than maintaining excellent signal-to-noise ratio (SNR). Nothing.

Keeping signal-to-noise ratio as high as possible is the most effective (although not the only) means for reducing dropouts and interference on wireless devices of any kind. There are many ways of increasing SNR, but the use and careful placement of remote antennas may be the most straightforward and accessible.

First, let’s define SNR.

Signal-to-noise ratio is the ratio of the amplitude of the signal of interest to the amplitude of the surrounding noise floor or competing signals. SNR measurements are used across many systems and components within an audio signal chain, like microphones and amplifiers, but with wireless we mean the signal-to-noise ratio of the amplitude of a radio signal (coming from a wireless microphone or IEM transmitter) in comparison to the amplitude of surrounding radio noise when measured at the receiver front-end (antenna input).

External antennas improve SNR in one of two ways, which may benefit the user independently, or combined.

(Here we should pause to define “external antenna” or “remote antenna” as any antenna that is attached to a coaxial cable and placed at a location other than directly connected to an antenna port at the receiver or distributor, whether that cable measures three feet or three hundred.)

External antennas improve SNR through proximity.

If traveling through air, radio signals lose amplitude (signal strength) in accordance with the inverse square law; if the distance between a transmitter is doubled, the amplitude of the signal seen by the receiver will be four times less strong.

inverse_square_illustration

Since remote antennas convey radio signals through coaxial cable back to the receiver, rather than air, a greater percentage of the signal received by the antenna is returned to the receiver, and consequently the signal-to-noise ratio at the receiver is usually improved if a remote antenna is placed via coaxial cable somewhere closer to your wireless microphone or IEM receiver.

There are important considerations here. Coaxial cable does cause loss of signal strength, too, which we call “in-line attenuation” or “transmission line loss.” The amount of signal lost in a cable depends upon the type of coaxial cable in use, as well as other factors like signal frequency, connectors, and damage along the cable’s length. Industry standard 50 ohm RG8X loses about 10 dB per 100 feet, give or take depending on frequency of operation. Often, the added gain of an external antenna is able to compensate for this loss, but when coaxial cable runs exceed 100 feet a cable type with less in-line attenuation is called for, or the use of in-line amplification—although amplifiers should be used with care, as they decrease SNR by adding noise, and can easily overload a sensitive receiver front-end.

(The inconvenience of coaxial cable in-line attenuation can now be overcome by using a low-cost RF over fiber-optic conversion system, like the RF Venue OPTIX platform. RF over fiber-optic devices convert radio waves into light, and transport wireless microphone or IEM signals through single-mode fiber-optic cable, which has almost no in-line attenuation at all.)

Antennas improve SNR through directionality.

Antennas are able to shape and concentrate fields of RF energy that travel between transmitter and receiver in powerful ways. An antenna that concentrates radio energy to a high degree is said to be “directional.” A directional antenna has increased sensitivity to radio waves in one direction, and decreased sensitivity in others. Directionality is usually quantified in decibels (dB) and described as having “antenna gain.” Thus, a high gain antenna (generally, 6 dB or greater) is more directional than a low gain antenna (generally 6 dB or less), but has a more spatially specific coverage area than a low gain antenna, which picks up radio energy more equally in all directions.

The most common directional external antennas in use today are the LPDA (also known as “paddle” or “shark fin” antenna) and the helical (examples include the Shure PASSIVE paddle and RF Venue CP Beam). Both of these types of high gain antennas can be thought of as creating beams of reception or transmission that are more concentrated than with low gain antennas. When pointed at talent, they increase the signal strength of the transmitters in that direction, and decrease the strength of signals and noise in other directions—which when correctly deployed on a reasonable length of coaxial cable dramatically improves signal-to-noise ratio and thus the hardiness of a system against dropouts.

Low gain antennas which have omnidirectional coverage patterns can also be remoted (provided they’re properly grounded) to increase SNR. Antennas like the Shure UA860SWB passive omnidirectional antenna or the RF Venue Spotlight antenna are often used in very close proximity to wireless microphones  for improved SNR, and also, if the coverage pattern is omnidirectional, as with the Shure UA860SWB, or hemispherical, as with the RF Venue Spotlight, they create a “bubble” of near field coverage that tends to include the transmitters inside that bubble and exclude sources of noise and interference outside the bubble. See the video below for a handy demonstration.

(The graph below demonstrates polar-plots for the low-gain RF Spotlight (left) and the high-gain CP Beam antenna (right).

Polar-plots-for-our-low-gain-RF-Spotlight-antenna,-left,-and-high-gain-CP-Beam-antenna,-right

The Bottom Line

Many sound professionals are perfectly content with using stock antennas attached to their receivers in a rack. Often, these professionals experience little to no interference at all, and don’t see an immediate reason to begin using external antennas.

While this is a fine assumption, the reality is that properly deployed external antennas and cabling will improve the performance of nearly any system over time. That is, if you experience one or two small dropouts per performance with stock dipoles, switching to external antennas can eliminate them altogether. Furthermore, in both the UK and USA, the UHF broadcast spectrum upon which wireless microphones and in-ear monitors operate is growing increasingly scarce. This year the FCC plans to sell off all or most of the 600 MHz band to be repurposed for mobile services, and OFCOM recently confirmed plans to clear 694-790MHz by 2022 at the latest, though this change will likely happen by 2019.

Even if your system functions well now, in the future that status is likely to change. If audio professionals wish to continue using wireless audio devices in the same quantity and with the same reliability as they do now in a future where spectrum is scarce and wireless devices of all types are more prevalent than ever, they’ll have to learn to use previously unfamiliar tools—like external antennas, filtration, and spectrum analyzers and coordination programs—to maintain the satisfaction of their clients.

About Our Guest Author:

Alex Milne writes for Audio Gloss, a blog by RF venue, Inc., a US based manufacturer of innovative products that make wireless audio systems work and sound better, specialising in remote antennas, RF distribution equipment and RF signal management and monitoring systems for audio/video installations and live sound events. Shure UK carries the full line of RF Venue antennas.