Fundamentals of RF Coordination for Live Sound Part 2 – Antennas

There seems to be a lot of discussion on various pro audio forums about choosing antennas and how to best configure your wireless systems for optimum performance. There are descriptions of many examples of setups that have worked well for specific situations, and different opinions about general rules of thumb. Anecdotal examples of success or failure of different antenna setups are helpful, but I hope to share some of the underlying reasons different antenna setups behave the way they do and how to use that knowledge to build well performing rigs.

On its most basic level, an antenna is a transducer designed to convert electrical voltage and current on conductors into electromagnetic waves, and/or reciprocally convert electromagnetic waves into voltage and current on conductors. This reciprocity applies to all antennas, where the inherent properties of the transducer are equal for applications of transmitting energy or receiving it.

It is worth noting that there is no such thing as an “active antenna”. The term is generally used to describe an antenna+amplifier system. You should not attempt to use an antenna that has a little box with an LED power indicator on the side to transmit your IEM signals! The amplifier in that box is usually a “one way” device, intended only to amplify signals received by the antenna. It’s also unwise to depend on such an active system to try to make your receivers more sensitive to distant transmitters, but rather it should be used to overcome cable losses when very long coaxial cables are used. This is because the active stage adds noise to the system, and amplifies the entire noise floor along with desired signals. The amplified signal and noise is then attenuated by the cable loss and ends up being acceptable at the receiver. It does nothing else to improve the signal to noise ratio of the received energy.

The most essential issue to consider with antenna selection and placement is maximizing RF signal to noise ratio at the receiver, whether it is a rack mounted mic system, com base, belt pack IEM, or IFB. It is important to understand that the noise floor is influenced by RF energy in the space around the equipment, by any active (amplification) stage including electronics inside the receiving equipment, and even by undesired energy radiated outside desired carrier frequencies by your transmitters (spurs and intermodulation products).

It’s also beneficial to consider how diversity reception works to overcome multipath interference and attempt to place your antennas in a manner that assures that multiple signal paths through space do not simultaneously cause dropouts in signal strength at both antennas. Signal degradation due to multipath is unavoidable, but you are reducing the chances that it will occur simultaneously when you separate your diversity receiver antennas by at least several wavelengths of distance. When dropouts occur at one antenna, an inaudible switch to the other antenna should maintain satisfactory performance.

Maintaining unobstructed line of sight between all transmit and receive antennas is indeed one of the most effective ways to maximize the transfer of desired carrier frequency energy from your transmitters to your receivers and keep the S/N ratio acceptable. When a radio frequency signal reflects off of a surface or refracts around an object, some energy is scattered and some is absorbed, and those portions don’t reach the receiver as useful signal. The human body, full of salty water, is exceedingly good at absorbing RF energy. Metallic scenic set pieces can scatter and reflect signals, contributing to multipath interference.

Panel mounted whip antennas suffer from being located very close to many obstructions and reflective surfaces created by the equipment and rack, and are usually located well out of any line of sight path. They are also located closer to potential RF noise sources inside the rack, including power supplies, local oscillators, and DSP’s. Mounting external antennas high above the performance area will usually assure that line of sight is maintained as well as possible, and you can place them far away from active electronic noise sources.

There is a delicate balance between the antenna’s inherent gain, the length and type of cable it’s driving, the antenna’s directional characteristics, the distance between transmit and receive antennas, the transmitter’s output power, the receiver’s sensitivity, and the direction and power level of noise sources. You can use these properties to your advantage. For example, you can use websites like tvfool.com to determine which direction strong DTV transmissions are coming from at your location, and place a directional antenna oriented with its sensitive axis pointed toward your performance area and away from the DTV transmitters. Usually, directional antennas like LPDA’s or helicals have inherently higher gain than a simple omnidirectional whip, and this gain can be used to drive long low-loss coaxial cables. When properly implemented, the desired signal coming out of the cable to your receiver may be stronger than that from an omni whip mounted directly to the receiver chassis connector! You can use online path loss calculators to determine how your external antenna systems should behave with different components and distances.

You could think of a drum overhead mic as an analogy for antenna placement. The mic is sensitive to nearby reflective and absorptive surfaces that alter the drum sound, and you want to minimize bleed from the instrument amplifiers in the room. So you place the mic in free space, above the kit, away from the other noise sources, with the mic’s sensitive axis pointed toward the drums and away from the amps.

As if this all isn’t complicated enough, antenna polarization is a major factor. This is completely different than polarity or polar pattern. RF signals propagate through space as linear, circular or elliptically polarized electromagnetic waves. Technically, linear and circular polarizations are variations of elliptical, but it’s more intuitive to think of them as individual types. You might imagine that linear polarized signals travel through space the way a snake slithers along flat ground, while circular polarization “corkscrews” through space.

Polarization of signals is determined by the characteristics of the transmitting antenna. Whip and LPDA antennas are linear polarized, while helicals or spirals like “top hats”, most “dome” antennas, and many flat panel types, are circularly polarized. This is important because receiver antennas are most sensitive to signals having polarization in the same linear (or elliptical or circular) plane as the transmitter antenna. If the transmit antenna is a whip, oriented vertically, it emits a vertically oriented linear polarization. If you orient the whip horizontal, the polarization follows. Let’s use the vertical transmitter whip as our example. When you orient the receiver’s whip antenna vertically, you achieve maximum reception of the transmitter’s signal. If you rotate either antenna into a horizontal orientation, you can lose over 20dB of signal due to polarization mismatch loss! If you rotate both antennas at the same time, maximum efficiency is maintained. These conditions hold true if either the transmit or receive antenna is any other kind of linear polarized antenna such as an LPDA.

Elliptical and circular polarizations require the transmit and receive antennas to have the same “polarization sense” for maximum efficiency. Usually, this is simply the direction of the “twist” in the antenna elements, similar to the way a standard right-handed screw thread fits into a standard nut, while a left-handed screw thread (like a gas pipe fitting) only fits into a left-handed nut. The advantages of using circular polarized antennas for our mic receivers and IEM transmitters result from the fact that most of the belt packs and handheld mics out in the performance area will generally have randomly oriented linear polarizations. Circularly polarized antennas are equally efficient at receiving linearly polarized signals in any orientation, with a predictable 3dB polarization loss. The reciprocal is true of linear antennas in any orientation being equally efficient at receiving circularly polarized signals at 3dB loss. We accept this predictable loss in all orientations as an acceptable alternative to random losses over 20dB when the performers’ antennas arbitrarily change orientation.

As you can see, there are many factors involved in choosing the right antennas and placements for your wireless systems. I have only touched on the fundamentals here, but understanding these concepts is crucial to dependable performance. I hope you’ll follow the links above and delve a little deeper into the science that makes reliable wireless systems possible.

© 2013 Jason G. Glass and Clean Wireless Audio LLC