Welcome to the third installment of All About Wireless. In this issue, we will focus on antennas, covering the design and performance characteristics of those most commonly utilized in the professional audio industry. An antenna is a transducer designed to convert the alternating voltage and associated current output by a transmitter into radiated electromagnetic waves. Passive antennas are reciprocal devices, which means they are equally capable acting as both transmitting and receiving transducers.
In a receiving antenna, the passing of electromagnetic waves through the antenna elements induces an alternating current and associated voltage at the antenna terminals. Due to the relatively low power of signals detected by wireless microphone and IEM receivers, many different antenna designs exist with elements arranged in such a way as to optimise transduction for a given application, compensating somewhat for the high losses that occur during wave propagation.
Size and weight are also important design considerations. The antenna types most commonly utilized in professional audio applications are the Monopole, Dipole, Log Periodic and Helical.
The Monopole is an unbalanced antenna consisting of a single quarter wavelength element. Reflection from a ground plane, which is usually provided by either the receiver chassis or the PC board to which the antenna is connected, provides a virtual second quarter wavelength element. The reflected image is in-phase with the physical element, so the two combine to produce a composite half wavelength response. Monopole antennas are often supplied with bodypack transmitters and entry level receivers. The directivity of the Monopole is omnidirectional in the ground plane, but exhibits a strong null at the top of the physical element.
The Dipole antenna consists of two physical elements, each of which is a quarter wavelength. Directivity is equal to that of the Monopole, but the use of two physical quarter wavelength elements, instead of one with assistance from a ground plane reflection, results in a theoretical gain of up to 3dB over the Monopole in some configurations.
Depending on the particular antenna design, a ground plane may still be required for correct Dipole performance. In this case, the absence of a ground plane may cause directivity to become asymmetrical, and changes in the input impedance may result in the generation of standing waves within the transmission line.
Operational bandwidth may be expanded by increasing the diameter of the antenna elements, or by using conical elements rather than cylindrical wire. Reflectors spaced one sixteenth to one quarter wavelength away from the Dipole may also be used to increase gain in a particular direction.
Log Periodic Antenna
The Log Periodic antenna consists of an array of Dipoles, the length and spacing of which varies. Bandwidth is determined by the frequencies at which the shortest and longest Dipoles in the array are a half wavelength. The length and spacing between elements varies in a logarithmic progression so that at any given frequency one or more Dipoles are active, while the others function as either reflecting or directing elements depending on their size and location relative to the active Dipole. A longer the antenna with more elements will exhibit wider bandwidth and narrower beamwidth. Log Periodic antennas typically exhibit 6-8dB of forward gain and a beamwidth of approximately 120 degrees.
Circularly Polarized Antenna
Some antennas are designed for circular polarization. An electromagnetic wave is said to be circularly polarized when equal electrical fields exist in the vertical and horizontal planes, 90 degrees out of phase. These two electric fields rotate as they are propagated, either clockwise or counter clockwise depending on the antenna design, by a full 360 degrees per wavelength of the carrier frequency.
Circularly polarized electromagnetic waves may be equally detected by both vertically and horizontally polarized receive antennas. Circularly polarized antennas are therefore a popular choice for IEM applications in particular. This is because they eliminate the risk of suffering up to 20dB signal attenuation caused by varying degrees of polarization mismatch between a statically polarized transmit antenna and IEM receive antenna, the polarization of which changes as the bodypack moves.
The Helical is the most commonly encountered antenna designed for circular polarization. The driven element of a Helical antenna is typically constructed from a wire or ribbon wrapped in a helix, positioned above ground plane. It may have a wide bandwidth with up to 12 dB of forward gain but with very narrow beamwidth of approximately 60 degrees.
Whilst the Monopole, Dipole, Log-Periodic and Helical antenna types are the ones most commonly encountered in professional audio applications, there are many, many more designs in production, some of which are quite well suited to wireless microphone and IEM use in the right situation.
Once such example is the Patch antenna. The primary advantage of the Patch antenna is that it is very low profile and may be mounted on a ceiling or wall. It is usually constructed from a half wavelength microstrip mounted on a substrate above a ground plane. Patch antennas are typically characterised by low gain and a narrow bandwidth, although wideband models are in production.
Many different Patch antenna designs exist, they are commonly utilized in cell phones for example, with various qualities and polarization options. In professional audio applications, Patch antennas are often used with wireless microphone systems operating in the DECT and 2.4GHz bands.
That covers the main antenna designs commonly encountered in the professional audio industry. Remember though, there are countless variation of each design, plus many more that we have not covered here. Log Periodic antennas, for example, are all based on the basic design principles discussed, but every model from each manufacturer will look slightly different.
The important thing is to have an understanding of the general characteristics of each antenna type to facilitate informed decisions regarding model selection and utilization for each specific application.
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