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Diversity Fin Antenna and Polarization Diversity
Diversity Fin Antenna and Polarization Diversity
Diversity Fin Antenna and Polarization Diversity
Diversity Fin Antenna and Polarization Diversity
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Diversity Fin Antenna and Polarization Diversity

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In many wireless microphone applications, even small signal dropouts are unacceptable. Multi-path fading typically causes dropouts, rather than weak signal strength caused by too great an operating …

In many wireless microphone applications, even small signal dropouts are unacceptable. Multi-path fading typically causes dropouts, rather than weak signal strength caused by too great an operating distance. One method to reduce the likelihood of dropouts is to use a diversity receiver with two antennas. While it is common practice to connect two physically separated antennas to these inputs, there are other antenna designs that achieve the same goal of reducing dropouts. One such antenna design is the Diversity Fin antenna. This white paper will explain how the Diversity Fin antenna works and what benefits it has over traditional wireless microphone antenna systems.

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  • 1. The Diversity Fin™ Antenna and Polarization Diversity for Wireless Microphone Applications by Michael BenonisIntroductionIn many wireless microphone applications, even small signal dropouts are unacceptable. Multi-path fading typically causes dropouts, rather than weak signal strength caused by too great anoperating distance. One method to reduce the likelihood of dropouts is to use a diversityreceiver with two antennas. While it is common practice to connect two physically separatedantennas to these inputs, there are other antenna designs that achieve the same goal of reducingdropouts. One such antenna design is the Diversity Fin antenna. This white paper will explainhow the Diversity Fin antenna works and what benefits it has over traditional wirelessmicrophone antenna systems.BackgroundSignal fades are generally caused by either path loss or multi-path interference. Since UHFwireless microphones are typically operated within a few hundred feet of the receiver, path lossis rarely the problem, even with low power transmitters. This leaves multi-path as the cause ofmost signal fades. Multi-path is a result of radio waves bouncing around in a room, much like anecho chamber. These echoes create a pattern of nulls, and if the receiver antenna is in one ofthese nulls, the signal is lost. While occasional signal fades might be tolerable in cellulartelephone systems, any dropout is unacceptable during concerts, corporate presentations,broadcasts, or worship services because it significantly distracts the audience.Diversity receivers help combat fades by using two antennas to reduce the odds of a total lossof signal. These receivers usually have two antenna ports and a circuit that compares the signalat each port. The receiver then either selects the better one (usually the strongest) orcombines the signals with the goal of making dropouts and other undesirable sounds inaudible.A fairly common diversity technique in wireless systems is switched or selection diversity [1]. Inthese systems, a sensing circuit determines the signal strength at each antenna and passes thebetter signal to the receiver circuit. A variant of this system forces a switch command when theselected antenna’s signal drops below a preset threshold, but so-called “blind switching” can leadto poor switching decisions that result in dropouts [2].A more complex antenna diversity technique is called Maximum Ratio Combining, or MRC. InMRC systems, both antennas are active at the same time, but the ratio of the two signals isvaried as a function of the signal to noise ratio [1]. In the best-case scenario, this results in a3dB boost in effective signal strength for a two-antenna system [3]. This technique is commonlyused in higher-end receivers. More exotic diversity systems exist as well, including systems thatuse two separate transmitters on separate frequencies, but these systems are typically reservedfor the most mission-critical applications and are decidedly more expensive.
  • 2. All diversity techniques are based on the principle that using multiple independently variablechannels reduces the chance of a total loss of signal [1]. In the event that the channels becomecorrelated, where they do not vary independently, the likelihood of a total dropout increasessignificantly. A typical method of ensuring independent channels exist is to space two or moreantennas apart by greater than few wavelengths, and is often very effective in providinguncorrelated channels, but it can be difficult to space antennas properly in many situations.How the Diversity Fin WorksThe Diversity Fin antenna uses a different approach to theproblem: Rather than spacing two antennas apart, it takesadvantage of the fact that radio waves are polarized—thatis, they have a specific orientation with respect to objectsin the environment. The Diversity Fin uses polarizationdiversity to achieve the same result as spaced antennasfrom just a single position, dramatically simplifying wirelessmicrophone installation and setup. Figure 1: The Diversity Fin AntennaTo understand how this is possible, it is necessary to understand how polarization plays a role inmulti-path environments. Most small wireless devices, such as wireless microphones, generatelinearly polarized radio waves. This means that the electric field is always pointed in the samedirection as the antenna. The receive antenna must generally be pointed in the same direction,or some fraction of power will be ignored. While this is simple to ensure for fixed transmitterantennas, it is a problem when the transmitters are constantly in motion in a mobile bodypackor handheld form.Cellular phone systems were a major driver of research into how UHF radio waves propagatein multipath environments where the transmitter is constantly moving. Bell Labs performedextensive research in this area [4] and found two important things: The first is that a vertically-polarized transmitter moving within a multi-path environment results in significant horizontally-polarized energy, on the order one-quarter the power of the vertical component, due to multi-path reflections that change the polarization of the signal. This is significantly different from thetheoretical result where a cross-polarized receiver antenna would not see any signal. Thesecond, more important finding was that a pair of cross-polarized antennas at the same locationnever resulted in a fade of more than 1 dB, on average.The reader may wonder why the signal does not fade in both polarizations if the antennas are atthe same location. The reason is that the strength of reflections depends to a significant degreeon the orientation of the wave with respect to the surface [5]. For a vertical wall, this meansthat a vertically polarized wave will reflect differently than a horizontally polarized wave, andthat the pattern of nulls in a room is different for horizontally polarized radio waves thanvertically polarized radio waves.Since wireless microphones transmit with randomly oriented linear polarization, the result is aconstantly changing pattern of nulls in a room that is different for vertical and horizontal waves.A horizontally and a vertically polarized antenna at the same point in space are statistically
  • 3. independent—the key requirement for a diversity system to work effectively. More simply put,there is almost never a time when both vertical and horizontal planes will fade in the samelocation! Hence, an antenna with both horizontal and vertical elements dramatically reducesthe chances of a fade.This is exactly the principle behind the Diversity Fin antenna [6].The Diversity Fin antenna combines a Log Periodic Dipole Array,or LPDA, with a perpendicularly mounted dipole antenna. TheLPDA antenna is a medium gain antenna that responds to linearpolarization. Figure 2 shows a plot of the antenna’s gain pattern,which shows it is more sensitive to radiation from the front. Inaddition, it attenuates radio waves from behind, and this helps to 270 90reduce the amount of multi-path seen by the antenna, which canreduce fades.The dipole antenna is mounted perpendicularly to the LPDA, andresponds well to radiation of the opposite polarization of theLPDA. If the LPDA is mounted vertically, then the dipole Figure 2: LPDA Gain Patternresponds well to horizontal radiation. The dipole’s mountinglocation on the LPDA affords it forward gain as well, as can beseen in Figure 3. This also helps to reduce multi-path seen by theantenna.Because the vertical and horizontal multi-path environments areessentially uncorrelated, it is highly likely that either the LPDA orthe dipole will have acceptable signal strength at any given time.The Diversity Fin has separate RF outputs for the LPDA anddipole components, and these two outputs are connected to thetwo antenna inputs of a diversity receiver for diversity reception.The receiver can then use diversity techniques with a singleantenna system, as opposed to two spaced antennas. Figure 3: Dipole Gain PatternAside from the advantages of polarization diversity, the Diversity Fin’s provided cover, orradome, allows the antenna to work well in the rain and other adverse conditions. This isbecause water cannot come directly in contact with the antenna, which may affect the patternof the antenna. The Diversity Fin can also be folded into a compact form factor for storage andtransport.ConclusionWireless microphones operate in environments rich with multi-path interference that can resultin deep signal fades at times due to constantly moving transmitters. It is imperative thatwireless microphone systems use diversity receiver systems to avoid dropouts that are a resultof fades. Diversity receive systems are based on the fundamental concept that multiple,statistically independent receive antennas exist within the environment. Traditionally, thiscriterion is met with two antennas spaced apart by multiple wavelengths. However, the
  • 4. Diversity Fin antenna uses the novel approach of perpendicularly mounted elements held in afixed relationship to achieve the same benefit as two antennas.ConclusionMichael Benonis is an Electrical Engineer with a focus on RF and analog design.  He works atWUVT-FM in Blacksburg and has nearly a decade of experience in live audio soundreinforcement. Mr. Benonis received his bachelors’ degree from The University of Virginia.References[1] Brennan, D. G., "Linear Diversity Combining Techniques," Proceedings of the IRE, vol. 47, no.6, pp. 1075-1102, June 1959.[2] Vear, T. “Selection and Operation of Wireless Microphone Systems,” Available http://www.shure.com/idc/groups/public/documents/webcontent/us_pro_wirelessmicrophonesy_ea.pdf[3] Balanis, C. A., Antenna theory: analysis and design, 3rd ed., Hoboken: Wiley-Interscience,2005.[4] Lee, W. C.Y., and Yeh,Y. S., "Polarization Diversity System for Mobile Radio," IEEE Transactionson Communications, vol. 20, no. 5, pp. 912-923, October 1972.[5] Harrington, R. F., Time-harmonic electromagnetic fields, New York: McGraw Hill, 1961.[6] Crowley, R. J., and Flyer, D. L., “Diversity fin antenna,” United States Patent Application2012/0032861, August 2011.

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