VERTICAL POLARIZATION AND FM TRANSMITTER ANTENNA PERFORMANCE OPTIMIZATION

© 1998 IBE Systems. All Rights Reserved.

Stephen Jon Blank
New York Institute of Technology
Old Westbury, New York

sblank@ibesystems.com

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Invited Lecture:

21st Brazilian Congress of Broadcasting

Iguacu, Brazil

September 10, 1998










This work was sponsored by:

IBE Systems Corporation

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Tel: 516-359-7947

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VERTICAL POLARIZATION AND FM TRANSMITTER ANTENNA PERFORMANCE OPTIMIZATION

Stephen Jon Blank

New York Institute of Technology

Old Westbury, New York

USA








Abstract: This paper reports on the results of a study to determine the optimum configuration for FM transmitter antennas. The study included a survey of antennas in current use for FM broadcast and revealed a significant gap between what could be done, in a practical way, to optimize FM transmission, and what is actually done in practice. The important antenna parameters of polarization, impedance, gain and radiation pattern coverage, bandwidth and "on-the-tower" performance are discussed. In addition, attention is given to the impact that the transmitter antenna characteristics have upon electric power consumption, equipment maintenance and radiation hazard. An optimized antenna configuration, the Linear Vertically Polarized Antenna, is described. The advantages of the Linear Vertically Polarized Antenna over horizontally or circularly polarized FM transmitter antennas are discussed in detail. This antenna has been tested in operation in both urban and rural areas and has been shown to provide significantly better performance than horizontally or circularly polarized transmit antennas. This is corroborated by data obtained in the United Kingdom, Israel and other countries around the world. Both the theory of operation and measured performance data are presented.

1. Introduction

This paper reports on the results of a study to determine the optimum configuration for FM transmitter antennas [1,2]. The subjects covered are: 1) the design of vertically polarized (VP) antenna systems for FM broadcasting, 2) the success of this system in achieving better penetration into service areas than had previously been achieved using either horizontal polarization (HP) or circular polarization (CP) and 3) the implications that this may have for countries that are establishing new or expanded FM broadcast services. In order to have a proper reference for this discussion, we will review both the general requirements for FM transmitter antennas and also the characteristics that are typical of FM transmitter antennas currently in use. Of special importance are the factors of polarization, impedance, gain, radiation pattern coverage, bandwidth and "on-the-tower" performance. In addition, attention is given to the impact that transmitter antenna characteristics have upon electric power consumption, equipment maintenance and radiation hazard.

2. FM Transmitter Antenna Requirements

The following is a very general, and necessarily brief, listing of the requirements that all FM transmitter antennas are required to meet.

2.1 Directional Gain and Pattern Coverage: - all antennas, to a greater or lesser extent, have directional radiation patterns. It is important to maximize radiation gain in the desired directions and to minimize gain in the unwanted directions. A desired three-dimensional radiation pattern is conveniently described in terms of the shapes of the vertical plane pattern and of the horizontal plane pattern. In the vertical plane, a relatively narrow beam of radiation is required, whereas, a very broad (sometimes, omnidirectional) radiation beam is required in the horizontal plane. The need for a narrow beam in the vertical plane follows from the requirement: that radiation on the horizon be maximized (so as to maximize signal to the service area, perhaps, with a small down-tilt in some cases); and that radiation be minimized in directions up to the sky and down to the tower base in order to minimize fading, radiation hazard, RF interference and sheer waste. In practice, the desired radiation pattern is achieved by mounting an array of antenna elements along the vertical length of a transmitter tower. The longer the vertical length of the array, the narrower is the radiation beam in the vertical plane, and the higher will be signal power density on the horizon as is desired.

2.2 Polarization: - The question of the optimum choice of polarization for FM has historically been troublesome and confusing. The original plans for FM broadcasts in the U.S., made prior to World War II, called for horizontal polarization. Shortly after WWII, it was recognized that horizontal polarization was not suitable for mobile, omnidirectional reception of FM broadcasts in automobiles. This is due to the fact that the ‘whip’ antenna, used for the omnidirectional reception of AM and FM broadcasts in automobiles, is primarily a vertically polarized antenna and has a weak response to a horizontally polarized signal, (see Fig. 1). Mounting a separate omnidirectional horizontally polarized antenna on automobiles for FM reception is not a practical option. A similar condition exists for portable FM receivers having telescoping ‘whip’ antennas. In 1946 in order to overcome the deficiencies of horizontal polarization, the FCC authorized the use of ..... supplemental vertically polarized effective radiated power..." [Ref. 3]. In practice, this has come to mean "circular" or "dual" polarization in which presumably equal amounts of horizontally and vertically polarized energy are radiated. From an engineering rationale, the polarization used for transmission should be chosen so as to best match the polarization of the receive antennas used for car and home stereos. One may therefore ask " How many car or home FM receivers have a circularly polarized receive antenna? ". The answer is – zero to none. If one asks " How many have a horizontally polarized receive antenna ? "; the answer is - a tiny percentage. If one asks " How many have a vertically polarized receive antenna? "; the answer is - most cars, the bulk of today’s market, have a vertical whip receive antenna. Most home FM receivers have a wire hanging out the back. The conclusion to be drawn from this is that: for optimum match between the polarization of transmitted and received signals in order to provide optimum area coverage, vertical polarization is best.

This conclusion is supported by the highly successful results achieved in practice using vertical polarization. These results showed increases in signal levels that exceeded 5 dB as compared to the levels with equivalent CP antennas in both urban and rural areas. Besides increased signal levels, there are other important advantages to be gained by the use of vertical polarization: a) a vertical dipole has a null in the tower base direction, thereby minimizing radiation hazard in accordance with OSHA regulations. Also RF interference to nearby studio equipment and telephone lines is minimized, b) vertical polarization provides an extra 10-20 dB interference rejection between Low VHF (Channel 6) and FM, and c) interference by power lines and telephone lines is reduced. 2.3 Broadband Operation: The FM band is 88 to 108 MHz, and the FM channel width is 200 KHz. It is desirable that the FM transmitter antenna have multi-channel, broadband impedance performance (i.e. low SWR) in order to minimize frequency sensitive performance degradation and to allow multiplexed operation. Ideally, the antenna should cover the entire 88-108 MHz band.

2.4 On-the-Tower Performance: It is important that specified radiation pattern and impedance performance be achieved in operation, on-the-tower, and not solely in some idealized free-space environment. This means that the design of the antenna should either account for tower effects or de-couple the antenna from tower effects or both.

2.5 Electric Power Consumption, Equipment Maintenance and Radiation Hazard: Maximizing antenna performance will minimize the need for higher transmitter power. Lower transmitter power means lower electric energy costs both to run the transmitter and to run the air conditioning that cools the transmitter. Lower transmitter power also minimizes equipment maintenance difficulties and the potential for radiation hazard. It is generally highly cost effective to trade improved transmitter antenna performance (i.e. better antenna / more efficient radiation / more signal gain, a one-time cost) against high transmitter power (a continual money sink).

3. The Linear Vertically Polarized Antenna

Vertically polarized transmitter antennas usually consist of an array of vertical dipole elements mounted along the vertical length of a tower. The vertical (half-wave) dipole, in free-space, has a radiation pattern that is directional in the vertical plane and omnidirectional in the horizontal plane. Generally, the effect of mounting a vertical dipole on a tower is that the tower acts as a reflector. This causes the radiated pattern to be slightly more directional in the vertical plane and to become a smooth, very broad cardiod shape in the horizontal plane; thereby increasing signal strength in the forward direction and decreasing it in the backward direction. When the transmitter tower is located near the edge of the coverage area, as is often the case, this is a very desirable result. In this configuration all the dipoles in the array are located along a single vertical axis on one side of the tower, Fig.2-a). The horozontal plane pattern (smooth cardiod ) is shown in Fig. 2-b).

There are cases where an omnidirectional (or a customized directional) horizontal plane pattern is desired. This can be achieved by using parasitic elements. This is a relatively low weight, low cost solution, but customized patterns generally require some engineering analysis and measurement. Several examples showing the performance of this configuration (with omni horizontal plane patterns) are shown in Figs. 3).

An alternative method of obtaining an omnidirectional horizontal plane pattern is using additional driven dipoles located circumferentially around the tower. But this significantly adds cost, weight and wind resistance.

Arbitrary spacing between the elements, including non-uniform spacing, can be used in VP dipole arrays. Usually, the spacing is one wavelength for maximum radiation efficiency. Due to its directionality in the vertical plane, the vertical dipole antenna does not radiate upward or downward. Therefore, radiation hazard in the tower base area is not a problem. The dipole antenna has multi-channel, broadband impedance performance (i.e. low SWR) which allows multiplexed operation. It is has low wind resistance and can be made light weight.

4. Horizontally Polarized FM Transmitter Antennas

Horizontally polarized transmitter antennas usually are an array of horizontal dipole elements. There are several difficulties with the horizontal dipole element. It has a radiation pattern that is very directional in the horizontal plane and very broad in the vertical plane, the opposite of what is normally required for a FM transmitter antenna. Horizontal loop antennas partially alleviate this difficulty, but they have less gain and narrower bandwidth than a vertical dipole (Fig. 4.).

If the array spacing is 1 wavelength, which gives maximum efficiency of radiation, then the horizontal component radiates strongly down to the tower base direction. This is a potential radiation hazard. If the array spacing is 0.5 wavelengths, then downward radiation is reduced but so is efficiency.

Beyond these difficulties, the fundamental problem with horizontal polarization is that the ‘whip’ antenna, used for the omnidirectional reception of AM and FM broadcasts in automobiles, is primarily a vertically polarized antenna and has a weak response to a horizontally polarized signal. Field measurements, made using automobile ‘whip’ antennas, have shown that the response to horizontally polarized signals can be 10 to 20 dB below the response to vertically polarized signals, see Fig. 1). Mounting a separate omnidirectional horizontally polarized antenna on automobiles for FM reception is not a practical option.

5. Circularly Polarized FM Transmitter Antennas

Circularly (or dual) polarized antenna elements presumably radiate equal amounts of horizontally and vertically polarized energy (Fig.5.). Their designs are usually based on the helical loop antenna [2], which can be viewed as a combination of the horizontal loop and the vertical dipole elements. The elements often have complicated, pretzel-like shapes; and usually have high Q, frequency sensitive performance. When arrayed and mounted on a tower, the horizontal and vertical components behave very differently due to strongly different coupling and reflection effects. If the array spacing is 1 wavelength, which gives maximum efficiency of radiation, then the horizontal component radiates strongly down to the tower base direction. This is a potential radiation hazard. If the array spacing is 0.5 wavelengths, then downward radiation is reduced but so is the efficiency of radiation (which means reduced signal strength). These antennas are relatively heavy and have high wind resistance. They often require radomes, which further adds to their weight and wind resistance.

But, once again, the main problem with circularly (or dual) polarized transmitter antennas is that, in practice, no one uses circularly (or dual) polarized antennas for reception. The ‘whip’ antenna, which is used for the omnidirectional reception of AM and FM broadcasts in automobiles, is primarily vertically polarized and has a weak response to a horizontally polarized signals. Therefore, half the power radiated by circularly polarized antennas is essentially wasted.

  1. Performance Data and Field Measurements
In this section, the measured performance of vertically polarized FM broadcast systems from three representative countries, Israel, the United Kingdom and the Netherlands, is discussed.

6.1 Israel

In 1987, IDF Radio (major radio network in Israel) installed its first vertically polarized FM transmitter antenna. The success of this installation convinced IDF Radio to use vertical polarization exclusively throughout its 8- transmitter FM network with transmitter powers ranging from 500W to 20kW.

IBA Radio, (Israel Broadcasting Authority, largest network in Israel) had been using horizontal polarization for 40 years. Recognizing that vertical polarization is more efficient, IBA is now using vertical polarization in all its last 15 installations.

The recently created 2nd Channel, Regional Radio Network (low power) uses vertical polarization in 14 of its 20 installations. The 6 sites that are using horizontal polarization are old, pre-existing sites inherited from the old PTT.

All the vertical polarized FM transmitter sites in Israel provide excellent coverage. There are many cases where a 1kW transmitter with a simple, two-element vertical dipole array antenna (6dBd forward gain) provides good city-grade coverage out to a radius of 35kM or greater. It is much simpler to obtain high antenna gain with vertical polarization than with either horizontal or circular polarization.

6.2 Great Britain

The BBC (Great Britain) has issued a detailed report, [4], which states that " Where a VHF-FM network is being established…vertical polarization is optimum…". For older, existing horizontally polarized FM networks, this report states that the addition of vertical polarization is necessary for the proper reception of FM in automobiles and portable receivers. It further states: " The disadvantage of MP (mixed or dual polarization) is the extra transmitter power required and the complexity of the transmitting antenna. This complexity is compounded when one considers the increased wind loading and the consequent mast requirements". The BBC has re-engineered its FM radio network for vertical polarization.

6.3 The Netherlands

An engineering study done by the Netherlands PTT describes propagation measurements for vertical polarization in Band II (FM) from 1 to 45 km in rural and urban areas in the western part of the Netherlands, [5]. The results show excellent agreement with CCIR Recommendations (370). See Fig. 6.
 
 

References

[1] S. J. Blank, "Optimizing the Performance of the FM Transmitter Antenna", 1990 NAB Engineering Conference Proceedings, pp. 263-265.

[2] S. J. Blank, R. Berkovits, T. Campbell, "FM Radio Stations Waste Energy and Can Be a Radiation Hazard", NARTE NEWS 12, Jan. 1996.

[3] FCC (10-1088 Edition).

[4] G. H. Taylor and D. S. Cox, "VHF-FM Radio Broadcasting, Tests to compare horizontal, vertical and mixed polarization", British Broadcasting Corporation, BBC RD 1986/13.

[5] J. Doeven, Propagation over Short Distances in Urban and Rural Areas in Band II, EBU review-Technical, No.224, pp. 191-197, August 1987.