Am Broadcast Antenna System

AM Broadcast Antenna Systems

AM Broadcast Antenna SystemsyNad_IntroductionThe chief purpose of a broadcast antenna system is to radiate efficiently the power supplied to it by the transmitter.A second purpose of an AM antenna system is often to concentrate the power in desired directions to cover populated areas and to suppress it in other directions to protect the coverage of other stations sharing the same or closely adjacent channelsIntroductionThe polarization of the transmitted waves is also a factor; for medium-wave broadcast stations, vertical polarization is used because of its superior groundwave propagation.Traditional and Modern Analysis MethodsAM antennas have been traditionally deemed to function as though the currents carried by their elements were purely sinusoidal in nature, theoretically be produced in a standing-wave pattern by two sinusoidal travelling waves of identical magnitude passing in opposite directions.For directional antennas, an additional layer of error is associated with the sinusoidal current distribution assumption due to the effects of mutual coupling between the towers of an array.

A common method for detuning a quarterwave tower, for instance, is to place a reactance at its base to produce a sharp minimum in tower current at approximately one-third of its heightwhere the current is near maximum in the transmitting mode.Moment Method ModelingIt is now possible to model an AM directional antenna (known as moment method modeling) as a large number of small conductor segments and to take into account the contributions of current that are both conducted from adjacent segments and induced through mutual coupling from all of the other segments. This makes it possible to calculate tower base impedances and drive currents using close approximations to their real-world current distributions,TRADITIONAL AND MODERNDIRECTIONAL ANTENNADESIGN METHODSThe process of determining what array geometry and field parameters are necessary to produce a desired directional antenna radiation pattern has become more automated with the advent of modern digital computers.

Most directional antenna patterns in use today were designed long before such computations were possible, however, using straightforward mathematical techniques. Independent FactorsStrength of the signal

Path attenuationAttenuation is determined by distance and the conductivity and dielectric constant of the ground along the propagation path.Field StrenghtField strength measurements are often graphically analyzed with conductivities that differ from the actual soil conductivity, when the dielectric constant differs significantly from 15, because it is more convenient to use the existing graphs published by the FCC than to develop alternative curves based on dielectric constants other than the assumed 15.Conductivity is normally the only term that is mentioned to describe the characteristics of earth for groundwave propagation analysis.10

FIGURE: Field strength versus distance for family of conductivity curves.11EfficiencyThe term efficiency is sometimes used to refer to unattenuated radiation. As applied for decades in FCC practice to define radiation, the word is utilized in an unconventional sense. It does not express an output/ input relationship of an antenna in percent as it is used to define amplifier efficiency. It expresses the unattenuated field strength in the horizontal plane of a nondirectional antenna or the root mean square (RMS) of the horizontal plane radiation pattern of a directional antenna pattern with a reference input power level of 1.0 kilowatt.12Single Tower Nondirectional AntennaFor simple tower, the current is deemed to be sinusoidal and to reach a maximum 90 electrical degrees down from the top. The distance along the height of a tower, measured in electrical degrees, differs from the physical distance slightly because the velocity of propagation along the tower structure is slower than the velocity of propagation in free space.Single Tower Nondirectional AntennaThe amount of delay depends on the cross section of the tower and the size and number of its cross members.

It is beneficial to consider velocity-of propagation effects when calculating tower impedances, antenna radiation characteristics have traditionally been calculated assuming thin wires equal in height to the towers they represent.The approximate shape of the current distribution on a thin wire of uniform cross section is given by:

ia = Iasin(G y)where:ia = current (in amperes) at height yIa = maximum current (in amperes)G = tower height (in degrees)y = height (in degrees) of the current element iaIt is important to visualize the shape of the voltage distribution along the tower because of the need for good insulators at the high-voltage points; otherwise, corona or arc-overs may result and disrupt broadcasting service.

FIGURE: Theoretical current and voltage distributionon a vertical radiator.Vertical Radiation CharacteristicsMaximum groundwave radiation occurs for a tower 225 electrical degrees high (5/8 wavelength). The variation in tower current distribution with increasing tower height defines the shape of the radiation characteristic in the vertical plane.

FIGURE: Radiation characteristics in a vertical plane.18Insulated Tower Base ImpedanceThe base impedance of a single nondirectional tower is determined principally by its electrical height, its cross section, the extent of the ground system, and the elevation of the feed point above ground.

For typical guyed towers of uniform cross section that are base insulated and fed 4 or 5 feet above ground level, the resistive and reactive components of the base impedance approximate the values shown in the figure.

FIGURE: Typical base input resistance and reactance of a uniform-cross-section, base-insulated, guyed tower.It must be remembered that the base input impedance of a tower, when measured at the output terminals of the antenna tuning unit that is used to feed it, includes the shunt effects of tray capacitance and any conductive circuits that are connected across its base insulator, as well as the series inductance of the conductor used to make the connection to the tower base.Grounded Towers and Shunt-fed and Folded MonopolesOccasionally, towers without insulated bases must be utilized as AM radiators. Such structures include towers that are also used for land-mobile communication and FM and TV stations. Although the impedance at the base of such a tower is necessarily essentially zero, the impedance rises with increasing height of the feed point.Shunt-FedA shunt-fed tower must be driven to provide a desirable input impedance. A common technique is a slant-wire feed, where a wire is attached to the tower at a selected height above ground and brought down to near ground level at an angle approximating 45 to serve as the antenna input terminal.

FIGURE: Shunt-fed grounded towers.Although the traditional method for matching both slant-wire and folded monopole antennas has involved experimentation with regard to the physical connection points of the feed wires, moment method modeling is sometimes used today to design optimized feed arrangements.Top loadingThe performance of an electrically short tower can be improved, both as to radiation efficiency and bandwidth, by means of top loading. Top loading is also sometimes used to provide vertical radiation characteristics that would otherwise require construction of taller towers where tower heights are sufficient such that radiation efficiency and bandwidth are not the major concerns.Top loading is accomplished by increasing the capacitance to ground from the top of the tower. The physical realization can take the form of either a flat, more or less circular horizontal disk attached to the top of the tower or sections of guy wires bonded to the top of the tower and extending down a useful distance.

FIGURE : Top-loading methods.Many variations of top loading are possible. Most recent installations use sections of the three upper guy wires for top loading, although some have used 6 or even 12 nonstructural wires for top loading. By interconnecting the lower ends of the top-loading wires, the capacitive loading is increased for a given guy wire length.Sectionalized TowresA utopian vertical radiator would have a constant current of unchanging phase throughout its height, but in real life the current must ultimately reduce to zero at the tower top or at the end of the top-loading cables.

The current can be made to diminish less rapidly by inserting an inductance in series with the tower at a point partway up its height.Towers approaching one wavelength in height can be employed to provide increased horizontal plane radiation and greater suppression of high-angle radiation when they are fed at approximately half of their physical height. Such center-fed towers are commonly known as Franklin antennas.

FIGURE : Sectionalized towers.It is also possible to use the technique of skirt-wire feeding for sectionalizing towers where it is not feasible to use insulators. It is also possible to eliminate the need for tuning boxes when skirts are used by adjusting the points at which the skirt wires are bonded to the tower to produce the required net reactances across the open skirt ends.

FIGURE : Sectionalizing with skirts.The FCC Rules contain formulas for calculating the vertical radiation characteristics of sectionalized and Franklin antennas and specify how the parameters that describe their physical characteristics must be specified in applications. Because most existing sectionalized and Franklin antennas were licensed before the current Rules were enacted many of them are grandfathered and require custom analysis to determine their vertical radiation characteristics.Top-Loaded Sectionalized TowerFor a simple vertical radiator, the radiation characteristic can be improved by increasing the tower height up to 225 for maximum groundwave signal where skywave self-interference from the high-angle lobe that is present for tower heights greater than 180 during nighttime.The purpose of top loading a sectionalized tower is to provide a means of further controlling the current distribution on the tower. Considering efficiency and stability, it is often possible to achieve a more favorable radiation characteristic of the whole tower by employing top loading and sectionalization together .

FIGURE : Theoretical current distribution on top-loaded sectionalized tower.Ground SystemThe current on a tower does not simply disappear. It returns to earth through the capacitance between the earth and each incremental element of the tower and the top-loading conductors, if used.For single towers, the ground currents are radial from the tower base. The ground losses are greatly reduced if the tower has a radial copper ground system, so the ground current will be in the low-loss copper ground system rather than in the earth, which has a much higher resistance.A solid copper sheet of infinite radius would be the ultimate ground system, but experiments and experience have defined the dimensions of an adequate ground system. A system of 120 radial ground wires, each 90 long and equally spaced out from the tower base, constitutes a standard ground system

FIGURE: Nondirectional antenna ground system.A system simply represents a reasonable balance between cost and radiation efficiency. The antenna system loss including the tower and ground system is normally assumed to be 2 and is added to the base resistance of the tower for simplified analysis. Most ground systems under directional antenna arrays consist of the usual 120 radials per tower truncated and bonded to traverse copper straps where the radials from the towers would otherwise intersect,Ground system losses are minimized if the radial wires are placed above ground; thus, the E-field voltage from the tower and top-loading cables (if any) terminate on these radial conductors so the H-field current can return to the tower base without penetrating the lossy earth.Two-Tower Directional AntennaThe protection requirements both daytime and nighttime, on the same and adjacent channels must be met in the directional antenna design, tend to define the shape and size of the required antenna pattern. Because the distances and directions to the other stations requiring protection are rarely the same, most directional antenna patterns are tailored to meet the specific requirements in various directions.A directional antenna functions by carefully controlling the amplitude and phase of the radiofrequency currents fed to each tower. The resulting field in any direction is the vector sum of the individual tower radiation components.The relative amplitudes from the individual towers remain unchanged, but the relative phases shift with azimuth because the signal from the closest tower arrives first.In a directional antenna system, one tower is defined as the reference tower, and the amplitude and phase of each other tower are measured relative to this reference. The ratio of the field from each other tower relative to the reference tower field is a fractional number sometimes expressed as a percent of the reference tower field.

FIGURE : Three simple directional antenna patterns.Multiplication of Two Tower PatternThe most widely used method of controlling pattern shape involves the multiplication of two-tower patterns and is known as pair multiplication.

When a two-tower pattern such as pattern 1 with nulls at n1 is multiplied by pattern 2 with nulls at n2, the result is pattern 3 in a three-tower array. The directions of the two-tower pair nulls are maintained in the three tower array. This is a very powerful design technique for protecting other stations and still serving a desired service area.

FIGURE : Multiplications of patterns to produce a three-tower inline array.Pattern Inversion (Moding)Directional antenna pattern designs using towers of identical height that do not have zero-field nulls often offer a choice of base impedances and power division.A pattern without any embedded design pairs can be inverted at least once by rotating all of the towers about the center point (adding 180 to each azimuth) and changing the sign of each phase angle.It is often beneficial from the standpoint of bandwidth performance to consider all possible parameter inversions before a pattern design is considered final. In general, better performance is achieved by selecting the design with the most nearly equal operating resistances and, where there are towers having negative power flow, with the minimum total negative power.Pattern Design using Modern Computer MethodsMost existing patterns were originally designed using the pairs multiplication process. Those designed prior to about 1970 may have had their parameters developed with the design engineers slide rule and had their radiation pattern calculations done on paper with the assistance of tables of trigonometric functions and mechanical calculators.In those days, complicated pattern shapes were developed by specifying where pattern nulls would be produced, and, if the radiation in other directions was found be satisfactory after the overall pattern calculations were completed, they were considered final. Pattern shapes were often biased toward meeting interference protection requirements without, for instance, optimization of null fill on the less critical protection azimuths.Radiation Pattern SizeThe pattern size is usually determined by integrating the energy flow outward through an imaginary hemispherical surface surrounding the directional antenna array. This method does not give information regarding the distribution of power radiated from the various towers of the directional antenna array; however, it is very useful for making comparisons of pattern size.This computation method is available in digital computer programs and is used by the FCC. There are other methods of determining pattern size, such as the mutual resistance method, which employs Bessel functions, and the driving point impedance method, which uses mesh circuit equations with self- and mutual impedance information.Moment method computer software, which has in recent years become available for determining current distribution on towers and top-loading cables, base driving point impedances, and the patterns of directional antenna arrays. It has found common use for predicting the drive characteristics of array elements to use in phasing and coupling system design and predicting tower base current ratios and phases to produce desired farfield pattern shapes for adjustment purposes.RSS to RMS RatioEach directional antenna pattern calculated to modern standards has specified for it both an RMS of its horizontal plane radiation, and an RSS of the individual field values radiated by the various towers to produce the pattern.The RMS corresponds to the area inside a directional pattern that is plotted to scale in millivolts per meter. It is a measure of how much radiation leaves the antenna system.The RSS, is a measure of how much field is required from the towers in aggregate to produce the far-field pattern.The RSS-to-RMS ratio for a given directional antenna pattern is the closest thing available to a quality factor for judging its characteristics relative to other patterns. A high ratio means that the combination of array geometry and pattern shape forces the individual tower fields to be high for the amount of power that is radiated into the far field.

FIGURE : High and low RSS design approaches.Seasonal Variation of Field StrengthField strength measurements on a previously licensed directional antenna may appear to indicate a change in pattern shape or size when the change was in fact due to changes in soil conductivity.In some areas, the conductivity is typically higher during winter and spring months when the soil is more moist than in summer and fall months, with the conductivity being the highest when the ground is frozen. Seasonal conductivity variations are not observable in some portions of the country, yet are extreme in other areas.Driving Point ImpedanceThe input impedance of each tower in an array (called the driving point impedance) is not what it would be if the tower were used as a nondirectional antenna. This is because of the effects of mutual coupling with the other towers of the array.

The driving point impedance contains the self-impedance plus the mutual impedances multiplied by the current ratios that exist with other towers in the array as driven to produce the desired pattern.Calculated driving point impedances are used in the design of new phasing and coupling equipment that is designed before the towers are erected, but it is sometimes desirable to measure the operating impedances of towers in an existing directional antenna system.Detuning Structures near AM antennasIt is sometimes necessary to detune a tower on a directional antenna systems property. For tower heights below one-half wavelength, this can usually be done by placing a reactance from the base feedpoint to ground to cause the current distribution on the tower to have the general shape.

FIGURE : Detuning with base termination.Monitoring Directional Antenna Operating ParametersAntenna monitoring systems are used by AM stations that employ directional antennas to monitor the ratios and phases of the currents flowing in the array elements so they can be maintained at the values that are known to produce the required pattern shapes. Antenna monitors are designed to meet the FCC requirements for accurate monitoring of the ratios and phases of the current samples that are fed into them, through the sampling lines, from the current sampling devices.

FIGURE: Antenna monitoring system.The sampling lines are typically 3/8 or 1/2 inch foam dielectric transmission lines and are semiflexible with solid outer conductors. The current sampling devices are either current transformers through which the tower base currents pass or tower-mounted inductive pickup loops.In either case, the sampling devices must be rated to produce voltages within the acceptable range of the antenna monitor with full power into the antenna system.

FIGURE: Base and loop sampling.

FIGURE : Insulated and uninsulated samplingloops.Electrically Short AntennasThere is considerable interest in AM transmitting antennas that are much shorter than the typical quarterwave tower. Such antennas are useful in situations where conventional towers cannot be constructed because of environmental or aeronautical concerns, or for emergency backup antennas at stations that have conventional towers.The difficulties with such antennas center on radiation efficiency and bandwidth issues, as they typically have low base resistances and, therefore, high base currents for the power that is fed into them.There are well-known methods for obtaining better efficiency than is expected for short conventional tower antennas. Several principles for optimizing the efficiency of very short antennas, a top loading can be used to maximize the value of the integral of antenna current over the vertical length of a short conductor, which is the condition for maximum radiation.

FIGURE : General principles for short antenna efficiency improvement.