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A GUIDE TO ANTENNA SELECTION
BY ANDREW CORPORATION
INTRODUCTIONSince the introduction and practical application of Log Periodic and frequency independent antennas, a wide range of truly broadband antennas has been developed to meet the varying demands imposed by specific requirements, such as area coverage and point-to-point circuits. The development of compatible terminal equipment such as baluns, impedance transformers and multicouplers has further enhanced the flexibility of these broadband radiating structures.
Choosing the right type of antenna for use on a HF link is very important if the best link performance is to be achieved. The choice of antenna types is large and, at times, baffling. A number of antennas that will more or less satisfy the requirements in question are available, finding the optimal solution is far from simple for the systems planner. It requires a knowledge of iononspheric behavior as well as antenna engineering, operating conditions and siting considerations. Because of this, the systems planner usually enlists the help of the antenna specialist to analyze the propagation conditions and find (or design, if necessary) an antenna to fit the requirement. In essence, the antenna specialist, to carry out this function effectively, needs to be a physicist, applications engineer, structural engineer and applied mathematician. This bulletin sets out to dispel that myth, explain how antenna types vary in performance and how these differences can be most effectively exploited.
BASIC PARAMETERSTo differentiate between the various antennas available, an understanding not only of the basic parameters of antenna performance, but the different ways in which this information is presented by various manufacturers is required.
The following notes are not intended as definitions but rather as comments which may be helpful when examining manufacturer's technical bulletins.
The Radiation Pattern of an antenna indicates the power (or field strength) radiated in any direction relative to that in the direction of maximum radiation. Both relative power and relative field diagrams are in use and often no clear statement is made as to which is presented.
The actual radiation pattern of any antenna is really a three-dimensional function, however, for the sake of simplicity, only cuts through this solid in the horizontal (azimuth) and vertical (elevation) plane as usually presented. These planes are referred to as the principle planes. For one important class of antennas, those used on short range paths via the ionosphere (i.e., vertical incidence), this practice is not observed for the following reasons.
When considering the behavior of an antenna operating over a short path and consequently required to direct energy at high angles of elevation, the conventional principle plane radiation pattern would be misleading, in so far that the normal "azimuth radiation pattern" will only indicate the variation in field strength at 90° to the zenith, a circle in the horizontal plane of the antenna, and not that being reflected at the ionosphere. A more useful set of data is obtained by traversing a path which joins all points with constant elevation angle and lie equidistant from the antenna. The radiation pattern so defined is always circular at the zenith as this is the trivial case, and the familiar patterns will be obtained normal to this. Patterns for a variety of elevation angles from 50° upwards are commonly published. It should be noted that the sense of polarization of the radiated field changes with bearing, therefore, the polarization direction of the antenna for such operations is not usually quoted, it is meaningless.
The radiation pattern in the elevation plane is strongly influenced by the presence of the ground beneath the antenna as the radiated signal is the sum of the signal radiated directly from the antenna and that reflected by the ground. The relative phase of these components changes with the antenna's height above ground, electrical properties of the ground and polarization, either adding to or cancelling the field due to the direct ray. Manufacturers' data generally refer to "average ground" usually taken to have a conductivity of 10m S/m and a relative permittivity of 10. These figures are typical of ground where grass or agricultural crops are growing. Data quoted for "ideal ground" should be viewed with reservation because, of course, no such ground exists, except perhaps sea water, and an extensive earthment of copper wires will be required to obtain an approximation to the published performance.
A further characteristic of radiation pattern performance often quoted is the beamwidth, or properly the half power beamwidth. This is the angle between the points on either side of the direction of maximum radiation at which the intensity of power radiated has fallen to half the maximum. Care is necessary as in some cases only half this angle is quoted. Specifying the beamwidth as ± degrees removes any ambiguity, unfortunately this is not always the case.
Power gain indicates how much the signal radiated in the direction of maximum radiation is increased over and above that that would be obtained if the antenna referred to is substituted for some standard reference antenna fed with the same input power. Here again there are possibilities for confusion. The reference antenna may be either a half wave dipole (usually used for VHF and UHF antennas) or an isotropic radiator (in the case of HF and SHF antennas). The isotropic radiator is a hypothetical device radiating energy uniformly in all directions in 3-dimensional space. As the power gain of an isolated halfwave dipole is 2.2 dB over that of an isotropic radiator, it is important to know which reference is used. Reference to an isotropic radiator is normally indicated by ndBi. Quoted gains normally allow for the enhancement of signal provided by ground reflection, but again the assumed ground conditions are often omitted.
Maximum input power will generally be determined by the onset of one of three effects:
- Dielectric losses causing overheating of insulators.
- Ohmic losses as a result of conductors carrying large currents.
- Corona discharge from insulators, element tips or other parts.
Input Impedance. As the transmission line feeding an antenna must have the same characteristic impedance as the antenna input impedance, it is often the economics and practicalities of transmission line design which determine suitable antenna input impedances. A wide range of efficient broadband transformers are available allowing changes in impedance level and from balanced to unbalanced line systems.
Input VSWR. The maximum permissible input VSWR for a transmitting antenna is generally determined by:
- The economic need to minimize the size of the transmission line
required, especially when large diameter semi-flexible coaxial cable is
used. The power rating of a cable reduces approximately 1/o where o is
- The ability of the transmitter output circuit to match a non-optimal impedance.
Receiving systems seldom require a closely specified VSWR. The VSWR data should be taken from actual antennas of the generic type when correctly installed under normal site conditions.
- Ground Type-Average soil as defined by CCIR
- Ground Constants-Conductivity 10 m S/m; Permitivity 10
- Flatness-±1 metre
- Obstructions-Presence of other antennas, towers, power lines,
metallic structures, etc., at a distance having negligible mutual
- Reference Point-Antenna input (ground level)
- Nominal-Signifies a value not exceeded throughout 90% of the specified frequency range.
- Max. or Peak-Signifies value not exceeded throughout worst 10% of frequency range.
ANTENNA SPECIFICATIONSIn order to select the optimum antenna for a given HF communication requirement, it is necessary to establish a number of important parameters; e.g.:
- Frequency range
- Elevation plane radiation pattern, location of beam maximum (take-off angle) and upper and lower half power points.
- Azimuth plane beamwidth
- Side-lobe level and front to back ratio
- Feedpoint impedance
- VSWR with respect to feedpoint
- Power handling capability (peak and average)
- Wind speed
- Ice loading
- Presence of corrosive atmosphere
- Soil conditions (load bearing capability and density)
- Land area requirements - limitations
- Tower height
It has already been pointed out that the performance of a HF antenna is dependent on the ground over which it is operating. If the electrical parameters of the soil (conductivity and permitivity) are known, these should be specified.
Having defined the basic requirements, we shall now consider the system requirements and methods for establishing specifications and their relationship.
Frequency Range. For predictions of usable frequencies reference can be made to a number of sources; e.g., monthly ionospheric predictions which give maps of the world for various times of the day on which contours of the maximum usable frequencies (MUF) are superimposed. By properly interpreting these charts which apply to the particular location and times of interest, the usable frequencies can be predicted. This method provides a long range average prediction and, although variations in actual conditions occur from day to day, they are useful for preliminary circuit planning. With the increased computing capacity of personal computers (PCs), programs such as IONCAP are now readily available to the antenna specialist. Such ionospheric models can predict circuit behavior statistically by taking into account a great many factors; where the old monthly charts gave a smoothed average of a relatively small number of observations, they are able to accommodate many more possible circuits. Taking due cognizance of such factors as sunspot activity and the behavior of the E and F layers of the ionosphere, including sporadic modes has increased our ability to predict the effects of seasonal and diurnal fluctuations in the ionosphere. The inclusion of ground reflectivity, noise levels, signal-to-noise ratios and circuit reliability further enhances the accuracy.
HF operation at 30° north latitude has been found to be reasonably indicative of operation throughout the northern hemisphere, with the exception of the region near the polar caps. Figure 1 shows the extreme values of Frequency of Optimum Traffic (FOT) for paths centered at 30° north latitude as a function of path length. This chart and the General Propagation Chart, Figure 2, provide the antenna designer with an approximation which serves as a useful guide in selecting a suitable antenna. To use the chart, a line from the desired path length is drawn down from the minimum and maximum FOT scales and across from the minimum layer height scales. The region enclosed by the rectangle on the chart determines the maximum frequency range and take-off angles for the circuit. As an example, assume a 1000 km point-to-point circuit. The frequency range (3.7 to 16.5 MHz) is obtained by selecting the corresponding frequency interval between 1000 km on the minimum FOT scale and 1000 km on the maximum FOT scale. The required take-off angle range (24° to 40°) is found by selecting the corresponding take-off angle interval between 1000 km on the 240 km layer height scale and 1000 km on the 450 km height scale.
Obviously it is generally desirable to incorporate maximum bandwidth capability to prolong the operational lifetime of the antenna system. The logarithmically periodic antenna and the application of the "angle condition" (such as conical and equi-angular antennas) have been the principle developments in the field of broadband high performance antennas over the past few decades. In fact, the very meaning of broadband changed with the introduction of frequency independent antennas of which the log-periodic class has been the principle embodiment. For this class of antennas, bandwidth limits are set by practical, not theoretical, limitations. The low end of the band having primary influence on the size of the antenna, dimensions of the active portion of the antenna being comparable to the wavelength at the lowest operating frequency; restrictions to the high end of the band are usually set by fabrication techniques and tolerances consistent with the structural requirements.
Polarization. When ionospheric paths are involved, the rotation of polarization which occurs within the ionosphere generally has the effect that the performance difference between vertical and horizontal polarizations is negligible, providing that the effective gain of the two antennas is identical. For transmitting, then, the antenna choice should be made on the basis of the elevation pattern which provides the highest effective gain at the expected take-off angles determined by geometry, without regard to polarization. For receiving antennas, the choice is complicated by an additional factor - the atmospheric noise pick-up. For locally generated noise; i.e., man-made or natural static arriving at the receiving site by ground-wave propagation, the noise pick-up is almost always somewhat higher with vertical than with horizontal polarization. For distant noise sources, the relative noise pick-up depends on the effective antenna gain, not on the polarization, as for any other signal source. The relative noise pick-up advantage of horizontal polarization depends on many factors, most of which are difficult to determine so that actual numbers are unavailable.
Horizontally polarized antennas are to some extent more versatile than vertically polarized antennas, because the elevation plane radiation pattern can be readily varied to suit the path requirements by changing the height of the radiator above the ground plane. In general, if the radiator is one-quarter wavelength or less above the ground, radiation is essentially upwards, and raising the antenna further above the ground tends to lower the radiation angle towards the horizon. The rapidly increasing side-lobe level (in the elevation plane pattern) for radiator heights greater than about one wavelength places a practical limit on this, and use of horizontally polarized log-periodics are not generally recommended where the nominal beam angle is less than 15°. Obviously the horizontal log-periodics are most useful for short and medium range circuits (requiring take-off angles in the order of 50° and 25°, respectively).
Vertically polarized antennas, on the other hand, tend to have their maximum radiation at lower angles in theory towards the horizon when the ground is perfectly conducting. However, the earth is not perfectly conducing and the ground parameters have considerable influence on the actual radiation pattern of the antenna, but nevertheless, a vertically polarized log-periodic with an adequate ground screen is found to be best suited for propagation at low elevation angles.
One further fundamental difference of operational significance is that a narrower beam is obtained in the principle plane which is parallel to the dipoles or radiators of the array. Nature has a way of trading beamwidth from one principle plane to another, so that the maximum gain obtainable from an optimized array of horizontally dipoles is about the same as that obtainable with an optimized array of vertical dipoles.
Elliptical polarization is a combination of the two fundamental planes, the ellipticity being determined by the ratio of horizontal to vertical components. If both were equal, the resultant wave would, in fact, be circular. This mixed polarization minimizes the loss effects which result due to the rotation of polarization which occurs within the ionosphere. The overall advantages can be likened to that achieved through polarization diversity where two antennas of opposite polarization ensure the reception of the maximum available field. Vertically polarized antennas cannot always provide reliable short range coverage because of limitations in the radiation pattern around zenith or excessive attenuation of the groundwave. Existing horizontally polarized antennas are limited in capability to provide reliable long range communications because of the lack of control of radiation patterns over an adequate frequency range. However, communication performance beyond that achieved with linear polarized antennas is possible using elliptical polarization.
Elevation Plane Radiation Pattern. The matter of take-off angles from the transmitting antenna and angle of arrival at the receiving antenna is very important in selecting an antenna for any particular circuit: so the natural question arises, "How do we know what this angle is?" One approach is to make a scale drawing of the ray path which may be done readily by adding distance and angular scales to diagrams of the type illustrated by Figures 3a and 3b, sketching in the ray for a given range and layer height and noting the elevation angle of the ray at the antenna location for this particular path. We frequently use for this purpose the Skywave Transmission Plot shown in Figure 4. The scales on the chart indicates the distance along the surface between the antennas or reflection points, the height of the reflection layer and the take-off angle. A HF Antenna Selector is available which has the elevation patterns for most antennas super-imposed onto Skywave Transmission Plots (request Bulletin 1401.)
A simple example will illustrate the use of this chart. Suppose we are concerned with a circuit of 1000 kilometres great circular distance. The ionospheric reflection point will occur halfway between the stations, and for F2 layer reflections the effective height may be assumed to occur at about 300 km. By laying a straight edge on the chart (Figure 4) between the antenna location (at the lower left corner) and this assumed reflection point, the take-off angle can be read on the scale at the top of the chart. In this case, the answer is 28°. In actual operation, of course, the elevation angle of the signal path changes from time to time as ionospheric conditions change. However, the usual situation can be bracketed by assuming F2 layer reflections at about 300 km. On longer range circuits where multihop modes will occur, the typical conditions can be obtained by following the procedures outlined above for various sub-multiples of distance. If the vertical angle is below 4°, repeat using an additional hop. The mode involving the least number of reflections will almost always incur the lowest attenuation, so this mode and perhaps the next one or two more complex ones will be of greatest interest. Examples of multiple hop transmission for E and F layer reflection can be found in Figure 5.
Figure 3b shows reflection of a signal from a lower layer in the atmosphere - the E layer, which occurs at a much lower height (about 100 kilometres) and is active primarily during the day. The elevation angle of the signal path is lower in this case than shown previously for reflections from higher layers. In addition to these simple modes, many more complex ones are possible on occasion, for example, those which involve E layer reflections along one part of the path and F layer reflections along the next. This might suggest that in antenna design there is a problem of meeting a wide range of conditions. This is certainly true, and a wide range of possible signal paths must be accommodated in order to ensure reliable circuit performance over a long period of time.
Azimuth Plane Beamwidth. Systems considerations that effect this parameter are, of course, azimuth plane coverage requirements, such as omnidirectional for some ground/air and shore/ship applications and possible use of multicouplers in a broad band antenna whose elevation pattern is suitable for simultaneous operation of various point-to-point circuits. Other factors affecting azimuth beamwidth specifications are gain, possible interference to (or from) other services, and off-great circle propagation effects (which place a lower limit in the order of 10° to 15° to azimuth beamwidth).
Broadband antenna types with different azimuth plane coverage are discussed below in the sections dealing with point-to-point and area coverage circuits.
The spread in azimuth beamwidth is from about 60° to 110°.
Side Lobe Level and Front-to-Back Ratio. Ideally an antenna should have no side lobes and infinite front-to-back ratio. This is, of course, unrealistic so the question is what should be specified and what can be attained. Firstly, however, it is useful to evaluate the effects of secondary lobes upon system performance. They are:
- Reduction in gain. Due account must be given to the power radiated
in the side lobes as it detracts from the directive gain of the main
beam. True directive gain of the antenna is established by integration
of the total power radiated.
- Interference to (or from) other services. In this matter of
interference caused by antenna side lobes, consideration must be given
to individual applications, proper evaluation of propagation factors and
antenna radiation pattern must be made to determine susceptibility to
interference. This is particularly true with regard to side lobes in the
elevation plane. The section regarding antenna polarization points out
the dependence of elevation plane side lobe level upon height of a
horizontally polarized antenna above ground to achieve low take-off
angles. If elevation plane side lobes are a major consideration,
take-off angles less than about 20° should not be attempted with
horizontally polarized structures of the log-periodic type. With
log-periodic antennas of not undue complexity, side lobe levels in the
order of -12 dB are attainable.
Practical applications of this fact can result in a reduction in size of the receiving antennas, however, the system planner must ensure that other factors such a logistics are not unduly complicated by requiring different types of transmitting and receiving antennas. This may be particularly true in transportable applications.
With broadband radiating structures of the log-periodic type when imaged above the ground, directive gain figures ranging from 10 to 15 dB are obtained. The effect of ground depends on various factors such as the electrical constant of the ground, height of the antenna above ground, antenna polarization, geometry of the ground screen and take-off angle. For the case of horizontally polarized antennas, the effect of ground is usually negligible for heights greater than about 0.2 wavelengths. It is difficult to generalize on the effect finite ground conductivity has upon the gain of vertically polarized antennas, however, a few general remarks can be made. For a vertically polarized antenna of the quarter-wave monopole type, a ground screen is required to provide a low loss ground return path to the current at the feed point and to provide a good reflection plane for radiation at angles close (in the order to 10°) to the horizon. In the case of vertically polarized antennas of the half-wave dipole type, the requirement for a ground screen is to provide radiation at low take-off angles. These facts must be kept in mind when considering antenna types for specific applications.
Feed point Impedance. From the standpoint of systems requirements, most cases involve either 50 ohms coaxial cable or 600 ohms balanced line, although at times 300 ohm balanced line is used for high power transmitting systems. Unfortunately, most of the practical and economical log-periodic antennas do not have input impedance values that are either 50 ohms coaxial or 600 ohms balanced, it is, therefore, necessary to provide baluns or transformers. Availability of these devices has allowed the antenna designer the freedom to design with relative abandon of input impedance level (provided VSWR with respect to this level does not exceed a predetermined point) in order to optimize the structure from other electrical and mechanical considerations.
VSWR. For the applications discussed herein, VSWR of the antenna is of importance primarily to the transmitting case. A low VSWR is essential for low power solid state transmitters whose output stage design is such that in order to keep voltages and/or currents to a minimum, the output automatically reduces in the event that a predetermined terminal VSWR is exceeded, it also facilitates the task of the tuning mechanisms in a fast-tuned transmitter. A low VSWR also permits near optimum utilization of the minimum size transmission line for the applied power and reduces losses along the line. A VSWR of 2:1 with respect to antenna feedpoint impedance is typical for broadband radiating structures of the log-periodic, spiral or conical types.
Power Handling Capability. A trend exists towards increasing the transmitted power level in HF systems. This trend is not entirely unjustified in view of the usage of multichannel or multimode transmission; the overall power capability of the transmission system must be sufficient to allow adequate power levels per channel. Another reason for operating at high power levels is to increase reliability, particularly in an emergency situation. Aside from cost, the main detraction from high power operation is the problem of interference; improvements in radiation pattern characteristics and in frequency management has, however, alleviated this situation.
Wind Speed and Ice Loading. On the matter of mechanical specification realism in regard to wind and ice loading is in keeping costs to a reasonable level. This is due, of course, to the fact that loading is proportional to the square of the wind velocity, and icing not only increases the weight of the structure, but also adds wind drag area without increasing strength. Many of the larger log-periodic arrays are capable of withstanding 1 cm radial ice simultaneously with winds of 160 kph.
Land Area and Tower Height. Undoubtedly, an important factor in use fulness of log-periodic antennas in HF communication systems is the reduced requirement for land, compared to that of the rhombic type antenna. It is true that large rhombic antennas have greater maximum gain (by about 5 dB to 10 dB) than practical log-periodic antenna designs; however, it must also be recognized that in the case of rhombics the maximum gain is orientated in the proper projection over a relatively narrow frequency range. Real estate is a major cost factor in HF antenna farms and the availability of multimode antennas, such as the SPIRA-CONE, help to minimize these costs by reducing the number of antennas required to accomplish a given operational requirement, especially when considering diverse range requirements of ship/shore, ground/air communications.
Tower height requirements are determined from specifications of low frequency cut-off and for horizontally polarized antennas, the additional specification of take-off angle. For high performance vertically polarized log-periodic arrays of the half-wave dipole type, the last radiating element is half-wavelength at the lowest operating frequency, tower height is about 0.7 wavelengths at the lowest operating frequency; for antennas of the quarter wave monopole type this figure is halved.
ANTENNA SITINGA usual problem in planning a HF communication system is the matter of antenna siting and to assist in this matter a few comments and useful references will be given. Firstly, it may be helpful to note certain basic distinctions between horizontally and vertically polarized antennas as each has to be treated a little differently.
A vertically polarized antenna, for example, may require a ground screen, and the nature of the terrain immediately around the antenna under which the ground screen must be buried may influence the exact placement and manner of installation of the antenna. The signal in the direction of interest will always be a combination of direct radiation from the antenna and energy reflected from the ground in front of it even beyond the ground screen, if one is used. Therefore, to ensure having a well directed beam in space, it is necessary that a fairly smooth area be available for a reasonable distance in front of the antenna. This distance may be as short as a few hundred metres for fairly high angle radiation or as long as a few thousand metres when low-angle radiation is of greatest interest. The distant terrain, up to several kilometres from the site, must also be considered, and the usual rule of thumb is that the angular elevation of the top of a distant range of hills in the direction of propagation should not be greater than one-half of the nominal take-off angle of the signal path. This may influence the selection of the site, although economic or political factors are probably the dominant factor.
Man-made objects near the antenna must also be considered, and for vertically polarized antennas, vertical objects such as steel towers and the like will naturally have the greatest potential for radiation pattern distortion. There are no general rules of thumb for required separation, but when the question arises a reasonable estimate can usually be made by estimating the mutual and self impedances of the elements involved and the currents which might flow in the parasitic radiator. The required separation is governed by the type of interference which is of greatest concern. Depending on the individual case, antenna VSWR, transmitter interaction, transmitter-receiver coupling, or radiation pattern distortion may set the criteria for clearance around the antenna.
In the case of horizontally polarized antennas, the immediate area underneath the antenna has very little effect on the antenna performance, particularly on the antenna impedance, and little grading is required. However, the reflection area in front of the antenna is still important, and the terrain must be relatively smooth for distances of a few thousand metres, depending on the frequency and the elevation angle of interest. Man-made objects will also be of concern; here the problem would primarily involve horizontal conducting objects such as power lines rather than vertical conducting objects. The establishment of minimum separations would be the same as was mentioned for vertical antennas.
A useful reference on siting of radio terminals is National Bureau of Standards Technical Note 139, dated April 1962, prepared by William F. Utlant.
The matter of relative spacing and orientation between antennas is important because it affects land area requirements and electrical performance. When transmitting, an antenna will transfer some of its radiated energy to any other antenna in relatively close proximity, and this transferred energy will affect the performance of the other antenna.
Ideally HF antennas of unlike functions, transmitting and receiving, should be separated by several kilometres (some authorities stipulate a minimum distance of 24 kilometres) if the latter's performance is not to be degraded due to interference from the former. This interference can be caused by adjacent channel operation, harmonics, keying transients and parasitic oscillations. Also, cross modulation products can be generated in HF pre-amplifiers and receivers by strong RF fields, even though normal receiving frequencies are widely separated from the frequencies of such fields.
The amount of energy coupled between antennas of like functions, all receiving or all transmitting, can be calculated accurately by solving the fundamental electromagnetic equations using a comprehensive antenna analysis computer program. However, the following criteria serve as a useful reference for planning purposes. All distances, unless otherwise noted, are based on the antenna's lowest design frequency. The larger of the two distances in each case is used as the spacing distance. The points of measurement are between the reference points listed for each type of antenna.
- Horizontal Log Periodic Antenna - space two wavelengths from
the main lobe and one wavelength outside the main lobe, measured from
the main supporting structure (midway between supporting structures for
two tower configurations).
- Vertical Log Periodic Antenna - spacing requirements are the same as for horizontal log periodic antennas.
- Rotatable Log Periodic Antenna - space two wavelengths
from horizontally polarized antennas. The separation requirement from a
vertically polarized antenna is determined by the spacing requirement of
the vertical antenna. In all cases, spacing must not be less than 45
- Inverted Cone - space one wavelength measured from the antenna center.
- Conical Monopole - spacing requirements are the same as for the inverted cones.
- Sector Log Periodic - space two wavelengths measured from the main supporting structure.