VHF and UHF Antennas
Describes a wide range of antenna designs and the fundamentals of their operation. Particular attention is paid to the effects of an antenna's ambient environment and the structure upon which it is mounted (permanent or mobile), and methods of predicting and measuring its performance.
Inspec keywords: slot antennas; broadband antennas; loop antennas; monopole antennas; directive antennas
Other keywords: slot antennas; directional antenna; VHF antennas; bodyborne antennas; monopole antennas; antenna measurements; UHF antennas; performance prediction; mobile antennas; feed systems; loop antennas; notch antenna; electrically-small antennas; direction finding antennas; broadband antennas
Subjects: Antennas
- Book DOI: 10.1049/PBEW035E
- Chapter DOI: 10.1049/PBEW035E
- ISBN: 9780863412691
- e-ISBN: 9781849193917
- Page count: 312
- Format: PDF
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Front Matter
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1 Introduction
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This chapter is dedicated to antennas that gives the reader new ideas for antennas. It is a book primarily for the antenna designer and user. Mathematics has been kept to the minimum necessary to provide guidance, particularly where the number of parameters involved would require a large number of graphs to give the information. In some instances a number of different formulae have been evolved for one particular antenna. Where possible, the merits and demerits of competing theories are discussed.
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2 The dipole
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This chapter discusses the dipole. For a straight thin dipole the current will be assumed to be sinusoidal falling to zero at the ends. If the dipole is not thin the zero current point is at the centre of each end; i.e. there is current on the end surface. If the dipole is fed on its axis the radiation pattern in the equatorial plane will be circular. If the diameter is sufficiently large for the surface currents to be considered as a ring of thin dipoles fed in phase, the radiation pattern then becomes dependent on Jo(kR) where R is the radius of the dipole cross-section and can be zero for some values of R.
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3 Monopole antennas
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In practice the monopole is not simply Haifa dipole; such a situation is only true when the ground plane is infinite and, as will be demonstrated below, even very large ground planes give radiation patterns significantly different from that on an infinite plane. The ground plane affects the performance of the monopole not only because it is finite in size but because the capacitance between the base of the monopole and the ground plane differs from that between two halves of a dipole.
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4 The loop antenna
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Much has been written about the theory of loop antennas but most of it refers to loops in which the current is constant round the loop and in phase. In practice this applies only to small loops with a single feed or to large loops with a multiplicity of in-phase feeds.
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5 Slot antennas
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Impetus to the development of the slot antenna was given by the need for VHF and UHF antennas with low aerodynamic drag for military aircraft in World War Two and subsequently for the first generation of civil airliners post war. The cavity-backed half-wave slot antenna was among the first 'suppressed' antennas, today generally referred to as 'flush mounted'. As aircraft engines have increased in power the desperate need to keep aerodynamic drag to a minimum has largely disappeared and the undoubted structural problems posed by slot antennas, particularly in pressurised fuselages, has led to a diminution in the use of slot antennas in this particular field. Nevertheless, slot antennas still have their uses, sometimes in surprising areas. If this chapter appears to have a large proportion of historical applications this is because much of the development work occurred between 1940 and 1960. The results are still valid and it is important that this body of knowledge should not be lost to today's engineers.
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6 The notch antenna
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Notch antenna is most probably used for HF communication on high performance aircraft but in fact its first use was for VHF telemetry and command on aircraft and missiles. These early antennas were all short, narrow-band devices; the self-resonant notch which is physically practical in the VHF and UHF bands is the main topic of this chapter.
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7 Directional antennas
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This chapter is concerned mainly with antennas of modest gain in the range 5 to 15 dBi. It does not consider shaped reflector or 'dish' antennas. Certain other directional antennas, i.e. slot, notch and log periodic types, are discussed in Chapters 5, 6 and 8 respectively. The main classes discussed here are: Aperiodic reflectors, Parasitic elements, Travelling wave antennas, Short backfire, Helix, Arrays of elements, and Steerable arrays.
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8 Broadband antennas
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What constitutes a broadband antenna is subjective, which is why some antennas so described by their inventors may not be considered so by potential users. An essential feature is that certain characteristics - usually including radiation pattern coverage and VSWR- should remain within specified limits over a. frequency range usually at least an octave. The limits will vary according to the specification; thus an antenna whose VSWR is less than 5:1 over an octave might be a satisfactory broadband receiving antenna for electronic surveillance monitoring (ESM) but could be unsuitable for a transmitting role in an electronic countermeasures system (ECM). It should be noted that the main bandwidth limitation is likely to be radiation pattern coverage rather than VSWR.
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9 Electrically-small antennas
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Few engineers would choose to use an electrically-small antenna on purely electrical grounds, except for the important characteristic of selectivity in connection with the short notch. There are, however, many reasons for using small antennas. There is no consensus on the definition of 'electrically small': some workers adopt a maximum dimension from the feed point of λ/30 but Schelkunoff and Friis suggest λ/8. Now it is perfectly possible to achieve large instantaneous bandwidths with a top-loaded λ/8 monopole or with Josephson's open folded monopole so this seems rather too large an upper limit. The A/30 criterion was probably chosen because it permits the use of two approximations: constant current in the small loop and linear current in the short monopole or dipole. In general the characteristic expected of an electrically short antenna is low radiation resistance and large reactance; hence very small instantaneous bandwidth with respect to the impedance of normal radio equipment. The short notch has a high radiation resistance at the mouth but may nevertheless have a reasonably high Q.
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10 Bodyborne antennas
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Initially most bodyborne radio equipment was used by the military, particularly the infantry, and the equipment was cumbersome necessitating the wearer being festooned with a number of radio units from one of which an antenna projected. The advent of transistorised equipment has made it possible to package the equipment in much smaller units allowing more flexibility in its positioning on the body. With the use of higher frequencies than the original 30-80 MHz, personal handsets were introduced. These would not have been practical for significant ranges without the use of elevated base stations; networks of these as in the cellular radio systems permit coverage over a wide area. Without such networks communication ranges are still limited by propagation considerations. For this reason operations in rural areas still rely largely on the lower VHF bands particularly in the military field where mobility may mean that there are no elevated base stations.
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11 Direction finding antennas
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This chapter discussed the direction finding antennas having two classes to distinguish this. These are (i) Terrestrial or airborne types requiring omniazimuth but limited elevation coverage (ii) Systems for tracking space vehicles and radio stars where essentially hemispherical coverage is required.
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12 Mobile antennas
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Many systems for communication and navigation involving vehicles of any kind use the VHF and UHF bands because propagation characteristics permit the use of simple, low-gain, omnidirectional antennas. In general the siting constraints of such antennas are less severe than with more complex systems and there is more scope to suit the antenna to the vehicle.In all instances the performance of the antenna, both as regards radiation pattern and impedance, will be modified to some extent by the vehicle on which it is mounted. The antenna itself may be simple but the combination of vehicle and antenna can be an electrically complex structure. A feel for the behaviour of an antenna on a vehicle can be obtained by considering its performance on appropriate regularly-shaped conductors such as flat sheets, cylinders or cubes. To get more precise information on radiation patterns recourse must be made either to measurements on, for example,scale models or to numerical methods. These are dealt with in Chapters 15 and 14, respectively, but some results will be shown in this chapter.
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13 Feed systems
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In many types of vehicle it is operationally undesirable to have only one cable from antenna to radio. In a civil airliner there may be the need to take the cable through pressurised bulkheads and this may be best achieved if a sealed junction is incorporated in the bulkhead. Very long cables are tedious to install and very difficult to replace so junctions may be necessary for maintenance. In military aircraft the need to take the vehicle apart for transport may similarly necessitate cable breaks. It must also be appreciated that it is rarely possible to run cables in a direct line. Thus from an upper VHF antenna on a wide-bodied jet to the radio equipment in a direct line below it could well involve 10 m of cable. From the aircraft extremities considerable lengths will be required. In calculating the installed gain of any system, therefore, some estimate must be made for cable attenuation and mismatch effects. The type of assessment necessary is outlined below for a typical IFF installation operating in the 960 1220 MHz band.
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14 Performance prediction
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Before undertaking any form of prediction of antenna performance it is essential to consider several aspects: (a) Accuracy (b) Time scale (c) Cost. These are very much interrelated so some trade-offs may be possible.
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15 Antenna measurements
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Accurate measurements are essential for successful antenna design, in the development stage, in qualification, and in production testing. The proliferation of automatic test equipment for impedance measurement and for radiation pattern recording, while making faster measurements possible, has not necessarily improved the accuracy of measurement or reduced the likelihood of errors being made. In fact, when less complex equipment had to be used more care was taken in ensuring that the measurement conditions were appropriate and in taking the measurements. Like computers, modern automatic test equipment is equally good at producing large quantities of rubbish if inappropriately used.
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Appendix 1: Calculation of loss resistance
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Although the loss resistance per unit length of an antenna remains constant, the power loss will vary if the current is not constant. This is the case, for example, when the antenna is open-ended since the current is zero at the open end. This is strictly true only for an infinitely thin element but is sufficiently true for antennas of large length/diameter. An equivalent total loss resistance at the base of the antenna is needed to add to the base radiation resistance.
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Back Matter
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