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The law of reciprocity
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Antennas possess a property called reciprocity That is a fancy way of saying that an antenna works on reception the same way it does for transmission Although articles occasionally appear in the literature claiming an HF or VHF design which violates the law of reciprocity, to date all have depended on either variable definitions, false premises, or faulty measurements There is even a school of thought that falsely argues against antenna reciprocity based on an ionospheric anomaly in which propagation depends on the direction traveled (see the end of Chap 2) The bottom line of reciprocity, for the SWL, is that every antenna described in this book can be also used with equal results on receive For example, a half-wavelength dipole works equally well as a receiver antenna, or as a transmitter antenna
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Which properties are important
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Selecting a receiver antenna is a function of several factors Assuming that you want more than a simple longwire (which we will deal with shortly), you will want to home in on the properties desired for your particular monitoring application Is the antenna to be fixed or rotatable Do you want omnidirectional or directional reception In which plane What about gain
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Copyright 2001 - the McGraw-Hill Companies
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272 Antennas for shortwave reception What about gain What is gain, for that matter The concept of antenna gain derives from the fact that directional antennas focus energy Two kinds of gain figure are often quoted: gain referenced to a dipole antenna on the same frequency, and gain relative to a theoretical construct called an isotropic radiator In the isotropic case, the reference radiator to which the antenna is compared is a spherical point source that radiates equally well in all directions The dipole exhibits a figure-8 radiation and reception pattern The gain of an antenna is merely the ratio (usually expressed in decibels) of the power radiated in a given direction by two antennas (ie, the reference antenna and the test antenna) If an antenna gain is listed as 8-dB gain over isotropic, this means that, in the direction specified, the power radiated is 8 dB higher than the same total power applied to an isotropic radiator So, of what use is antenna gain Two answers immediately present themselves First, by accumulating more signal, the antenna essentially makes your receiver more sensitive Note that the gain of the antenna does not create a higher powered signal, it merely increases the apparent signal power by focusing energy from a given direction And, note well that gain implies directivity Any antenna that claims to provide gain, but that is truly omnidirectional in all planes, is a fraud The fundamental assumption is that gain implies directivity The concept of directivity (hence also of gain) is often taken to mean horizontal directivity, which is the case of a dipole antenna But all forms of antenna radiate in three-dimensional space Azimuth angle of radiation, and elevation angle of radiation, are both important Certain 2-m vertical antennas are listed as gain antennas, yet the pattern in the horizontal direction is 360 implying omnidirectional behavior In the vertical plane, however, lost energy is compressed into a smaller range of elevation angles, so gain occurs by refocusing energy that would have been radiated at a higher than useful angle The second application of directivity is in suppressing interfering signals On the regular AM and FM broadcast bands, channelization permits receiver selectivity to overcome adjacent channel interference in most cases But in the HF amateur radio and international broadcast bands, channelization is either nonexistent, poorly defined, or ignored altogether In these cases, interfering adjacent channel signals can wipe out a weaker, desired station Similarly, with cochannel interference (ie, when both stations are on the same frequency), two or more signals compete in a dog fight that neither will ever totally win Consider Fig 13-1A Assume that two 9540-kHz signals, S1 and S2, arrive at the same omnidirectional vertical antenna Either both signals will be heard, or the stronger signal will drown out the weaker signal Now, consider Fig 13-1B Here, a dipole is used as the receiving antenna, so a little directivity is obtained The main lobes of the dipole are wide enough to provide decent reception of signal S1, even though the antenna is positioned such that S1 is not along the maximum line (dotted) But the positioning shown places the interfering cochannel signal (S2) in the null off the ends of the dipole, so it weakens it considerably The result will be enhanced reception of S1 In Fig 13-1B, the idea is not to exploit the ability of the gain antenna to increase the level of S1 Indeed, by placing the antenna as shown, we are not getting S1 levels as high as might otherwise be
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