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Vertically polarized HF antennas
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IN PREVIOUS CHAPTERS, YOU HAVE FOUND THAT THE POLARITY OF AN ANTENNA IS THE
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direction of the electrical (E) field Because the transmitted signal is an orthogonal electromagnetic wave, the magnetic field radiated from the antenna is at right angles to the electric field The direction of the electric field, which sets the polarity of the antenna, is a function of the geometry of the radiator element If the element is vertical, then the antenna polarity is also vertical The signal propagates out from the radiator in all directions of azimuth, making this antenna an omnidirectional radiator Figure 7-1A shows the basic geometry of the vertical antenna: an RF generator (transmitter or transmission line from a transmitter) at the base of a radiator of length L Although most commonly encountered verticals are quarter-wavelength (L = /4), that length is not the only permissible length In fact, it may not even be the most desirable length This chapter covers the standard quarter-wavelength vertical antenna (because it is so popular), and other-length verticals (both greater and less than quarter-wavelength) The quarter-wavelength vertical antenna is basically half of a dipole placed vertically, with the other half of the dipole being the ground Because of this fact, some texts show the vertical with a double-line ghost radiator, or image antenna, in the earth beneath the main antenna element Figure 7-1B shows the current and voltage distribution for the quarter-wavelength vertical Like the dipole, the quarterwavelength vertical is fed at a current loop, so the feedpoint impedance is at a minimum (typically 2 to 37 , depending upon nearby objects and design) As a result, the current is maximum and the voltage is minimum at the feedpoint As you will see, however, not all vertical antennas are fed directly at the current loop As a result, some designs require antenna tuning units to make them match the antenna impedance to the transmitter output impedance Figure 7-1C and 7-1D show the two basic configurations for the HF vertical antenna Figure 7-1C shows the ground-mounted vertical antenna The radiator
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174 Vertically polarized HF antennas
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7-1A Basic elements of the vertical antenna
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7-1B Current and voltage distribution along vertical
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Vertically polarized HF antennas 175
/4 Radiator 7-1C Simple coaxial-fed vertical antenna There is a slight mismatch, but it usually is within tolerable limits
Coax to XMTR
7-1D Mast-mounted vertical uses radials as a counterpoise ground
/4 Radiator
Transmission line
/4 Radials
element is mounted at ground level, but it is insulated from ground Because the antenna shown is a quarter-wavelength, it is fed at a current loop with 52- coaxial cable The inner conductor of the coaxial cable is connected to the radiator element, and the coaxial cable shield is connected to the ground As you will see shortly, the ground system for the vertical antenna is critical to its performance Normally, the feedpoint impedance is not 52 , but rather is somewhat lower (37 ) As a
176 Vertically polarized HF antennas result, without some matching there will be a slight VSWR, but in most cases, the VSWR is a tolerable tradeoff for simplicity If the antenna has a feedpoint impedance of 37 , which is the value usually quoted, then the VSWR will be 52 /37 , or 141:1 A vertical mounted above the ground level is shown in Fig 7-1D This antenna is equally as popular as the ground mounted Amateurs and CB operators find it easy to construct this form of antenna because the lightweight vertical can be mounted at reasonable heights (15 to 60 ft) using television antenna slip-up telescoping masts that are reasonably low in cost A problem with the non-ground-level vertical antenna is that there is no easy way to connect it to ground The solution to the problem is to create a counterpoise (artificial) ground with a system of quarterwavelength radials In general, at least two radials are required for each band, and even that number is marginal The standard wisdom holds that the greater the number of radials, the better the performance Although that statement is true, there are both theoretical and practical limits to the number of radials The theoretical limit is derived from the fact that more than 120 radials returns practically no increase in operational effectiveness, and at more than 16 radials, the returned added effectiveness per new radial is less than is the case for fewer radials That is, going from 16 to 32 radials (doubling the number) creates less of an increase in received field strength at a distant point than going from 8 to 16 radials (both represent doubling the density of the radial system) The practical limit is 16 radials The radials of the off-ground-level vertical antenna can be at any angle In Fig 7-1D, they are drooping radials (ie, the angle is greater than 90 relative to the vertical radiator element) Similarly, Fig 7-1E shows a vertical antenna that is equipped with radials at exactly 90 (no common antenna has radials less than 90 ) Both of these antennas are called ground plane vertical antennas The angle of the vertical s radials is said to affect the feedpoint impedance and the angle of radiation of the vertical antenna Although those statements are undoubtedly true in some sense, there are other factors that also affect those parameters, and they are probably more important in most practical installations Before digging further into the subject of vertical antennas, take a look at the subjects of angle of radiation and gain in vertical antennas
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