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CHAPTER
Directional phased vertical antennas
THE VERTICAL ANTENNA IS A PERENNIAL FAVORITE WITH RADIO COMMUNICATIONS
users The vertical is either praised, or cursed, depending upon the luck of the owner DXability is usually the criterion for judging the antenna s quality Some amateurs can t get out of their backyards with a vertical, and they let everyone within earshot know that such and such a brand is no good Yet, another person routinely works New Zealand or Australia on 15 m using exactly the same brand of vertical The proper installation of vertical antennas is dealt with in another chapter, so, for the present, let s look at another problem attributed to vertical antennas That problem is that vertical antennas are omnidirectional in the azimuth aspect; that is, they send out and receive equally well from all directions Some people moan that this pattern dissipates their power, and gives them a weaker signal out where it counts (true) However, the main disadvantage of the omnidirectional pattern is noise (QRN and QRM) QRN is natural noise from thunderstorms and other sources QRM is man-made noise, and can consist of other stations or the many assorted forms of electrical filth that pollute the airwaves All forms of noise, however, have one thing in common: they are directional with respect to the station In other words, if you could null signals coming from the direction of the noise source (or undesired station), you would be able to hear desired stations much better A directional antenna performs this task, so let s look at some vertically polarized directional antennas Although most amateurs seem to think that the effective radiated power (ERP) increase that the directional antenna gives them is the real reason to own one, the main benefit is actually on receive Think about it for a moment With anywhere from 100 to 1500 W available, the increase or decrease in signal strength (due to the directivity of the antenna) results in a minimal difference on the receive end, especially during good DX conditions If we rotate the directional pattern, to null out interference, then we usually find that the change in our signal strength perceived by the other guy is small; the S meter reading of the desired station is minimally affected; but the amplitude of the interference source is greatly attenuated! The overall effect is an apparent increase
Copyright 2001 - the McGraw-Hill Companies
246 Directional phased vertical antennas in the other guy s signal, even though the S meter tells a slightly different story The improvement of signal-to-noise ratio (SNR) is tremendously improved
Directivity and phasing
So, how does a vertical antenna owner get the benefit of directivity without the kilobuck investment that a beam or quad costs The usual solution is to use phased verticals AM broadcast stations, with more than one tower, are using this type of system (although for different reasons than hams) The idea is to place two or more antennas in close proximity and feed them at specific phase angles to produce a desired radiation pattern A lot of material is available in the literature on phased vertical antenna systems, and it is far too much to be reproduced here There are standard patterns dating from before World War II that are created with different spacings and different phase angles of feed current In this chapter, we will consider only one system Figure 11-1 shows the patterns for a pair of quarter-wavelength vertical antennas spaced a half-wavelength (180 ) apart Without getting into complex phase shifting networks, there are basically two phasings that are easily obtained: 0 (antennas in phase) and 180 (antennas out of phase with each other) When the two antennas (A and B) are fed in phase with equal currents (Fig 111A), the radiation pattern (shown somewhat idealized here) is a bidirectional figure 8 that is directionally perpendicular to the line of centers between the two antennas; this pattern is called a broadside pattern A sharp null exists along the line of centers (A-B) When the antennas are fed out of phase with each other by 180 (Fig 11-1B), the pattern rotates 90 (a quarter way around the compass) and now exhibits directivity along the line of the centers (A-B); this is the end fire pattern The interference cancelling null is now perpendicular to line A-B It should be apparent that you can select your directivity by selecting the phase angle of the feed currents in the two antennas Figure 11-2 shows the two feeding
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