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558 General antenna mechanical construction techniques
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28-14 Two alternate pulley systems for wire antenna mounting
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ability to raise and lower the antenna But do not let a loose end slip, or you will be chasing the rope up the mast as it slips through the pulley Pulleys work nicely, but they are mechanical contraptions that tend to fail occasionally When they fail, the rope gets stuck Pulleys also can corrode rather badly, with the same end result All in all, a pulley is not the best solution for most people Figure 28-15 shows a better way In this approach, a U-bolt is fastened to the top of the support mast If the U-bolt is made of brass, then all the better, for it will not corrode The rope can be passed through the U-bolt as in the pulley system but without the mechanical failure problem Be sure to install the U-bolt all the way up to the threads so that the rope will not chafe against them The method shown in Fig 28-16 can only be used by a horse lover! This method uses a brass stirrup in place of the U-bolt If you have switched to cars or trucks and no longer need your horse stirrups, then perhaps this is a viable means of attaching the support rope to the top of the mast
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Masts and other supports for wire antennas 559
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28-15 U-bolt end mounting
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560 General antenna mechanical construction techniques
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Installing vertical antennas
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Despite continuous (and mostly untrue) stories of very poor performance, the fouror five-band commercially manufactured trap vertical antenna remains one of the most popular amateur radio antlers in the HF bands They are economical compared with directional rotating beam antennas, easy to erect, and do not have to occupy a lot of real estate (the footprint of the vertical can be very small, especially if you bury the radials) Unfortunately, a misinstalled vertical is both dangerous and a very poor performer This section covers both problems in order to give you a good chance of success
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Figure 28-17 shows the usual form of multiband trap vertical antenna Each trap (TR1 TR3) is a parallel resonant LC tank circuit that blocks a certain frequency but passes all others In Fig 28-17, TR1 is the 10-m trap, TR2 is the 15-m trap, and TR3 is the 20-m trap No 40-m trap is needed because the antenna resonates the entire length of the tubing on 40 m Each section (except perhaps the 10-m section) is actually a little shorter than might be expected from the standard quarter-wavelength formulas This is because the traps tend to act inductively and so lessen the length required to resonate on any given band The vertical antenna manufacturer might give suggested lengths for the various segments between traps Do not make the mistake of assuming that these are absolute numbers They are only recommended starting points, even though the literature packed with the antenna may suggest otherwise Loosely (meaning do not tighten the clamps too much) but safely install the antenna, and then adjust each segment for resonance Start with the 10-m band, and then work each lower-frequency band in succession: 10-15-20-40, etc After each lower band is adjusted, recheck the higher bands to make sure that nothing has shifted, because there might be a little interaction between bands Once the antenna is properly resonant, tighten the clamps, and make the final installation I know this is a pain in the neck, and means putting the antenna up and taking it down a couple of times, but it pays dividends in the end I failed to do this once and found that the 15-m band was useless; it resonated at 192 MHz Radials make or break a vertical antenna; they form the ground plane for the antenna AM broadcast stations typically use vertical antennas and must have 120 radials for the ground plane Antenna reference books usually contain a graph plotted to show the effectiveness of radials and demonstrate a decreasing return on investment after about 32 radials For amateur work, I recommend not less than 2 radials per band, and preferably 4, arranged so that they are equally spaced around the antenna If you cannot space them correctly, never fear; they will work anyway On a four-band antenna, this means 16 radials, which really is not a lot Of course, the general rule is that the more radials, the better is the antenna, at least up to the point where diminishing returns are realized The radials are made of no 14 wire and must be a quarter wavelength (246/FMHz) Figure 28-18 shows how to mount the radials on a mast-mounted (or roof-mounted) installation, and Fig 28-19 shows radials in a ground-mounted situa-
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