Practical_Antenna_Handbook_0071639586
C h a p t e r 2 8 : S u p p o r t s f o r W i r e s a n d V e r t i c a l s 643 In theory, a pulley is the best way to change the direction of a support line’s path, but few users are willing to pay for the right pulley for the task. The wrong device will tend to rust if made of ferrous metal, or crack if made of plastic. Equally important, the size of the pulley and the diameter of the cordage running through it must be correctly matched to avoid jams. The smaller the diameter of the pulley sheave, the shorter the life of the rope that lies in its groove. When selecting a pulley, make sure the groove is neither too wide nor too narrow for the size of line to be run through it. All in all, a pulley is not always the best solution if you have neither the time nor the inclination to select and pay for the right one. Figure 28.14 shows a “poor man’s” pulley, suitable for temporary installations or lightweight (i.e., higher-Âfrequency) dipoles. In this approach, a U-Âbolt is fastened to the top of the support mast. If the U-Âbolt is made of brass or galvanized steel, all the better, for it will not corrode. (In general, automobile muffler U-Âbolts and inexpensive plated U-Âbolts sold at the local hardware store will rust almost immediately; do not use them!) The rope can be passed through the U-Âbolt as in the pulley system but without fear of jamming. Be sure to install the U-Âbolt all the way up to the threads so that the rope has no chance to chafe against them. The method shown in Fig. 28.15 can only be loved by equestrians! A brass stirrup replaces the U-Âbolt. If you have switched to cars or trucks and no longer need your Figure 28.14 U-Âbolt end mounting.
644 p a r t V I I I : M e c h a n i c a l C o n s t r u c t i o n a n d I n s t a l l a t i o n T e c h n i q u e s Figure 28.15 Stirrup end mounting. horse stirrups, then perhaps this is a viable means of attaching the support rope to the top of the mast. Strain Relief of the Antenna For long dipoles with heavy center insulators and coaxial cable feedlines, the ideal installation includes a high center support capable of supplying strain relief and thus minimizing sag at the dipole center. Such a support also reduces longitudinal forces (tensions) on the dipole wires, end insulators, and halyards or hoist lines at both ends. Most commonly it is used with inverted-Âvee antennas, where the center of the antenna is pulled up high on the only available tall support. Figure 28.16 shows an alternate method of strain relief for the center insulator or balun to reduce total sag from the height of the dipole ends to its center. A strong messenger cable is run between two end supports (which must themselves also be quite strong), and tensioned for minimal sag. By suspending the center of the dipole from the messenger, forces on the various electrical and mechanical components of dipole are substantially lessened. If the messenger cable is a conductor (stranded galvanized steel guy wire material is a favorite), it should be broken up with insulators to prevent detuning the wire antenna. However, there’s no reason the messenger cable can’t be run at right angles to the antenna wire (using an entirely different pair of end supports), in which case the need for insulators disappears.
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644 p a r t V I I I : M e c h a n i c a l C o n s t r u c t i o n a n d I n s t a l l a t i o n T e c h n i q u e s<br />
Figure 28.15 Stirrup end mounting.<br />
horse stirrups, then perhaps this is a viable means of attaching the support rope to the<br />
top of the mast.<br />
Strain Relief of the <strong>Antenna</strong><br />
For long dipoles with heavy center insulators and coaxial cable feedlines, the ideal installation<br />
includes a high center support capable of supplying strain relief and thus<br />
minimizing sag at the dipole center. Such a support also reduces longitudinal forces<br />
(tensions) on the dipole wires, end insulators, and halyards or hoist lines at both ends.<br />
Most commonly it is used with inverted-Âvee antennas, where the center of the antenna<br />
is pulled up high on the only available tall support.<br />
Figure 28.16 shows an alternate method of strain relief for the center insulator or<br />
balun to reduce total sag from the height of the dipole ends to its center. A strong messenger<br />
cable is run between two end supports (which must themselves also be quite<br />
strong), and tensioned for minimal sag. By suspending the center of the dipole from the<br />
messenger, forces on the various electrical and mechanical components of dipole are<br />
substantially lessened. If the messenger cable is a conductor (stranded galvanized steel<br />
guy wire material is a favorite), it should be broken up with insulators to prevent detuning<br />
the wire antenna. However, there’s no reason the messenger cable can’t be run<br />
at right angles to the antenna wire (using an entirely different pair of end supports), in<br />
which case the need for insulators disappears.
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