The parasitrc etements are lnsulated from the boom by small phenolic blocks hetd to the boom with U-bolts. The dr(ven element (s attached to the bopm dlreetly The length of the driveri.element may be adjusted to aehleve torvest SWt; after the initial adjustments are made to the tength of the gamma matching rod Ga(n of this dessgn drops off qt,ta quickly on the high side of the desrgn frequency but much more slowly. on the low-frequency sde Plan vtew of the entenna is shown. in A and details of the gamma mazching section are shown at B:.
L
Figure 13. L-P Yagi Antenna for six Meters c This design combines bandwidth of log-periodic strudure with gain of Yagi antenna. L-P Yagi may be built on h" (4.0 i cm) diameter boom, about 19' (5.8 m) long. L-P elements are insulated from boom by mounting on insulating blocks. Yagi elements are grounded to boom at their center point. The antenna is fed with a balanced 70-ohm ribbon line at the feedpoint and the L-P transmission line is made up of No. 8 aluminum clothesline wire, criss-cross conneded between the elements.. Rear element is shorted with 6" loop of aluminum wire. The spacing between the inner tips of the L-P elements is 3.5" (8.9 cm).
Figure 8. The J-Pole Antenna for 50 and 144 MHz The half-wave vertical antenna is a popular omnidirec- tional installation for the vhf bands. A-50 MHz J-pole antenna. The coaxial cable is brought up inside the aluminum pipe, which serves as antenna and mast. The outer shield of the line is grounded to the pipe 6" above the mounting clamp for the matching sedion. The inner condurtor is tapped on the matching sedion as shown. B-144 MHz J-pole antenna. A coaxial receptacle is mounted on the shorting bar. The inner terminal is tapped on the vertical radiator through a 20-pf variable capacitor. Adjustment of the capacitor and the tap point (about 2" above the bar) permit a very low value of SWR to be achieved on the transmission line. This is a simple version of the gamma match.
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