Practical_Antenna_Handbook_0071639586
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Practical_Antenna_Handbook_0071639586

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426 P a r t V I : A n t e n n a s f o r O t h e r F r e q u e n c i e s Caution A 14-element 2-mYagi is so lightweight it can be carried up the tower in one hand by one person. But long-boom beams, even at VHF/UHF, have a relatively high “windsail area”, and even relatively light winds can apply a lot of force or torque to them. One of the authors once witnessed a muscular technician blown off a ladder by a sudden puff of breeze acting on a modest, “suburban”-sized TV antenna he was holding. It can happen to you, too. Always install antennas with a helper, and use hoists and other tools to actually lift and support the array while it is being attached to the mast or rotor. To speak of a “VHF” or “UHF” antenna is somewhat misleading because virtually all forms of antenna used on the HF or MW bands can also be used at VHF and UHF. The main factors that distinguish supposedly VHF/UHF designs from others are mechanical: Some things are simply much easier to accomplish with small antennas. For example, consider the delta match. At 80 m, the delta-match dimensions are approximately 36 × 43 ft, and at 2 m they are 9.5 × 12 in. Clearly, delta matching is a bit more practical at VHF! Although VHF/UHF antennas are not substantially different from HF antennas, for various practical reasons there are several forms that are especially well suited to the VHF/UHF bands. In this section we will take a look at some of them. Ground-Plane Vertical At VHF and above, the ground-plane antenna is a vertical radiator situated immediately above an artificial RF ground consisting of quarter-wavelength radiators or a sheet of highly conductive material such as copper. Most ground-plane antennas are either ¼-wavelength or 5 ⁄8-wavelength (although for the latter case impedance matching is needed—see the example later in this chapter). Figure 19.1 shows how to construct an extremely simple ground-plane antenna for 2 m and above. The base of the antenna is a single SO-239 chassis-type coaxial connector. Be sure to use the type that requires four small machine screws to hold it to the chassis, not the single-nut variety. The radiating element is a piece of 3 ⁄16-in or 4-mm brass tubing usually obtained at hobby stores that sell model airplanes and modeling supplies. The sizes quoted just happen to fit over the center pin of an SO-239 with only a slight tap from a lightweight hammer. If the inside of the tubing and the connector pin are pretinned with solder, sweat-soldering the joint will make a good electrical connection that is resistant to weathering. Cover the joint with clear lacquer spray for added protection. Tubing is also used for the radials. Alternatively, solid rods can also be used for this purpose. At least three radials equally spaced around a circle are needed for a proper antenna. (Only two are shown in Fig. 19.1.) For this method of construction, four might be a better number, with one radial attached to each of the four SO-239 mounting holes. Gently flatten one end of the radial with the same small hammer, and drill a small hole in the center of the flattened area. Mount the radial to the SO-239 using hardware of the appropriate size (typically 4-40 screws and nuts for this particular SO-239). The SO-239 can be attached to a metal L-bracket that is mounted to a length of 2- × 2-in lumber that serves as the mast. While it is easy to fabricate such a bracket from aluminum stock, it is also possible to buy suitable brackets in any well-equipped hardware store.

C h a p t e r 1 9 : V H F a n d U H F A n t e n n a s 427 Brass tubing or brazing rod element L-bracket SO-239 Radials Wooden support Figure 19.1 Construction of VHF ground-plane antenna. Coaxial Vertical The coaxial vertical is a quarter-wavelength 0111057 vertically FIG 17-19polarized antenna that is popular on VHF/UHF. It is really a l/2 dipole, however, since the exposed quarter-wave center conductor is working against an equal length of exposed shield or braid in line with it, rather than in a ground-plane configuration. The only thing unusual about this antenna is that its method of construction forces the feedline to come away from the antenna inside the braid—an asymmetrical arrangement that has no particular merit and which probably would benefit from the addition of common mode chokes at the bottom of the antenna. Two varieties of this antenna are usually encountered, depending on how permanent it is supposed to be. In Fig. 19.2A we see the coaxial antenna made with coaxial cable. Although not suitable for long-term installations, it is very useful for short-term, portable, or emergency applications. The length of either the exposed insulation or the exposed shield is found from Eq. (19.1):

C h a p t e r 1 9 : V H F a n d U H F A n t e n n a s 427<br />

Brass tubing<br />

or brazing<br />

rod element<br />

L-bracket<br />

SO-239<br />

Radials<br />

Wooden<br />

support<br />

Figure 19.1 Construction of VHF ground-plane antenna.<br />

Coaxial Vertical<br />

The coaxial vertical is a quarter-wavelength 0111057 vertically FIG 17-19polarized antenna that is popular<br />

on VHF/UHF. It is really a l/2 dipole, however, since the exposed quarter-wave center<br />

conductor is working against an equal length of exposed shield or braid in line with it,<br />

rather than in a ground-plane configuration. The only thing unusual about this antenna is<br />

that its method of construction forces the feedline to come away from the antenna inside<br />

the braid—an asymmetrical arrangement that has no particular merit and which probably<br />

would benefit from the addition of common mode chokes at the bottom of the antenna.<br />

Two varieties of this antenna are usually encountered, depending on how permanent<br />

it is supposed to be. In Fig. 19.2A we see the coaxial antenna made with coaxial<br />

cable. Although not suitable for long-term installations, it is very useful for short-term,<br />

portable, or emergency applications. The length of either the exposed insulation or the<br />

exposed shield is found from Eq. (19.1):

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