Practical_Antenna_Handbook_0071639586
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232 P a r t I I I : H i g h - F r e q u e n c y B u i l d i n g - B l o c k A n t e n n a s Grounded Vertical Monopole The vertical dipole is a perfectly fine antenna, and it has formed the basis for many VHF and UHF broadcast antennas for decades. The reader can build it by using exactly the same dimensions as required for the horizontal dipole of Chap. 6. Its “real life” pattern is as shown in Fig. 9.3 as long as it is kept far (many wavelengths) from conducting objects with dimensions comparable to, or larger than, itself. That turns out to be fairly easy (for fixed stations, at least) at VHF and UHF frequencies, and not so easy at MF, HF, and microwave frequencies. It is, in fact, the mechanical difficulty of keeping vertical dipoles far from conducting surfaces that makes the grounded quarter-wave vertical so popular. To suspend a vertical dipole for 80 m, for instance, above the earth requires a support more than 150 ft high! And just bringing the transmission line away from the center of the dipole at right angles would present a completely separate challenge. We saw in Chap. 5 (“Antenna Arrays and Array Gain”) that we can use the earth beneath us as a substitute for one side of a dipole. So, as the saying goes, “If you can’t beat ’em, join ’em!” Over the range of frequencies where we would have to incur tremendous cost and effort to lift a vertical dipole into the air high enough to avoid unbal- Horizontal polar pattern Antenna Horizontal plane Antenna (inside) Lobe Null Vertical pattern Lobe Solid pattern Figure 9.3 Vertical antenna radiation pattern. Vertical plane Null

C h a p t e r 9 : V e r t i c a l l y P o l a r i z e d A n t e n n a s 233 anced currents in the two sides, we typically choose to use a quarter-wave monopole operated against the ground beneath. For the MF and lower HF bands (up through perhaps 5 MHz or so), the simplest approach is to mount the vertical on the earth’s surface. For still higher frequencies, lifting the vertical into the air (to get above any nearby obstructions) works well as long as we also lift an artificial ground plane—consisting of a finite number of (usually equally spaced) radials—with it. The equivalence of grounded l/4 verticals to l/2 vertical dipoles is easily seen if you think of the bottom half of the original vertical dipole as being made of stranded wire. Starting with the dipole, unwrap the strands of the lower wire all the way back to the center insulator and spread them out horizontally, equally spaced around a circle. You have created a ground-plane antenna. Next, take each of the strands and with a very sharp (imaginary) knife, slice it into hundreds of thinner strands. Now spread all of those out equally, as well. Ultimately, you will have a perfectly conducting ground underneath your l/4 vertical. Figure 9.4A shows the basic geometry of the vertical monopole antenna. Here a source of RF is applied at the base of a radiator of length L. Although most commonly encountered verticals are a quarter-wavelength (L = l/4) long, that length is not the only permissible length. For now, however, we will limit our discussion to l/4 verticals. Figure 9.4B shows the current and voltage distribution in a quarter-wavelength vertical. Like the half-wave dipole, the l/4 vertical is fed at a current node, so the feedpoint impedance is at a minimum—typically less than 37 Ω, depending upon nearby objects, diameter of the radiating element, and other factors. Figures 9.4C and 9.4D show the two configurations previously discussed for a l/4 HF vertical antenna. In Fig. 9.4C the radiator element is mounted at ground level but electrically insulated from ground and fed with 52-Ω coaxial cable. The inner conductor of the coaxial cable is connected to the radiator element, while the cable shield is connected to ground at the base of the vertical. For a l/4 radiator, the feedpoint impedance will be lower than 52 Ω, but in most cases, the resulting voltage standing wave ratio (VSWR) is an acceptable tradeoff for simplicity and allows elimination of a matching net- L work or ATU. If the antenna has a feedpoint impedance of 37 Ω, the VSWR will be 52 Ω/37 Ω, or 1.4:1 at the design frequency. An elevated ground-plane vertical is shown in Fig. 9.4D. This antenna is equally as popular as the ground-mounted quarter-wave vertical, especially on 40 m and above. Amateurs and CB operators find it easy to construct this form of HF antenna because the lightweight vertical can be mounted at a reasonable height (15 to 60 ft) using a television antenna slip-up telescoping mast that is reasonably low in cost. As discussed earlier, this antenna replaces the lower half of a vertical dipole with an artificial ground comprised of quarter-Â wavelength radials. Figure 9.4A Basic vertical monopole.

232 P a r t I I I : H i g h - F r e q u e n c y B u i l d i n g - B l o c k A n t e n n a s<br />

Grounded Vertical Monopole<br />

The vertical dipole is a perfectly fine antenna, and it has formed the basis for many VHF<br />

and UHF broadcast antennas for decades. The reader can build it by using exactly the<br />

same dimensions as required for the horizontal dipole of Chap. 6. Its “real life” pattern<br />

is as shown in Fig. 9.3 as long as it is kept far (many wavelengths) from conducting<br />

objects with dimensions comparable to, or larger than, itself. That turns out to be fairly<br />

easy (for fixed stations, at least) at VHF and UHF frequencies, and not so easy at MF,<br />

HF, and microwave frequencies.<br />

It is, in fact, the mechanical difficulty of keeping vertical dipoles far from conducting<br />

surfaces that makes the grounded quarter-wave vertical so popular. To suspend a vertical<br />

dipole for 80 m, for instance, above the earth requires a support more than 150 ft<br />

high! And just bringing the transmission line away from the center of the dipole at right<br />

angles would present a completely separate challenge.<br />

We saw in Chap. 5 (“<strong>Antenna</strong> Arrays and Array Gain”) that we can use the earth<br />

beneath us as a substitute for one side of a dipole. So, as the saying goes, “If you can’t<br />

beat ’em, join ’em!” Over the range of frequencies where we would have to incur tremendous<br />

cost and effort to lift a vertical dipole into the air high enough to avoid unbal-<br />

Horizontal<br />

polar<br />

pattern<br />

<strong>Antenna</strong><br />

Horizontal<br />

plane<br />

<strong>Antenna</strong><br />

(inside)<br />

Lobe<br />

Null<br />

Vertical<br />

pattern<br />

Lobe<br />

Solid<br />

pattern<br />

Figure 9.3 Vertical antenna radiation pattern.<br />

Vertical<br />

plane<br />

Null

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