Practical_Antenna_Handbook_0071639586

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.