Practical_Antenna_Handbook_0071639586

248 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
Elevated Ground-Plane Antenna
There are three key differences between a ground-mounted vertical and an elevated
ground-plane vertical:
• Radials located some distance above ground must be treated as resonant
elements.
• Fewer radials are needed to ensure that most of the transmitter power is
delivered to the radiating element instead of being dissipated in lossy earth.
• An elevated GP vertical may have fewer nearby obstacles (e.g., trees and
buildings) and thus enjoy a cleaner shot at low elevation angles.
Once the base of an elevated ground-plane vertical is raised above a certain height
(many consider l/8 a reasonable minimum), its radials act less like long, skinny capacitors
and more like the other half of a vertical dipole. As such, it is important for
maximum radiation plus ease of matching that their lengths to be approximately l/4 at
the operating frequency, although the exact length will vary somewhat with radial
“droop”—the vertical angle at which the radials come away from the base of the monopole.
However, the farther away the lossy earth is from these elevated radials, the less
effect it has and the easier it is to capture the bulk of the return currents of the vertical
radiating element in a small number of radials. Recent experiments based on detailed
modeling using NEC-4 (see Chap. 25) indicate that somewhere between two and eight
radials per band is quite adequate for elevated antennas, although unless at least three
equispaced radials are used, there will be some variation in field strength with azimuth
(compass heading). The improvement in radiated field characteristics for more than
three or four radials is quite subtle, however, as long as the GP antenna is kept at least
l/8 off the ground. Models of a l/4 antenna show little difference between horizontal
and drooping radials other than to raise the feedpoint impedance from 25 to 40 Ω.
Keep in mind that these discussions of height above earth ground refer to the height
of the electrical ground, not the sod. Depending upon ground conductivity and groundwater
content, the effective height of earth ground may lie some distance beneath the
surface. The actual depth is best found from experimentation and may, unfortunately,
vary with precipitation and with the season—especially if the ground freezes and/or
the local water table changes greatly.
Vertical versus Horizontal Polarization
A question frequently asked is: “Would I be better off putting up a horizontal dipole or
a vertical?” As usual, the answer is: “It depends.” Here are some factors often found to
be helpful in arriving at an answer:
• Is there a “convention” regarding antenna orientation (polarization)? Usually
because of historical patterns of usage that have evolved, certain groups have
standardized on specific polarizations. The 11-m citizens band, for instance,
uses vertical polarization because of the high percentage of vehicular mobile
users. As a result, almost all house- or tower-mounted antennas—even
multielement Yagis and quads—for 27 MHz are vertically polarized. The same