Practical_Antenna_Handbook_0071639586

C h a p t e r 6 : D i p o l e s a n d D o u b l e t s 203
Extended Double Zepp Antenna
As the length of each side of a half-wave dipole is lengthened beyond l/4, the gain (or
directivity) of the main lobe increases. This continues until each side is 5l/8 long, at
which point current reversal in adjacent l/2 sections begins to progressively subtract
from the main lobe. Thus, the maximum gain of a standard dipole “doughnut” or figure
eight pattern occurs when the total length of the antenna is 2 × 5/8 l, or 5/4 l. This
configuration (Fig. 6.17) is known as the extended double Zepp (EDZ) antenna, and it
provides a gain of about 2 dB over a l/2 dipole at right angles to the antenna wire
plane. It consists of two sections of wire, each one of a length
600
L1
(feet) =
F(MHz)
(6.11)
Typical lengths for L 1 (or half the total wire length) are 20.7 ft on 10 m, 28 ft on 15 m, 42
ft on 20 m, 84 ft on 40 m, and 168 ft on 80 m.
Because the feedpoint of the extended double Zepp is midway between a current
node and a voltage node, the input impedance is also midway between high and low,
and contains reactance as well as resistance. The antenna can be fed directly with 450-Ω
open-wire line, especially if a balanced antenna tuner is available at the equipment end
of the line. Alternatively, it can be fed from a quarter-wavelength matching section
(made of 450-Ω twin-lead or equivalent open air parallel line) L 2 , as shown, or a balun
if coaxial cable is preferred. The length of the matching section should be
103
L2(feet)
=
F(MHz)
(6.12)
The extended double Zepp will work on several different bands. For example, a
20-m EDZ will work as an EDZ on the design band, as a slightly overlength l/2 dipole
on 40 m, as a slightly short l/2 dipole on 80 m, and as a four-lobe cloverleaf-patterned
antenna on frequencies above the design band. If multiband use is intended, the best
feed system is open-wire line back to an ATU.
L 1 L 1
Rope
EI
EI
Rope
L 2
(Z L 150 )
Figure 6.17 Extended double Zepp (EDZ) antenna.