Practical_Antenna_Handbook_0071639586

214 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
When Eq. (7.4) is applied to a system having Z s = 52 Ω and Z L = 110 Ω:
Z
0
= (110)(52) = 76Ω (7.8)
This is a very good application of 75-Ω coaxial cable or CATV hardline.
Half-Delta Sloper (HDS)
The half-delta sloper (HDS) antenna (Fig. 7.7) is similar to the full delta loop, except that
(like the quarter-wavelength vertical) half of the antenna is in the form of an “image” in
the ground. Gains of 1.5 to 2 dB are achievable. The HDS antenna consists of two elements:
a l/3-wavelength sloping wire and a l/6 vertical wire on an insulated mast (or
a l/6 metal mast). Because the ground currents are very important, much like the vertical
antenna, either an extensive radial system at both ends is needed or a base ground
return wire must be provided.
The HDS will work on its design frequency, plus harmonics of the design frequency.
For a fundamental frequency of 5 MHz, a vertical segment of 33 ft and a sloping section
of 66 ft are needed. The approximate lengths for any frequency are found from
d
1
(feet) = λ 3 = 328
F(MHz)
(7.9)
Good electrical
and mechanical
connection
d 2 =
6
Metal mast
or wire
d 1 =
3
Earth
A
52
Coax
Ground return wire
Figure 7.7 Half-delta loop (HDS) antenna.