362 ; - PROPAGATION THROUGH THE STANDARD ATMOSPHERE 
primary element caused by the presence of the screen 
is appreciable. 
Reflecting screens are used primarily in connection 
with broadside arrays (curtains) to eliminate one 
of the two main lobes in opposite directions. An 
adequate screening effect is produced by a set of 
wires parallel to the direction of the radiating dipole 
with spacings somewhat less than a tenth of a 
wavelength. 
Corner-Reflector Antenna 
A simple directional device that gives an appre- 
ciable power gain (of the order of 10 to 20) is a 
corner reflector, which is essentially a combination 
of two reflecting sheets and a dipole. In the case 
shown in Figure 41 where the angle subtended by 
et 
e DIPOLE BA i 
0 IMAGES = ”°N\ ce 
GROSS SECTION 
Ficure 41. Corner reflector antenna. 
the corner is 90°, the corner is equivalent to the 
combined radiation of three image antennas. The 
' reflector can also be made of wires parallel to the 
‘direction of the radiating dipole. The reflecting 
wires do not, however, act as parasitic antennas 
but are taken so long that they are practically 
equivalent to conductors of infinite length. 
These should not be confused with corner re- 
flectors which are extensively used as targets and 
consist then of three mutually perpendicular con- 
ducting planes (see page 472.) 
PARABOLIC ELEMENTS 
Parabolic Reflectors 
These reflectors are the devices most commonly 
used to produce highly directive radiation patterns 
in the microwave region. The three main types are 
shown in Figure 42; they are the parabolic cylinder, 
the paraboloid of revolution, and the truncated 
paraboloid, the latter being a rectangular section 
cut from a paraboloid of revolution. If the parabolic 
cylinder is relatively short and provided with flat 
metallic covers at the top and bottom, its shape and 
DIPOLES AON 
ai \ 
| 
NA / 
“\\ / 
Si 
PARABOLIC B PARABOLOID € TRUNCATED 
CYLINDER OF REVOLUTION PARABOLOID 
Ficure 42. ‘Types of parabolic reflectors. 
its electrical properties resemble those of a sectoral 
horn (see page 363). 
The directive action of the parabolic reflector 
depends on two gecmetrical properties of the parab- 
ola (Figure 43). A ray coming from the focus is 
reflected into a direction parallel to the axis of the 
parabola, and the distance from any point P on the 
parabola to the line called the directrix is equal to 
the distance from P to the focus. Consequently, the 
effect of the parabola in the forward direction is 
equivalent to that of a distribution of sources in the 
directrix that all oscillate in phase (but usually have 
varying intensities over the directrix). 
DIRECTRIX | 
Figure 43. Properties of a parabola. 
The parabolic cylinder produces a directive pattern 
only in a plane perpendicular to the generating line 
of the cylinder (horizontal plane in A of Figure 42). 
In order to concentrate the beam in a plane parallel 
to the generating line of the cylinder (vertical plane 
in Figure 42), an additional directive device must be 
employed. Usually this is a colinear array of dipoles, 
as shown; the direction of polarization is parallel 
to the focal axis. In microwave work this type of 
antenna offers advantages over the two-dimensional 
curtain of dipoles employed in VHF directional 
antennas. 
For the paraboloid of revolution or the truncated 
paraboloid, a simple source of radiation at the focus 
is used. Often this is a half-wave dipole, sometimes 
combined with a parasitic dipole which acts as a 
reflector (page 360). 
In other types, the energy is brought to the focal 
point by a wave guide and is then reflected back onto 
the parabolic surface. 
If the wavelength is small compared with the 
dimensions of the parabolic reflector, the following 
approximate formula holds for the radiation pattern 
produced by a parabola: 
a (fm. @ 
sin E sin =| 
ADAG (1 + cos 6), (40) 
where D is the aperture of the reflector and @ the 
angle from the axis. The half-power points corre- 
spond approximately to 
E = constant 
snd 2@= geen (41) 
