Practical_Antenna_Handbook_0071639586
482 P a r t V I : A n t e n n a s f o r O t h e r F r e q u e n c i e s y ' B Parabolic (y 2 4ux) reflector surface y N M MN MF u u (0,0) F (Focal point) x Advancing wavefronts D Directrix A d Figure 20.27 Ray tracing shows operation of parabolic antenna. Feed illumination refers to how evenly the feed element radiates to the reflector surface. For circular parabolic dishes, a circular waveguide feed produces optimum illumination, and rectangular waveguides are not as good a match. The TE 11 mode is preferred. For best performance, the illumination should drop off evenly from the center to the edge, with the edge being 210 dB down from the center. The diameter, length, and beamwidth of the radiator element (or horn) must be optimized for the specific F/d ratio of the dish. The cutoff frequency is approximated from f cutoff = 175,698 d (20.36) where f cutoff = cutoff frequency in MHz d = inside diameter of circular feedhorn in mm
C h a p t e r 2 0 : M i c r o w a v e W a v e g u i d e s a n d A n t e n n a s 483 The gain of the parabolic dish antenna is found from ξ ( πD) 2 G = 2 λ (20.37) where G = gain over isotropic D = diameter l = wavelength (same units as D) x = reflection efficiency (0.4 to 0.7, with 0.55 being most common) The –3-dB beamwidth of the parabolic dish antenna is approximated by and the focal length by l BW = 70 (20.38) D 2 D F = 16 d (20.39) For receiving applications, the effective aperture is the relevant specification and is found from 2 A = xπ( D / 2) (20.40) e The antenna pattern radiated by the antenna is similar to Fig. 20.22B. With horn illumination, the sidelobes tend to be 23 to 28 dB below the main lobe, or 10 to 15 dB below isotropic. Of the energy radiated by the parabolic dish, 50 percent is within the –3-dB beamwidth, and 90 percent is between the first nulls bracketing the main lobe. If a dipole element is used for the feed device, then a splash plate is placed a quarterwavelength behind the dipole in order to improve illumination. The splash plate must be several wavelengths in diameter and is used to reflect the backlobe back toward the reflector surface. When added to the half-wave phase reversal inherent in the reflection process, the two-way quarter-wavelength adds another half-wavelength and thereby permits the backwave to move out in phase with the front lobe wave. Parabolic Dish Feed Geometries Figure 20.28 shows two methods for feeding parabolic dish antennas, independent of the choice of radiator (horn, dipole, etc.). In Fig. 20.28A the radiator element is placed at the focal point, and a waveguide (or transmission line) is routed to it. This method is used in low-cost installations such as home satellite TV receive-only (TVRO) antennas. Figure 20.28B shows the Cassegrain feed system modeled after the Cassegrain optical telescope. The radiator element is placed at an opening at the center of the dish. A hyperbolic subreflector is placed at the focal point, and it is used to reflect the wavefronts to the radiator element. The Cassegrain system results in lower-noise operation for several reasons:
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482 P a r t V I : A n t e n n a s f o r O t h e r F r e q u e n c i e s<br />
y '<br />
B<br />
Parabolic (y 2 4ux)<br />
reflector surface<br />
y<br />
N<br />
M<br />
MN MF<br />
u<br />
u<br />
(0,0)<br />
F<br />
(Focal point)<br />
x<br />
Advancing<br />
wavefronts<br />
D<br />
Directrix<br />
A<br />
d<br />
Figure 20.27 Ray tracing shows operation of parabolic antenna.<br />
Feed illumination refers to how evenly the feed element radiates to the reflector surface.<br />
For circular parabolic dishes, a circular waveguide feed produces optimum illumination,<br />
and rectangular waveguides are not as good a match. The TE 11 mode is preferred.<br />
For best performance, the illumination should drop off evenly from the center to the<br />
edge, with the edge being 210 dB down from the center. The diameter, length, and<br />
beamwidth of the radiator element (or horn) must be optimized for the specific F/d ratio<br />
of the dish. The cutoff frequency is approximated from<br />
f<br />
cutoff<br />
=<br />
175,698<br />
d<br />
(20.36)<br />
where f cutoff = cutoff frequency in MHz<br />
d = inside diameter of circular feedhorn in mm
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