Antennas
5.7 ANTENNA HARDWARE LOSSES
Measure of suppression ' eell2 % 1 eell2 & 1
ratio or 20 log10 eell2 % 1 eell2 & 1
dB
10 12 14 16 18 20 22 24 26 28 30
0 0.5 1 1.5 2 2.5 3
Weighted mean ellipticity, dB
(5.93)
Figure 5.66 The effects of ellipticity on rain echo reduction.
5.6.7 Rain echo suppression
The amount of rain echo reduction [17, p. 7.6] is given by the measure of suppression with e as the average ellipticityell
If (5.93) is weighted by the two-way antenna pattern and integrated, it is called the integrated cancellation ratio, where e is the average ellipticity weighted by the two-way antenna voltage characteristic.ell
Figure 5.66 shows this in graphical form.
blocked sidelobe ratio, p) '
p % wb w 1 & wb
w
voltage ratio
Effective width ' width % depth 8 2
(5.94)
(5.95) This is a so-called loss because it is a reduction of the gain in a practical antenna compared with a uniformly illuminated antenna of the same size. Losses for aperture functions are given in Appendix B. The antenna gain can be restored to the desired value by increasing the size of the antenna or changing the illumination function.
5.7.2 Blocking loss
Any obstacle placed in front of the reflector affects the antenna pattern. Effectively, looking perpendicularly at the directrix plane, there is a hole in this plane. Using the fact that the Fourier transform is the sum of the Fourier transforms of the components, the pattern is the Fourier transform of the illumination of the directrix minus the Fourier transform of the illumination of the “hole”. These effects are shown for one dimension of a rectangular antenna with cosine-squared illumination for 1% and 3% linear blockage in Figure 5.67. The sidelobe levels with and without blocking are shown in Figure 5.68. The equivalent sidelobe envelope for 1% blockage is nearly constant, while that for 3% shows less gain at wider angles. When the linear blocking pattern is subtracted from the linear antenna pattern, the odd sidelobes are increased, and the even sidelobes are decreased. The result in decibels is also shown.
There will be a loss of (coherent) gain caused by the blocking. The loss is 1 - w /w in amplitude, where w is theb b effective blocking width. There is a formula in [3, p. 101] for the increase in odd (1, 3, 5, ...) sidelobes:
where p is the unblocked sidelobe voltage ratio.
For structures that are short along the antenna axis, the area and the shape of the blocking structure may be used.
For horns and similar radiators, there is a rule of thumb [5, p. 144; 22, p. 158] that states that this area should be multiplied by two. Longer structures must be treated as end-fire arrays with an appropriate, larger blocking area extending many times the physical width [17, p. 6.38]
The effects of increased blockage between 0.5% and 10% are shown in Figure 5.68.
5.7.3 Spillover loss
Not all the energy from the transmitter radiated by a horn hits the reflector. Some of the energy passes by around the edges and is called the spillover, causing extra clutter as the echoes enter the horn by the same path. In military radars, this path may be a way for active jamming to enter the antenna. The amount of spillover represents a loss.
-40 -30 -20 -10 0
-20 -10 10 u´ 20
3% blockage modified sidelobes
1% blockage modified sidelobes
Pattern for 1% blockage
Pattern for 3% blockage
Original cosine squared antenna pattern with sidelobes
u´=w sin 2 8
dB
-40 -30 -20 -10 0 -50 -40 Sidelobe level without blocking dB-30 -20 -10
10%
8%
6%
5%
0.5%
1%
1.5%
2%
3%
4%
.
Figure 5.67 The effects of 1% and 3% linear blockage for an antenna with cosine-squared illumination.
Figure 5.68 The effects of blocking on the antenna sidelobes.
Loss ' exp(rms phase error in radians²)
' exp 2 B *
8
2
' 4.3429 2B* 8
2
' 171.4526 * 8
2
dB
-7 -6 -5 -4 -3 -2 -1 0
0.1
0 0.02 0.04 0.06 0.08
Loss caused by the (two-way) tolerance in a reflector Loss caused by the (one-way) tolerance in a plane
Standard deviation or rms value of the tolerance
*
*
*
* 888 8
2B*/8
(5.96)
Figure 5.69 The loss caused by surface errors in an array plane or a reflector.
5.7.4 Surface tolerance loss
The surfaces of a planar array or a reflector always have tolerances. If the standard deviation or root mean square (rms) tolerance of a planar radiator is *, then the phase error standard deviation is radians. Ruze in [17, p. 6-41] has developed the relationship for the loss when radiating from a surface:
Tolerances in reflectors have an effect on the incident and reflected portions of the energy, so the tolerance figure for * must be doubled. These losses are shown in the graph in Figure 5.69.
5.7.5 Losses in power dividers, phase shifters, and other beam-forming network components
The gain and patterns of an antenna are measured on a test range at a particular reference point or waveguide flange.
There are losses between the points where the returning echoes land on the antenna and the point where the gain is measured, as is shown in Figure 5.70.
5.7.6 Other effects giving losses
Reflector antenna Planar array
Reference flanges for gain measurements Path for losses
Path for losses Feed horn
Beam- forming network
GOne way, voltage(2) ' exp &2 ln(2) 22 223
Figure 5.70 Losses up to the reference point for antenna gain measurement.
(5.97) Other effects that are recorded in the patterns during antenna testing are:
• Leakage through the mesh forming the reflector which gives a backlobe diffraction around:
The edges of a planar array;
The edges of a reflector;
The edges of a lens;
• Energy that avoids the reflector or lens and enters the feeder system directly, called spillover.
Leakage and diffraction account for the backlobe of the antenna. Nearby interference can enter the feed system directly which is shown by a small sidelobe at between 90 and 120 degrees. Antenna patterns at elevation angles below zero are rarely made, so screens to reduce interference entering the feed horn under the reflector are exceptional.