The equation above shows antenna size is not the only factor affecting its gain. Efficiency is also important. Antenna efficiency is a catchall parameter that includes the following effects:
Surface accuracy. A measure of the RF smoothness of the reflecting surface of the antenna (which may not be visible if the dish surface has a protective coating).
Profile accuracy. A measure of how closely the shape of the dish approximates a true parabolic shape
Physical size. For larger dish sizes (say above 2.4 m ) it becomes impractical to manufacture and more importantly to transport dishes as a single object. So they tend to be built in petals. The shape and surface accuracy become dependent on supporting struts. This tends to mean that it becomes harder to achieve high efficiency from larger dishes.
Deformation of the dish's profile. Profile deformation can be caused by the effects of wind, aging and structural effects or damage
Aperture blockage. The effect where the LNC and supporting spars (or subreflector of a Cassegrain dish) reduce the amount of signal that reaches the parabolic collecting area of the dish.
Deformation of the dish profile
Losses between feed and LNC
Mismatch effects
A related issue is feed illumination efficiency. This is a measure of how much of the energy reflected from the parabolic surface of the dish is used by the feed. This is explained further in section 4.
The importance of RF smoothness and profile accuracy increases with frequency. As frequency increases the effect of small deviations from the ideal parabolic dish shape become more significant with respect to the wavelength of the signal.
In satellite antenna design there is a trade off between the gain and the efficiency of the antenna. Varying dish efficiency from, say, 65% to 50% will reduce gain by 1.1 dB. Where dish size is relatively small this may not make too much difference - but for larger dish sizes a 1 dB gain reduction could be important if it has to be made up by an increase in dish diameter.
As a general rule of thumb antenna efficiency decreases with increasing dish size. Indicative values of dish performance derived from a recent catalogue are as follows:
band diameter gain calculated Comments Cat No. Retail Cost
(dBi) efficiency (dish only),
($A)
Ku 60 x 65.1 cm 37 75% offset fed D1000 90
Ku 90 x 101 cm 40 65% offset fed D1100 140
Ku 120 x 132 cm 42.5 66% offset fed D1200 245
Ku 1.6 m 44 57% prime focus, D1300 320
C 34 56% solid dish
Ku 1.8 m 45 57% prime focus, D1400 490
C 35 56% solid dish
Ku 2.3 m 46.5 54% prime focus, D1490 750
C 38.3 66% 6 petals
Ku 3.0 m 47.1 35% prime focus, D1550 1450
C 40.2 60% mesh, 4
petals
Ku 3.65 m 49.8 44% prime focus, D1600 1890
C 42.3 66% mesh, 8
petals
Ku 5.0 m 51.2 33% prime focus, D1700 4250
C 44.5 58% mesh, 18
petals
Table 1a: Antenna data (Source: AV/COMM 1995 Catalogue).
band diameter gain calculated Comments
(dBi) efficiency
Ku 1.8m 45.1 59% offset fed
Ku 2.4m 47.6 59% offset fed
Ku 3.6m 51.2 60% prime focus
Ku 3.7m 52.3 73% Gregorian
Ku 4.5m 53.1 60% prime focus
Ku 4.6m 54.3 75% Gregorian
Ku 5.6m 56.2 78% Gregorian
Ku 7.6m 58.5 72% Gregorian
Table 1b: Antenna data (Source: NTA data December 1994).
The values listed in table 1b for antenna gain and efficiency are slightly higher than those listed in table 1a.
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