To judge whether a pair of antennas is good or bad, you need some of the major technical parameters of antenna to help you. The main technical parameters of the receiving antenna include the efficiency, gain, main lobe width, and ratio of focus diameter.
- Antenna efficiency
Antenna efficiency means that the ratio of the maximum output power the antenna actually sends to the matched load and the maximum output power the antenna sends to the matched load when it is assumed lossless. Antenna efficiency is normally 40% to 70%, and the efficiency of offset reflector antenna is up to 80%.
- Gain
Antenna gain is the radiant power density of the antenna at a distance in the direction of the maximum radiation. The weaker the signal, the higher requirements is needed for the antenna gain. Antenna gain is related to the aperture and in direct proportion with the square of the antenna radius. That means the larger the aperture is, the more the gain will be. The gain is also related to signal frequency and in direct proportion with the square of the signal frequency. That means the higher the signal frequency is the greater gain will be got.
- Main lobe width
The center radiation lobe called main lobe, the rest are side lobes. It will be better if there is less and smaller side lobe. Main lobe includes the radiation lobe which is in the direction of the largest radiation. And in this direction the antenna will have the best radiation performance. The major signals for the receiving antenna are mainly from main lobe. Main lobe width means its signal reception angular region. The narrower the lobe is, the narrower the signal receiving angle will be. But if the lobe is too wide, it is likely to receive the nearby satellite signal that will cause interference. Lobe width is inversely proportional to the antenna aperture and signal frequency, so accordingly, the larger the antenna aperture, the narrower the lobe width; and the higher the frequency of the signal, the narrower the lobe width.
Half-power angle is used to reflect the width of lobe. The smaller the half-power angle is, the greater the directivity and the radiation power will be. The half-power is inversely proportional to the diameter of the antenna, but in direct proportion to the work wavelength. That is to say, the half-power angle of a small aperture antenna is comparatively large. So the antenna pointing is relatively easy. And it is not sensitive to the drift of the satellite. But the large aperture antenna is opposite, difficult of pointing and sensitive to the drift of the satellite, because of the comparatively smaller half-power angle.
- F/D
The focal length of prime focus antenna is the distance from the center of the antenna to the main focus. The normal prime focus antenna F/D is between 0.3 and 0.4. F means the parabolic focus of the antenna, and D means the diameter of the antenna actinal surface. According to the mathematical calculation, when F/D=3.8, the performance of the rotating parabolic reflector antenna is the best. So the common finished prime focus parabolic reflector antennas are mainly taking F/D=3.8.