Introduction to the analysis of the offset antenna

The biasing antenna is relative to the feedforward antenna, and means that the feeding source of the biasing antenna and the mounting position of the tuner are not on a line perpendicular to the center plane of the antenna and passing through the center of the antenna. Therefore, there is no influence of the so-called feed shadow, and the gain of the offset feed antenna is larger than that of the positive feed antenna under the premise of the antenna area, the processing accuracy, and the receiving frequency. But whether it is a feedforward antenna or a biasing antenna, they are all cross-sections of a paraboloid of revolution, but the positions of the intercepts are different.

The biasing antenna is relative to the feedforward antenna, and means that the feeding source of the biasing antenna and the mounting position of the tuner are not on a line perpendicular to the center plane of the antenna and passing through the center of the antenna. Therefore, there is no influence of the so-called feed shadow, and the gain of the offset feed antenna is larger than that of the positive feed antenna under the premise of the antenna area, the processing accuracy, and the receiving frequency.

But whether it is a feedforward antenna or a biasing antenna, they are all cross-sections of a paraboloid of revolution, but the positions of the intercepts are different. The feedforward antenna is the part of the curved surface of the rotating paraboloid that is concentric with the rotating parabolic rotating axis. The offset feeding antenna is the part of the curved surface that the rotating paraboloid is cut by the cylindrical surface that is different from the rotating parabolic rotating axis.

In addition, the feed and the high frequency head of the feedforward and offset feed antennas must be mounted at the focus of the paraboloid of rotation. This is determined by the nature of the rotating paraboloid. That is, when the rotation axis of the paraboloid of revolution is directed to the satellite, the electric wave is reflected by the paraboloid and then concentrated in the focus, and the strokes are equal. Here, the mathematical proof of this conclusion is given, and the paraboloid of revolution is provided.

Y2+Z2=4fX, the rotation axis of the paraboloid of revolution, that is, the X axis points to the satellite. The coordinates of the satellite in the space rectangular coordinate system are (a, 0, 0). Since the value of a is very large, the incident direction of the electric wave can be considered as X. The axes are parallel. Without loss of generality, take the curve Y2=4fX where the surface intersects the XOY plane as an example, as shown in Figure 3. If you take a point A (X1, Y1) on this curve, the satellite wave will be reflected by point A to the focus of the stroke L = AF + AC, that is,

L=a-X1+[Y12+(X1-f )2]1/2

= a-X1+[4fX1+X12-2X1f+f2]1/2

= a-X1+[X12+2X1f+f2]1/2

= a-X1+X1+f

= a+f

It can be seen that the travel of the electric wave emitted from the satellite through any point of the paraboloid and then to the focus is a fixed value. It can also be seen that "only when the ratio of the focal length to the aperture of the antenna, that is, f/D = 1/4, the travel of the radio wave emitted from the satellite through any point of the paraboloid and then to the focus is a fixed value." The argument is a wrong concept. However, f/D is an important parameter for designing the antenna. If the value of f/D is too large, the ambient noise behind the antenna will enter the feed; if the value of f/D is too small, The electric wave reflected by the edge of the antenna cannot enter the feed, and the effective area of ​​the antenna is reduced, as shown in FIG. 4 .

The biasing antenna as a section of the paraboloid of rotation must also obey the above conclusions. Therefore, when the rotation axis of the rotating paraboloid is directed to the satellite, the electric wave will be concentrated in the focus after being reflected by the offset antenna, and the electric wave travel is equal. Since the electric wave travel is equal, the electric waves reaching the feed are in phase, so that the waveguide enters the waveguide. The amplitude of the electric wave is increased, thereby playing the role of energy convergence.

Based on this principle, a multi-focal plate antenna was developed, but due to the bandwidth limitation of this antenna, the application is not widespread. Its working principle will be introduced in a future article.

Today, due to the increase in production technology and the increase in production, although the price of a biasing antenna accounts for a smaller proportion of the entire receiving system, it plays a key role in the entire receiving system. Therefore, it is correctly recognized. The working principle has a positive guiding significance for the scientific "fever".

At present, there are many brands that bias the antenna on the market. Most of the products may lack comprehensive use and installation instructions due to the manufacturer's reasons, so they do not provide a parameter and focal length that the fans care about. Since the parameters of the antennas produced by different manufacturers are not consistent, a method for calculating the focal length of the offset antenna by measurement is given here.

First, measure the distance AF and BF of the two intersections A and B of the long-axis of the offset antenna and the outer edge of the antenna to the feed F, and then measure the length of AB;

Second, substituting AF and BF into the equations:

{AF = AFcosθ+2fBF = BFcos(θ+Δθ)+2f

Where Δθ= arccos[(AF2+BF2-AB2)/2AF*BF]

Finally, to solve this system of equations, the obtained result f is the antenna focal length.

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