[Photo] Cross-type transmitting antenna connection
In some small and medium-power high-frequency transmission equipment, multi-layer cross-type half-wave folded oscillator transmission antennas are usually used. The characteristic of this type of antenna is its simple structure and convenient installation. Its disadvantages are its low gain and relatively narrow bandwidth. Here, the double-layer cross-type antenna is used as an example to explain its connection principle. Figure 1 shows the structure of this type of antenna. We know that the input impedance of a half-wave vibrator is balanced 300 ohms, and the output impedance of the transmitting device is usually 50 ohms unbalanced, so first we must balance the 300 ohms equivalent to the oscillator. Convert to 75 ohms unbalanced (for the conversion principle, please refer to my previous articles), then two 75 ohm coaxial cables with length λ (wavelength) (called sub-feeders) will be two layers of east-west oscillators Use two 75-ohm cables of length λ + λ / 4 to connect the north-south vibrators in the upper and lower layers into a node 1 (as shown in Figure 2). The impedance of node 1 is 75/4 = 18.75 ohms, because The output impedance of the transmitting device is 50 ohms, so impedance conversion must also be performed. We use 
This formula is used for calculation, where Z0 is the required impedance characteristic of the matching cable of λ / 4 length, Z1 is the impedance of node 1, and Z2 is the output impedance of the transmitting device (node ​​2). Calculated 
To this end, we can use a 50-ohm cable with a length of λ / 4 and a 75-ohm cable of the same length in parallel to approximate replacement. Why should the east-west and north-south subfeeders differ by λ / 4? This is to make the electromagnetic wave uniformly radiated in the horizontal plane. As the length of the sub feeder, its radiation pattern in the horizontal plane is shown in Figure 3. From the figure, it can be seen that the radiation field is in the southwest.
Northeast, southeast, and northwest are weaker. If the north-south and east-west oscillators differ in phase by π / 2 on the feed, then the radiation field formed is a rotating magnetic field, and the radiation pattern is as shown in the figure As shown in Fig. 4, it can be seen from the figure that the radiation field is relatively uniform in all directions. According to the principle of the transmission line, to generate a phase difference of π / 2, it is only necessary to increase the sub-feeder in a certain direction (such as north-south) by λ / 4. . 
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[Photo] Fabrication of cross antenna ![[Photo] Fabrication of cross antenna]()
Many netizens have made many launch circuit versions, and also used ...
Posted at 2006-04-15 21:07 • 1306 times read
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