The influence of SWR on the operation of the radio station. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Antennas. Measurements, setup and matching

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Most shortwaves use feeders made of coaxial cable to feed the antennas. Such a cable is not always easy to match with an antenna, especially if it is designed to operate on multiple bands. With poor matching, standing waves occur in the cable, which degrades the operation of the radio for the following reasons:

- efficiency decreases. feeder, and, consequently, the overall efficiency. transmitter; the real sensitivity of the receiver decreases;

- the maximum power that can be supplied to the antenna-feeder device is reduced;

- it is difficult to coordinate the transmitter and receiver with the antenna-feeder device.

However, it can be concluded that the existing antenna-feeder device will work noticeably better if the SWR is reduced to 1 only after comparing its characteristics with the existing SWR and SWR=1.

efficiency feeder and the proximity of its regime to the maximum allowable can be determined by calculation. Shown in fig. 1-5 graphs allow you to carry out the necessary calculations for feeders made from the following coaxial cables: RK 50-2-11 (RK-19), RK 75-4-11 (RK-1), RK 50-9-11 (RK -6) and RK 75-9-13 (RK-3). The design data of these cables are shown in the table.

With sufficient accuracy for practice, it is possible to apply graphs for coaxial cables of other types (with a characteristic impedance of 50 and 75 ohms), taking the average values ​​​​between the data obtained for two cables, the outer diameters of which are smaller and larger than the outer diameter of the cable used. On fig. 1, 2, 3 shows the dependence of the efficiency of the feeder on its length for amateur shortwave bands with SWR = 1. On fig. 4 shows the dependence of the efficiency of any cable on the SWR value.

Ris.1

Consider an example of using charts. On the range of 20 m, a feeder is used from the PK 75-4-11 cable, 32 m long; SWR=3. From the graph in Fig. 1 we determine that with SWR=1 this feeder would have h=80%. Therefore, from the graph in Fig. 4 we determine the real efficiency - 73%.

Ris.2

Consideration of the graphs shows that when using cables with an outer diameter of 12 mm, up to 30 m long, with a change in SWR from 1 to 3-5 k.p.d. feeder will not differ significantly from unity, therefore, the power radiated by the antenna will be constant. This conclusion seems to contradict the results of an experiment known to many: if the degree of matching is changed with the transmitter turned on with the help of matching elements of the antenna, then the field strength will change. The thing is that when the SWR changes, the input impedance of the feeder changes, therefore, with the positions of the tuning and communication elements of the output stage of the transmitter unchanged, the power delivered to the feeder also changes accordingly. If, after each manipulation with the matching elements, the transmitter output is adjusted, then the field strength will not change significantly.

Ris.3

When using a cable longer than 30 m (or a cable with an outer diameter of less than 7 mm), an increase in SWR to values ​​exceeding 2-3 noticeably worsens the efficiency. With a short feeder, the SWR limiting factor is the power that can be supplied without the danger of overheating the cable.

Ris.4

On fig. 5 shows the dependences of the allowable power on the frequency for various cables with SWR=1. With SWR>1, the allowable power is equal to the value of P divided by the SWR in the feeder. For example, if the antenna is powered on a range of 14 m by a PK 50-2-11 cable with SWR = 3, then, in accordance with Fig. 5 P=400 watts, and the allowable transmitter power is 133 watts. When using a cable with an outer diameter of 12 mm and a maximum power supplied to the transmitter output stage of 200 W (which corresponds to an output power of no more than 150 W), the cable will not overheat at very high SWR values.

Ris.5

Thus, in many cases, the only consideration that determines the acceptable value of the SWR in the feeder is the convenience of matching it with the transmitter or receiver. With SWR = 1, the input impedance of the feeder (regardless of its length) is equal to the input impedance of the antenna and the wave impedance of the cable. With SWR>1, the input impedance of the feeder is equal to the input impedance of the antenna only if the electrical length of the cable is equal to an integer number of half-waves of the used range. Usually, the feeder can be easily matched to the transmitter output and receiver input at any length if SWR<2. This value should be considered the limit, which in most cases it is advisable not to cross.

Author: Ya. Lapovok; Publication: N. Bolshakov, rf.atnn.ru

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