SWR meter. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Measuring technology
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Most of the SWR meters used by shortwavers are made on the basis of a coaxial directional coupler made by pulling an additional thin wire under the coaxial cable sheath (K. Rothammel. Antennas - 3rd ed. - M .: Energy, 1979, p. 307). With all its advantages, such a reflectometer is usually asymmetric (due to the irregular arrangement of the additional wire). To verify this, it is enough to measure the SWR of any load, first at one position of the input and output of the SWR meter, and then swapping them. The resulting values usually do not match.
The SWR meter described below, which has two measuring lines and the possibility of full balancing during the tuning process, was developed and built on the basis of the design given by K. Slomczynski (SP5HS) in the book "Shortwave ABC". WKL edition 1988, Warsaw.
Ris.1
The KBC-meter is placed in a box made of foil textolite with a thickness of 1,5...2,0 mm (Fig. 2).
Ris.2
The main component of the SWR meter is the measuring line located between the input connector and the output connector X2. Two rods are connected to the measuring line: L1, in which a voltage proportional to the direct wave amplitude is induced, and L2, in which a voltage proportional to the reflected wave amplitude is induced. These voltages are rectified by VD1 and VD2 and transmitted through a switch and a potentiometer to the measuring head with a total deviation of 100 μA (type M24).
You can also use a less sensitive measuring head, but then you will need to use a DC amplifier made according to any known scheme.
The measuring line is made of a copper tube with a diameter of 6 mm and a length equal to the distance between the protruding pins of the RF connectors X1 and X2 ( 110 mm). The screen for the measuring line is three strips of foil getinaks, the length is equal to the internal distance between the side walls of the box. On both sides of the central tube, two rods L1 and L2 are placed on two insulating spacers, made of copper wire with a diameter of 1,5 ... 2,0 mm and a length of about 75 mm. The rods are approximately 2 - 3 mm from the central tube.
The section of the measuring line is shown in Fig. 3, and the termination of the ends of the tube is shown in Fig. 4. A microammeter is placed on the front wall of the box. When installing the circuit, you should try to make all connecting conductors as short as possible.
Ris.3
The device is calibrated as follows. We connect the transmitter to connector X1, and to connector X2 the equivalent (75 Ohm) of the antenna.
Ris.4
We set switch S1 "to the antenna" - "direct", turn on the transmitter and set the instrument pointer to the end of the scale with a potentiometer, i.e. to point oo. After that, the switch S1 is moved to the "reflected" position and the point "1" is marked, it will be slightly higher than "0" on the scale of the measuring head. After that, without touching the potentiometer knob, we swap the connection of the transmitter and the equivalent antenna to the SWR meter. We put S1 in the "reflected" position and turn on the transmitter. The pointer of the device should be at point oo. If there is a deviation from this point, then by bending the rod L2 we achieve that the arrow is at the point oo.
Depending on the power of the transmitter, it may be necessary to select resistors R1 and R2. With more power, the value of the resistors R1 and R2 must be reduced. The values of these resistors must be the same.
The SWR meter with the ratings indicated in the diagram (Fig. 1) works with RA on one GU29 lamp.
Author: UC2AR; Publication: N. Bolshakov, rf.atnn.ru
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