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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING VHF reflectometer (100-600 MHz). Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Measuring technology Figure 1 shows the design of a VHF reflectometer on a flat coaxial line (operating range 100-600 MHz). The SWR introduced by the device itself into the transmission line is about 1,1-1,13 in the specified range. The device consists of a segment of a flat line 1 and a bare measuring line 2 with a directional coupler 3.
Figure 2 shows the main vertical section of the reflectometer. The outer surface of the flat line is made of two duralumin plates 5 115x195x2 mm in size, interconnected by two segments of channel 4 2x18x25,04 mm in size, 115 mm long. The inner conductor of the line 6 is made of a piece of brass tube with a diameter of 9,4 mm, a length of 160 mm, elongated at both ends with stepped transitions 7, which compensate for the unevenness of the line itself and its transition to external coaxial connectors 8. The connectors are attached to the channel 4 with four M3 screws, their connection with the inner conductor 6 is done depending on the design of the connector itself.
A hole 5 mm in diameter is made in the center of one of the plates 10, and the measuring head of the device is attached above it. Mechanically, the head consists of two sections of the sleeve N 20 and serves as the base 9 for the rotary part of the head 10 from the sleeve N 24. All parts of the directional coupler are mounted in the rotary part of the head: communication loop 3, load resistance 11, detector 12 and detector holder 13. Disc 10 made of brass 14-0,8 mm with a diameter of 1,2 mm is soldered to the bottom of the sleeve 26; the disc rim is corrugated, as it also serves as a handle for turning the entire head. On the smooth surface of the disk 14, a mica gasket 0,8-0,1 mm is laid, on top of which a brass disk 15 is also superimposed, which serves as the second lining of the decoupling capacitor of the head. The planes of the capacitor are pulled together through the mica with a screw 16 passing through an insulating sleeve 17. The M2 thread for the screw 16 is made in the central part of the bottom, where the primer is usually located. In the prototype of the reflectometer, it is desirable to make the resistance 11 replaceable, so its grounded end is fixed in the bottom of the sleeve using a locking screw 18 with an M2 thread. The thickness of the bottom for this purpose is quite sufficient. In repeated designs, this assembly can be simplified and the resistance R1 = 120-130 ohm of the MLT type can be soldered into the thin side wall of the sleeve approximately as shown in Fig. 2. The detector holder 13 has an M2 external thread and an M3 internal thread, into which a DKI-type detector is screwed. The thin leg of the holder passes through a hole with a diameter of 4,2 mm in the bottom of the sleeve 10 and is screwed into the M2 thread in the disc 15 of the decoupling capacitor. After selecting the desired height of the holder 13, its position is fixed with a lock nut, under which a petal is simultaneously placed for connection with a microammeter. Loop 3 of the Lc coupler is made of a wire with a diameter of 0,6 mm, has a length of 12-13 mm and a distance between centers of 2,6-2,8 mm. Its left end is soldered to the R1 resistance output wire, the right end, going to the detector, to a small ring 2,0-2,5 mm in diameter, 2-2,5 mm high, bent from thin bronze or brass. The ring is tightly put on the cylindrical output of the detector. It is desirable to limit the rotation of the head 10 in any way within the range of 0-180°, since the counting is carried out only in two extreme positions. The use of a reflectometer. The main purpose of the device is to measure the standing wave ratio (SWR), loads and control matching. To measure the SWR, the device is switched on using high-frequency connectors between the transmitter output and the antenna cable. The head of the coupler is placed in the position of measuring the incident wave (IW), i.e. loop in the direction of the generator, and the connection with the transmitter is selected so as to obtain a convenient reading on the scale of the device a1. The head is then rotated towards the load to measure the reflected wave a2. P=Uneg/Upad=Sqr(a2/a1) where Uotr and Upad are the voltage values to which the reflectometer responds; Knowing the reflection coefficient P, one can also determine the SWR in the measured line:
K=(1+P)/(1-P) Let, for example, the antenna gives a1=20, a2=5, what will be the SWR and power loss? P=Sqr(5/20)=0,5 Consequently, K=(1+0,5)/(1-0,5)=3,0 Such calculations are needed only in the case when, for some reason, it is impossible to achieve agreement and find out the power that the antenna actually radiates, taking into account all losses. However, most often the reflectometer is first used as a mismatch indicator, comparing a1, a2, the first should be large. If it is possible, for example, by moving the reflector in the "wave channel" antenna to achieve that a2 will be 10 times less than a1 with a slight change in the antenna gain, then a further decrease in the reflected wave must already be achieved by a matching transformer or by changing the diameters and distances of complex loop vibrators. Ratios a2/a1=10, <- 15, <- 20 correspond to SWR=1,93, 1,7, 1,57 and power loss Рp=10%, 8%, 5%. Therefore, the ratio a2/a1=10 should be considered acceptable, since higher ratios require accuracy from the reflectometer itself. Its accuracy is estimated by the ratio a2/a1 without load on connector P2. In this case, the entire power of the incident wave should be reflected back, i.e. a2=a1 or a2/a1=1. The deviation from 1, expressed as a percentage, can be considered as the error b of the instrument. In the described design, b = 1,3% at 400 MHz, 1,6% at 600 MHz, 2,2% at 900 MHz. The error in the desired narrow part of the range can be reduced by selecting the length of the communication loop Lc and the value of the load resistance R1 of the loop. For example, for the range of 120-450 MHz, Lc=19 mm, d=4,0 mm with R1=160-170 ohm, Rp=5-6% gives a smaller error. Literature
Publication: N. Bolshakov, rf.atnn.ru
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