RF current indicator on the cable sheath. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Measuring technology
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Parasitic currents on the braid of the coaxial cable going from the radio station to the antenna occur when the latter is unbalanced, in case of any accident, for example, when the counterweight or the braid itself breaks at the point of connection with the antenna, when the antenna or counterweight comes into contact with other objects, and in many other cases . Among the consequences caused by parasitic currents on the cable sheath, it is necessary to mention the appearance of RF voltage on the body of the radio station, interference on neighbors' TV screens, and a decrease in the efficiency of the antenna-feeder path. The commonly used SWR meter cannot always determine the asymmetric cable operation mode, and besides, it does not provide operational control of the situation.
A simple and very convenient current indicator on the cable braid is offered to the attention of radio amateurs. Its scheme is shown in the figure.
The basis of the device is a current transformer T1, made on a ring magnetic circuit made of ferrite with a magnetic permeability of 50...1000. The dimensions of the ring are chosen such that it (with the wound secondary winding) fits snugly on the cable passed through its hole. The cable is a single-turn primary winding. Since the current is indicated precisely on the outer surface of the braid, the integrity of the cable is not violated, even the insulation does not need to be removed.
The secondary winding of the transformer T1 contains 15 turns of PEV or PELSHO wire with a diameter of 0,3 ... 0,5 mm. The RF voltage from the secondary winding is rectified by the VD1 diode and fed to the VD3 LED. Diode VD2 protects the LED from random reverse voltage pulses. Diodes VD1 and VD2 can be any high-frequency, for example, KD503A, the LED is suitable for any type.
The device does not need to be adjusted. To check it, a coordinated equivalent load is connected to the controlled cable and the transmitter is turned on at an output power of 15 ... 20 W - the LED should not glow. If you connect any antenna only to the central conductor of the cable, the LED will light up.
This device also responds well to "antenna barbarians", of course, when the transmitter is on.
The sensitivity of the device can be increased by winding the symmetrical secondary winding of the transformer T1 (2x15 turns) and connecting the right LED output according to the scheme with the midpoint - you get a push-pull rectifier. A bridge rectifier with four diodes will give the same effect.
Author: A.Prokudin (RN9APE)
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