Notch circuit made of coaxial cable. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Antennas. Technology

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When using multi-band antennas, it often happens that on a separate amateur band, the SWR in the feeder is more than 2,5. In this case, almost 20% of the power is reflected at the input of the transceiver. Depending on the reasons for the occurrence of SWR, part of the reflected current flows inside the antenna cable, and the other part of the current (1arr) flows through the outer sheath of the cable, this is especially evident when SWR occurs due to antenna misbalance (Fig. 1). The current flowing through the outer sheath of the cable is partially shorted to ground (I1, Fig. 1), and also flows through the transceiver case (I2, Fig. 1), which creates a "hot" contact with the device. In this case, it is possible to excite the transceiver when transmitting on separate ranges.

Further, getting into the power network, the high-frequency current reflected by the antenna will cause TVI. The author managed to eliminate the above phenomenon by installing a notch circuit, the inductance L1 of which is made of the same coaxial cable as the feeder (Fig. 2).

The circuit was tuned by the capacitor C1 to the rejection frequency in a certain range, where an increased SWR occurs, according to the maximum glow of the VD1 LED, which is connected with the rejector circuit by one turn (L1). If the reflected power is not enough to light the LED, the circuit can be tuned using the RF field strength indicator. The data of the circuits made on the basis of a coaxial cable are given in Table 1.

Table 1

Range, MHz	Inductance, uH	Frame diameter, mm	Winding length, mm	Number of turns
3.6	30	100	100	30
7	10	50	100	18
10	5	50	50	12
14	2,4	40	50	10
18	1,5	50	50	7
21	1,2	50	50	6
24	0.9	50	40	5
27-30	0,6	30	35	5

The notch circuits were wound on frames, which were empty plastic bottles for drinks and detergents, the coils were fixed with electrical tape. It is possible to make a coil on a frame, the diameter of which differs by 20% up or down from that indicated in Table 1.

The circuit was in a cardboard shoebox not far from the input of the coaxial cable into the room. In some cases, a noticeable decrease in TP1 occurred when the "ground" was connected directly to one of the sides of the circuit with the switch S1. The "earth" can be a heating battery or a wire 5-20 meters long connected to an "Artificial Earth" type device. If "hot" contact and excitation of the transceiver are present on several bands, it is necessary to put several notch circuits in series one after the other.

Author: Igor Grigorov, RK3ZK, Belgorod; Publication: N. Bolshakov, rf.atnn.ru

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