136 kHz band antenna. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / HF antennas

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The low activity of radio amateurs on the 136 kHz band is due to the lack of appropriate equipment and antennas. It is the antennas on this range that determine the efficiency of the radio station, since it is realistic to use only very shortened radiators with a very low efficiency.

As an experimental transmitting antenna in the range of 136 kHz, a wire antenna 41 m long was tested with a capacitive load at the end and an extension coil at the base (Fig. 1). The upper end of the antenna was fixed on a tree at a height of about 12m above the ground (Fig. 2). Capacitive load - five radially divergent conductors 5 m long. They are made of coaxial cable with a diameter of 12 mm. The hanging ends of the cable must be insulated with resin.

Ris.1

A metal fence around the home garden served as a counterweight in these experiments.

The wire going to the vertical part of the antenna and passing through the window frame is a coaxial cable with a diameter of 12 mm with the outer braid removed.

Ris.2

The capacitance of the antenna, measured relative to the fence - the counterweight, turned out to be 260 pF, i.e., to tune the antenna to resonance, an extension coil with an inductance of about 5,26 mH is required.

It was made up of two parts - coils L1 'and L1 ". Coil L1' is wound on a glass three-liter jar and contains 120 turns of PEV-2 1,0 wire. The length of the winding is 16 cm. The taps are made every 10 turns. The inductance of the coil is 1,4 mH. Parquet varnish was used to fix it on the bank. The L1 "coil is wound on a plastic bucket with a diameter of 25 cm. 29 turns of PEV-180 2 wire are placed on a length of 1,0 cm. The inductance of this coil is 4,8 mH. It is adjusted by a short-circuited coil from a strip of copper foil 5 cm wide, suspended on a nylon thread (Fig. 3).

Ris.3

To match the amplifier with the antenna, a coupling coil L2 is used, located at the bottom of the coil L1'. The choice of L2 parameters is quite critical. The maximum antenna current was obtained with the number of turns of the L2 coupling coil equal to 40. To measure the antenna current, a self-made small-sized RF voltmeter was used, connected in parallel with the conductor that goes from the L1 "coil to ground (Fig. 4). It was included along the length of the wire 20 cm. The active resistance of two coils connected in series, measured at direct current, turned out to be 4,5 ohms.

Ris.4

Tuning the transmitting antenna to resonance is quite critical. If the resonance of the antenna lies below 136 kHz, then, by reducing the inductance of the coil L1', adjust the antenna to resonance by smoothly changing the inductance L1". If the resonance lies above, then, on the contrary, increase the inductance L1'. transmitter and be sure to solder the end from L1 "to L1'. To indicate the tuning of the antenna, it is useful to use a VL1 neon lamp, attaching it with tape near the middle turns on the inside of the can on which the coil L1' is wound. The transmitter in these experiments was a low-frequency oscillator connected through the UMZCH, which gave a power of 1 W at a frequency of 136 kHz into a load of 15 Ω.

For reception in the range of 136 kHz, a rebuilt radio receiver "Ishim-003" was used with an EMF installed in the IF path with a bandwidth of 500 Hz. It turned out that it was impossible to use a transmitting antenna for reception, since the receiver input had an input impedance of 75 ohms and the antenna was detuned. I had to use a separate tunable magnetic loop antenna for reception. It is made on a wooden cross with dimensions of 140x140 cm and contains 140 m of a coaxial cable with a diameter of 6 mm closed at the ends.

The antenna is tuned to resonance using a built-in variable capacitor from an old tube receiver. Communication coil - 10 m of the same cable. The bays are fastened together with electrical tape.

The antenna was located on the window, which made it possible to choose the direction of reception - from southeast to west. It was clearly tuned into resonance, and when it was rotated, directionality was clearly felt when receiving lightning discharges and impulse signals of unknown origin.

Since nearby radio amateurs did not have equipment and antennas for the 136 kHz band, an attempt was made to receive their own signals. The Ishim-003 receiver was taken to the dacha, where the signals were confidently received at a distance of 20 km. At the next monitoring point, at a distance of 35 km from the transmitting antenna, only a very weak signal was detected.

Since the antenna has a good quality factor and a high RF voltage arises at the ends of the conductors of both the antenna itself and its counterweights, the design of the antenna and the counterweight system should completely exclude the possibility of accidentally touching it.

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

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