Active KB antenna. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Active KB antenna. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / HF antennas

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This small antenna (Joachim Schwender. Aktive Schleifanenne fur Empfang. - Funkamateur, 1999, No. 7, S. 787 - 789) operates in the frequency band from 6 to 30 MHz, which provides reception of radio stations in several broadcasting and amateur bands.

Active KB antenna

The output impedance of the antenna is 50 ohms. It is a frame (see figure), which is tuned to the operating frequency by a variable capacitor. An amplifier with a differential input is connected to the frame, made according to a cascode circuit using field-effect and bipolar transistors. The use of field-effect transistors provides a high input impedance and a low input capacitance of the amplifier. The first allows you to fully connect the frame to the amplifier and thereby realize a high transmission coefficient of the device as a whole. The second - makes it possible to cover a large frequency band without switching.

The amplifier uses high-frequency field-effect transistors and bipolar microwave transistors with a cutoff frequency of about 5 GHz. With a well-made output transformer T1, this allows you to get the frequency band of the amplifier itself 1 ... 100 MHz. The transfer coefficient from its input to a load of 50 ohms is about 1. The inductor L1 in the drain circuit of field-effect transistors VT1 and VT3 is turned on to increase the input impedance of the amplifier at the high-frequency edge of the antenna operating frequency band.

The supply voltage at the bases of bipolar transistors (about 4 V) is stabilized by a chain of diodes VD1-VD6. It is impossible to use zener diodes here, since the high-frequency noise generated by them in the stabilization mode can negate all the advantages of the amplifier.

The amplifier consumes a current of no more than 3 mA, so it can be powered by a small-sized 9 V battery (domestic equivalent of "Krona". "Korund").

Transformer T1 is made on a ring magnetic circuit of size K 13x7,9x6,4 mm made of ferrite with an initial magnetic permeability of 800. Winding I contains 3 turns, and windings II and III - 20 turns each. The wire is a high-frequency litz wire.

The frame is made of a copper tube with a diameter of 16 mm and has the shape of a ring with a diameter of 1 m with a slit in which a variable capacitor C1 is placed. It is taken from a broadcast receiver. The conclusions from the stators are connected to the frame, and the rotor is not connected to anything. This minimizes the influence of the hand when tuning the antenna to the operating frequency. Since the frequency overlap of the antenna is large, and the quality factor of the frame is high, the variable capacitor must be equipped with a good vernier device and at least a simple scale.

Structurally, the frame is fixed vertically on a wooden base, on which capacitor C1 and other elements of the amplifier and a power battery with a switch are installed (they are not shown in the diagram). The top of the frame is supported by a vertical wooden post. Along it to the amplifier there is a lead wire from the frame (it must be made exactly from its middle).

The quality factor of the frame at a frequency of 6 MHz is about 1000. This ensures a high transmission coefficient of the device as a whole and good filtering of signals from interfering radio stations. However, if the interfering station receives a strong signal and operates at a frequency close to the resonant frequency of the antenna, non-linear effects will appear in the amplifier. Since the frame has spatial selection, such problems can be partially eliminated by the optimal orientation of the frame.

Author: Joachim Schwender

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