Small loop antenna. Scheme, description

Small loop antenna. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / HF antennas

An interesting version of a small-sized loop antenna for amateur KB radio stations was proposed by the West German shortwave DF91V. Small-sized frames - with a perimeter significantly smaller than the operating wavelength - are referred to as the so-called magnetic antennas, since they respond (if we are talking about radio reception) to the magnetic component of the electromagnetic wave. This results in a number of advantages. Firstly, such an antenna does not require a good (in the radio engineering sense of the word) "ground", i.e., appropriate "balances", and secondly, the magnetic component of the wave penetrates deeper into the room. Its small size allows it to be used as a balcony or even indoor transmitting antenna.

The quality factor of the frame is very large (several hundreds), so when changing the operating frequency, it must be rebuilt. This shortcoming is compensated to a certain extent by the suppression of signals from interfering stations during reception, as well as harmonics and other spurious emissions during transmission (up to 35 dB for the second harmonic). The OF9IV antenna (see figure) is an open ring of copper tube, inside which there is a frame of insulated copper wire with a cross section of 8 mm2 in an insulating sheath (sheath diameter is slightly smaller than the inner diameter of the tube). Do not use a tube and a frame made of other materials, otherwise the efficiency of the antenna will noticeably decrease.

Small Loop Antenna

The frame is tuned with a variable capacitor C1; its rotor is connected to a small electric motor through a gearbox. With the dimensions indicated in the figure and the capacitor C1 with a capacity of 15 ... 220 pF, the antenna can be tuned within 10 ... 30 MHz, which allows it to be used in four amateur bands. The gap between the plates of the rotor and stator of the capacitor must be at least 1,5 mm with a transmitter power of up to 100 W and, accordingly, more if the transmitter is more powerful. It is desirable that the capacitor does not have rubbing contacts. With the help of short metal or insulating racks, the antenna is fixed on a wooden board, on which a capacitor and an electric motor with a gearbox are installed.

The communication loop with the antenna is made from a coaxial cable feeding the antenna with a wave impedance of 50 ohms. From the end of the cable and from the section 400 mm away from it, the outer insulating sheath is removed, and in the middle of this segment, both the sheath and the braid are removed at a length of 10 mm. The inner conductor at the end of the cable is soldered to the braid, and then to the area where the outer insulation was removed from it. The resulting ring is attached with electrical tape to the top of the frame, as shown in the figure.

The standing wave ratio of the described antenna for all amateur bands in the frequency range 10 ... 30 MHz does not exceed 1,3.

With a transmitter power of 5 W with an antenna mounted on a window, DF9IV made QSOs in the 14 MHz band with many countries in Europe, and with a power of 60 W with other continents.

Author: K. Hagenbuchner; Publication: N. Bolshakov, rf.atnn.ru

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