An interesting variant of the small loop antenna for Amateur LW radio stations proposed West German shortwave DF91V. Compact frame with a perimeter that is substantially less the working wavelength is attributed to the so-called magnetic antennas, as they react (if it comes to radio) on the magnetic component of the electromagnetic wave. This leads to several advantages. First, this antenna does not require a good (in the electronic sense) to the ground (i.e. appropriate "checks and balances", and secondly, the magnetic component of the wave penetrates deeper into the room. Small size allow you to use it as a balcony room or even transmitting antenna.
The quality framework is very large (several hundreds), so when you change the operating frequency needs to be rebuilt. This disadvantage to a certain extent compensated by the suppression of interfering signals stations at the reception, as well as harmonics and other spurious emissions during transmission (up to 35 dB for the second harmonic). Antenna OF9IV (see figure) a semi-circular ring of copper tube, which is located inside the frame of copper insulated wire cross section 8 mm2 in an insulating sheath (sheath diameter slightly less than the inner diameter of the tube). Apply the tube and the frame of other materials should not be, otherwise the efficiency of the antenna decreases significantly.
Frame tune variable capacitor C1; the rotor via a gear connected to a small motor. When specified on drawing size and capacity of the capacitor C1 220 15... pF antenna can be rebuilt within 10...30 MHz, which enables its use in four Amateur bands. The gap between the plates of the rotor and stator of the capacitor should be at least 1.5 mm with transmitter power up to 100 W, and accordingly, if the transmitter is more powerful. It is desirable that the capacitor had no rubbing contacts. With the help of short metallic or insulating stands antenna is fixed on a wooden Board that includes a capacitor and a gear motor.
A loop connection with an antenna made from a feed antenna coaxial cable with a characteristic impedance of 50 Ohms. With the end of the cable and from the site, separated from it by 400 mm, remove the outer insulation sheath, and in the middle of this cut over a length of 10 mm is removed and the shell, and braid. The inner conductor at the end of the cable is soldered to the braid, and then to the area where discharged outside insulation. The resulting ring is attached to the insulating tape to the top of the frame, as shown in the figure.
The standing wave ratio described antenna for all Amateur bands in the frequency range 10...30 MHz does not exceed 1.3.
When the transmitter power of 5 watts with an antenna that is installed on the window, DF9IV QSO in the range of 14 MHz with a lot of European countries, and at a power of 60 W and with other continents.
Author: K. Hagenbuchner; Publication: N. Bolshakov, rf.atnn.ru