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Fig. 1
The first loop built for 137 kHz (fig. 1), was made with TV cable, 18 turns, square shape 1.2 x 1.2 m, one turn secondary winding, single stage amplifier based on J310 FET. This loop was usable only indoor and obviously was very sensitive to the local noise. To overcome this disadvantage I have built in October 2000, but installed on the roof only in February 2001, a new loop suitable for outdoor installation.
Fig. 3
Fig. 2
Fig. 2 shows the loop in its definitive location while fig. 3 refers to its layout, where T1 is the toroidal transformer located at the base of the loop, L is the coaxial line, C the tuning condenser, A the safety relais, T2 the output transformer and O the output connector. The diameter of the loop is 1.5 meters, only one turn, made with aluminium tubing 8 mm OD. The loop is secured to the metallic (aluminium) container, using two suitable clamps, entering the container, insulated with TEFLON. To avoid damages due to the wind blowing transversely, a plastic pole, supporting the center of the turn, and a couple of Nylon guys as shown in the picture, have been necessary. In the above-mentioned container are located the impedance transformer T1 and the connector for the coaxial line. Over the container a plastic screen prevent the rain to accumulate around the clamps; for the same reason, gummi washers are inserted, under the screen, in the aluminium tubing and in the rafters of the guys, for purpose of intercept the rain flowing along them.

The primary of T1 is wound with a thick wire (about 1 mm), 2 turns, and its center is connected to the container and therefore to the ground, necessary condition to achieve the symmetry of the loop. The number of turns in the secondary winding, with some limits, is not critical; after some tests, I have chosen 18 turns, meaning a turn-ratio of 1:9 and an impedance ratio of 1:81. With these values, the tuning capacitance results around 4500 pF, accounting for the capacitance of the coaxial cable, that, since it works at an impedance very higher than its characteristic impedance, can be simply considered a capacitor, with a capacitance, for usual 50 ohm cables like RG 58 U, of around 100 pF/meter. The core of this transformer is made with 8 toroids Mullard type A4, 1/2", the best I have found in my shack, put side by side to form a tube. In any case, the ferrite must have a great permeability and low losses in the considered frequency range, since the reactance of the primary winding (2 turns) must reach a value at least 4 -5 times greater than the reactance of the loop itself, that results 4.3 ohm.

The layout of the amplifier may be obviously changed, according with the available components; I have preferred to feed the amplifier through the output cable, since my station was arranged that way. The relais A, when d.c. is switched off, (during transmission), shorts the coaxial line, avoiding damages to the amplifier, in the case that the diodes are not able to support the strong induced current. T2 is a toroidal transformer, wound with 15 turns, bifilar, connected in series; also in this case, the toroid used must have great permeability and low losses, showing, with 30 turns, at least a reactance of 1000 -1500 ohm. I have used an unlabeled toroid that, after some measurements, appeared to be suitable for this purpose.

This loop shows a great efficiency, some measurements give a loss, in respect to an isotropic antenna, around 33 - 34 dB, obviously accounting for the above-mentioned amplifier. An article with a description of this antenna was already sent to our official magazine RadioRivista and published in the issue of July/August 2001 (see also Articles).

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© Cesare Tagliabue I5TGC