A dipole with an off-centre feed point. Scheme, description

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A dipole with an off-centre feed point. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Antennas. Measurements, adjustment, coordination

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Many shortwavers are interested in simple HF antennas that provide operation on several amateur bands without any switching. The most famous of these antennas is Windom with a single-wire feeder. But the price for the simplicity of manufacturing this antenna was and remains the inevitable interference with television and radio broadcasting when powered by a single-wire feeder and the accompanying showdown with neighbors.

The idea behind Windom dipoles seems to be simple. By shifting the feed point away from the center of the dipole, one can find such a ratio of arm lengths that the input impedances on several ranges become quite close. Most often, they look for dimensions at which it is close to 200 or 300 Ohms, and matching with low-resistance supply cables is carried out using balancing transformers (BALUN) with a transformation ratio of 1:4 or 1:6 (for a cable with a wave impedance of 50 Ohms). This is how, for example, the FD-3 and FD-4 antennas are made, which are produced, in particular, in series in Germany.

Radio amateurs construct similar antennas on their own. Certain difficulties, however, arise in the manufacture of balancing transformers, in particular, for operation in the entire short-wave range and when using power exceeding 100 W. A more serious problem is that such transformers normally only work on a matched load. And this condition is certainly not met in this case - the input impedance of such antennas is really close to the required values of 200 or 300, but obviously differs from them, and on all ranges. The consequence of this is that, to some extent, in such a design, the antenna effect of the feeder is preserved despite the use of a matching transformer and coaxial cable. And as a result, the use of balun transformers in these antennas, even of a rather complex design, does not always completely solve the TVI problem.

Alexander Shevelev (DL1BPD) managed, using matching devices on the lines, to develop a variant of matching Windom-dipoles that use power through a coaxial cable and do not have this drawback. They were described in the journal "Radioamateur. Vestnik SRR" (2005, March, pp. 21, 22).

As calculations show, the best result is obtained when using lines with wave impedances of 600 and 75 ohms. A 600 ohm line adjusts the input impedance of the antenna on all operating bands to a value of approximately 110 ohms, and a 75 ohm line transforms this resistance to a value close to 50 ohms.

Dipole with an off-centre feed point
Fig. 1

Consider the implementation of such a Windom-dipole (ranges 40-20-10 meters). On fig. 1 shows the lengths of the arms and dipole lines on these ranges for a wire with a diameter of 1,6 mm. The total length of the antenna is 19,9 m. When using an insulated antenna cord, the lengths of the arms are made slightly shorter. A line with a characteristic impedance of 600 ohms and a length of approximately 1,15 meters is connected to it, and a coaxial cable with a characteristic impedance of 75 ohms is connected to the end of this line. The latter, with a cable shortening factor equal to K = 0,66, has a length of 9,35 m.

The reduced line length with a wave impedance of 600 ohms corresponds to a shortening factor K=0,95. With such dimensions, the antenna is optimized for operation in the frequency bands 7 ... 7,3 MHz, 14 ... 14,35 MHz and 28 ... 29 MHz (with a minimum SWR at a frequency of 28,5 MHz). The calculated SWR graph of this antenna for an installation height of 10 m is shown in fig. 2.

Dipole with an off-centre feed point
Fig. 2

Using a cable with a wave impedance of 75 ohms in this case is actually not the best option. Lower SWR values can be obtained using a cable with a characteristic impedance of 93 ohms or a line with a characteristic impedance of 100 ohms. It can be made from a coaxial cable with a wave impedance of 50 ohms (for example, dx.ardi.lv/Cables.html). If a line with a wave impedance of 100 ohms from a cable is used, it is advisable to include a BALUN 1: 1 at its end.

To reduce the level of interference from a part of the cable with a wave resistance of 75 ohms, a choke should be made - a coil (bay) with a diameter of 15 ... 20 cm, containing 8-10 turns.

The radiation pattern of this antenna is practically the same as that of a similar Windom dipole with a balancing transformer. Its efficiency should be slightly higher than that of antennas using BALUN, and tuning should be no more difficult than tuning conventional Windom dipoles.

Author: B.Stepanov

See other articles Section Antennas. Measurements, adjustment, coordination.

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