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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Multi-band half-wave antenna. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / HF antennas Since the development of short waves began, radio amateurs have been of constant interest in wire antennas, the length of the emitter of which is equal to or a multiple of half the wavelength, and its excitation is carried out from the end of the emitter. In the English literature, such antennas are called EFHW, which stands for "end-fed half wave" (end fed half wave) antenna. Perhaps the most famous of them is the Fuchs antenna, in which the excitation of the emitter is carried out by means of an additional parallel oscillatory circuit tuned to the operating frequency. Many people are attracted by the fact that, according to Fuchs, it does not require a good "ground" or "radio ground" (balances) unlike most simple antennas (many wire antennas, GPs, etc.). This statement is erroneous, although this antenna really turned out to be efficient without obvious counterweights. It's just that she has low requirements for them (not the same as, for example, for GP), and their role is often played by what is connected to the matching circuit (feeder, transmitter case). Although the EFHW antenna, in fact, is a multi-band antenna, but today it has a small drawback - it works without problems only on multiple ("old") HF bands. And now there are already several of those that do not fall into this grid. The second disadvantage is that on different ranges such antennas with a constant electrical length of the emitter have different radiation patterns on different ranges. But absolutely all such antennas have this drawback, starting with WINDOM. However, they always turn a blind eye to this, since in real urban conditions it is not always possible to install even one wire antenna. The output impedance of modern transceivers and transmitters is low (usually 50 ohms), which means that a matching device is needed to drive a half-wave antenna, which has a high input impedance (up to several kilo-ohms). This can be a parallel oscillatory circuit, as in the Fuchs antenna, and various LC circuits. The disadvantage of such matching devices in a multi-band antenna is the need for switching and tuning when moving from range to range. Broadband high-frequency transformers on ferrite magnetic cores have long been used in transistor amplifiers, in particular, in broadband power amplifiers. Therefore, one should not be surprised that the idea arose to feed a half-wave emitter from the end through such a transformer. The gain is clear - when changing ranges, switching in the matching device is not required. One of the options for such an antenna was proposed by the Dutch shortwave PD7MAA [1]. He used it for work in the field, but it is also suitable as a stationary in the city. After all, many shortwavers are forced to limit their "antenna farm" to a wire antenna that goes out of the apartment window to a nearby pole or tree. He implemented two versions of the antenna - one for the bands 80, 40, 20, 15 and 10 meters, and the other for the bands 40, 20 and 10 meters. They differ only in the design of the emitter. A variant of the antenna for 40, 20 and 10 meters and its matching device is shown in fig. 1. For her, A = 10,1 m, B = 1,85 m.
Its emitter is formed by a half-wave (for a range of 20 meters) piece of wire, an inductor L1 and a relatively short piece of wire connected after this coil. The inductance of the coil L1 is chosen such (34 μH) that, together with the second piece of wire, the electrical length of the emitter is close to half the wavelength on the range of 40 meters. On the bands of 20 and 10 meters, this inductor works as a choke, practically "cutting off" the additional segment from the main part of the radiator, and its length becomes equal to half a wavelength on a range of 20 meters and one wavelength on a range of 10 meters. As a result, "half-wave" emitters are connected to the matching device on all three ranges. The distribution of currents over the emitter for these ranges is shown in Fig. 2.
The inductor L1 is wound on a plastic frame with a diameter of 19 mm and has 90 turns of wire with a diameter of 1 mm. The matching device turned out to be extremely simple - a broadband RF transformer T1 and a correction capacitor C1. It is housed in a small plastic box (Fig. 3). The transformer is made on the FT 140-43 magnetic circuit from Amindon. Its primary winding is 2 turns, the secondary is 16 turns. The windings are wound with a wire with a diameter of 1 mm.
The secondary winding, as shown in Fig. 3 is divided into two parts, 8 turns each, spaced along the ring. A feature in the design of this transformer is that the wire of the primary winding and the wire of the first two turns of the secondary winding (lower in Fig. 3) are intertwined with each other. This is also clearly seen in Fig. 3. Capacitor C1 serves to correct the frequency response of the matching device on the 28 MHz band (10 meters). Its capacitance can be in the range of 100 ... 150 pF. It must be rated for 1000 V. The coaxial RF connector XW1 is installed on the body of the matching device for connecting the cable coming from the transceiver, and the E1 terminal for connecting the antenna radiator. This matching device is designed for a transceiver power of approximately 100 watts. Another version of the PD7MAA antenna, designed to operate on the bands 80, 40, 20, 15 and 10 meters, differs only in the size of the emitter and the inductance of the L1 coil. For him, the dimensions are A \u20,35d 2,39 m and B \u110d XNUMX m, and the coil has an inductance of XNUMX μH. It is also wound on a frame with a diameter of 19 mm - 260 turns of wire with a diameter of 1 mm. It is necessary to install a cable choke on the feeder at the transceiver (put on, for example, a ferrite "latch"), and it is desirable to connect short counterweights to the matching device. Their length is not critical - for a Fuchs antenna, a length of approximately 0,05λ is recommended in the literature. The radiator tuning for both antenna options begins with high-frequency ranges. The inductor L1 is not a good "rejector" (trap, as in the antenna type W3DZZ), so the second segment of the radiator (B) may slightly affect the resonant frequency of the radiator. Accordingly, some correction of its inductance may be required. On the lowest frequency range, tuning is reduced to selecting the length of segment B so that the electrical length of the emitter (its resonant frequency) in this range (40 or 80 meters, respectively) is close to "half a wave". The American company PAR Electronics produces several antennas of this type, including an antenna called EF-10/20/40 MKII for 40, 20 and 10 meters [2]. Interesting test data is available on the Internet [3, 4]. This antenna has a matching device rated for lower power handling (25 watts) but is otherwise very close to the PD7MAA antenna. On fig. 4 shows a photo of the kit for installing this antenna.
According to the company, its bandwidth on the range of 20 meters in terms of SWR \u1,5d 500 is approximately 40 kHz. On a range of 140 meters it is about 2 kHz in terms of SWR = 10, and on a range of 900 meters - about 1,5 kHz in terms of SWR = 50. These data correspond to a feeder with a characteristic impedance of XNUMX ohms. In other words, these are very decent bandwidth values for a simple multi-band antenna. The antenna description contains data that may be useful when tuning the PD7MAA antenna. Changing the length of the main part of the radiator and its additional segment (A and B in Fig. 1) by 1 inch (2,5 cm) leads to a shift in the bandwidth by 30...35 kHz. Literature
Author: Boris Stepanov (RU3AX)
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