EP0065973B1 - Directional antenna with a plurality of elements - Google Patents

Description

There are a number of arrangements which allow electromagnetic waves to be emitted in a preferred direction or received from a preferred direction. As long as such systems are only to be designed for a certain frequency or a relatively narrow frequency range, they are easy to implement, e.g. B. according to the principle of Uda-Yagi with reflector and director elements designed for the desired frequencies. However, such arrangements become more difficult and technically complicated if they are to operate at multiple frequencies with a gain and a good forward-backward ratio.

It is known that the amateur radio areas are distributed over practically the largest part of the shortwave and ultra-shortwave range. Since the known broadband log-periodic antennas require a great deal of mechanical effort and, from an antenna gain point of view, are not an ideal solution, there has been no shortage of attempts to design the principle of the Uda-Yagi antenna system in such a way that it is based on several usually 3 shortwave ranges gives an optimal directivity.

So far, however, this result could only be achieved if compromises were made for the desired frequency ranges with regard to both the mechanical design and the electrical effect. A typical compromise antenna of this type is e.g. B. the so-called trap or blocking antenna, in which the individual elements are divided by blocking circuits so that only certain antenna parts can be effective electrically depending on the frequency range used. The main disadvantage here is the shortening of the element due to the insertion of coils, which inevitably leads to a reduced profit. In addition, a certain loss of radiated energy has to be accepted, which is converted into unwanted heat by the blocking circuits. In addition, blocking circles are generally sensitive to the weather and lose quality over time due to weather influences, which further increases losses.

A significant improvement was achieved in that blocking circles were dispensed with and the antenna was set to three independent frequencies by means of parallel and series circuits (Auerbach, Amateurfunk-antennas, Franzis-Verlag München 1971, pp. 162-167). In order to insert the tuning circuits into the antenna system, so-called hairpin loops had to be installed as inductors and capacitors in the form of coaxial cables of a certain length had to be built into the antenna system as capacitors. The disadvantages of this development are also of a constructive nature. Similar to trap antennas, the tuning circuits are subject to weather influences and can only be operated with a limited RF power, since they can be destroyed when heated.

The mechanical structure is also complicated with all the advantages compared to blocking-circuit antennas and they require tuning work which has hitherto made the series production of such antennas difficult. (Auerbach, loc. Cit. P. 167).

The object of the invention is to realize an antenna system in which, as far as possible, the entire antenna system can be used for reception and radiation and, in contrast to previously known antennas, all additional tuning aids, such as separate resonant circuits, can be dispensed with. The advantage of such a system lies in addition to its unprecedentedly simple mechanical structure, which is realized exclusively by means of metallic conductive parts, such as aluminum pipes and insulating brackets, in excellent electrical properties, e.g. B. a complete insensitivity to weather influences.

The object is achieved in that no more additional inductive and capacitive components are used to tune the different frequency ranges, but rather the "blind components" required for tuning by deliberately lengthening and shortening elements in considerable deviation from the typical resonance length of the elements and producing them Dummy components within the system are compensated for by coupling the elements together by approximation.

It has surprisingly been found that by simply approximating the radiating elements, a coupling which has not yet been fully clarified is achieved, which makes the insertion of separate tuning circles unnecessary and at the same time causes the entire system to vibrate, as a result of which the "radiating area" is considerably enlarged in the desired manner .

From US-A-3 503 074 an antenna system operating on the basis of log periodic antennas is known, which also has antenna elements that are very well-fed. This system - like all log periodic antennas - can only be used to a small extent, since only the few antenna parts are active which, due to their physical dimensions and electrical properties, resonate with the working frequency, while all other elements are dead . In contrast, in the antenna system of the invention all elements are in action so that almost the entire antenna area is effective on each of the band areas provided. The system of US-A-3 503 074 is completely different because of the phase-infeed and the resultant Coupling relationships with the directional antenna according to the invention, which makes use of an in-phase feed, cannot be compared. An antenna with closely spaced elements is also known from GB-A-937 686. However, this antenna is broadband and should cover the entire FM radio range with a bandwidth of 8 MHz. According to GB-A-937 686, it is not possible to manufacture an antenna which works according to the invention on narrowband bands which are approximately 1: 2 apart.

The basic unit of such an antenna system is an antenna for z. B. three frequency ranges from a combination of three elements. This basic unit mainly serves to tune the overall system to the desired frequency ranges and consists of a% / 2 radiator for the lowest frequency, which is approximately matched to the resonance length. At a relatively short distance from this, a greatly extended a X / 2 radiator for the next lowest frequency is attached. A further shortened?. / 2 radiator of the highest frequency is now attached at a further short distance from this element. Conveniently, the two radiators are fed in together for the lower frequency ranges, the middle radiator being able to be bent in the middle towards the common connection point of the feed line. Due to the deliberate extension of the central radiator, it has a relatively high inductive reactive component, which in itself brings it outside the desired resonance range. Likewise, the shortening of the radiator for the highest frequency per se causes a severe detuning due to the additional capacitive shortening component. Due to the closely approximated arrangement of the three emitters in a range of less than 0.01 - 0.02λ in relation to the lowest frequency, all three emitters can now be coupled with one another in such a way that the reactive components and the entire system of all three emitters are fully compensated vibrates at the desired resonance frequencies. It is particularly astonishing that not only the two longest elements fed in carry considerable high-frequency voltages and currents during operation, but also that the only parasitically excited third element is live and live, although it should not be due to its mechanical dimensions.

As a result of the approach of the elements according to the invention, the radiating area of the system is considerably increased for all three frequency ranges, as a result of which an unexpected increase in antenna gain is already achieved. The spatially close arrangement of the radiating elements additionally results in a forward-backward ratio of approximately 2-5 dB for the radiated or radiated RF energy for the two higher bands, which has a favorable influence on the directional effect of the antenna. Of course, the antenna gain produced by a Yagi antenna is not yet to be expected from this spatially closely spaced basic unit, but the two higher frequency ranges are already emitted by about 1-2 dB above a reference dipole. The longest element acts like a dipole.

The basic unit described according to the invention now makes it possible to arrange one or more elements of the lowest frequency on a longer "boom" at a greater distance (cf. Auerbach loc. Cit. P. 164). Surprisingly, adjustment circles can also be completely dispensed with here. These elements are also fed via the crossed phase line.

The additional elements placed in front of or behind the 3-element arrangement described increase the antenna gain and the forward-backward ratio dramatically on all three frequency ranges. An antenna gain of 5 - 8 dB over a reference dipole and a forward-backward ratio of 20 - 30 dB can be easily achieved.

A further significant increase in the forward-backward ratio can be achieved if the three-way combination of the fed-in elements described above is arranged again at certain intervals on the boom.

The adaptation of the entire antenna system to a given supply cable (e.g. 50 ohm coaxial cable) which should be connected via an iron-free balun (balun), is carried out by means of an additional line, the electrical length of which is adapted to the entire antenna system. The additional line is preferably connected in the manner of an open Lech line to the part of the crossed phase line facing away from the antenna base and brought to a length by means of an impedance measuring device which brings the overall system to the desired connection value at the base of the antenna.

The subject of the application is described in the patent claims.

The mode of operation of the invention will now be described in more detail with reference to FIG. 1.

Figure 1 shows a typical antenna for the frequency ranges 28-30 MHz, 21-21.5 MHz and 14-14.4 MHz (10, 15 and 20 m amateur band)

The basic unit labeled G is fed in at point 4 via an iron-free balun via coaxial cable. With the point 4, the elements 2 and 3 are connected. Element 3 has a mechanical length of 9.9 m, which corresponds approximately to the electrical length of a dipole rated for 14 MHz. Element 2 is arranged at a distance of 40 ₌ 5 cm and bent over. bent legs of 40 cm connected to the feed point 4. Element 1 is arranged at a distance of 20 cm + 5 cm. Element 2 has a length of 2 x 3.6 m (measured from entry point 4) and is considerably longer than a dipole for the resonance frequency 21 MHz. Element 1 is again considerably shorter than a dipole for 28 - 30 MHz (4.6 m).

The elements 6 and 8 with a length of 10.6 m and 11.6 m are arranged at a distance of 2 m each. Both elements correspond approximately to the electrical length of a Uda-Yagi system for 14 MHz.

The antenna gain in all three frequency ranges is at least equal to the gain of an antenna system that is only calculated for one frequency band. It is assumed that both antenna systems have the same boom length. In the specialist literature, the gain of monoband antennas with three elements and a boom length (distance between the two outermost elements) of 0.2 to 0'3λ is given as 7 dB (S. Auerbach, page 152).

The suppression of the backward unwanted signal is between 15 and 25 dB. The bandwidth is around 1 MHz within the set bands, so that no additional matching devices have to be used. The standing wave ratios reach values of 2 or better.

In addition to the properties described, the triple combination of intentionally inductive or capacitive loaded dipole elements has the ability to avoid the unwanted splitting of the radiation diagrams of whole-wave dipoles or dipoles that are longer than 0.5 λ. The z for the lowest frequency. B. 14 MHz designed dipole excited collinear when operating with the highest frequency and thus generates only one radiation lobe (Auerbach p. 133 and 131).

An extension of the system by a further basic unit (G) and an additional reflector (6 ') is shown schematically in FIG. This arrangement increases the gain of the system by about 20% or 1 dB, but at the same time the forward-back ratio is greatly improved, namely by about 10-15 dB. This is of great importance with regard to the "overcrowded amateur bands" because it enables largely trouble-free operation.

Of course, it is possible to further increase the gain of the system by means of additional parasitic elements or further radiator elements, provided that this makes mechanical sense.

In addition, the system according to DE-A-3 010 688 can be adapted to any other frequencies by adapting the additional line 10 accordingly and, if necessary, adding further elements which have been brought into resonance.

When using several reflector elements connected via a crossed phase line, it is necessary for adaptation reasons not to connect the phase line directly to the entry point to which the feed line coming from the transmitter or receiver is connected, but via a "detour line". This is formed by a line running parallel to the element tube and the element tube itself. The inductance formed in this way has the effect that when operating at the lowest frequency there is only a slight influence on the overall system, while when operating at the medium and higher frequency there is a desired inductive one Influences. This makes it possible to compensate for undesired capacitive components of the phase line caused by the tubes of the phase line. This also makes it possible to better adapt the antenna system to the power cable and to increase the usable bandwidth. It turns out that especially in the 10 m band ranging from 28,000 to 29,700 KHz. B. even at the frequency 29,700 KHz there is still good SWR adjustment (less than 2). In addition, the detour lines (they are also used on the reflectors) enable an increase in the front-to-back ratio because the exact phase relationship of the elements to one another can be set without changing the distance between them.

Depending on the desired antenna parameter, the length of the detour line is measured, it is typically between 0.3 and 1 m long if frequencies from 14 - 21 - 28 MHz are used. This corresponds to a length of 0.015 to 0.05 k in relation to the lowest frequency.

The detour line is made of pipe, the diameter of which is 10 - 15 mm. It is therefore much thinner than the element pipes, which typically have 30 - 50 mm 0.

Claims (3)

1. Multi-element directional antenna for several for VHF bands consisting of a basic unit (G) of at least three dipoles (1, 2, 3) electrically coupled with each other, one or more reflectors (6) co- excited via a crossed phasing line (7) and, if desired, additional parasitically excited reflector-or director-elements without trap-like tuning elements, characterized in that the basic unit (G) consists of three dipoles which are given mechanical dimensions of a size that the dipole (3) being destinated for the lowest frequency has about the length of an independently oscillating dipole, while the dipole (2) for the next higher frequency is physically 5 to 20 % too long, whereas the dipole (1) for the highest frequency is correspondingly cut too short, while the three dipoles are electrically coupled solely by approximation to a spacing of 0,01 to 0,03 corresponding to the lowest frequency, in a way that the inductive and capacitive reactances caused by the shortening and lengthening of the elements is mutually compensated, the desired resonance frequencies being thereby simultaneously achieved while the dipoles (2, 3) for the lowest and middle frequency range are commonly directly fed, whereas the dipole (1) for the highest frequency range is excited parasitically.

2. Multi-element directional antenna according to claim 1 characterized by an additional transmission line (10) constructed as an open Lecher line connected through the phasing lines (7) with the antenna at a place remote of the feedpoint, the length of which brings the feedpoint of the antenna to the desired impedance.

3. Process for tuning a multiband antenna according to claim 1 characterized in that the inductive reactance of the too-long dipole (2) of the basic unit and the capacitive reactance of the too-short dipole (1) are compensated solely through approximation of the elements while the antenna is tuned to the desired frequency ranges deviating from the physical lengths of a corresponding dipole, which is oscillating independently.

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