Dielectric Directional Antenna

Abstract

A dielectric directional antenna using a wedge shaped dielectric with conducting exciters on each of the angular sides. One or both of the exciters can be triangular with side indentations and one exciter can be a ground conductor with a coaxial line feeding the exciters. A three-terminal amplifier can be connected to the conductors and the feed line.

Description

BACKGROUND OF THE INVENTION

This invention relates to directional antennas, and more particularly to dielectric antennas having a uniform dielectric constant.

A dielectric directional antenna is an antenna whose directional effect is produced by the incorporation of one or more dielectric members in its near field. If several dielectric members are present in the antenna, then the different members can have different dielectric constants. An example of such a directional effect produced by dielectric members are lenses and lens combinations, but a true lens effect in the sense of geometric optics occurs only when the dimensions of the lenses are a large multiple of the wavelength. However, in the present invention, the dimensions of the dielectric members are on the order of magnitude of a wavelength (between a quarter-wavelength and a few multiples of the wavelength) and consequently the rules of geometric optics no longer apply.

When a directional effect is to be produced by means of relatively small dielectric members, a wave field is generated in the dielectric by means of an exciter or primary radiator and the wave field is radiated from the dielectric into the surrounding air space. The exciter can be located outside the dielectric in the surrounding air, inside the dielectric, or on the surface of the dielectric. Ordinary lenses are a well known case in which the exciter is outside the dielectric. A disadvantage of such excitation is that the waves emanating from the primary radiator are partially reflected at the dielectric boundary surfaces. Furthermore, with small dielectric directing members there is the danger of pass-by radiation and therefore the useful rays contain only a portion of the available power. The remaining power strays around in the space and generates undesirable secondary radiation. An exciter inside the dielectric is described by P. Mallach in Fernmeldetechnische Zeitschrift, Vol. 2 (1949), pp. 33-39. The mounting of an exciter and the replacement of defective exciters in the interior of the dielectric without an air gap between the exciter and dielectric results in engineering difficulties if the exciter has a complicated shape. From an engineering viewpoint, the simplest exciters are those which have a flat construction and are pasted or pressed onto the surface of the dielectric. Such flat exciters which are pasted onto the surface of a dielectric hemisphere are described in G. Niedermair, Dielectric Spherical Antenna, Dissertation, Technische Universitat Munchem, May 1971. However, due to internal resonances of the dielectric member, this spherical antenna has a very frequency-dependent directional effect and large frequency ranges with pronounced secondary radiation, which makes the spherical antenna unusable for many applications.

The present invention overcomes the above-mentioned disadvantages.

SUMMARY OF THE INVENTION

In the present invention the dielectric member and the flat exciter are attached to the surface of the dielectric member in such a manner that the frequency dependence of the directional effect of the antenna becomes extremely small. For a given total volume of the dielectric member, the focusing of the radiated energy around the principal radiation direction is greatly reduced and undesirable side lobes of the radiation pattern are kept very small in the largest possible frequency range. A very useful criterion for the fulfillment of these requirements is that the antenna has no appreciable backward radiation in a very large frequency range, i.e., no appreciable radiation in a direction which is opposed to the principal radiation direction.

It is therefore an object of this invention to provide a novel directional antenna.

It is another object to provide a dielectric antenna that is independent of frequency.

It is still another object to provide a dielectric antenna that has limited secondary radiation.

These and other objects, features and advantages of the invention will become more apparent from the following description taken in connection with the illustrative embodiments in the accompanying drawings.

DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram of a wedge-shaped dielectric with a pair of exciters;

FIG. 2 is a diagram of a nonsymmetrical antenna having a wedge-shaped dielectric with different exciters;

FIG. 3 is a diagram of the wave condition along the exciter line of that shown in FIG. 2;

FIG. 4 is a diagram showing how the antenna of FIG. 2 is fed with a coaxial line;

FIG. 5 is a diagram showing the antenna feed system of FIG. 4 integrated with an amplifying device;

FIGS. 6 and 7 are diagrams showing variations in the shapes of the conductor or exciter;

FIGS. 8 and 9 are diagrams showing the use of additional conductors on the surface of the dielectric to form parallel capacitances; and

FIGS. 10-11 are diagrams showing a technique for a low reflection front as a termination for the dielectric antennas of FIG. 3 by adding additional wedges.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, dielectric member 3 has a wedge shape in the vicinity of the feed points a and b of the two flat exciters 1 and 2. Two approximately or exactly plane boundary surfaces 4 and 5 are inclined at an angle with respect to each other. The two flat exciters 1 and 2 are mounted to these two surfaces. The surfaces 4 and 5 intersect approximately or exactly in a straight line 6. At least one of the two exciters has a triangular apex a or b which lies in the immediate vicinity of the intersection line 6 and is a connecting terminal of the antenna. In a symmetrically fed antenna, the antenna has a symmetrical construction in which the two exciters have the same size, shape, and symmetrical positions on the dielectric wedge. In nonsymmetrical antennas, the exciters 1 and 2 differ. For example, with coaxial feeding as shown in FIG. 2, the exciter 2 can be a large conducting surface and can project over the boundaries of the dielectric wedge, such as in the conducting surface of an aircraft. The shape of the boundary surfaces 4 and 5 of the dielectric can very greatly. However, in the present invention it is preferred that the two exciters do not both project over their corresponding boundary surfaces of the dielectric.

A symmetrical antenna can be produced by symmetrically combining two equal nonsymmetrical antennas with a conducting base surface 2 as shown in FIG. 2 and then neglecting the intermediate conducting base surface. Therefore, only the nonsymmetrical form of FIG. 2 is described in the following, it being assumed that the conductor 2 is a very large plane surface and that the principal radiation direction is directed along this plane.

If the antenna is to have less frequency-dependent characteristics in a larger frequency range, then there must be less frequency-dependent variation of the currents on the exciter surface. Such a current variation is obtained if the two exciter surfaces together form a line of nearly constant characteristic impedance, called an exciter line. With a wedge shaped dielectric as in FIG. 2, exciter 1 should have approximately a triangular shape with the antenna fed between the conducting base surface and the triangular apex a which lies in the immediate vicinity of the intersection 6. The above-mentioned exciter line is a dissipative line, since along its entire length it gives off energy to a wave field which is built up in the dielectric and from there is radiated into the surrounding space. If the exciter line is given a minimum length of a quarter-wavelength and the described line losses are large enough, a nearly progressive wave whose shape is only slightly frequency dependent travels on this line in a direction away from the feed point a. The input impedance of the exciter at the feed point is then nearly equal to the characteristic impedance of the exciter line, independent of frequency. For example, if the characteristic impedance of the exciter line is made equal to the characteristic impedance of coaxial feed line 7 of the antenna shown in FIG. 4, the feed line will have a nearly matched termination in a large frequency range by the connected exciter system.

The special effect of an exciter line with a progressive wave which is applied directly to the dielectric is explained in the following. It is sufficient to consider the effect of a transmitting antenna, since the wave processes of a receiving antenna obey the reciprocity law and travel in the same manner but in the opposite direction. A generator connected between the feed point a and the surface 2 in FIG. 2 generates between exciters 1 and 2 a wave which travels along the exciter line. The wave condition along line 1 is illustrated schematically in FIG. 3. The wave along line 1 travels in air above the line and in the dielectric below the line. In the immediate vicinity of the line there is formed a near-field wave with a phase velocity v.sub.p which, in accordance with the mixed dielectric, lies between the phase velocity of a pure air-wave (velocity of light c.sub.o) and the phase velocity c.sub.o /.sqroot..epsilon..sub.r of a wave completely in the dielectric (.epsilon..sub.r is the relative dielectric constant). Then at a certain distance from the line 1, the wave in air and the wave in the dielectric is very different and no longer has the form of the known TEM conduction waves. In the air space, this wave is delayed relative to its natural velocity, which would be the velocity of light. Such delayed waves have the nature of surface waves in which all field strengths decrease approximately exponentially with increasing distance from the conductor. FIG. 3 shows the characteristic shape of the electrical field lines E of such surface waves in air. See, for example, H. Meinke, Introduction to Electrical Engineering at Higher Frequencies, Vol. 2, FIG. 27, Springer-Verlag, Berlin, 1966. The power radiated directly from the antenna conductor into the surrounding air space is very small. The delaying effect of the conduction wave is the reason for the low lateral and backward radiation of the directional antenna, according to the invention, which occurs almost independently of frequency if faults due to internal reflections in the dielectric, which will be described later, are avoided.

Furthermore, because v.sub.p > c.sub.o /.sqroot..epsilon..sub.r the near field wave of the conductor 1 is faster inside the dielectric than a plane wave completely in the dielectric. The result is that in the dielectric there is formed a wave in which the preferred direction of energy movement is directed obliquely into the dielectric. This oblique wave direction results for a conductor segment shown in FIG. 3. The phase fronts of the wave in the dielectric always travel with the velocity c.sub.o /.sqroot..epsilon..sub.r. The phase velocity in the direction of conductor 1 is v.sub.p and, together with the velocity c.sub.o /.sqroot..epsilon..sub.r forms the illustrated right triangle with the angle .alpha., where

c.sub.o /.sqroot. .sub.r = v.sub.p . cos .alpha. (1)

or

cos .alpha. = c.sub.o /v.sub.p . .sqroot..sub.r (2)

Thus, such an exciter pressed onto the surface of a dielectric member is capable of radiating the energy of its conduction wave obliquely into the dielectric and therefore is specially suited for the transmission of wave energy from a conduction wave into a dielectric member. Accordingly, the exciter line has high radiation damping so that reflections at the end of the exciter line are hardly detectable even with short lines and the input impedance of the exciter line is equal to the characteristic impedance of the exciter line.

It is especially desirable if the wave traveling in the dielectric of FIG. 3 is already traveling in the principal radiation direction of the antenna, since then this wave need undergo no further change of its direction. With the nonsymmetrical antenna of FIG. 2 the dielectric should travel parallel to the conductor 2, i.e., the wedge angle between conductor 1 and conductor 2 should be approximately equal to the angle a given by Equation (2). The desirability of this wedge angle has been confirmed by measurements on antennas.

In FIG. 4 it is shown how the antenna is fed by the conducting base surface 2 with a coaxial line 7, wherein the inner conductor of the feed line is connected to point a of the exciter 1 and the outer conductor is connected to the conducting base surface 2. If the antenna is to be used for a very large frequency range, the characteristic impedance of the exciter line will thus be made equal to the characteristic impedance of the feed line so that the feed line will have wide band matching to the imput impedance of the antenna.

The dielectric antenna can be integrated with an amplifying electronic circuit having a three-terminal character. A three-terminal electronic circuit is illustrated symbollically in FIG. 5 by three-terminal transistor T. The first terminal of the transistor is connected to the exciter 1, the second terminal to the base surface 2, and the third terminal to the inner conductor of feed line 7.

It is desirable to have the components required for the electronic circuit and its frequency band limitation mounted wholly or partially on the surfaces of the dielectric. A frequency band limitation is often necessary in order to prevent the radiation of undesired harmonics during utilization as a receiving antenna. Furthermore, it is known that optimization of an active antenna, e.g., in regard to noise matching for receiving, is possible only in a system with a bandpass filter action of the antenna. Therefore, in many applications the input impedance of the antenna in the complex impedance plane must with increasing frequency traverse a loop curve having the character of the input impedance of a two-circuit resonance-band filter. The loop should be close to a given optimal value Z.sub.opt throughout the entire operating frequency range. In passive systems, this Z.sub.opt is the characteristic impedance of the connected feed line and, in active systems, the optimal impedance for the electronic circuit.

An antenna impedance with a loop structure of the previously described type can be obtained by the addition of a circuit at the base of exciter 1 and/or by introducing certain suitable modifications of the impedance of the exciter itself, in departure from the triangular basic form. Flat conductors of a special shape are applied to the outer surfaces of the dielectric member. It is well known that thin and/or meander-shaped elongated conductors act as conductances, while larger flat conductors act as capacitances. Variations of conductor 1 on the surface 4 can result, for example, as shown in FIG. 6, by making conductor 1 somewhat thinner in the vicinity of the feed point and somewhat thicker at the other end than in the simple triangular shape of FIG. 2. Such conductor shapes yield impedances which in certain frequency ranges are very similar to those of a series-resonant circuit. FIG. 7 shows triangular conductor 1 having a symmetrical indentation. The resulting constriction of the current path in the center of conductor 1 acts as a series-connected inductance and likewise generates resonance effects in interaction with other wider parts of the conductor 1 which therefore acts as a capacitor.

As shown in FIG. 8, the surfaces of the dielectric member can be attached to conductors 8 and 8a which are connected to the conducting base surface 2. Parallel capacitances arise between the conductor 1 and these additional conductors in a filter circuit. If the edges of the conductors 8 and 8a lie near conductor 1 then slots result, as shown in FIG. 9, which can have a position-dependent width and, if long enough, can have slot resonances which have a strong effect on the impedance of conductor 1 and can contribute to the formation of defined bandfilter effects.

When there is effective coupling between the exciter and the dielectric, additional measures are essential in order to produce between the wave field in the dielectric and the wave field in the adjoining free space a transition for directional radiation but has little frequency dependence.

The wave entering into the dielectric from the exciting conductor as shown in FIG. 3 strikes the wall of the dielectric and is partially reflected into the dielectric, partially exits from the dielectric into the outside air, and also travels as a wave into the surrounding space. Therefore, inside the dielectric there is a complicated superposition of waves in many directions. The dielectric member acts similarly to a cavity resonator but as a resonator of low Q, since its outer walls are not a completely reflecting metal but rather a more or less transparent boundary between dielectric and air. The higher the dielectric constant of the dielectric, the greater are the reflections and the higher is the Q of the resonator. There are two different engineering embodiments of the directivity-generating dielectric member; first, antennas with small bandwidth in which a suitable resonance of the dielectric member at the operating frequency is intentionally introduced and the filtering action of this resonance is used, and second, antennas for a broad frequency band in which resonances of the dielectric are substantially eliminated.

In the wide band case, the wave generated in the dielectric as shown in FIG. 3 and in the configuration of FIG. 2 should emerge into the adjoining space with as little reflection as possible and should be radiated. For this purpose, the dielectric wedge requires a low-reflection front as termination. Therefore, in the wide band case, this front must create a continuous transition between dielectric and air. In the examples of FIGS. 10 and 11, this transition is achieved by the addition of several small wedges 9. The transition regions shown in FIGS. 10 and 11 can have differently shaped outer boundaries. As shown in FIG. 11, they can also be a direct continuation of the wedge of the feed zone.

Claims

What is claimed is:

1. A directional antenna comprising:

a. a wedge-shaped dielectric member having a pair of surfaces inclined to each other and joined at an intersection line;

b. a first conducting surface mounted on one of the pair of dielectric surfaces and having the general shape of a triangle;

c. a second conducting surface mounted on and extending beyond the other of the pair of dielectric surfaces with the first and second conducting surfaces forming a line of constant characteristic impedance; and

d. means for feeding the antenna, the feeding means being connected to the apex of the triangle of the first conducting surface and on the second conducting surface in the immediate vicinity of the intersection line.

2. A directional antenna according to claim 1 wherein one of the conducting surfaces is at least a quarter of a wavelength long.

3. A directional antenna according to claim 1 wherein the characteristic impedance formed by the pair of conducting surfaces is equal to the characteristic impedance of the feeding means.

4. A directional antenna according to claim 1 wherein the wedge angle of the dielectric member is equal to arc cos c.sub.o /v.sub.p .sqroot..epsilon..sub.r where c.sub.o is the speed of light, v.sub.p is the phase velocity, and .epsilon..sub.r is the relative dielectric constant.

5. A directional antenna according to claim 1 which further comprises a three-terminal amplifier with one terminal connected to the feed point of one conductor, one to the feed point of the other conductor, and one to the inner conductor of the coaxial line.

6. A directional antenna according to claim 5 wherein the triangle has indentations at the two sides that form the apex thereof.

7. A directional antenna according to claim 1 which further comprises a pair of flat conductors mounted on the dielectric member adjacent to the triangle but spaced therefrom and extending to the intersection line and connected to the extended conducting surface, the triangle and the flat conductor forming slots to resonate in the operating frequency range.

8. A directional antenna according to claim 7 wherein the surface of the wedge opposite the angle of the intersection line includes angular indentations.