WO1990010959A1

WO1990010959A1 - Combination dipole array and slot array antenna

Info

Publication number: WO1990010959A1
Application number: PCT/AU1990/000104
Authority: WO
Inventors: Neil Henry Hill
Original assignee: Neil Henry Hill
Priority date: 1989-03-15
Filing date: 1990-03-15
Publication date: 1990-09-20

Abstract

An antenna (5) having driven colinear dipole array (18) and passive slot array (14). Dipole array (18) comprises active elements (34) and phasing bars (36). Slot array (14) comprises two passive elements (35) having slots (28, 30, 32). Active elements (34) and phasing bars (36) are stamped from metal plates wherein the remainder forms slot array (14), slots (28, 30, 32) corresponding to stampings of elements (34) and bars (36). Dipole array (18) receives or transmits signals. Slot array (14) is located behind dipole array (18) with respect to a received incident wavefront. Slot array (14) acts as a resonator to absorb an incident wavefront, thereby to minimize reflected signals, and also acts to shield dipole array (18) from signals incident generally from the rear of the slot array (14).

Description

COMBINATION DIPOLE ARRAY AND SLOT ARRAY ANTEN_NA

Field of the Invention

This invention relates to antennas, and relates particularly but not exclusively to antennas for use in the reception or transmission of television broadcast transmissions.

Background of the Invention

Many forms of antenna are known for use in receiving or transmitting broadcast transmissions. Well known broadcast transmissions include television, radio, two way radio communications, navigational aids and telecommunications. The present discussion is directed to television broadcasting.

In most countries, television transmission takes place in well defined frequency ranges. The usual frequency ranges are the VHF band and the UHF band. Frequencies above UHF are typically reserved for satellite transmission.

Early VHF antennas included linear arrays of both the broadside or endfire types, and other angled arrays (V-shaped) , wherein all the elements were driven and could be thought of as forming an array of dipole elements.

In this specification, a driven antenna element has its usual art meaning of an element which is adapted to be either a receiving transducer or transmitting transducer. In the former case the driven element is sourced by the received signal, while in the latter, it is sourced by a transmitter which generates the signal to be broadcast.

Loop or circular driven elements have also been used for VHF applications, these forming pseudo-folded dipole elements, occasionally being configured in conjunction with passive elements. A different approach to VHF antennas which has received much contemporary support is the reflector/director or Yagi-Uda type antenna. Typically these antennas have a number of passive (or parasitic) director elements spaced equally apart and -succeeded by a driven element, which may be a dipole or folded dipole. Spaced behind the driven element is a passive reflector (parasitic) element.

The relatively late introduction of the higher frequency UHF broadcasting over the conventional VHF has meant that UHF antennas for commercial and domestic application have evolved quickly. The most common form of UHF antenna is the Ygi-Uda structure, described above. One problem with such configurations, however, is that a wavefront impinging on the antenna from behind will only be partially reflected by the reflector element, resulting in the well-known ghosting effect on a television receiver connected thereto by virtue of time delayed signals arriving at the driven element.

Other efforts to improve the noise rejection performance (noise is to be understood as any unwanted signal) include the addition of more complex reflector structures in the form of quasi-parabolic arrays or meshed half or quarter hemispherical constructions. These advances still suffer from noise interference. In addition, such antennas often become unwieldy in size and awkward to install.

Another drawback of this type of UHF aerial is that it is only of use over a limited frequency range close to the resonant frequency of its various elements. Outside this range, there will be a dramatic reduction in gain and directivity.

Other types of UHF antennas are known, such as phased arrays, which comprise stacking a larged number of dipole pairs connected alternatively in an in-phase and out-of-phase arrangement.

Mesh screens or resonant or corner reflectors are also occassionally incorporated in this type of antenna to improve directivity, however, this is only because unwanted signals are physically obstructed. There is then mis-matching due to the fact that it is most difficult to reach a compromise length in reflector elements to achieve adequate directivity over UHF frequencies. A phased array with mesh screen also has a performance limitation in terms of its noise rejection at the extremes of its pass band. It is also known for UHF antennas to incorporate an amplifier to amplify the received signals to improve of the signal-to-noise ratio of the signal passed to a television receiver or the like. Intrinsic and extrinsic losses have an order of magnitude greater significance at UHF frequencies over cables carrying signals in the VHF range. As a result, specially shielded co-axial cable, which may be double or triple shielded, is usually required. It is an object of the present invention to attempt to overcome one or more of the disadvantages of known antennas.

Summary of the Invention

Therefore, according to one aspect of the invention, there is provided an antenna comprising a colinear dipole array in adjacent spatial relationship to a slot array with respect to a plane normal to the colinear dipole array, and wherein the colinear dipole array is driven. According to a further aspect of the invention, there is provided a method for construction of an antenna having a colinear dipole array and a slot array, the method comprising the steps of: forming the colinear dipole array and the slot array from the same piece of electrically conductive material; and arranging the colinear dipole array and the slot array in an adjacent spatial relationship with respect to a plane normal to the colinear dipole array. According to yet a further aspect of the invention, there is provided a method of fabricating components of an antenna having a colinear dipole array and a slot array, comprising the step of: forming at least two active members and at least one phasing bar from one piece of eletrically conductive material, the active members and phasing bar forming components of the colinear dipole array, the remaining metal forming part or all of the slot array, and wherein there is no wastage of the electrically conductive material.

Brief Description of the Drawings

In order that the invention may be more clearly understood, a preferred embodiment for use in television reception will be described with reference to the examples shown in the accompanying drawings wherein:-

Figure 1 shows an isometric view of the preferred antenna in accordance with the invention; Figure 2 shows a view of the mounting section forming part of the antenna of Figure 1;

Figure 3 shows a view of the supporting section forming part of the antenna of Figure 1; and

Figure 4 shows a view of the sections of Figures 2 and 3 as fitted together.

Detailed Description of Preferred Embodiments

Whilst the embodiment will be described with reference to the reception of television broadcasts, it is to be understood that the invention is equally applicable to many transmission types in various frequency ranges, and can also be applied as an antenna for a transmitter of such transmissions.

Figure 1 shows an antenna 5 adapted for television reception comprising the general elements of a mast mounting section 12 for mounting the antenna to a mast 10, a passive slot array 14 and a supporting section 16 which supports the slot array 14. A driven colinear dipole array 18 is mounted from the supporting section 16. The antenna orientation shown is suited only to horizontally polarized transmissions, and for the purposes of this discussion, the orientation of the mast 10 defines the vertical direction.

The mast mounting section 12 is shown in more detail in Figure 2, wherein base component 22 and fastening component 20 can be seen. Fastening component 20 is clamped to the mast 10 by the action of bolt and nut combinations 25 (shown in part in Figure 1) which pass through holes 27. The four U-shaped grooves 23 on the base component 22 allow for alternate horizontal or vertical polarisation orientation of the antenna 5 when installed on the mast 10. It is therefore simple for the installer to afford correct orientation of the antenna, or indeed, for the opposite orientation to be effected should the antenna be moved to an area where opposite polarisation is used.

The mast mounting section 12 is constructed of plastic material which is lightweight, strong, environmentally robust to resist weathering and which can be injection moulded into the two separate components 20,22.

The base component 22 also contains a tab 24 and two clip legs 26 for attachment to the supporting section 16 as will be shown in Figure 4. The passive slot array 14 is not shown in Figure 2, however, it is noted that the constituent plate elements 35 forming the slot array 14 are to be fixed to the base component 22 via screws (not shown) entering receiving bores 21.

Referring again to Figure 1, it can be seen that the active elements 34 of the colinear dipole array 18 are supported in the sleeve fixtures 42. The innermost end of each active element 34 is fixed at the driving point 50 to a phasing bar 36 by means of a metallic or other electrically conductive fixture, such as a metal screw. In the present example, the dipole elements 34 are slightly U-shaped in cross-section to improve rigidity in twist.

The phasing bars 36 extend from respective feed points 49 to a driving point 50, and are fixed at feed points 49 by insulated connectors 48 which are a metallic or other eletrically conductive fixture such as a metal screw.

The colinear dipole array 18 forms a pair of dipole halves, with each half being that arrangment of phasing bars 36 and active elements 34 respectively above or below a line drawn between the two feed points 49.

The spatical relationship between the colienar dipole array 18 and the slot array 14 is one of being adjacent with respect to the horizontal direction, which is the direction of the normal to the colinear dipole array 18.

The passive slot array 14 is shown as two constituent plate elements 35 each having three slots 28,30,32.

The plate elements 35 lie in a common place with respect to the vertical direction and the slots 28,30 are spaced adjacent to the respective elements 34 in the horizontal direction. The effect of the slot array 14 is primarily as a resonator. The slots 28,30,32 have resonant performance in the frequency range of the colinear dipole array 18, providing a resonant characteristic which results in an incoming wavefront being effectively absorbed with only very little reflection back towards the active elements 34.

A small proportion of the incoming wavefront will nevertheless be reflected, however, the spacing between the slot array 14 and the colinear dipole array 18 is such that the reflected wavefront is presented at the active elements 34 being 180 degrees out-of-phase with respect to further incoming signals, thereby providing cancelling in a conventional sense.

A similar effect is achieved for wavefronts impinging on the antenna 5 from behind the slot array 14. The slot array 14 provides both a physical blocking of the wavefront due to the dimension of the plate elements 35 with respect of the receiving active elements 34, as well as providing significant resonant effect in the frequency range of interest, thus absorbing the unwanted signals that could cause ghosting or other interference effects. Prior art devices such as mesh screens may provide a degree of blocking, however, they do not afford the significant resonant performance. It is of economic benefit to avoid any waste or scrap metal offcuts resulting from the fabrication process. The antenna 5 is fabricated by pressing or stamping the active elements 34 and phasing bars 36 of the colinear dipole array 18 from one of two metal blanks; the remainder forms the plate elements 35 of the slot array 14. Therefore, the slots 28,30 are a result of removal of the active elements 34, while the slots 32 result from removal of the phasing bars 36. If the stamping were of one metal blank, then that blank would need to be further cut into the two plate elements 35.

The stamping should be such that the dimension widthwise and lengthwise of the slots 28,30,32 should be only up to 15 percent greater than the corresponding active element 34 or phasing bar 36. Referring now to Figure 3, there is shown the supporting section 16 in an inverted orientation. This section 16 is preferably of plastic, and injection moulded into two components forming the top 38 and the base 40. The base 40 comprises four sleeve fixtures 42, which, when viewed in conjunction with Figure 1, form part of the supporting arrangement for the active elements 34. The active elements 34 and phasing bars 36 are not shown in Figure 3.

On the upward facing side of the base 40 is an enclosure 44, which has the purpose of housing any electrical components required for the correct operation of the antenna. The mounting posts 45 are adapted to locate the electrical components within the enclosure 44.

It is quite common to incorporate a balun impedance matching and balancing device in the antenna 5.

Often baluns are used in conjunction with pre-amplifiers.

A problem associated with commonly used pre-amplifiers is that the output cabling therefrom is required to be heavily shielded to avoid interference from the driven elements, typically by the use of double or triple shielded co-axial cable.

In the present arrangement, the need for such expensive cabling is alleviated, in that the outgoing cable exits the enclosure 44 through duct 46, which is formed upon fixture of the top 38 to the base 40. The incoming signals enter the enclosure 44 via insulated connectors 48 which, in turn, are connected to the feed points 49 of the phasing bars 36 (shown in Figure 1). In this way, the outgoing cable is arranged remotely from any driven elements of the colinear dipole array 18 thus decreasing the effects of capacitively coupled interference. Figure 4 shows a view of the mounting section

12 fixed into supporting section 16 by tab 24 fitting into slit 47, and clip legs 26 cooperating with opening 43. The slot array 14, colinear dipole array 18 and fastening component 20 are not shown. This modular construction is fundamental in alleviating the need for heavily shielded signal cabling from the electrical components within the enclosure 44, and providing easy access for installation purposes such that the cabling can be connected without the need for dismantling a large part of the antenna structure. The cabling can also be performed with the mast mounting section 12 and slot array 14 fixed to the mast 10.

A further advantage of such a configuration is that the injection moulding process of fabrication is simple and requires no undercut dyes, thereby reducing the unit cost of manufacture and equipment maintenance time. Further, the plastic construction of the mounting section 12 and supporting section 16 avoids resonances which may affect the performance of the antenna.

In order that the antenna function correctly, the dimensions of the colinear dipole array 18 must be carefully chosen, and are dependant upon the carrier frequency of the television broadcast. There is also a need to impedance match the colinear dipole array 18 with the reception system and associated cabling. The characteristic driving impedance of such an array is dictated by a number of factors, including the separation and length of the phasing bars 36, the length of the active elements 34, and the spacing between the colinear dipole array 18 and the slot array 14. Such a determination can only be made using known mathematical approaches in conjunction with detailed experimentation and testing. In one example, various crucial dimensions of an antenna suitable for use in UHF applications are as follows -

(i) the distance between the phasing bars 36 is approximately equal to 30mm, (ii) the distance between a feed point 49 and a driving point 50 is approximately 95mm, (iii) the length of a dipole element 34 is approximately 145mm, and iv) the distance between an active element 34 and a corresponding slot 28,30 is approximately 70mm. Preferably, all the metallic components, being the slot array 14 and colinear dipole array 18, will be treated as protection from the effects of atmospheric corrosion or other chemical or weather effects. A suitable process is chromate conversion. The antenna may be industrially applied by using known metal stamping and forming techniques and by using known plastics molding techniques as would be apparent to persons skilled in the art.

Claims

CLAIMS :

1. An antenna comprising a colinear dipole array in adjacent spatial relationship to a slot array with respect to a plane normal to the colinear dipole array, and wherein the colinear dipole array is driven.

2. An antenna as claimed in claim 1, wherein the colinear dipole array comprises a pair of dipole halves, the dipole halves being in colinear relationship and having common connection at two feed points, each dipole half comprising a pair of phasing bars, wherein each of the phasing bars is connected at one end to an end of an active element forming a right angle therebetween, and the other end of each of the phasing bars is connected to a respective one of the feed points.

3. An antenna as claimed in claim 2, wherein the slot array comprises a plurality of slot elements, each of which is disposed coincident to a one of the active elements of the colinear dipole array with respect to the said normal.

4. An antenna as claimed in claim 3, further comprising a supporting member and a mounting member which are, in use, engageable; the supporting member providing for fixing of the colinear dipole array thereto, and the mounting means providing for fixing of the slot array thereto and further providing for fixed mounting of the antenna into its operational orientation.

5. An antenna as claimed in claim 4, wherein the supporting member and the mounting member are of plastic material.

6. An antenna as claimed in claim 4, wherein the supporting member further comprises a slit and an opening, and the mounting member further comprises a tab and one or more clips such that in engagement between the supporting member and the mounting member the tab is received in the slit and the one or more clips is received in the opening.

7. An antenna as claimed in claim 2, being adapted for the reception of UHF broadcasts, and wherein, in use, the colinear dipole array is disposed normal to the direction of propogation of received wavefronts.

8. An antenna as claimed in claim 3, wherein the colinear dipole array has electrical connection from the two feed points.

9. An antenna as claimed in claim 8, wherein there is electrical connection between the feed points and electrical signal conditioning apparatus.

10. An antenna as claimed in claim 3 wherein each of the slot elements is lengthwise and widthwise dimensioned so as to be in the range of 100 to 115 percent of the respective dimension for the respective active element.

11. A method for construction of an antenna having a colinear dipole array and a slot array, the method comprising the steps of: forming the colinear dipole array and the slot array from the same piece of electrically conductive material; and arranging the colinear dipole array and the slot array in an adjacent spatial relationship with respect to a plane normal to the colinear dipole array.

12. A method as claimed in claim 11 comprising the further steps of: fixing the colinear dipole array to a supporting member; fixing the slot array to a mounting member; and engaging the supporting member with the mounting member by means of cooperating parts thereof.

13. A method as claimed in claim 12, further comprising the step of: disposing the mounting member fixedly to a fixture so as to provide the antenna in operational orientation.

14. A method of fabricating components of an antenna having a colinear dipole array and a slot array, comprising the step of: forming at least two active members and at least one phasing bar from one piece of eletrically conductive material, the active members and phasing bar forming components of the colinear dipole array, the remaining metal forming part or all of the slot array, and wherein there is no wastage of the electrically conductive material.