GB2397696A

GB2397696A - Co-linear antenna

Info

Publication number: GB2397696A
Application number: GB0227211A
Authority: GB
Inventors: Henry O'tani
Original assignee: Individual
Current assignee: Individual
Priority date: 2002-11-21
Filing date: 2002-11-21
Publication date: 2004-07-28
Also published as: GB0227211D0

Abstract

A co-linear antenna comprises a plurality of full wavelength dipoles fed by at least one resonant open wire feed line. This aspect of the invention combines the concept of a co-linear antenna having a plurality of high impedance full wave dipoles with an open wire feed line arrangement for feeding the dipoles. The use of an open wire feed arrangements permits full wavelength dipoles to be coupled together in parallel to provide parallel high impedances at very low losses. In embodiments where a large number of full wave dipoles are connected to the open wire feed line, matching the antenna to a standard 50 Ohm constant impedance transmission line, for example coaxial cable, can be readily achieved. For example a full wave dipole element having an impedance of 2.2 KOhms provides an antenna impedance of 50 Ohms if connected in parallel (end fed) in an array of 44 such elements. When the dipoles are arranged in a Vee a high gain omni-directional antenna is achieved. The dipoles and at least one parallel feed line may be printed on a circuit board (pcb) and arranged in a V or X configuration with the feed line(s) at the apex of the V or the cross over point of the X configuration.

Description

1 2397696

CO-LINEAR ANTENNA

This invention relates to co-linear antennas and in particular high gain omni- directional antennas.

0 Multi element co-linear antenna ar ays usually comprise a plurality of half wave dipoles vertically slacked at one wavelength intervals, with the complementaryhalves of each dipole fed 180 out of phase with each other and adjacent dipoles. With this arrangement it is possible to increase the gain of the antenna by adding additional dipoles to the antenna stack. For instance, a co-linear antenna having four half wave dipoles has a gain approximately four times that of a single dipole (6dB) with an extra 1.5 dB stacking gain. Antennas of this type are directional having broadside azimuth on both sides of the antenna. Typically a single transmission line, for example, a coaxial cable, feeds the four half wave dipoles. In a broadside ar ay a plural ity of half wave dipoles are stacked at one half wavelength intervals with the dipole elements 2 0 being fed by a conventional transmission line, the conductors of which feed alternate dipole elements on opposite sides of the stack in a cries-cross manner such that complimentary dipole element pairs are fed 180 out of phase with each other and with adjacent dipole elements. The broadside ar ay has a directive radiation pattern in the horizontal plane similar to the co-linear array previously described but is also 2 5 directive in the vertical plane, that is to say along the axis of the antenna perpendicular to the plane of the dipole element pairs.

Half wavelength dipoles are preferred in co-linear antenna arrays because it is relatively easy to match low impedance of wavelength dipole elements to standard 500hm impedance feed lines such as coaxial cable and the like. A half wave dipole is nominally 750hms, but If made with a very thick 14:1 diameter/length the dipole can give a 500hm match. The gain advantage of using full wave dipoles over half wave dipoles is only 3dB, which can be negated by matching losses of 2 or 3dB when matching a standard 500hm feed line to a full wave dipole element having an impedance of I OOOOhms, for instance. However, there is a limit to the number of low impedance half wave elements that can be added to an antenna stack since resistive 0 losses, due to for example skin effects and metal junctions, become significant in large arrays where the dipole elements are effectively connected in parallel with each other.

Current developments of domestic and commercial wireless telecommunication services above 1 GHz such as third generation telecoms and community WLAN has created a requirement for a high gain antenna that combines high gain with broad band, omni-directional and sector- directional characteristics.

According to an aspect of the invention there is provided a co-linear antenna comprising a plurality of full wave dipoles fed by at least one resonant open wire feed 2 0 line. This aspect of the invention combines the concept of feeding parallel high impedance full wave dipoles by an open wire feed line. The use of an open wire feed permits the full wave dipoles to be coupled together in parallel to provide parallel high impedances at very low losses. Open feed lines exhibit the lowest losses of all known transmission systems (other than free space). In embodiments where a large number of full wave dipoles are connected to the open wire feed line matching the antenna to a standard 500hm constant impedance transmission line, for example coaxial cable, can be readily achieved. For example a full wave dipole element having an impedance of 2.2kOhms would provide an antenna impedance of 500hms if connected in parallel (end fed) in an array of 44 such elements. In this way it is possible to vary the number of full wave dipoles in an array, where the dipole elements are end fed and connected to an open wire parallel feed line, to provide an overall impedance substantially the same as the standard 500hm impedance used by most equipment suppliers. This way it is possible to avoid significant matching losses from high ratio impedance matching 0 circuits which would otherwise be required to match a high impedance full wave dipole to a 500hm transmission line.

For the avoidance of doubt the term "open wire feed line" referred to herein concerns a particular type of resonant feed line where the actual impedance and phase angle of the feed line at a given frequency is the same at positions spaced one or more multiple wavelengths apart. The use of an open wire feed is generally considered to be an obsolete method of joining a source to a load particularly since the introduction of constant impedance transmission lines and coaxial cable.

2 0 In one embodiment the antenna comprises a plurality of open wire feed lines each feeding a respective array of complimentary dipole element pairs. This arrangement readily enables separate dipole arrays to be fed by a separate open wire parallel feed line. This can be used to increase the gain of the antenna or direction by varying the phase relationship between a respective dipole arrays.

Preferably, the antenna comprises a plurality of dipole arrays each fed in phase relation to each other by respective open wire parallel feed lines.

In preferred embodiments, the antenna comprises a plurality of dipole arrays each feed in phase relation to each other by respective parallel feed lines. In this respect the dipole elements are connected to the open transmission wires with adjacent elements on the same transmission wire being fed l 80 out of phase with respect to each other so that adjacent elements combine with corresponding elements connected to the other 0 transmission wire so that the spacing between adjacent dipole pairs connected to a respective feed line are positioned at half wavelength intervals along the feed line. In embodiments where more than one feed line is used further dipole arrays can be arranged between the dipole complementary pairs of the other array or arrays.

In one embodiment each parallel wire and associated dipole elements connected to that wire are arranged in a single plane. This arrangement is particularly convenient for embodiments where the wire and its associated elements are fabricated on a PCB, that is to say printed in a single plane on one side of a PCB with the other parallel wire and its associated dipole elements printed on the other side of the PCB.

In another embodiment each parallel wire and its associated dipole elements are arranged into angled planes. This arrangement is appropriate where the complementary dipole elements are arranged in a Vee antenna arrangement for enhanced omni-directional performance characteristics. In an embodiment ofthis type for instance, adjacent dipole elements are arranged in different planes such that the adjacent elements are angled with respect to one another.

In some embodiments the parallel feed line and dipoles are printed on a PCB. This has the advantage that the position of the elements can be accurately and repeatedly fabricated using known PCB printing methods.

In preferred embodiments the dipoles are arranged in a V or X configuration with the parallel open feed lines arranged along the apex of the V or the cross over point of the 0 X. With a V or X configuration the antenna is inherently omni- directional and this arrangement readily provides for a compact antenna with the feed line arranged along one side of the antenna rather than along the centre between the dipole element pairs.

In preferred embodiments the open wire feed line is connected to a common feed transmission line and the impedance of the antenna is substantially matched to the impedance of the transmission line. For example, where the impedance of the transmission line is stay SOOhms it is possible to substantially match the impedance of the antenna without the use of a matching circuit by adjusting the impedance characteristics of the radiating dipole elements and also the number of such elements 2 0 in the antenna fed by the common source. It will be understood to those skilled in the art that the number of dipole elements is invertedly proportional to the overall antenna impedance when the dipole elements are end fed and effectively connected in parallel to the open wire transmission line as further described herein.

Preferably, the antenna comprises n dipole elements, where n is the number of dipoles required to be coupled by the said open wire feed line to substantially match the transmission line impedance: In one embodiment the antenna comprises between 40 and SO dipoles having an impedance in the region of lOOOhms each, preferably between 44 and 48 dipoles, and most preferably 8 sections of 6 dipoles fed from a common source.

In some embodiments the transmission line is connected to the open wire feed line by 0 a quarter wavelength transformer to trim the antenna array to obtain a favourable VSWR at the operating frequency. In one embodiment a quarter wave transformer is used to match a 250hm antenna to a SOOhm transmission line and in other embodiments to fine-tune a nominal SOOhm antenna to a SOOhrn coaxial transmission line.

In preferred embodiments the dipole elements and open feed line(s) are supported along the length of the antenna by a dielectric support means. The dielectric may comprise a plastics material and in embodiments where the antenna is configured as a V antenna the dipole elements may be supported by an L-shaped angle section with 2 0 the individual dipole elements passing through and being supported by apertures in the angle section. This can further avoid losses in the antenna support structure since the elements are supported as near as possible at low voltage nodes along their length.

(because of the prior possibility of efficiently matching a small number offull wave V dipoles to a 50 or 750HM line).

In another aspect of the invention there is provided an antenna comprising a plurality of dipole arrays, each array comprising a parallel feed line including a pair of parallel elongate transmission elements connected to respective dipole radiating elements, the said arrays being arranged such that the respective elongate transmission elements extend parallel to each other with the radiating elements ofthe respective transmission elements being positioned in respective adjacent planes.

Claims

1. A collinear antenna comprising a plurality of full wave dipoles fed by at least one resonant open wire (parallel) feed line.

0

2. An antenna as claimed in Claim 1 comprising a plurality of open wire parallel feed lines each feeding a respective array of complementary dipole element pairs.

3. An antenna as claimed in Claim 2 comprising a plurality of dipole arrays each fed in phased relation to each other by respective open wire parallel feed lines.

4. An antenna as claimed in Claim 3 wherein each feed line comprises a pair of parallel transmission wires and complementary dipole elements are connected to the wires for end feeding 180 out of phase with respect to each other and 180 out of phase with respect to adjacent dipoles.

5. An antenna as claimed in Claim 4 wherein each parallel wire and associated dipole elements are arranged in a single plane.

6. An antenna as claimed in Claim 4 wherein each parallel wire and its associated 2 5 dipole elements are arranged in two angled planes.

7. An antenna as claimed in any preceding claim wherein the said dipoles and the said at least one open wire parallel feed line are printed on a PCB.

8. An antenna as claimed in any preceding claim wherein the said dipoles are arranged in a V or X configuration with the parallel open feed line(s) arranged along the apex of the V of the cross-over point of the X.

9. An antenna as claimed in any preceding claim wherein the at least one open 0 wire feed line is connected to a common feed transmission line and the impedance of the antenna is substantially matched to the impedance of the transmission line.

lO. An antenna as claimed in Claim 9 wherein the antenna comprises n dipole elements, where n is the number of dipoles required to be coupled by the said open wire feed line to substantially match the transmission line impedance.

An antenna as claimed in Claim 10 comprising between 40 and SO dipoles.

12. An antenna as claimed in Claim 11 comprising 44 or 48 dipoles, preferably eight sections of six dipoles.

13. An antenna as claimed in any one of Claims 9 to 12 wherein the said transmission line is connected to the said open wire feed line by a i/. wavelength transformer.

14. An antenna as claimed in any preceding claim wherein the dipole elements and open feed line(s) are supported along the length of the antenna by a dielectric support means.

15 À An antenna comprising a plurality of dipole arrays, each array comprising a parallel feed line including a pair of parallel elongate transmission elements connected to respective dipole radiating elements, the said arrays being arranged such that the respective elongate transmission elements extend parallel to each other with the radiating elements of the respective transmission elements being positioned in lo respective adjacent planes.

16. An antenna substantially as hereinbefore described and/or with reference to the accompanying drawings.