GB2174249A

GB2174249A - Dual-polarised conical spiral antenna

Info

Publication number: GB2174249A
Application number: GB08607128A
Authority: GB
Inventors: Stephen Joseph Foti
Original assignee: ERA Patents Ltd
Current assignee: ERA Patents Ltd
Priority date: 1985-04-26
Filing date: 1986-03-21
Publication date: 1986-10-29
Also published as: GB8607128D0; GB8510709D0

Abstract

A dual-polarised conical spiral antenna comprises two pairs of symmetrical electrically conductive arms 2 and 3 (only one arm of each pair is shown) which are wound spirally around the inner and outer surfaces of a thin truncated conical support member 1 made of an electrically insulating material, the outer pair of arms 2 being wound in the opposite circumferential direction to the inner pair of arms 3. Preferably the wrap angle of each arm 2, 3 is 45 DEG . <IMAGE>

Description

SPECIFICATION Dual-polarised conical spiral antenna This invention relates to a dual-polarised conical spiral antenna.

Conical spiral antennas have been well known for many years, and basically comprise a number of similarly spirally wound electrically conducting arms which follow the surface of a truncaed cone, the arms usually being mounted on an insulating conical support member. The operating characteristics of such antennas depend on the antenna symmetry and its basic parameters, namely the cone angle, the wrap angle of the arms, the angular width of the arms, and the height or length of the antenna. The conventional conical spiral antenna is generally a wide band device, but is capable of radiating or receiving only one hand of circular polarisation, i.e. either the lefthand or right-hand depending on the direction in which the arms are wound around the cone.Attempts have been made to make a dual-polarised conical spiral antenna, i.e. an antenna capable of radiating or receiving both hands of circular polarisation, such as by modifying the shape of the arms or by arranging to energise the arms from opposite ends of the cone, but these efforts have not been very successful in that the operating characteristics and band-width of the antenna are seriously degraded.

According to our invention we propose a dual-polarised conical spiral antenna comprising electrically conducting arms which are conically spirally wound in opposite circumferential directions and which axially overlap but are electrically insulated from each other.

In effect the antenna in accordance with the invention is equivalent to a pair of oppositely wound conventional conical spiral antennas which are co-axially overlaid and electrically insulated from each other, and we have found that such an antenna can be constructed which is capable of simultaneously transmitting or receiving both hands of circular polarisation over a wide operating frequency band and which is virtually the same size as a corresponding conventional conical spiral antenna operative with only one hand of circular polarisation.

Preferably the oppositely wound arms of the antenna in accordance with the invention are mounted on the inner and outer surfaces of a thin conical support member of insulating material, and preferably there are at least two balanced (i.e. symmetrically arranged) arms wound in each direction.

Preferably the wrap angles of the oppositely wound arms are complementary to each other, i.e. are such that the arms cross each other substantially orthogonally, and preferably also the wrap angles are substantially equal to each other. Arranging the wrap angles of the oppositely wound arms to be complementary should minimise the coupling between the oppositely wound arms since their surface current vectors will be orthogonal, and making the wrap angles equal and complementary will not only minimise the coupling between the oppositely wound arms but will also provide the antenna with similar operating characteristics and radiation patterns for each hand of circular polarisation.The wrap angle is the angle between a conically spirally wound arm and any straight line on the surface of the cone (i.e. any line which is co-planar with the cone axis) at the point of intersection between the two.

A particular example of a dual polarised conical spiral antenna in accordance with the invention will now be described with reference to the accompanying drawing which is a diagrammatic perspective view of the antenna and from which some of the components of the antenna have been omitted for the sake of clarity.

The antenna illustrated comprises a thin walled truncated conical support member 1 made of an electrically insulating material. On the outer surface of this support member 1 the antenna has a pair of electrically conductive strips or arms 2 (only one is shown) which are wound spirally around the conical outer surface with a wrap angle of 45 . The arms 2 expand exponentially as they progress away from the apex of the member 1 and are evenly spaced to form a balanced two arm logarithmic spiral antenna on the outside of the member 1. On the inner surface of the support member 1 the antenna has a further pair of electrically conductive strips or arms 3 (again only one is shown) which are wound spirally around the conical inner surface, also with a wrap angle of 45 , but in the opposite direction about the cone axis to the arms 2.

As with the arms 2, the arms 3 expand exponentially as they progress away from the apex of the member 1 and are evenly spaced to provide a second balanced two arm conical logarithmic spiral antenna. Because the two conical spiral antennas formed by the inner and outer arms 2 and 3 respectively spiral in opposite directions about the cone axis and have wrap angles which are equal and complementary, they will radiate and receive opposite hands of circular polarisation from each other with similar radiation patterns and minimum coupling between them.

The antenna feed, which is not shown, may be formed in any conventional manner, for example by means of a transmission line (preferably balanced) along the cone axis to the apex and connecting wires to each arm, or by means of transmission lines along the individual arms themselves.

1. A dual-polarised conical spiral antenna

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Dual-polarised conical spiral antenna This invention relates to a dual-polarised conical spiral antenna. Conical spiral antennas have been well known for many years, and basically comprise a number of similarly spirally wound electrically conducting arms which follow the surface of a truncaed cone, the arms usually being mounted on an insulating conical support member. The operating characteristics of such antennas depend on the antenna symmetry and its basic parameters, namely the cone angle, the wrap angle of the arms, the angular width of the arms, and the height or length of the antenna. The conventional conical spiral antenna is generally a wide band device, but is capable of radiating or receiving only one hand of circular polarisation, i.e. either the lefthand or right-hand depending on the direction in which the arms are wound around the cone.Attempts have been made to make a dual-polarised conical spiral antenna, i.e. an antenna capable of radiating or receiving both hands of circular polarisation, such as by modifying the shape of the arms or by arranging to energise the arms from opposite ends of the cone, but these efforts have not been very successful in that the operating characteristics and band-width of the antenna are seriously degraded. According to our invention we propose a dual-polarised conical spiral antenna comprising electrically conducting arms which are conically spirally wound in opposite circumferential directions and which axially overlap but are electrically insulated from each other. In effect the antenna in accordance with the invention is equivalent to a pair of oppositely wound conventional conical spiral antennas which are co-axially overlaid and electrically insulated from each other, and we have found that such an antenna can be constructed which is capable of simultaneously transmitting or receiving both hands of circular polarisation over a wide operating frequency band and which is virtually the same size as a corresponding conventional conical spiral antenna operative with only one hand of circular polarisation. Preferably the oppositely wound arms of the antenna in accordance with the invention are mounted on the inner and outer surfaces of a thin conical support member of insulating material, and preferably there are at least two balanced (i.e. symmetrically arranged) arms wound in each direction. Preferably the wrap angles of the oppositely wound arms are complementary to each other, i.e. are such that the arms cross each other substantially orthogonally, and preferably also the wrap angles are substantially equal to each other. Arranging the wrap angles of the oppositely wound arms to be complementary should minimise the coupling between the oppositely wound arms since their surface current vectors will be orthogonal, and making the wrap angles equal and complementary will not only minimise the coupling between the oppositely wound arms but will also provide the antenna with similar operating characteristics and radiation patterns for each hand of circular polarisation.The wrap angle is the angle between a conically spirally wound arm and any straight line on the surface of the cone (i.e. any line which is co-planar with the cone axis) at the point of intersection between the two. A particular example of a dual polarised conical spiral antenna in accordance with the invention will now be described with reference to the accompanying drawing which is a diagrammatic perspective view of the antenna and from which some of the components of the antenna have been omitted for the sake of clarity. The antenna illustrated comprises a thin walled truncated conical support member 1 made of an electrically insulating material. On the outer surface of this support member 1 the antenna has a pair of electrically conductive strips or arms 2 (only one is shown) which are wound spirally around the conical outer surface with a wrap angle of 45 . The arms 2 expand exponentially as they progress away from the apex of the member 1 and are evenly spaced to form a balanced two arm logarithmic spiral antenna on the outside of the member 1. On the inner surface of the support member 1 the antenna has a further pair of electrically conductive strips or arms 3 (again only one is shown) which are wound spirally around the conical inner surface, also with a wrap angle of 45 , but in the opposite direction about the cone axis to the arms 2. As with the arms 2, the arms 3 expand exponentially as they progress away from the apex of the member 1 and are evenly spaced to provide a second balanced two arm conical logarithmic spiral antenna. Because the two conical spiral antennas formed by the inner and outer arms 2 and 3 respectively spiral in opposite directions about the cone axis and have wrap angles which are equal and complementary, they will radiate and receive opposite hands of circular polarisation from each other with similar radiation patterns and minimum coupling between them. The antenna feed, which is not shown, may be formed in any conventional manner, for example by means of a transmission line (preferably balanced) along the cone axis to the apex and connecting wires to each arm, or by means of transmission lines along the individual arms themselves. CLAIMS

1. A dual-polarised conical spiral antenna comprising electrically conducting arms which are conically spirally wound in opposite circumferential directions and which axially overlap but are electrically insulated from each other.

2. An antenna according to Claim 1, in which the oppositely wound arms of the antenna are mounted on the inner and outer surfaces of a thin conical support member of insulating material.

3. An antenna according to Claim 1 or Claim 2, in which there are at least two balanced arms wound in each direction.

4. An antenna according to any one of the preceding Claims, in which the wrap angles of the oppositely wound arms are such that the arms cross each other substantially orthogonally.

5. An antenna according to any one of the preceding Claims, in which the wrap angles of the oppositely wound arms are substantially equal to each other.

6. An antenna according to any one of the preceding Claims, in which each of the arms expands (i.e. it width increases) as it progresses away from the apex of its conical spiral.

7. An antenna according to Claim 6, in which each of the arms expands exponentially.

8. An antenna according to Claim 1, substantially as described with reference to the accompanying drawing.