GB2234859A - Spiral antenna - Google Patents

Spiral antenna Download PDF

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Publication number
GB2234859A
GB2234859A GB9014536A GB9014536A GB2234859A GB 2234859 A GB2234859 A GB 2234859A GB 9014536 A GB9014536 A GB 9014536A GB 9014536 A GB9014536 A GB 9014536A GB 2234859 A GB2234859 A GB 2234859A
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United Kingdom
Prior art keywords
spiral
port
antenna
antenna according
spiral arm
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Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Granted
Application number
GB9014536A
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GB2234859B (en
GB9014536D0 (en
Inventor
Stephen Joseph Foti
Timothy Richard Waterfield
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ERA Patents Ltd
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ERA Patents Ltd
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Publication date
Application filed by ERA Patents Ltd filed Critical ERA Patents Ltd
Publication of GB9014536D0 publication Critical patent/GB9014536D0/en
Publication of GB2234859A publication Critical patent/GB2234859A/en
Application granted granted Critical
Publication of GB2234859B publication Critical patent/GB2234859B/en
Anticipated expiration legal-status Critical
Expired - Fee Related legal-status Critical Current

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Classifications

    • HELECTRICITY
    • H01ELECTRIC ELEMENTS
    • H01QANTENNAS, i.e. RADIO AERIALS
    • H01Q1/00Details of, or arrangements associated with, antennas
    • H01Q1/36Structural form of radiating elements, e.g. cone, spiral, umbrella; Particular materials used therewith
    • HELECTRICITY
    • H01ELECTRIC ELEMENTS
    • H01QANTENNAS, i.e. RADIO AERIALS
    • H01Q9/00Electrically-short antennas having dimensions not more than twice the operating wavelength and consisting of conductive active radiating elements
    • H01Q9/04Resonant antennas
    • H01Q9/16Resonant antennas with feed intermediate between the extremities of the antenna, e.g. centre-fed dipole
    • H01Q9/26Resonant antennas with feed intermediate between the extremities of the antenna, e.g. centre-fed dipole with folded element or elements, the folded parts being spaced apart a small fraction of operating wavelength
    • H01Q9/27Spiral antennas

Abstract

A spiral antenna 1 for circularly polarised radiation has a single regular or irregular spiral arm 2 and a port A, B is formed by a truncation 3 at a minimum or maximum circumference of the spiral 2. The port is arranged to be directly coupled to an RF system. In the preferred example, the antenna has ports A and B at both the minimum and maximum circumferences respectively and is arranged to be coupled to the RF system at one or both of the ports. The antenna may be formed as a microstrip circuit. The diameters at which the antenna is truncated by the ports may be such that the phase difference between each port and the next adjacent turn of the spiral 2 is approximately (2n+1) pi . An inner disc shaped ground plane 5 and an outer annular ground plane 6 define the minimum and maximum circumferences. <IMAGE>

Description

SPIRAL ANTENNA The present invention relates to a spiral antenna for transmitting or receiving circularly polarised radiation.
A typical spiral antenna consists of two spiralling conducting arms both in the same plane but angularly displaced by 1800. A requirement of such a spiral is that the currents excited in the two arms are equal in amplitude and 1800 of out phase. To achieve this the spiral must be fed in balanced fashion, typically accomplished by connection to some form of twin lead transmission line and a 'balun'. The balun connects the balanced line to the unbalanced coaxial transmission line used to connect the antenna system to the RF transmitter or receiver. This balun represents a significant part of the manufacturing cost of spiral antennas and if not designed properly, some imbalance of the currents in the two arms of the spiral can occur causing the radiated beam of the spiral antenna to'distort with consequent degradation of the inherent circular polarisation of the spiral.A secondary purpose of the balun is to provide an impedance matching function between the balanced and unbalanced transmission lines which make up a part of the composite two arm spiral antenna structure.
Although it is desirable to provide two ports, one at the spiral centre and one at the outer truncated edge which produce opposite senses of circular polarisation, a standard two arm spiral is only a one port device which provides only one sense of circular polarisation and attempts by designers to achieve two port antennas have met with little success.
According to this invention, a spiral antenna for circularly polarised radiation comprises a single regular or irregular spiral arm having at least one port formed by a truncation at a minimum or maximum circumference of the spiral and arranged to be directly coupled to an RF system.
Preferably the antenna includes a pair of ports formed at the minimum and maximum circumferences respectively and arranged to be coupled to the RF system at one or both of the ports.
The present invention provides an antenna capable of similar performance to that of a multiple arm spiral but using only a single arm with one or preferably two ports.
This structure avoids many of the disadvantages of multiple arm spirals and in particular makes it possible to couple the antenna directly to a transmitter or receiver without using a balun.
Preferably the diameter of the or each port is such that the phase difference between a port and the next adjacent turn of the spiral is substantially (2n+l)s, where n=o,1,2, .
The phase number n may be different for the inner and outer ports.
The principles of operation of the one-arm spiral antenna apply to any geometrical form of the spiral pattern of the conducting arm, whether it be Archimedean, equiangular, planar, non-planar of regular or of irregular configuration. The spiral conductor pattern is truncated at some minimum circumference and some maximum circumference. These truncations form the inner and outer ports of the antenna, which provides circularly polarised radiation with opposite hands of polarisation depending upon the input port used.
The one arm spiral may be used equally in either receive or transmit mode as a nominally circularly polarised one-port device with the port at either the inner or outer truncation of the spiral, providing corresponding opposite hands of polarisation.
Alternatively the one arm spiral may be used as a dual-circularly polarised two-port device with the port at each truncation of the spiral receiving either hand of polarisation, the inner port having polarisation of opposite hand to the outer port.
The cross-sectional geometry of the spiral arm may be varied along its length as required to tailor its performance.
Preferably the antenna is formed on a microstrip circuit board.
When the antenna is formed on a microstrip circuit board microstrip circuits can be formed on the same substrate, providing a means of directly feeding the antenna.
An antenna in accordance with the present invention will now be described in detail with reference to the accompanying drawings in which: Figure 1 is a plan; Figure 2 is a side elevation and plan; and, Figure 3 shows side elevations of the generic form and microstrip example of the invention.
An antenna 1 comprises a planar spiral 2 of regular geometry.
An inner port A is formed at a minimum circumference of the spiral 2 and an outer port B at the maximum circumference. In this preferred example the spiral 2 is formed as a microstrip circuit. An inner disc-shaped ground plane 5 and outer annular ground plane 6 define the minimum and maximum circumferences and hence the position of the ports A,B. At the point of the truncation the microstrip extends onto the region overlying the ground plane to form a micros trip transmission line for coupling RF to or from the respective port A,B.
The spiral conductor pattern is shown in Figure 1.
The diagram depicts a clockwise winding up of the spiral, though there is no preference over an anticlockwise winding. The inner truncation is the inner port A of the antenna and the outer truncation is the outer port B of the antenna. The region from which radiation is received or transmitted lies in the annular region bounded by the circumferences of these two truncations. The direction of radiation maximum is orthogonal to the plane of the spiral. Radiation maxima occur within this zone at circumferences which are whole multiples of the wavelength in the transmission medium, so that the fundamental mode is radiated at a circumference of one wavelength, and higher order modes at circumferences of two or more wavelengths.
The means by which a travelling wave, injected at either port, is guided to the radiating region, is by truncating the spiral at a circumference which yields the desired phase relationship between the injection point and the nearest point to it on the first following turn of the spiral arm (ie points A and A' for the inner port, and B and B' for the outer port). This phase difference affects the complex impedance at the port. If the phase difference is (2n +1) ir (where n = 0,1,2,...), then the two adjacent points on the consecutive turns of the arm will be in antiphase, and these turns will thus act as a "twin-lead" transmission line. This initial phase difference may conceivably be chosen as a value different from (2n+1)a, in order to match the input impedance to the output of the device connected to it, but the preferred phase difference would be (2n+1)o to provide a purely real impedance.
In common with other spiral antennas, a planar one-arm spiral radiates opposite hands of circular polarisation into the two half-spaces bounded by its plane. If radiation is required into just one of these half-spaces, one may simply influence the radiation into the other half-space by backing the spiral with absorbent material (if the radiation energy is unwanted), or, with a reflecting structure or cavity in the backward half-space, to reflect the energy into the forward half-space to combine with that radiated directly.For example, a parallel reflecting plane placed an effective 90" electrical distance (ie. accounting for the refractive index of the medium) behind the spiral will invert the phase of incident wave, and the total on-axis path will result in the reflected wave passing through the spiral and adding coherently (with the same hand of polarisation) on axis with the forward wave. This "end-fire" effect will improve the directivity of the absorber-backed antenna by up to 3dB, depending upon the construction of the spiral and any ohmic losses in its support medium. The shape and position of the reflecting surface may be tailored to create a range of beam shapes if required.
If fed at the inner port, the spiral would therefore yield radiation of the (n+l) th order mode, and if fed at the outer port it would yield radiation of the n th order mode. A typical design would use n=0 for the inner port, and n=l for the outer port.
An important aspect of this invention is therefore the geometry of the conductor pattern at the truncated edges. This geometry simultaneously provides an impedance matching function and the appropriate phasing of the currents on adjacent turns of the spiral near these edges. The latter can be considered a balun function as required by multiple arm spirals in that a structure is provided which facilitates the interconnection between, for example, an unbalanced circuit and a one-arm spiral wherein the adjacent conductor pattern turns at the edge can be considered to be twin lead transmission line. Note that the circumferences shown in Figure (1) in terms of wavelengths represent the effective wavelengths with any dielectric media (if present) taken into account and are therefore not necessarily free space wavelengths.Also, Aol and A02 are the effective wavelengths at the centre frequencies f01 and f02 of the operating bands of the inner and outer ports respectively, which may have equal or unequal bandwidths and may coincide, overlap or be separated.
In order to achieve optimum performance from the composite structure described above, by creating a purely real impedance at the ports, certain design constraints should therefore be adhered to, these are: 1. The circumference of the inner boundary should be made (2n+l) Rot/2 at the inner port design centre frequency (f01). This will produce radiation of the (n+l) th order mode.
2. The circumference of the outer region should be made (2n+l) By2/2 at the outer port design centre frequency (f02). This will produce radiation of the n th order mode.
In the example shown in Figure 2 there are certain design points which illustrate the means by which the spiral may be fed at the inner A and outer B truncated edges.
1. The spiral (which is printed on the upper surface of the substrate) is bounded by two microstrip circuit regions, the edges of whose ground planes (printed on the lower surface) define the inner and outer truncated edges of the spiral. In order to provide connections at each end of the spiral arm, it is printed such that it continues into these two microstrip regions, to become simple microstrip transmission line. The spiral conductor geometry should continue into the central and outer microstrip regions at least until the full width of the conductor lies above the ground plane. Depending upon the size and geometry of the spiral, there is therefore the option to integrate it with microstrip circuits in either or both of these regions, on the same substrate 8.
2. The conductor width to conductor gap ratio will determine the radiation impedance of the spiral which will also be affected by the thickness and permittivity of the dielectric substrate upon which all conductors reside. The conductor width chosen for the spiral may or may not be of the correct width in the microstrip circuit regions to yield a matched impedance at the boundaries.
If for this reson, an impedance match does not occur, matching circuits must be included in the microstrip circuit regions.
Taking the one-arm spiral in its general conceptual form, the maximum circumference at the truncated edge for the fundamental mode is two wavelengths at the highest operating frequency. In this situation the asymptotic bandwidth limits for the composite structure operating in the fundamental radiation mode are 3:1 for the inner port and 2:1 for the outer port, based upon the radiation mechanism of the structure. However, in the example realised on the microstrip, the impedance matching function of the structure at the inner and outer edges of the spiral where the balun transition to the microstrip circuits occur will be degraded over these wide asymptotic bandwidth limits. A more practical bandwidth limitation for the composite structure when impedance matching is considered is of the order of 25%.
The presence of the dielectric substrate and the ground planes for the peripheral microstrip regions in this example would also affect, to a certain degree, the performance of the antenna with a rear reflecting plane as compared with the generic from. Figures 3a and 3b illustrate the difference between the generic form of such an arrangement and its physical realisation on microstrip, for an antenna radiating in the fundamental mode. The radiating zone of the spiral (being towards the centre-line of the annulus) is sufficiently removed from the microstrip regions for the reflecting plane to be limited to the annular cylindrical space immediately behind the turns of the spiral, creating a coaxial cavity 7. This therefore allows rear access to the two microstrip regions for interconnection.

Claims (10)

1. A spiral antenna for circularly polarised radiation comprising a single regular or irregular spiral arm having at least one port formed by a truncation at a minimum or maximum circumference of the spiral and arranged to be directly coupled to an RF system.
2. An antenna according to claim 1, including a pair of ports formed at the minimum and maximum circumferences respectively, and arranged to be coupled to the RF system at one or both of the ports.
3. An antenna according to claim 1 or 2, in which the diameter of the or each port is such that the phase difference between a port and the next adjacent turn of the spiral is substantially (2n+1)s, where n = 0,1,2...
4. An antenna according to any one of the preceding claims, in which the spiral arm is formed on a microstrip circuit board.
5. An antenna according to claim 4, in which at least part of an RF system directly feeding the spiral arm is formed as a microstrip circuit on a common substrate with the spiral arm.
6. An antenna according to claim 4 or 5, further comprising an annular reflective cavity having radial dimensions generally matched to those of the spiral arm positioned coaxially with the spiral arm adjacent the microstrip substrate.
7. An antenna according to claim 6, including central and peripheral ground planes defining the minimum and maximum circumferences of the spiral arm formed integrally with the reflective cavity underlying the microstrip substrate.
8. A system for receiving or transmitting circularly polarised radiation, comprising an antenna according to any one of the preceding claims coupled to an RF system at one port only.
9. A system for transmitting or receiving dual-circularly polarised radiation comprising an antenna according to any one of the preceding claims coupled to an RF system at both of an inner and outer ports so as to receive radiation polarised in opposite senses at either port.
10. An antenna substantially as described with respect to the accompanying drawings.
GB9014536A 1989-06-30 1990-06-29 Spiral antenna Expired - Fee Related GB2234859B (en)

Applications Claiming Priority (1)

Application Number Priority Date Filing Date Title
GB898915047A GB8915047D0 (en) 1989-06-30 1989-06-30 Spiral antenna

Publications (3)

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GB9014536D0 GB9014536D0 (en) 1990-08-22
GB2234859A true GB2234859A (en) 1991-02-13
GB2234859B GB2234859B (en) 1993-10-20

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GB9014536A Expired - Fee Related GB2234859B (en) 1989-06-30 1990-06-29 Spiral antenna

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Cited By (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP0606514A1 (en) * 1993-01-15 1994-07-20 Rolan Wu Microstrip element phase-shift array antenna
EP0707388A1 (en) * 1994-10-12 1996-04-17 Dai Nippon Printing Co., Ltd. Signal transmission device using a fixed and a rotatable body
US5668514A (en) * 1994-10-12 1997-09-16 Dai Nippon Printing Co., Ltd. Signal transmission device

Citations (8)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
GB1204929A (en) * 1968-02-23 1970-09-09 Gen Electric & English Elect Improvements in or relating to aerials
US3656168A (en) * 1971-05-25 1972-04-11 North American Rockwell Spiral antenna with overlapping turns
GB1568436A (en) * 1976-12-17 1980-05-29 Transco Prod Inc Broadband spiral antenna
GB1595121A (en) * 1977-01-25 1981-08-05 Rapem Sarl Apparatus for emitting high-frequency electromagnetic waves
US4525720A (en) * 1982-10-15 1985-06-25 The United States Of America As Represented By The Secretary Of The Navy Integrated spiral antenna and printed circuit balun
GB2159334A (en) * 1984-05-25 1985-11-27 Revlon Method of manufacturing a conical spiral antenna
GB2176057A (en) * 1985-05-17 1986-12-10 Marconi Co Ltd Radar antenna array
US4636802A (en) * 1984-10-29 1987-01-13 E-Systems, Inc. Electrical connector for spiral antenna and resistive/capacitive contact therefor

Patent Citations (8)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
GB1204929A (en) * 1968-02-23 1970-09-09 Gen Electric & English Elect Improvements in or relating to aerials
US3656168A (en) * 1971-05-25 1972-04-11 North American Rockwell Spiral antenna with overlapping turns
GB1568436A (en) * 1976-12-17 1980-05-29 Transco Prod Inc Broadband spiral antenna
GB1595121A (en) * 1977-01-25 1981-08-05 Rapem Sarl Apparatus for emitting high-frequency electromagnetic waves
US4525720A (en) * 1982-10-15 1985-06-25 The United States Of America As Represented By The Secretary Of The Navy Integrated spiral antenna and printed circuit balun
GB2159334A (en) * 1984-05-25 1985-11-27 Revlon Method of manufacturing a conical spiral antenna
US4636802A (en) * 1984-10-29 1987-01-13 E-Systems, Inc. Electrical connector for spiral antenna and resistive/capacitive contact therefor
GB2176057A (en) * 1985-05-17 1986-12-10 Marconi Co Ltd Radar antenna array

Cited By (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP0606514A1 (en) * 1993-01-15 1994-07-20 Rolan Wu Microstrip element phase-shift array antenna
EP0707388A1 (en) * 1994-10-12 1996-04-17 Dai Nippon Printing Co., Ltd. Signal transmission device using a fixed and a rotatable body
US5668514A (en) * 1994-10-12 1997-09-16 Dai Nippon Printing Co., Ltd. Signal transmission device

Also Published As

Publication number Publication date
GB8915047D0 (en) 1989-08-23
GB2234859B (en) 1993-10-20
GB9014536D0 (en) 1990-08-22

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PCNP Patent ceased through non-payment of renewal fee

Effective date: 19940629