EP2916389A1

EP2916389A1 - Wide band antenna

Info

Publication number: EP2916389A1
Application number: EP14275039.7A
Authority: EP
Inventors: designation of the inventor has not yet been filed The
Original assignee: BAE Systems PLC
Current assignee: BAE Systems PLC
Priority date: 2014-03-04
Filing date: 2014-03-04
Publication date: 2015-09-09

Abstract

A wide band antenna having an upper and a lower operational frequency and comprising an array (100) of antenna elements (102), each antenna element comprising an elongate portion substantially occupying a plane and comprising a pair of tapering antenna blades, each of which narrows as it extends outwardly, the pair thereby defining between their inner edges a notch which opens outwardly from a feed region, and wherein antenna elements having elongate portions in a first plane are separated from antenna elements having elongate portions in an adjacent plane by a distance less than or equal to half of the wavelength of the upper operational frequency, thereby providing a spatially over-sampled array.

Description

This invention relates in a first aspect to a wide band antenna and in a second aspect to an antenna element for a wideband antenna.
Wide band technology is increasingly being developed for communications and other applications. Unlike narrow band systems, which operate at specific frequencies, wide band systems can transmit and receive sequences of very short pulses - i.e. pulses generated from a broad range or bandwidth of frequencies (typically several MHz to several GHz) of the electromagnetic spectrum. The input to a wide band antenna is typically from one or more pulsed sources, and the antenna is required to radiate incident energy into free space.
Clearly, maximising efficiency is a consideration in antenna design. It is well known in the art that, in order to maximise transmission efficiency, the impedance of the source can be matched, via the antenna, to that of the medium in which the signals are to be transmitted. The medium in which signals are to be transmitted is often free space.
Horn antennas have been used for many years as a means of matching the impedance of a transmission line to that of free space and directing the radiated energy in a controlled manner by virtue of their gain characteristics. The horn antenna can be considered as an RF transformer or impedance match between the waveguide feed (supplying the input signal) and free space which has an impedance of 377 Ohms.
An accepted method of broadening the range of frequencies over which a horn antenna is impedance-matched is to introduce ridges within the horn. These are often combined with a dielectric lens or tapered periodic surface in order to aid in limiting diffraction from the horn edges, thus helping to limit the beamwidth at low frequencies. The use of ridges essentially extends the upper frequency limit over which the antenna remains well matched, since this is a function of the aperture dimensions.
A horn antenna of the types described above could be designed which permits a significant proportion of the incident energy to be radiated over a broad band. However, for the proposed application, which may involve several high-power input sources, for example several signal generators such as microwave frequency oscillators (MFOs), the inputs may first need to be combined before being fed to the single horn antenna. This is not generally considered to be feasible at high powers, principally due to the increased risk of dielectric breakdown at the combined high power, and losses in the combination process. To overcome this problem, the available antenna aperture can instead be sub-divided into a number of smaller regions, with sources attached to each region.
Alternative antenna designs comprise arrays of elements where the radiation from a number of such elements can be coherently summed in a particular direction to form the main beam. The aim in such an antenna design is to generate a single lobe from the antenna array, substantially uncorrupted by so-called grating lobes, which are spurious lobes resulting from standing waves in the elements. To minimise such grating lobe corruption, it is common for the element spacing to be greater than half the wavelength. Thus, by convention, arrays are constructed so as to maximise the element spacing (thereby using a minimum number of elements whilst maintaining a sufficient impedance match for a specified area or aperture), to avoid the onset of grating lobes at particular scan angles. Such a spacing of elements tends to decrease efficiency due to compromised impedance matching.
The present invention seeks to provide an efficient wide band antenna that radiates energy, possibly input from at least one high power pulsed source and fed via a co-axial line, into free space, which can be designed to enhance the efficiency with which the incident energy is transmitted and/or to give a free space flux of enhanced intensity in a defined region of space, for the aperture size available.
According to a first aspect of the present invention, there is provided a wide band antenna having an upper and a lower operational frequency and comprising an array of antenna elements, each antenna element comprising, an elongate portion substantially occupying a plane and comprising a pair of tapering antenna blades, each of which narrows as it extends outwardly, the pair thereby defining between their facing edges a notch which opens outwardly from a feed region, and wherein antenna elements having elongate portions in a first plane are separated from antenna elements having elongate portions in an adjacent plane by a distance less than or equal to half of the wavelength of the upper operational frequency, thereby providing a spatially over-sampled array.
Thus a feature of the present invention is that the elements are closely packed, tending to increase the number of input sources that can be accommodated within the available aperture area, thereby tending to optimise the output power from a number of distributed sources over a given area. Further, the resultant array aperture is highly over-sampled spatially for all but the highest frequencies. This very close-packing is the opposite of conventional array design as described previously.
Indeed the wide band antenna according the antenna elements having elongate portions in a first plane may be separated from antenna elements having elongate portions in an adjacent plane by a distance less than or equal to a quarter of the wavelength of the upper operational frequency.
Such a provision allows further input density and can tend to improve the RF coupling between elements and thereby can further raise the efficiency of the array in radiating input signals.
The antenna may further comprise a plurality of driven elements, each driven element comprising: an input for electrically connecting the driven element to a signal generator, and a plurality of passive elements, wherein the passive elements are not configured to be driven by connection to a signal generator. Thus in certain embodiments, only a proportion of said antenna elements are configured to be driven, as such each driven (active) element is associated with a respective transmission line input. Said driven antenna elements provide an impedance transformation between each respective transmission line input and the medium in which the antenna is to be used. The passive elements of the array, which are not configured to be driven, may be designated as dummy elements for preserving the active match of the driven elements by maintaining a mutual coupling environment.
The driven elements may be located generally centrally within the array of antenna elements. This can provide a convenient arrangement for connection to the antenna input.
It will be appreciated that the term "wide band" antenna used herein refers to an antenna suitable for use over a broad range of frequencies such as the range of frequencies typically output by a microwave frequency oscillator, specifically 60:1 or 193% bandwidth, or more. Accordingly, certain embodiments have a bandwidth between lower and upper frequencies of 20MHz and 1.2GHz. However, other embodiments may have an upper operational frequency of 1GHz and a lower operational frequency of 50MHz. Further, the operational frequency range may extend between 20MHz and 3GHz.
A plurality of antenna elements may have an aspect ratio defined as the ratio of the distance between the feed point and the ends of said notch, to the distance between the outer edges of the blades, wherein said aspect ratio is at least 10, or at least 15.
Thus, given such an aspect ratio and the form of the antenna blades, the elements have a form which is suited to provide an impedance match over a wide range of frequencies. The length of the elements may be in the region of 2.5m plus or minus 0.5m so as to tend to be suitable for operation at the operational frequency range.
In general, the array will be arranged such that there are rows of antenna elements, formed by mounting antenna elements such the for a given antenna element in that row: the antenna element blades are parallel with intra-row antenna element blades, the upper edge of the upper blade is co-planar with intra-row antenna upper blade upper edges, and the tips of the antenna element are coplanar with intra-row blade tips. In addition, the antenna elements may be arranged in columns, where the blades of the intra-column elements are coplanar. The number of rows of elements may be less than the number of elements in a row.
For example, in one exemplary embodiment of the invention, there is provided a wide band antenna comprising seven rows of 22 Vivaldi antenna elements.
Where such an exemplary embodiment is provided, a central 100 elements may be driven elements, arranged in a contiguous 20 by 5 sub-array. Thus only the peripheral elements of the array are passive elements.
The overall shape of the antenna array may tend to be governed by the vertical and horizontal beam widths required.
The present invention thus provides a wide band antenna in which the antenna elements are closely packed within the available aperture, such that the antenna aperture is highly over-sampled for all but the highest frequencies. For the avoidance of doubt, the term "over-sampled" refers to a spatial sampling rate of the antenna elements which is significantly greater than twice, or even four times, the frequency of the output radiation - i.e. element separation significantly less than half the wavelength. Thus the overall sampling rate of the aperture will be greater than twice, or even four times, the frequency for all but the highest frequencies output by a microwave frequency oscillator (MFO).
According to a second aspect of the invention, there is provided an antenna element for a wide band antenna according to the first aspect of the invention, the antenna element comprising an elongate portion substantially occupying a plane and comprising a pair of tapering antenna blades, each of which blades narrows as it extends outwardly, the pair thereby defining between their inner edges a notch which opens outwardly from a feed point, said element having an aspect ratio defined as the ratio of - the distance between the feed point and the ends of said notch, to - the distance between the outer edges of the blades, wherein said aspect ratio is at least 3.
Further, said aspect ratio may be at least 5, and still further, said aspect ratio may be at least 17.
In the case of the specific exemplary embodiment of the invention as defined above, wherein the wide band antenna comprises an array of seven rows of 22 Vivaldi antenna elements, each Vivaldi element may have an aspect ratio between 15 and 19, to give an active impedance match of 82 or 50 Ohms.
Embodiments of the present invention will now be described by way of examples only and with reference to the accompanying drawings, in which:

Figure 1 illustrates a typical spectrum of frequencies as generated by an MFO and suitable for being fed into; an antenna according to an exemplary embodiment of the present invention;
Figure 2 is a schematic diagram of a wide band antenna array according to a first exemplary embodiment of the present invention;
Figure 3 shows, schematically, side and end views of a Vivaldi antenna element;
Figure 4 is a schematic perspective view, giving an impression of the relative dimensions, of a Vivaldi antenna element for use in the embodiment of Figure 2;
Figure 5 is a graphical representation of the magnitude of the reflection coefficient of the element of Figure 4 when modelled as immersed in an infinite array;
Figure 6 illustrates the peak realised gain at 200MHz of the wide band antenna of Figure 2;
Figure 7 illustrates the peak realised gain at 1GHz of the wide band antenna of Figure 2;
Figure 8 is a schematic perspective view of a Vivaldi antenna element, giving an impression of the relative dimensions, for use in a wide band antenna array according to a second exemplary embodiment of the present invention;
Figure 9 is a schematic diagram of a wide band antenna array according to a second exemplary embodiment of the present invention;
Figure 10 is a graphical representation of the magnitude of the reflection coefficient of the element of Figure 8 when modelled as immersed in an infinite array;
Figure 11 illustrates the peak realised gain at 200MHz of the wide band antenna of Figure 9; and
Figure 12 illustrates the peak realised gain at 1GHz of the wide band antenna of Figure 9.

In the following exemplary embodiments, an antenna is configured to be driven by microwave frequency oscillators (MFOs). However, it will be appreciated that the present invention is not intended to be limited in this regard and that other multi-frequency pulsed energy sources can be used.
Throughout the specification, references are made to components being 'outward' or 'inward'. The term 'outward' has been used to indicate a direction that is towards the medium into which the antenna radiates (often referred to as the boresight), and 'inward' is used to indicate the opposite direction, i.e. away from the medium into which the antenna radiates.
Terms such as 'upper' and 'lower', and 'row' and 'column' are used for convenience and to distinguish between components so as to better explain the invention, no absolute orientation is intended from the use of such terms alone.
Referring to Figure 1 of the drawings, the output spectrum from a microwave frequency oscillator (MFO) comprises significant energies at the fundamental RF input frequency, in this case, 50MHz, in addition to a range of harmonics extending up to around 3GHz. The MFO comprises a ferrite which is driven in the so-called 'spinwave' regime, leading to the onset of harmonics, and the peak in the region of 1GHz is associated with so-called "ringing" in the MFO. These are desirable features, with the purpose being to up-convert as much of the power delivered into the device at the fundamental frequency into the higher microwave frequencies associated with the harmonics.
This broad band signal may be efficiently radiated by an equally wide band antenna into free space. However, the development of an antenna which radiates across the full width of the input spectrum is infeasible for many applications, given the likely dimensions of such and antenna and given the finite space into which the antenna must fit in operation. Therefore a region from, 20MHz to 1.2 GHz has been selected for the present embodiments, since this encompasses the frequencies containing the majority of the energy.
An antenna according to an exemplary embodiment of the present invention comprises an array of closely-packed Vivaldi antenna elements, as illustrated in Figure 2.
Referring to Figure 3 of the drawings, a conventional Vivaldi antenna element 2 comprises a conductive material 4, particularly a metal, which forms a pair of separated antenna blades 5a and 5b extending outwardly in generally the same direction, and an interconnecting link 15 extending inwardly from the inward side of the blades 5a, 5b and connecting them. The antenna blades 5a,b define an elongate portion 7 of the element 2 and are substantially coplanar.
The conductive material 4 defines a tapered slot 8 between the opposing (or facing) edges 9a and 9b of the blades 5a, 5b. The slot 8 has a minimum separation 10 at the inward end of the blades 5a, 5b. The distance between the opposing edges 9a, 9b of the blades 5a, 5b widens as the blades extend outwards, and thus the width of the tapered slot 8 increases from a minimum at the narrow end 10 of the slot 8 to a maximum at an open end 12 of the slot 8. Adjoining and inwardly of the narrow end 10, a square, or circular, or other-shaped region 13 without conductor is defined within the bounds of the interconnecting link 15.
In conventional Vivaldi antenna elements, the slot 8 is symmetrical about a central axis 14 and the blades 5a, 5b are the mirror image of one another reflected in this central axis 14. The interconnecting link 15 extends from the distal edges 11 a, 11 b (or upper and lower edges respectively) of the blades 5a, 5b. The minimum separation 10 defines a feed gap through which an input signal may be fed to the antenna element 2, at the conductive material 4 which defines exponentially tapered blades. The region 13 defines a cavity or short-circuit gap which prevents energy from flowing back away from the feed gap 8 to the back end of the element 2.
The aspect ratio - i.e. the ratio of the blade taper length to the height as measured between the distal blade edges 11a, 11b,-of a conventional Vivaldi antenna element is typically less than 2.
Referring back to Figure 2, the array 100 of closely-packed Vivaldi elements 102 defines a highly spatially over-sampled aperture, given the operating frequencies of 20 MHz to 1.2GHz.
The array is formed from a plurality of rows of elements, and in this particular embodiment there are 7 rows, each row comprising 22 elements.
In the array, each row is formed by arranging a plurality of elements 102 such that the element axes 14 are generally coplanar.
Further, within each row of elements, each planar elongate portion (comprising the blades) is parallel to and spaced apart from the planar elongate portions of the other elements. The pitch between neighbouring elements is constant across the array and is selected to promote the over-sampling at the aperture. Specifically, adjacent elongate portions (effectively the blades) are offset from the equivalent plane of the nearest neighbouring element by 52mm. Unlike conventional arrays, this is significantly less than half the wavelength of the highest operational frequency required and so facilitates the spatial over-sampling across the full wideband operational range. With an upper operational frequency of 1 GHz, the half-wavelength would be 150mm. The quarter wavelength would be 75mm, which would be a feasible separation at this upper frequency.
Alternatively, the upper operation frequency may be extended to make use of the over-sampling provision. Indeed, an upper operation frequency of 3GHz, equivalent to a half-wavelength of 50mm, could be substantially accommodated by this array.
Each row has an element axes plane defined by the aligned element axes. Within the array, the rows are arranged such that each element axes plane is parallel with the element axes planes from other rows. Further, in this particular embodiment the rows are arranged such that the distal edge of the upper blade from each element in a first row extends towards, and is electrically connected to, the distal edge of a lower blade of an element in the row above. Thus the array defines columns of inter-row elongate portions where the blades generally occupy a common blade plane.
Further in this particular embodiment, a planar conducting sheet 101 (not all references applied in the figures, for clarity) is placed between each row, such that the distal edges of neighbouring inter-row elements abut the sheet 101 so as to electrically connect such neighbouring inter-row elements. Further planar sheets are provided at the top, bottom and sides of the array.
A conducting planar sheer 101, formed for example from metal, has been determined through simulation to have suitable characteristics.
Alternative embodiments may have dielectric planar sheets between rows and/or at the top, bottom and sides of the array. Other possible embodiments may alternatively or additionally have elements that are not aligned between rows. Other embodiments may alternatively or additionally have elements that are not electrically connected.
The planar sheet, whether conducting or not, should provide structural support for the elements.
An active or 'driven' antenna element is provided for each transmission line input to the antenna, and additional 'dummy' (or passive) elements are provided to preserve the active match of the 'driven' elements by promoting a uniform mutual coupling environment for said driven elements, where this is needed to obtain the desired match. In this case, 100 driven elements are located generally centrally within the array, with 'dummy' edge elements being provided around the edge of the array.
Referring to Figure 4 of the drawings, a Vivaldi antenna element 102 suitable for use in an antenna according to the present invention is illustrated. In this instance, the overall length of the array - and hence of any individual element - was constrained to be less than 2.5m. Although the physical structure of the element is similar in many respects to that illustrated in Figure 3 of the drawings, it can be seen that the aspect ratio, i.e. the length-to-height ratio, of the element 102 in Figure 4, is significantly larger than that of a conventional Vivaldi element, in order to maximise the physical length over which the impedance transformation between the antenna input and free space takes place. Thus, a first exemplary Vivaldi antenna element 102, used in the embodiment illustrated in Figure 2 of the drawings, has a width of 52mm, a height of 143mm, a short-circuit gap 13 of 219mm and a total length of 2452 mm. The aspect ratio of the element is defined as the ratio of the length from the feed gap (the minimum separation 10) to the outward end of the element, which in this case is 2233mm (2452 - 219mm), to the height (between the distal edges of the blades) which is 143mm, giving an aspect ratio of 15.6. The feed for the element is based upon a traditional co-axial Marchand balun, which is known in the art and will not be discussed further here. It will be appreciated, however, that the present invention is not intended to be limited in this regard, and other feed configurations are envisaged.
The antenna elements 102 may be formed entirely from metal so as to provide beneficial structural and electrical properties.
Thus, an array of 22 by 7 closely-packed antenna elements in this case would have an overall aperture size of 999mm by 1138mm. The active match of an antenna element modelled in an 'infinite array' - i.e. a simulated array consisting of infinite rows and columns of elements - is shown in Figure 5, expressed as the magnitude of a reflection coefficient, relative to an input impedance of 82 Ohms.
During simulation of the first exemplary embodiment described above, the resultant boresight gain at 200MHz and 1 GHz was found to be 5.5dBi and 18.7dBi as illustrated in Figures 6 and 7 respectively.
It will be appreciated from the above that by maximising the physical length over which the impedance transition from the antenna element feed point to free space takes place, the structure of the Vivaldi elements used in various embodiments of the present invention can be optimised, in order to meet the antenna size limitations (length, height, width) and frequency requirements imposed by the specific application.
Thus, referring to Figure 8 of the drawings, a Vivaldi antenna element 202 for use in an antenna 200 according to a second exemplary embodiment of the present invention is illustrated.
Once again, 100 central 'driven' elements 202 are provided as a 20 by 5 sub-array, each one associated with a respective transmission line input, with 'dummy' edge elements being provided to preserve the active match of the central 'driven' elements by promoting a uniform mutual coupling environment.
The second embodiment is configured for a 50 Ohm input impedance and the element width is fixed at 136mm to ensure that a 22 by 7 element array can fit within an aperture of 3m width. Thus, the element height in this case is 127mm, with a short-circuit gap of 392mm and overall length of 2424mm, giving an aspect ratio of 16. The resultant antenna array 200 is illustrated in Figure 9, and the reflection co-efficient, relative to a 50 Ohm input is shown in Figure 10, for the element of Figure 8 modelled in an 'infinite' element array.
It can be seen that there is a number of frequencies (i.e. c.0.37 GHz, c.0.76 GHz and c.1.14 GHz) at which the magnitude of the reflection coefficient is 1 and the antenna presents a short circuit. However, these are relatively narrow band and thus the energy lost at these frequencies is offset by the overall bandwidth over which the antenna functions. Moreover, a good match is maintained at the fundamental frequency and in the 1 GHz 'ringing' band.
During simulation of the second embodiment, the resultant boresight gain at 200MHz and 1 GHz was found to be 8.8dBi and 23.5dBi as illustrated in Figures 11 and 12 respectively of the drawings.
A third embodiment of the Vivaldi antenna element, having an equivalent form to the first and second embodiments but having a height of 122mm, a width of 136mm, a short circuit gap of 232mm and a total length of 2453mm (and thereby having an aspect ratio of 18.2) was found to be capable of radiating up to 51 % of the input energy fed to the element when modelled in an infinite array.
However, there is a trade-off between the range of output frequencies, particularly in the upper band, that are required, and the proportion of energy successfully radiated. Longer Vivaldi antenna elements may permit higher overall radiative efficiencies, but have reduced efficiency in higher frequency bands considered critical for the specific application.
Thus, in the present invention, Vivaldi elements, which are inherently wide-band, are used as a means of transforming the impedance of the antenna inputs to that of free space, with one element being associated with each of the high power transmission line inputs employed (the 'driven' elements). Additional elements (the 'dummy' elements) are then used to promote a uniform mutual coupling environment for all driven elements; in particular those driven elements close to the array boundary. When optimising the physical dimensions of the antenna elements, firstly determine the number of elements required - at least one per input source - and the maximal dimensions of the antenna array - i.e. the aperture size, which constrains the element height and separation (width), and the array depth, which constrains the element length. Then, one of the dimensions, for example the element separation (width), can be fixed accordingly to provide sufficient spatial sampling for the highest operational frequencies, and the remaining dimensions optimised to give the desired impedance matching within the size constraints of the antenna array. In the case of the present invention, this is achieved by providing for the relatively large aspect ratio of the individual antenna elements, which enables the required "input to free space" impedance transformation to be achieved over a maximised taper length.
Thus, the present invention provides an ultra-wide band antenna array with carefully managed mutual coupling between elements in what is, for all but the highest frequencies, a spatially over-sampled aperture. This very wide impedance match permits the use of new waveforms and, importantly for this application, narrow pulses with high peak powers.
It will be appreciated by a person skilled in the art that modifications and variations can be made to the described embodiments without departing from the scope of the invention as claimed.

Claims

A wide band antenna having an upper and a lower operational frequency and comprising an array of antenna elements, each antenna element comprising,
- an elongate portion substantially occupying a plane and comprising a pair of tapering antenna blades, each of which narrows as it extends outwardly, the pair thereby defining between their facing edges a notch which opens outwardly from a feed region, and wherein antenna elements having elongate portions in a first plane are separated from antenna elements having elongate portions in an adjacent plane by a distance less than or equal to half of the wavelength of the upper operational frequency, thereby providing a spatially over-sampled array.
A wide band antenna according to claim 1 wherein antenna elements having elongate portions in a first plane are separated from antenna elements having elongate portions in an adjacent plane by a distance less than or equal to a quarter of the wavelength of the upper operational frequency
A wide band antenna according to claim 1 or claim 2, comprising
- a plurality of driven elements, each driven element comprising:
- an input for electrically connecting the driven element to a signal generator, and

- a plurality of passive elements, wherein the passive elements are not configured to be driven by connection to a signal generator.
A wide band antenna according to claims 1, 2 or 3, wherein said driven elements are located generally centrally within the array of antenna elements.
A wide band antenna according to any of the preceding claims wherein the antenna elements are dimensioned such that the upper operational frequency is 1 GHz or more, and the lower operational frequency is 50MHz or less.
A wide band antenna according to any one of the preceding claims wherein a plurality of antenna elements have an aspect ratio defined as the ratio of
- the distance between the feed point and the ends of said notch, to

- the distance between the outer edges of the blades, wherein said aspect ratio is at least 10.
A wideband antenna according to claim 6 wherein the aspect ratio of the plurality of antenna elements is at least 15.
A wideband antenna according to any of the preceding claims wherein the distance between the feed point and the end of the notch is between 2 and 3 metres.
An antenna element for a wide band antenna according to any of the preceding claims, the antenna element comprising an elongate portion substantially occupying a plane and comprising a pair of tapering antenna blades, each of which blades narrows as it extends outwardly, the pair thereby defining between their inner edges a notch which opens outwardly from a feed point, said element having an aspect ratio defined as the ratio of
- the distance between the feed point and the ends of said notch, to

- the distance between the outer edges of the blades, wherein said aspect ratio is at least 3.
An antenna element according to claim 9, wherein said aspect ratio is at least 5.
An antenna element according to claim 9, wherein said aspect ratio is at least 17.