WO2014009697A1

WO2014009697A1 - Antennas

Info

Publication number: WO2014009697A1
Application number: PCT/GB2013/051762
Authority: WO
Inventors: Yiannis Costas VARDAXOGLOU
Original assignee: Antrum Ltd
Priority date: 2012-07-11
Filing date: 2013-07-03
Publication date: 2014-01-16
Also published as: GB201212340D0

Abstract

An antenna (10) comprising a substrate (12) and an active element (13) in the form of a microstrip circuit disposed on the substrate to form a dipole, wherein the antenna further includes one or more microstrip parasitic elements (19) disposed on the substrate.

Description

Antennas

The present invention relates to antennas, and particularly but not exclusively to antennas for use in the Ultra High Frequency (UHF) band.

Conventionally, the UHF denotes a range of electromagnetic frequencies between 300 MHz and 1000MHz. The simplest antennas for the UHF band are the λ₀/2 dipole, the bowtie dipole, the folded dipole, and the loop. However, these types of antennas are low gain when they serve as a driven element in an antenna system. In order to increase the directionality and the gain of such an antenna, dipoles are commonly stacked horizontally (collinearly), vertically (broadside) and in echelon (end-fire). To increase the gain even more (up to 3dB), ground planes of various size and type can be used, such as flat metal sheet reflectors, corner reflectors, screen reflectors, and parabolic reflectors.

The most popular high gain antenna systems in the UHF band are the Yagi-Uda antenna (YUA), the vertically stacked dipole antenna (VSDA) and the Log periodic dipole array (LPDA). These antenna systems achieve a high gain, broad bandwidth and narrow beamwidth mainly due to the number of dipole elements, their structure and alignment. However, these antenna systems also have disadvantages. The main deficiencies of the YUA, LPDA and VSDA systems are their limited bandwidth (e.g not covering the whole TV band of 470MHz-850MHz with 10dB return loss). They also have radiation patterns that are not consistent over the whole band, and their behaviour is not perfectly balanced through whole band. Furthermore, such antennas typically have large complex structures which can be expensive to make, difficult to install, and requiring of regular maintenance. It is therefore an object of the invention to provide an alternative antenna.

According to a first aspect of the present invention there is provided an antenna comprising a substrate and an active element in the form of a microstrip circuit disposed on the substrate to form a dipole, wherein the antenna further includes one or more microstrip parasitic elements disposed on the substrate.

Antennas comprising a microstrip active element disposed on a substrate are known as printed dipole antennas (PDA) and can be produced using printed circuit board technology. Such antennas typically have narrow bandwidth and limited power handling capabilities, making them unsuitable for use in the UHF band. However, we have determined that including one or more parasitic elements (i.e. passive radiators) in the antenna greatly increases the bandwidth.

The antenna may further comprise a transmission line including a balun as part of the microstrip circuit, wherein the transmission element is operable to couple the active element to a signal source/signal receiver when the antenna is in use. As used herein a balun refers to a transformer employed to balance signals supplied to the active element. The balun is preferably tapered.

The parasitic element may be located between the balun and the active element. Alternatively, the parasitic element may be located on a side of the active element which is remote from the balun.

The parasitic element may be spaced from the active element. The spacing of the parasitic element from the active element may be small when compared to the width of the active element (for example: less than 50%, less than 20%, such as approximately 10%, of the width of the active element). The spacing of the parasitic element from the active element may be less than 2mm, and may be 1 mm. The active element preferably comprises a pair of arms. Each arm may be disposed on an opposite side of the substrate. The longitudinal axes of the arms may be substantially parallel and may be substantially coincident when viewed in plan. Alternatively, the longitudinal axes of the arms may be non-parallel and angled with respect to one another (for example, the longitudinal axes of the arms may be at an angle with respect to one another of between 180 and 90 degrees, such as approximately 120 degrees). Likewise, the longitudinal axis of each arm may be angled with respect to a longitudinal axis of the transmission line, such that the arms are at an angle of approximately 120 degrees to the transmission line. Such a Ύ" shaped configuration of the active and parasitic elements and the transmission line enables used of the antenna for both horizontal and vertically polarised signals, such as TV signals.

Each arm may be substantially identical. The arms may each define a length x (for example 12 cm). The balun may comprise a length which is between 1/4 and 3/4Λ, for example substantially 1/2 where λ is the wavelength, or guided wavelength of the signal in the dielectric.

The transmission line may include a pair of parallel strips, which may be substantially identical, on opposed side of the substrate, each connecting a respective arm to the balun. The balun may taper linearly or exponentially.

The parasitic element may comprise two pairs of arms, each pair of arms being disposed on an opposite side of the substrate. Each pair of arms may share a common longitudinal axis. The longitudinal axes of the two pairs may be substantially parallel and may be substantially coincident when viewed in plan. The antenna could be used such that the arms of the active element and parasitic element are parallel to a first direction, e.g. a horizontal direction as shown in figures 3 and 4, or alternatively, the antenna could be used such that its arms perpendicular to the first direction, e.g. aligned along a vertical direction. Accordingly, the antenna could be suitably orientated in use so as to receive horizontal or vertical signals. The antenna may further comprise a non-dielectric ground, which may comprise a metamaterial (for example a plurality of conducting (resonant or non resonant) elements or a plurality of apertures (resonant or non resonant) in a conducting plane), and may be formed of a metal or other conductive material.

The ground may be solid or may comprise perforations, and may comprise a mesh, a grid or rods. The grid may be formed by wire in a regular cross assembly comprising two wires perpendicular to each other.

The ground may be generally planar, and may be substantially elliptical when viewed in plan. Alternatively, the ground may have a cross sectional shape that is generally "v" shaped. For example a ground having such a generally "v" shaped cross-section may comprise a number of rods arranged to form a corner reflector.

The dipole may be mounted to the ground substantially at a central point of the ground. The dipole may be mounted to the ground such that a plane defined by the substrate is substantially perpendicular to a plane defined by the ground. The dipole may be mounted to the ground such that the plane of the substrate is substantially parallel to the plane of the ground.

The ground may be at an acute angle to the plane of the substrate. The ground may be curved. The ground may be substantially "v" shaped in cross section.

The dipole may be matched to a known wavelength λ in the desired frequency band, which may be a selected portion of the UHF frequency band. The desired frequency band may be 300MHz to 1 GHz, and may be 470MHz to 860MHz in the TV band. The length of the arms (cf) of the active element may be selected to match the dipole to the wavelength λ. For example, the length of each arm x may be substantially λ/4. The perforations may have a spacing which is a known proportion of the wavelength λ. In particular, the perforations may be much smaller than λ in their largest dimension, (e.g. λ/16 or less) so as to form a reflector at that wavelength.

The present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 illustrates the structure of a printed dipole antenna;

Figure 2 shows the simulated return loss of a printed dipole antenna similar to that of Figure 1 over the frequency range 100MHz to 1200MHz;

Figure 3 shows a view of an alternative antenna from a first side;

Figure 4 shows the antenna of Figure 3 from a second side, opposite to the first side;

Figure 5 shows an alternative antenna from A. a first side and B. a second side;

Figure 6 shows the simulated return loss of the antenna of Figure 5 over the frequency range 100MHz to 1200MHz;

Figure 7 shows an alternative antenna from A. a first side and B. a second side;

Figure 8 shows the simulated return loss of the antenna of Figure 7 over the frequency range 100MHz to 1200MHz;

Figure 9 schematically shows an antenna including a ground from A. a side view and B. a plan view;

Figure 10 is a front view of the ground of Figure 9, including a detail showing a perforation;

Figure 1 1 shows an alternative ground;

Figure 12 shows the ground of Figure 1 1 with a dipole A. perpendicular to the ground and B. parallel to the ground; and

Figure 13 shows an alternative antenna with a curved ground.

Referring firstly to Figure 1 , the general structure of a printed dipole antenna 1 is shown. The antenna is formed on both sides of a generally planar substrate (which is not shown in Figure 1 for clarity) from a conducting material such as copper. Components formed on a first, upper side are shown in grey, whilst components formed on a second, lower side, are shown in black. The antenna is formed using conventional printed circuit board technology, and so the process of its manufacture is not described in detail herein.

The antenna 1 includes three main parts. An active or driven element 3 comprises two substantially in-line arms. The arms act as a dipole, and are able to transmit and receive electromagnetic signals. Each arm is located on an opposite side of the substrate. The length of each arm is selected to couple the antenna to a selected wavelength, in a similar way that the radius of a wire dipole is selected to couple such a dipole to a particular wavelength. In particular, the length of each arm is selected to be approximately one quarter of the selected wavelength (λ/4), so that together the arms measure approximately λ/2. The selected wavelength could refer to the waveguide wavelength Ag « Kl^e_eff , where e_eff is the effective dielectric constant of the substrate in-between the arms. In this example, the selected wavelength equated to a frequency of 550MHz in the UHF band.

A transmission line including balun 5 and a parallel strip line 7 feeds power to the active element 3. The purpose of the balun is to convert unbalanced signals emitted from an electrical signal source (not shown) to balanced signals which are carried by the parallel strip line to the active element. It is important that the input to the active element is well balanced, to ensure a regular radiation pattern.

In this instance the balun comprise a portion which generally tapers inwardly (i.e. reduces in cross section) towards the parallel strip line on the lower side of the substrate, and a portion which is generally untapered on the opposite side of the substrate. The antenna shown in Figure 1 is not ideal for use in the UHF band for two main reasons. Firstly, the balun does not balance signals input to the active element well at high frequencies (e.g. 850MHz +), resulting in an irregular radiation pattern at these frequencies. Secondly, the antenna does not have sufficiently high return loss across the desired frequency band. Simulated results for an antenna similar to that of Figure 1 are shown in Figure 2. Figure 2 shows that the simulated antenna achieves a return loss of greater than 10dB for only a small proportion of the tested frequency band (0.1 GHz to 1 .2GHz), mainly about the selected frequency of 550MHz and about a resonance at 790MHz.

Figures 3 and 4 show an improved antenna 10 with two main structural changes over the antenna of Figure 1 . As before, the antenna comprises a microstrip circuit disposed on a suitable substrate 12.

Firstly, the antenna of Figures 3 and 4 includes a 'stretched' balun 15a and 15b such that the transmission line (i.e. balun 15 and parallel strip 17) is approximately the same length as an arm of the active or driven element 13 of the antenna.

In this example, the transmission line is approximately λ/4 in length, so that when in use the active element 13 is located λ/4 from the ground plane (which will be located at the end of the balun remote from the active element). This reduces the effect of destructive interference due to reflections from the ground plane on the radiation pattern. In some embodiments the ground is purely reflective and has no affect on the current distribution that flows in the arms. In some embodiments the balun acts as an isolator of currents flowing back to the ground. In certain embodiments the absence of the balun would distort the antenna pattern and reduce its bandwidth.

The balun 15 has been extended by adding a generally straight sided portion to the tapered balun adjacent the ground. In addition the balun tapers outwardly on a first side of the substrate, and inwardly on the opposite side of the substrate, towards the parallel strip line. The balun itself comprises approximately half the length of the transmission line. It has been found that increasing the length of the balun, rather that the length of the parallel strip line, improves the balancing of the signals input to the antenna.

The antenna 10 additionally includes a parasitic element 19 located between the active element 13 and the ground.

The parasitic element 19 comprises two pairs of parallel arms which are spaced from (i.e. not physically coupled to) either the active element 13 or the transmission line. Each pair of arms shares a common longitudinal axis, which is substantially parallel to a longitudinal axis of the active element. The arms of the parasitic element are of substantially the same width as the arms of the active element, but are of different lengths. In particular, for a given side of the antenna, the parasitic arm 19 adjacent the active arm 13 is slightly longer than the active arm, whilst the parasitic arm remote from the active arm is slightly shorter. This adjusts the overall bandwidth of the antenna. A spacing 108 of the parasitic element from the active element may be small when compared to the width of the active element (for example: less than 50%, less than 20%, such as approximately 10%, of the width of the active element). The spacing of the parasitic element from the active element may be less than 2mm, and may be 1 mm.

The parasitic element 19 is disposed close to (i.e. less than 1 mm from) the active element 13, and acts as a passive radiator/receiver when the active element is in use. The parasitic element has been found to increase the return loss of the antenna at higher frequencies, significantly increasing the operational bandwidth of the antenna. The dimensions of the antenna 10 are given in the following table (as a proportion of d, a wavelength scaling factor equal to 0.055A), with reference to numerals 101 to 124: 101 102 103 104 105 106 107 108 109 1 10 1 1 1 1 12

1 .4 0.6 1 .5 1 .6 0.4 0.08 0.06 0.04 0.5 10.2 2.1 1 .4

1 13 1 14 1 15 1 16 1 17 1 18 1 19 120 121 122 123 124

1 .5 0.55 1 .9 0.4 0.4 4.3 4.7 0.06 0.3 0.2 0.05 3.5

Figure 5 shows first and second sides of an alternative antenna. The antenna is similar to that of Figures 3 and 4, and some dimensions are given in the table below (for an active element tuned to 550MHz):

The antenna of Figure 5 includes an extended balun which tapers linearly and gradually, rather than a balun having a straight portion and a steep tapered portion. This further improves the balancing of the signals fed to the active element.

Figure 6 shows the return loss simulated for such an antenna. The bandwidth of the antenna is calculated to be 52% (at 10dB or more, over the band of 0.47GHz to 0.860GHz. This is twice the bandwidth of the antenna shown in Figure 1 .

It will be noted that the antenna produces a gain of greater than 10dB for a large portion of the 470MHz to 860MHz range, making the antenna suited to use for a terrestrial TV aerial.

Figure 7 shows first and second sides of another antenna. The antenna is similar to that of Figure 5, except that the parasitic element 39 has been placed on a side of the active element 33 which is remote from the ground. This has been found to achieve a more regular radiation pattern across the whole bandwidth, as the parasitic element of Figure 5 was found to produce irregularities in the radiation pattern at high frequencies.

Figure 8 shows that the antenna of Figure 7 has a bandwidth of 52.9%, slightly better than that of the antenna of Figure 5.

The antennas described above are suitable for use as an internal or external TV aerial. The antennas are hardwearing, and may be produced from any conductor disposed on or within a suitable substrate. The antennas described above were produced from copper printed on a fibreglass reinforced epoxy laminate of the sort commonly used in printed circuit boards, but it will be appreciated that other non-conductive substrates such as resin or plastics might be used. The antenna can be painted or otherwise coated to improve its appearance or increase its resistance to weathering without substantially adversely affecting its performance.

The antennas described above produce a generally omni-directional radiation pattern, and may be coupled directly to or from a signal source. Thus they are particularly suitable for use as an indoor TV aerial, where received signals might include multiple reflections from various angles.

To produce an outdoor aerial, it is sometimes desirable to achieve a more directional antenna. To improve the directionality of the antenna, the antenna might be used with an artificial ground.

Referring to Figures 9 and 10, an antenna 40 is shown. The antenna uses a dipole antenna of the type described above, having an active element 43, transmission line 44 and parasitic element 49 disposed on a substrate 42. The dipole antenna is disposed on a ground 50, perpendicular to the ground.

The ground 50 is better shown in Figure 10. The ground is generally planar and rectangular. The ground includes a plurality of perforations 52 which in this case are formed by a plurality of crossed wires 54 which together make a mesh. Such a material functions as a metamaterial, such as an inductive grid so as to act as a reflector.

The mesh spacing d is a small fraction of the selected wavelength (in this case less than λ/16, at approximately 0.055A, which ensures that signals at and around the selected wavelength are reflected and directed by the ground back towards the active element, rather than being transmitted through the ground. This improves the directivity of the antenna significantly. Furthermore, the perforated ground is both light and simple, making it easy to manufacture and install. The perforations allow air, and in particular, wind to pass through the antenna without substantially affecting its performance, but enabling a reduction in the loading on the antenna due to wind.

An alternative ground 60 is shown in Figure 1 1 . Again, the ground comprises a plurality of perforations 62 formed in a mesh 64. The ground 60 is generally elliptical in shape. Since the peripheral and extreme edges of the ground have minimal affect on the antenna function, the ground may be configured in a curved or other aesthetically pleasing/desirable shape.

The active element can be mounted to either ground 50 or 60 such that the substrate is generally perpendicular to the ground (Figure 12A) or generally parallel to the ground (Figure 12B). Slightly improved directionality has been found with the substrate located parallel to the ground.

Directionality of a perpendicular dipole can be further improved by providing a curved ground 70, of the type shown in Figure 13. The ground 70 curves away from a plane 71 which is substantially perpendicular to the plane of the substrate carrying the dipole. Angles of up to 60° have been found to improve the directionality of the antenna, with angles of between 20° and 50° being most effective. An angle of substantially 45° has been found to give the best directionality over the entire bandwidth of the antenna. Various modifications may be made without departing from the scope of the invention. Whilst endeavouring in the foregoing specification to draw attention to those features of the invention believed to be of particular importance it should be understood that the applicant claims protection in respect of any patentable feature or combination of features hereinbefore referred to and/or shown in the drawings whether or not particular emphasis has been placed thereon.

Claims

An antenna comprising:

a substrate, and

an active element in the form of a microstrip circuit disposed on the substrate to form a dipole,

wherein the antenna further includes one or more microstrip parasitic elements disposed on the substrate.

An antenna as claimed in claim 1 , further comprising:

a transmission line including a balun as part of the microstrip circuit, wherein the transmission line is operable to couple the active element to a signal source/signal receiver when the antenna is in use.

An antenna as claimed in claim 2, wherein the balun is tapered.

An antenna as claimed in any one of claims 2 or 3, wherein the parasitic element is located either:

between the balun and the active element, or

on a side of the active element which is remote from the balun.

5. An antenna as claimed in any one of the previous claims, wherein the parasitic element is spaced from the active element. 6. An antenna as claimed in claim 5, wherein a spacing of the parasitic element from the active element is at least one of:

less than 50% of the width of the active element, less than 20% of the width of the active element, substantially 10% of the width of the active element,

less than 2mm, and

substantially 1 mm. An antenna as claimed in any one of the previous claims, wherein the active element comprises a pair of arms.

An antenna as claimed in claim 7, wherein each arm is disposed on an opposite side of the substrate.

An antenna as claimed in claim 7 or 8, wherein longitudinal axes of the arms are either:

substantially parallel and optionally substantially coincident when viewed in plan, or

non-parallel and angled with respect to one another.

An antenna as claimed in claim 7 or 8, wherein longitudinal axes of the arms are at an angle with respect to one another of at least one of: between 180 and 90 degrees, and

substantially 120 degrees.

An antenna as claimed in any one of claims 7 to 10, wherein a longitudinal axis of each arm is angled with respect to a longitudinal axis of the transmission line.

An antenna as claimed in any one of claims 7 to 1 1 , wherein the arms of the active element are at an angle of approximately 120 degrees to the transmission line.

An antenna as claimed in any one of claims 7 to 12, wherein each arm is substantially identical.

An antenna as claimed in any one of claims 7 to 13, wherein the balun comprises a length which is at least one of:

between 1 /4λ and 3/4λ, and

substantially 1/2λ, where λ is a wavelength or a guided wavelength of a signal in a dielectric.

15. An antenna as claimed in any one of the previous claims, wherein the transmission line includes a pair of parallel strips, on opposed side of the substrate, each connecting a respective arm to the balun.

16. An antenna as claimed in any one of the previous claims, wherein the balun tapers linearly or exponentially.

17. An antenna as claimed in any one of the previous claims, wherein the parasitic element comprises two pairs of arms, each pair of arms being disposed on an opposite side of the substrate.

An antenna as claimed in claim 17, wherein each pair of arms of the parasitic element share a common longitudinal axis.

An antenna as claimed in claim 18, wherein the longitudinal axes of the two pairs of arms are substantially parallel and optionally are substantially coincident when viewed in plan.

An antenna as claimed in any one of claims 17 to 19, wherein the pair of arms of the parasitic and active elements are at least one of:

parallel to a first direction,

perpendicular to the first direction, and

configured to form a Ύ" shaped configuration with respect to the transmission line.

An antenna as claimed in any one of the previous claims, further comprising a ground.

An antenna as claimed in claim 21 , wherein the ground comprises at least one of: metamaterial elements,

perforations,

a plurality of apertures in a conducting plane,

a mesh,

a grid, and

rods.

23. An antenna as claimed in claim 21 or 22, wherein the ground is at least one of:

formed of a non-dielectric, metallic or other conductive material, solid

generally planar

substantially elliptical

curved

configured to have a cross sectional shape that is generally "v" shaped

24. An antenna as claimed in any one of claims 21 to 23, wherein the

dipole is mounted to the ground substantially at a central point of the ground.

25. An antenna as claimed in any one of claims 21 to 24, wherein the

dipole is mounted to the ground such that a plane defined by the substrate is either:

substantially perpendicular to a plane defined by the ground, or substantially parallel to the plane of the ground.

26. An antenna as claimed in any one of previous claims 21 to 25, wherein the ground is at an acute angle to the plane of the substrate.

27. An antenna as claimed in any one of the previous claims, wherein the dipole is matched to a known wavelength λ in a desired frequency band. An antenna as claimed in claim 27, wherein the desired frequency band is at least one of:

a selected portion of the UHF frequency band,

300MHz to 1 GHz, and

470MHz to 860MHz.

An antenna as claimed in any one of claims 7 to 29, wherein the length of the arms of the active element is selected to match the dipole to the wavelength λ, optionally, wherein the length of each arm is

substantially λ/4.

An antenna as claimed in any one of the previous claims, further comprising a ground comprising perforations and wherein the perforations have a spacing which is a predetermined proportion of a wavelength λ.

An antenna as claimed in claim 30, wherein the perforations are smaller than λ in their largest dimension so as to form a reflector at that wavelength.