GB2568280A

GB2568280A - Log-periodic antenna with a passband and a stopband

Info

Publication number: GB2568280A
Application number: GB1718617.2A
Authority: GB
Inventors: Lazaridis Pavlos
Original assignee: University of Huddersfield
Current assignee: University of Huddersfield
Priority date: 2017-11-10
Filing date: 2017-11-10
Publication date: 2019-05-15
Also published as: GB201818423D0; GB201718617D0; GB2570038A

Abstract

A log-periodic antenna comprises dipole elements not in a monotonically increasing sequence of length which are arranged on metallic booms. The dipole elements and/or the booms may be cylindrical, rectangular or thin metal sheet type formations. The lengths of, and/or spacing between, the dipole elements, may differ from those present in a classical log-periodic antenna design according to Isbell and Carrel. The elements of the antenna may be optimized to achieve a desired passband or stopband. The optimized dipole elements can be in the short, middle or long elements of the array to reject frequencies, respectively, at high, medium or low frequencies. The passband may be that of a UHF TV reception antenna and the stopband may be that used for mobile cellular devices. The antenna may be produced on a flat circuit board (PCB). The antenna may avoid the need for any external frequency band filters and can provide highly directional characteristics with a relatively flat gain between 7dBi and 12dBi, depending on the number of dipoles used.

Description

The recent move from analogue to digital broadcasting television systems has led to a more efficient utilization of the frequency spectrum, that in turn, benefited mobile communication frequency allocations. Followed by this transition, and in order to ensure the efficient usage of the spectrum, the European Parliament urged European Member States to reallocate some TV broadcasting services in the UHF band to mobile communications. EU COM (2009) 586, “Transforming the digital dividend into social benefits and economic growth”, October 2009, advocates the benefits of allocating a socalled first digital dividend band of 790 MHz -862 MHz (or the so-called 800 MHz band) to Long-Term Evolution (LTE), also known as fourth generation (4G) mobile broadband wireless services based on a research analysis of the cost/benefit ratio.

As described in: Dec. 2010/267/EU “On harmonised technical conditions of use in the 790-862 MHz frequency band for terrestrial systems capable of providing electronic communications services in the European Union”, May 2010, the harmonized plan to deploy LTE-4G, to the UHF TV broadcasting band of 800 MHz was adopted in Europe (ITU region 1) to effectively utilize the spectrum for mobile services because of the superior radio wave building penetration capabilities at these frequencies in comparison to the 3G 2.1 GHz band or the 4G 2.6 GHz band. The spectrum for uplink and downlink transmission is made up of 6 blocks of 5 MHz bandwidth in the frequency band of 791 MHz-821 MHz and 832 MHz-862 MHz respectively with a duplex gap of 11 MHz. The broadcasting of Digital Terrestrial Television (DTT) is carried up to 790 MHz, leaving a gap of only 1 MHz between the DTT and the LTE 800 MHz band, which makes them both subject to interference. A detailed study of the interference effects caused due to the coexistence of TV broadcasting services with LTE mobile systems in the 800 MHz band, resulting in degrading the quality of service (QoS) of the TV broadcasting services can be found in: M. Ferrante, G. Fusco, et al., “Experimental results on the coexistence of TV broadcasting services with LTE mobile systems in the 800 MHz band, “2014 Euro Med Telco Conference (EMTC), Naples, 2014, pp. 1-6, 2014. Thereafter, a worldwide resolution on the allocation of the 700 MHz band between 694 MHz and 790 MHz (UHF TV channels 52-69) was passed in WRC-12 (World Radiocommunication Conference) and the provisions to use this band in Region 1 was made in WRC-15. The 700 MHz band is already in use in the United States of America (698 MHz - 806 MHz) since 2008 in order to provide an extra 108 MHz of spectrum to mobile broadband services operators. Similar to the 800 MHz, the 700 MHz band penetrates buildings and walls easier and covers larger geographic areas with less infrastructure (relative to frequencies in higher bands). The use of the 700 MHz UK band (694 MHz - 790 MHz) is planned for 2020. A detailed research on the interference problems due to the coexistence of TV broadcasting services with LTE mobile systems in 700 MHz has been presented in: M. Fuentes, et al., Coexistence of digital terrestrial television and next generation cellular networks in the 700 MHz band, IEEE Wireless Communications, vol. 21, no. 6, pp. 63-69, 2014. Therefore, and in order to avoid interference and overloading effects from the mobile cellular band into the remaining UHF TV band, some filtering has to be used to filter-out the 700 MHz and 800 MHz band signal received by TV reception antennas. An external or integrated filter can be used, however, this is usually costly and adds unwanted attenuation of TV band signals. A more cost-efficient solution would be to specifically design novel geometry TV reception antennas with the filtering action produced by the antenna itself. Thus, this new antenna would present a stopband in the region of the 700 MHz or 800 MHz bands similar to that of an external stopband filter. An improved log-periodic antenna has been proposed in European Patent Application EP2549587A1, by N. Andrea, Improvements of antennas, particularly log-periodic antennas, including resonant stubs at the front, middle, or rear parts of the antenna in order to filter out the unwanted frequency bands. Normally, shorter stubs placed at the front part of the antenna should be used in order to filter out higher frequencies and producing a high-cut filter characteristic. In contrast to the Yagi-Uda antenna, log-periodic antennas operate at a wide frequency range and provide relatively lower gain but better front-to-back ratio. In US patent application US3210767 D. E. Isbell proposed a basic design procedure for a Log-Periodic Dipole Array (LPDA) where a combination of cylindrical dipoles was arranged in an angular sector of 2a as shown in Fig. 1. The length of the dipoles active at the highest frequency will be the shortest, whereas the longest dipoles will be active at the lowest operating frequency. Since LPDAs are designed to operate at a wide frequency range, a large difference is observed between the lengths of the shortest and of the longest dipoles of the antenna. Generally, LPDAs consist of two conducting longitudinal supports, also known as “booms” or “cradles”, which are responsible for feeding the half-dipole elements of the antenna. Each conducting boom, feeds the alternate half-dipoles of the antenna, in a crisscross fashion. This configuration provides a 180-degree change in phase of the feeding source between two consecutive dipoles. The change in phase ensures that the radiation pattern of the antenna points in the forward direction, thereby providing directional characteristics. The arrangement of dipoles is made in such a manner that all the dipoles are in contact with the conducting boom, thereby making all the dipoles active. The number of dipoles used in the antenna has a significant impact on its gain. The gain significantly increases by increasing the number of dipoles used. In most cases, the feeding to the boom is provided using a coaxial cable through the front end of the antenna in order to avoid pattern distortion. A fastener is usually attached at the rear part of the antenna, acting as a short circuit stub as well as a point of support for the antenna. The apex angle of the LPDA is defined as the half angular sector in which the dipoles elements are embedded, which can mathematically be represented as:

a - tan

1-τ

4σ (1) where, τ is a scaling factor and σ is a spacing factor. The spacing between the dipoles is determined using the spacing factor, which can be represented as:

(2) where, s_n is the spacing between the n^th dipole and its consecutive (n+l)^th dipole and L_n is the length of nth dipole. The ratio of the lengths or diameters of two consecutive dipoles is defined as the scaling factor τ, which can be represented as:

_r= 4+l. ₌ ^2+1./

L d ηn where, L_n and d_n are respectively the length and the diameter of the n^th dipole.

The scaling factor, spacing factor and apex angle play an important role in the design of LPDAs, and a complete design procedure according to Carrel and Isbell can be found in many textbooks, e.g. C. A. Balanis, Antenna Theory, Analysis and Design, 2nd edition, John Wiley & Sons, pp. 551-566, 1997, as well as an initial report by L. Carrel: L. Carrel, Analysis and design of the Log periodic dipole antenna” Technical Report No. 52, Elec. Eng. Dept., Univ, of

Illinois, 1961.

This basic design with monotonously increasing dipole lengths, spacings, and diameters has been successfully used in designing antennas over the past 50 years. A detailed study of optimization of this classic design using various algorithms has been made in K. Mistry, P. Lazaridis, et al. “Measurement, simulation and optimization of wideband log-periodic antennas”, XXXIInd URSI General Assembly and Scientific Symposium (URSI GASS), Montreal, Canada, pp. 1-8,2017.

Furthermore, recently and in order to mitigate problems of interference caused due to the coexistence of the LTE 800 MHz and 700 MHz bands with the UHF TV band, various TV reception antennas have been designed, which can operate in the UHF TV band of while rejecting the mobile-cellular bands with the inclusion of external band stop filters.

This invention relates to a cost-efficient design of dipole log-periodic dipole antenna capable of operating in the UHF TV reception band and rejecting the mobile-cellular 700MHz and 800MHz bands without using any external filter. The design provides good matching of antenna in the passband. The invented design exhibits highly directional characteristics in the passband with a relatively high gain. The proposed EPDA design is obtained by varying the length of only the few shorter dipoles, eventually using an optimization algorithm, and following the classical Isbell and Carrel log-periodic design described above for the rest of the dipoles. This EPDA design is very simple and avoids the use of costly external filters, thereby reducing the actual cost of the antenna.

Description

A log-periodic antenna is a unique type of antenna which approximates frequency independence possessing highly directional characteristics and relatively good gain. It generally consists of a pair of conducting cradles or booms, that extend longitudinally, used to support usually cylindrical dipoles of identical or varying diameter embedded in an angular sector from the front end of the conducting boom. The dipoles are arranged in such a fashion that some half-dipoles are attached to the upper cradle whereas other half-dipoles are attached to the lower boom in a gradually increasing spacing pattern. This ensures that the feeding is provided to all the half dipoles in a crisscross fashion by the cradles or booms, which form a feeding transmission line, thereby making all the dipoles active. This configuration provides a phase shift of 180 degrees between two consecutive dipoles. The change in phase between two consecutive dipoles is responsible for the antenna’s highly directional characteristics as it ensures that the radiation always points towards the forward direction of the antenna. The number of dipoles used in the log-periodic antenna plays a significant role in determining the gain of the antenna. The number of dipoles used in the antenna is proportional to the antenna gain. Generally, the feeding to the conducting cradles or booms of the antenna is provided through the front end using coaxial cables to avoid pattern distortion. The rear parts of both cradles are short-circuited using a cuboidal fastener, which also acts as a point of support for the antenna.

BRIEF DESCRIPTION OF THE DRAWINGS

The antenna design according to the invention can be clearly visualized in Figures 1 to 6. However the design may vary, depending on the number of dipoles used to meet desired goals.

- Figure 1 shows a schematic of the classical prior-art log-periodic antenna design by Isbell and Carrel’s method.

- Figure 2 shows a schematic of an improved log-periodic antenna according to the invention.

- Figure 3 shows a perspective view of an antenna design with 14 dipoles in accordance with the present invention, seen from a particular angle.

- Figure 4 represents a top plan view of the antenna of Figure 1; the dipoles with the boom can be clearly visualized using this view.

- Figure 5 is a left-hand side plan (LHS) view of the antenna of Figure 1; the crisscross pattern of half dipoles embedded into the boom along with the spacings can be visualized using this view.

- Figure 6 shows the frequency response of a designed log-periodic antenna according to the invention. The passband and stopband are clearly visible.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The geometry of the invented log-periodic design is slightly different but motivated by the classic log-periodic antenna design of Figure 1. Referring to the attached Figures 3 to 5, the design consists of two cuboidal metallic cradles of rectangular cross-section 15 and 16, with 28 cylindrical half-dipoles (14dipoles numbered from 1 to 14) embedded in a crisscross pattern. However, metallic cradles can also be of another shape, e.g. cylindrical, or even thin metallic sheet. In contrast to the conventional log-periodic antenna design, the dipole lengths do not gradually increase throughout the proposed antenna design as seen in Figures 2 to 5. Generally, the lengths of the first few dipoles, and in a particular embodiment of five dipoles 1 to 5, are optimized and are responsible for providing a high frequency stopband. The first five dipoles of the antenna are arranged in such a fashion that the second dipole 2 is smaller than the first and third dipole, 1 and 3 respectively. Whereas, the fourth dipole 4 is bigger than the third dipole 3 and fifth dipole 5. Thereafter, the dipole lengths of the antenna gradually increase towards the rear part of the antenna following the classic logperiodic antenna theory.

The feeding with a coaxial cable is provided at the front end of the cradles 15 and 16. The feeding impedance can be 50 Ohms, 75 Ohms, or any other value depending on the specific design. Both cradles 15 and 16 are short circuited using a fastener 17, to provide the antenna a rigid support as well as maintain the electrical continuity between the cradles.

The cylindrically shaped dipoles can have a solid or hollow cross-section and their shape may also vary. In one preferred embodiment the dipoles are cylindrically shaped solid wires. However, dipoles can be of a rectangular crosssection, or even of thin metallic sheet form. These dipoles can be embedded into the holes of the cradles by welding, screws, or any other fixing method. This ensures a mechanically stable fastening of dipoles to the cradles. A typical performance graph showing the distinct passband and stopband of a design according to the invention is shown in Figure 6. Passband stretches from 470 MHz to 790 MHz, while the stopband extends from 790 MHz to 1000 MHz.

Claims

1. Log-periodic antenna for receiving/transmitting electromagnetic waves, including at least one passband and at least one stopband, comprising a number of cylindrical, rectangular, triangular, or thin metallic sheet, etc. dipoles not in a monotonically increasing sequence of length, on metallic booms of cylindrical, rectangular, triangular, or thin metallic sheet crosssection.

2. Antenna according to claim 1, and designed according to Isbell and Carrel’s classical design method using tau and sigma parameters, where a number of dipoles are optimized according to their lengths and relative spacings in order to provide a stopband characteristic.

3. Antenna according to claim 2, where said dipoles to be optimized are in the front, middle, or rear part of the antenna in order to reject higher, middle, or lower frequencies respectively.

4. Antenna according to claim 3, where said dipoles to be optimized are in the front part of the antenna, and wherein the length of the first dipole is approximately between 35% to 45% longer than the classical design method of Isbell and Carrel. In a preferred embodiment of the invention, the first dipole is 40% longer than the one calculated by the classical design method.

5. Antenna according to claim 4, wherein the length of the second dipole is between -10% to +10% of the one calculated by the classical design method.

6. Antenna according to claim 5, wherein the length of the third dipole is between -10% to +25% of the one calculated by the classical design method.

7. Antenna according to claim 6, wherein the length of the fourth dipole is between -20% to +20% of the one calculated by the classical design method.

8. Antenna according to claim 7, where an optimisation algorithm is used to optimise the lengths and spacings of the first few dipoles by taking into account the limit values for these lengths.

9. Antenna according to claim 8, where said optimised antenna is a UHF TV reception antenna and the stopband is the 700 MHz and/or 800 MHz mobilecellular bands.

10. Antenna according to claim 9, produced on a flat printed circuit board (PCB).

11. Antenna according to claim 3 where a number of dipole lengths and spacings are optimised by an optimisation algorithm in order to produce a desired passband and stopband characteristic.

12. Antenna according to claim 11, produced on a flat printed circuit board (PCB).