EP2639882A1

EP2639882A1 - Device for receiving and frequency filtering radio signals

Info

Publication number: EP2639882A1
Application number: EP12382096.1A
Authority: EP
Inventors: Jesús Mª San José Damboriena
Original assignee: Angel Iglesias SA
Current assignee: Angel Iglesias SA
Priority date: 2012-03-15
Filing date: 2012-03-15
Publication date: 2013-09-18

Abstract

The present invention relates to a device for optimal reception of television signals, especially when there are problems caused by new services (such as mobile internet services, LTE telephony...). The present invention solves in a simple manner the problems they present due to existing systems especially in scenarios (digital dividend) where the signals of new services are very close to television channels. The present invention proposes an antenna whose natural frequency response presents a very important and discriminating rejection of these nearby interfering signals.

Description

Technical Field of the Invention

The present invention has its field of application in the interferences reduction in signals received by radio signal receiving devices and more specifically, in the interferences reduction in antennas receiving television signals.

Background of the Invention

The compression systems in current digital television systems allow transmitting several normal digital television channels (usually up to six, according to the coding and modulation techniques used) with acceptable quality in the radio frequency space previously used by a single analogue channel. There are usually four or five terrestrial analogue services per region and, therefore, the use of a single digital television channel for all of them will considerably reduce the use of the spectrum. The difference between the amount of spectrum used in analogue systems (11) and that used in digital systems (12) to cover existing analogue services, and which has accordingly been freed when going from analogue to digital television (after a transition period (15) in which the 2 technologies were used), is referred to as the digital dividend (13), which is illustrated in Figure 1. Said figure shows a comparison of the spectrum (14) occupied by previous analogue systems with respect to current digital systems.
New digital terrestrial television broadcast coding, compression and modulation techniques (such as those proposed in various recommendations of the International Telecommunication Union, ITU) have indirectly contributed to the process of creating this so-called digital dividend. For example, in precursor Recommendation ITU-R BT.798 it is stipulated "that digital terrestrial television broadcast be adapted in channels (with bandwidths of 6, 7 and 8 MHz) intended for emitting analogue television in metric and decimetric wavebands". That Recommendation, in which it is prohibited that the bandwidth used for digital programs is greater than the analogue channel bandwidth, has opened up the way for the development of advanced digital compression techniques. In other words, the digital dividend is due to the fact that digital compression systems allow multiplexing the transmission of several television programs in the spectrum previously used by a single analogue television channel. This means that the possibilities of accessing the digital dividend spectrum continue to increase as more advanced terrestrial television standards are developed and progressively introduced for the infrastructure and compression (for example, the second generation of digital terrestrial television, DTT, broadcast transmission systems), offering a higher binary capacity per hertz than existing systems.
The amount of spectrum freed by changing from analogue to digital transmission primarily depends on national particularities such as country geography and topography, degree of penetration of digital transmission services, needs of regional or minority television services, and use of the spectrum in neighboring countries. This amount also depends on the digital television technology adopted to replace analogue services. Accordingly, the size of the digital dividend changes from one region to another and from one country to another. Although, the specific location of the digital dividend also varies from one country (or region) to another, since it depends on the assignment of frequencies of each country/region, it is usually located between 200 MHz and 1 GHz. In Europe in particular, this freed band (dividend) is located in the range between 790 and 862 MHz.
The spectrum created with this digital dividend can be used for services of any type, such as additional terrestrial radio television services (which could even include the delivery of new interactive and high-definition television programs), mobile multimedia applications, mobile communications, wireless broadband access systems (for example, it could be used to offer ubiquitous broadband Internet access in areas where terrestrial lines have not yet arrived, which would help reduce the digital gap), etc. In other words, it can be said that this spectrum is the chance to respond to the growing demand for new wireless communication services.
However, it is the mobile telephony sector the most interested in using this digital dividend given the number of new mobile services that are being offered in this sector (mobile television, Internet access through mobile terminals, massive data transmission...). Furthermore, the freed frequencies in the digital dividend (which, as previously stated, are usually in the band between 200 MHz and 1 GHz) have signal propagation characteristics greater than, for example, 2.4 GHz, and the sector has declared that it is interested in using these lower frequencies to facilitate coverage and, accordingly, achieve optimal equilibrium between transmission capacity and the operating range. Therefore less infrastructure would be needed to obtain broader mobile coverage, with the subsequent reduction of costs for communication services, especially-in rural areas. As a result, in many countries where television transmission has changed from analogue to digital, most part of the freed spectrum has been assigned to mobile telephony communications and particularly to new generation mobile telephony transmissions, known as LTE (Long Term Evolution) mobile communication, or also for other types of technologies such as 4G mobile telephony or WiMAX.
This freed spectrum (digital dividend) resulting from converting from analogue to digital television (as can be seen in Figure 1) and which is going to be assigned to this other type of services mentioned above (wireless internet, mobile telephony...), is going to be very close to the frequency band used in the service of terrestrial digital television (in some cases the signals used by these new telephony services are in the band between 791 and 821 MHz; very close to the television signals that can occupy up to 790 MHz). Here an important problem will arise because digital terrestrial television (DTT) signals are going to be interfered from the signals of these new services, with the subsequent degradation of quality in the television service. This will be especially problematic in those television channels which are located at the end of the band dedicated to television (close to 790 MHz), i.e., in the area bordering with the freed spectrum band.
The interference caused by these new signals in digital television signals is going to be significant because they are a type of signals very different from digital television signals, and furthermore they are very close together (in some cases, at 1 MHz), causing the following unwanted effects:

On one hand, since they are high-powered signals compared with television signals, they usually produce saturation in broadband amplifiers and in the tuners of the television receiving system.
On the other hand, these signals usually have a bandwidth occupying frequencies outside the assigned channel causing interference (cochannel interference) in television channels, therefore deteriorating the signal-to-noise ratio of the television signal received.

To improve reception of digital television signals, filters separating digital television signals from those unwanted signals (the signals of the new services) could be used. It would also be possible to solve this problem (or at least reduce its severity) by increasing the power with which the television signal is received (for example, using repeaters antennas), but this would be a very expensive solution.
Therefore, it is necessary a receiving system which solves in an inexpensive and effective manner, the problem of the worsening of the digital television service reception quality in the scenario considered above, and this is the purpose of the present invention.

Summary of the Invention

The present invention proposes an antenna which solves in a simple manner the problems presented in existing devices. Said antenna has an amplitude-frequency response the efficacy of which can be greater than that of a traditional filter made up of coils and capacitors, providing vast selectivity and low through pass losses at an advantageous cost.
In a first aspect, the present invention describes an antenna which receives radio signals, said antenna comprising a reflective element (reflector) and a dipole, where the reflective element and the dipole are assembled on a first boom, said antenna being further characterized by comprising an element made of conductive material, referred to as main parasitic element, located on the boom between the reflective element and the dipole.
The radio signals received may be television signals, for example, in the UHF frequency band.
Said antenna can further comprise an assembly of director conductive elements placed along the first boom, following the dipole in the side opposite that of the reflector, and where the director element closest to the dipole has an electrical length equal to the electrical length of the main parasitic element.
In one embodiment, the electrical length of the main parasitic element is half the wavelength corresponding to the desired cutoff frequency, where the desired cutoff frequency is a design parameter of the antenna which will be the frequency from which signals are considered interfering signals and are therefore desired to be rajected.
To more fully understand the invention, its objects and advantages, reference can be given to the following specification and the attached drawings.

Description of the Drawings

To complement the description being made and for the purpose of aiding to better understand the features of the invention according to several preferred practical embodiments thereof, a set of drawings is attached as an integral part of this description where the following has been depicted with an illustrative and nonlimiting character:

Figure 1 shows an explanatory graph of the digital dividend concept.
Figure 2 shows a television receiving antenna according to an embodiment of the present invention.

Detailed Description of the invention

The present invention proposes an antenna that would reduce the problems occurring in television signal reception; especially in cases where telephony signals are very close to the television channels (such as in the digital dividend scenarios mentioned above) as a very relevant reduction of the ratio of the telephony signal (interfering) received by it and the TV signal is allowed.
In one embodiment of the present invention, the antenna would be a television signals receiving antenna (although it could also be applied to another type of antennas) in the UHF band frequency range either the complete band or parts of the band, and more specifically in the frequency range comprised between 470 and 790 MHz.
In one embodiment, the antenna is a Yagi-type antenna. Antennas of this type are usually made up of a main active element, in most cases a dipole (20), preceded at a certain distance by a reflector formed by an assembly of conductive rods (21) (forming reflective grids or screens 22 in many cases), and on the side opposite that of the reflector, followed by a number of director elements (23) (elements made of conductive material which can be cylindrical metal rods made of metal, for example aluminum). The director and reflective elements are called parasitic or passive elements (since they are not active, i.e., are not powered or electrically feed). All these components are assembled on one or several bars (24) (also called support bars or booms). One of these bars there is a central boom on which the dipole is assembled and to which the reflective grids are attached (in fact, this central boom is usually built in the bisector of the angle formed by the reflective grids). If there are more bars besides the central boom (as in Figure 2), these bars can be located on both sides of the central boom, the reflector (reflective grids) being supported on them and the director elements placed along said bars (perhaps perpendicular to them though other orientations are possible) and in front of the dipole. According to their position, all these elements determine an axis which will be the direction in which the antenna has maximum sensitivity to the signals received (that is, those signals the propagation direction of which coincides with this direction will be received with maximum sensitivity), i.e., the axis of the main lobe of the antenna or in other words, the direction in which the antenna is pointed (it can therefore also be called antenna axis or antenna pointing axis). Logically, this axis must ideally be aligned with the trajectory of the radio signals desired to be received (the direction of signals reception being the direction going from the directors towards the reflector, passing through the dipole).
For the sake of clarity, many of the examples shown refer to television signal receiving antennas and to Yagi-type antennas, however, this invention is also applicable to other types of antennas (television signal receiving antennas or not) and generally to all those antennas having at least one active element or dipole and at least one reflective element or reflector.
Television signal receiving antennas used until now have usually frequency responses with a certain bandwidth, but the slopes of this gain response according to the frequency in the cutoffs are low (i.e., they are not very discriminative); so they are not very effective for the scenario described above (digital dividend) where the unwanted communication signals are so close to television channels.
The proposed antenna solves some of the problems of existing antennas. To that end, said antenna incorporates parasitic elements (i.e., non-active or non-powered) made of conductive material having certain electrical dimensions which are placed in the nearby environment of the dipole and provide a suitable frequency response (these elements can be, for example, rods made of electrically conductive material). In one embodiment of the invention, the electrical dimensions of said elements will be tuned to half the wavelength corresponding to the cutoff frequency of interest (e.g. 790 MHz), obtaining a frequency response defined by letting pass in the desired reception band (for example, in the case of digital television signals in Spain, the desired reception band would be 470 to 790 MHz) with an important rejection after the cutoff frequency and with a steep slope in its response that is much greater (more discriminatory) than in many filters. All this is done without introducing losses in the pass band and with a much lower cost given the simplicity of said structure. These features make this antenna extremely valid in TV signal reception after applying the digital dividend, as it can reject the interfering LTE signals in the 790 to 862 MHz band. In other words, by simply incorporating in suitable positions of a conventional signals receiving antenna (e.g. Yagi) parasitic elements having very specific dimensions, a frequency response with an important rejection of unwanted signals is achieved. In the example above, a cutoff frequency (790 MHz) has been chosen to reject LTE signals, but in the event that other cutoff frequencies are needed, for example, for dividend 2, or for current mobile telephony in the 900 MHz band, it would be sufficient to tune said elements (i.e., choose their electrical dimensions) to the required frequencies and thus having the desired frequency response.
These parasitic elements incorporated in this invention will generally consist of a conductive element located between the dipole and the reflector, in the boom where the dipole is located, which is referred to as the main parasitic element, and optionally one or several elements each of which is located in each of the director element booms forming the antenna, which are referred to as secondary parasitic elements. All these parasitic elements can be rods (for example cylindrical) made of conductive material (for example metal), placed on the booms with a certain inclination with respect to said booms (which can be perpendicular or any other).
Figure 2 shows an example of a television receiving antenna according to an embodiment of the present invention (Yagi-type antenna with three director element booms) and the added parasitic elements can be seen, indicated with letters A, B, C and D.
The secondary elements (B, C and D) located in the support booms give the antenna a low-pass filter response at its desired cutoff frequency and the main element (A) located between the dipole and the reflector, resonating in the same' cutoff frequency as them, gives an even more high-pitched resonance causing a drop in the amplitude-frequency response of the antenna at the cutoff frequency much more quickly. Therefore, the main parasitic element is the one adding an even more discriminatory and effective filtering, giving the antenna a response with a more abrupt frequency cutoff (more discriminatory), maintaining the features in the pass band. In the example described above, this would allow a rejection of the frequencies not only from the LTE uplink band of (832 to 862 MHz) but also the LTE downlink band (791 to 821 MHz).
As stated, this effect will occur provided that the dimensions of these elements are the required which will of course depend on the desired cutoff frequency. In one embodiment, the electrical length that these elements must have is half the wavelength of the desired cutoff frequency. So if the desired cutoff frequency is 790 MHz, the wavelength would be approximately c/f_cutoff, i.e., 3*10⁸/790*10⁶=0.38 meters. In other words, the electrical length of these elements must be approximately 0.38/2=0.19 meters (19 centimeters).
Electrical length is a widely known concept in the antennas. The electrical length of an element is the length of said element expressed as the number (or fraction) of wavelengths of a radio signal being propagated in said element (or in other words, the number of wavelengths that "fit" in said element). Therefore, the electrical length of an element with a specific physical length will vary according to the speed of the radio signal in said element. In other words, the physical length equivalent to a specific electrical length will depend on the speed of the signal in said element.
In certain cases, said "optimal" dimensions (producing the desired effect) can slightly vary around (slightly above or below) half the wavelength of the desired cutoff frequency, depending on various factors (radio situation, material of the antenna,....). In one embodiment, the exact length that the elements must have and their specific position can be adjusted by means of simulations or empirical tests.
These elements can be added to an already constructed antenna, or the antenna can be designed and constructed from the start with these elements incorporated.
In terms of the arrangement of said elements in the antenna to achieve the desired effect, it will be the following: The main element (A) is located between the dipole and the reflector. The position of said element can be equidistant between the reflective element and the dipole, although any other intermediate position of the main parasitic element not equidistant between the reflective element and the dipole is also possible. In the event that the reflective element is formed by reflective grids or screens attached to the boom where the dipole is located (Figure 2), the main parasitic element will be in the boom where the dipole is located, in any intermediate position between the dipole and the point of the boom where the reflective screens are attached (for example, in an approximately equidistant position as in Figure 2).
The secondary elements will be located in each of the support booms, in front of the dipole and before the remaining director elements (with respect to the dipole). Actually, these secondary elements will simply be another director element (the closest to the dipole in each boom) but with specific dimensions different from the remaining director elements. Said secondary elements can be at the height of the dipole or further ahead and can be aligned between them though it is not necessary.
In a specific example (Figure 2), it is assumed that the antenna is a Yagi-type antenna with three assemblies or groups of directors located in 3 booms or levels (24) one on top of the other, a dipole and a dihedral-shaped reflector. In this case, the arrangement of these elements would be the following: The secondary elements B, C and D in each boom will be the director element (23) closest to the dipole and as mentioned, its electrical dimensions will be half the wavelength of the cutoff frequency that is sought in the response of the antenna (so it will have a dimension that is a little larger than the remaining director elements). In turn, the main element A (which will have the same electrical length as B, C and D) is located in the central boom, between the dipole and the reflector.
For cases of antennas of another type with a different number of levels (or booms), the operation is the same, the dimensions of the elements, like in the previous case, are determined by half the wavelength corresponding to the desired cutoff frequency. The position of these elements will be in the area surrounding of the dipole: One of them will at least be between the reflector and the dipole (in an equidistant position between both or in another intermediate position), and the other one or ones will be in front of the dipole in the axis of the antenna before the remaining director elements or also above and below the dipole (if these levels exist).
Although only some elements (A, B, C and D) have been described, other parasitic elements can optionally be added in other parts of the antenna to enhance and improve the desired filter effect.
In summary, with this antenna structure TV signal reception is optimized even in situations where interfering signals (telephony or another type of signals) are at frequencies very close to those of the television channels. Having an antenna like the one proposed, having a natural frequency response with important rejection and being highly discriminatory, without the need for filters, in the unwanted frequency band (for example LTE band) has enormous advantages because it solves the problems that may occur in TV signal reception in a simple, effective and inexpensive manner.
Even though for the sake of clarity many of the examples shown refer to Yagi-type antennas, this invention of course is not only applicable to Yagi-type antennas but to any other type of antennas.
Some preferred embodiments of the invention are described in the dependent claims included below.
In this text, the word "comprises" and its variants (such as "comprising", etc.) must not be interpreted in an excluding manner, i.e., they do not exclude the possibility that what is described can include other elements, steps, etc.
Having sufficiently described the nature the invention as well as the way of being carried out into practice, the possibility that its different parts could be made in a variety of materials, sizes and shapes is herby stated, also being able to introduce in the construction or method those variations suggested by practice, provided that they do not alter the fundamental principle of the present invention.
The description and drawings merely illustrate the principles of the invention. Therefore, it must be taken into account that persons skilled in the art could conceive of several arrangements which, though not explicitly described or shown herein, represent the principles of the invention and are included within its scope. Furthermore, all the examples described herein are mainly provided for teaching purposes to help the reader understand the principles of the invention and the concepts provided by the inventor or inventors for improving the art, and must not be considered as limiting with respect to such examples and conditions specifically described. Furthermore, all that has been described herein in relation to the principles, aspects and embodiments of the invention, as well as the specific examples thereof, cover equivalences thereof.
Although the present invention has been described in reference to specific embodiments, persons skilled in the art must understand that the preceding and other various changes, omissions and additions to the form and detail thereof can be done without departing from the scope of the invention as defined in the following claims.

Claims

Radio signals receiving antenna for reducing interference, comprising a reflective element and a dipole, where the reflective element and the dipole are assembled on a first boom, being said antenna characterized by further comprising an element made of conductive material, referred to as main parasitic element, located in the boom between the reflective element and the dipole.
Antenna according to any of the preceding claims, where the antenna further comprises a group of director conductive elements placed along the first boom, following the dipole in the side opposite that of the reflective element, and where the director element closest to the dipole has an electrical length equal to the electrical length of the main parasitic element
Antenna according to any of the preceding claims, where the electrical length of the main parasitic element is half the wavelength corresponding to the desired cutoff frequency, where the desired cutoff frequency is a design parameter of the antenna which will be the frequency from which signals are considered interfering signals and are therefore desired to be rejected.
Antenna according to claim 3, where the desired cutoff frequency is 790 MHz.
Antenna according to any of the preceding claims, comprising in addition to the boom where the dipole is assembled, other side booms located on the sides of said boom, with a group of director elements placed along each boom and where the director element closest to the dipole in each boom has an electrical length equal to the electrical length of the main parasitic element.
Antenna according to any of the preceding claims, where the reflective element is formed by an association of rods made of conductive material forming one or more reflective grids (22).
Antenna according to any of the preceding claims, where the main parasitic element is a metal rod.
Antenna according to claims 2 or 5, where the director elements are metal rods.
Antenna according to claims 6-8, where the rods are cylindrical rods.
Antenna according to any of the preceding claims, where the main parasitic element is located in the first boom between the reflective element and the dipole in an equidistant position between the reflective element and the dipole.
Antenna according to any of the preceding claims, where the radio signals received are television signals.
Antenna according to any of the preceding claims, where the antenna is a Yagi-type television signals receiving antenna.
Antenna according to claim 5, where the antenna is a 3-boom Yagi-type antenna, where the reflector is made up of 2 reflective grids forming an angle with one another and where the direction of the first boom coincides with the bisector of the angle formed by the reflective grids.