The present invention relates generally to radio frequency electromagnetic signal (RF) broadcasting. More particularly, the present invention relates to techniques for broadcasting electromagnetic signals in the 26 MHz Short Wave band with sky wave suppression.
There is a trend toward adoption of digital technology in radio and communications, especially for distribution and transmission. Digitization offers substantial advantages to national and international broadcasters in the short wave bands from 2 to 30 MHz. Analog transmissions are often of poor quality because of fading and interference from both human-made and natural sources.
The Digital Radio Mondiale (“DRM®”) consortium has developed a world-recognized standard for a digital modulation of short wave transmissions that can produce signals of “FM” quality—that is, signals comparable to frequency-modulated analog signals in the familiar VHF entertainment broadcasting band at 88 MHz to 108 MHz, hereinafter the FM band—in their resistance to variations in signal level due to ghosting and other forms of interference and in their resistance to extraneous noise from electrical sparks, lightning, and other sources. While it is to be understood that the FM band is also undergoing digital enhancement, the perceived quality of performance of FM since its inception remains a standard of excellence. Except as noted, the electrical engineering (EE) term of art “RF” is used herein in its usual senses—that is, to refer either to radio frequency (broadly, subsonic to terahertz) electromagnetic signals or to the frequencies of the signals, as implied by the context. Other EE terms of art are likewise used in their usual senses except as noted.
Short wave broadcasting typically directs a signal toward the ionosphere, which, by reflecting and/or refracting the signal, and generating a so-called sky wave, allows the signal to reach audience areas many hundreds or thousands of kilometers from the transmitting station. To achieve reception at these distant targets, transmitter power typically has to be 50 kilowatts (kW) or higher, with many short wave transmitters providing output power of 250 kW to 500 kW.
At the upper end of the short wave band, as at substantially all other frequencies, broadcast signals can propagate directly to receivers in the line of sight, and, if a transmitting antenna is mounted on a suitably tall structure, can be received at distances on the order of 60 miles (100 km) from the antenna. Line of sight transmission, also termed transmission by terrestrial wave herein in contradistinction to transmission by sky wave, can use both familiar analog and DRM® and other, equivalent digital modulation methods, with the digital methods allowing signals to be received with very high quality. Such signal quality, comparable to that of FM band broadcasting, can be achieved while using only relatively low broadcast power, namely, a hundred watts to a few thousand watts. Unlike VHF-FM broadcasting, DRM® and other digital transmission methods have been developed for operation at frequencies ranging from approximately 200 kHz to 30 MHz. Of relevance for the instant invention is the upper short wave band from 25.67 MHz to 26.10 MHz, hereinafter the 26 MHz band, currently little used.
While legacy (primarily non-digital) operators within this narrow 330 kHz-wide band have been assigned 9 kHz- or 10 kHz-wide channels, and have been allowed very wide but irregular (time-of-day and sunspot-cycle dependent) geographic coverage with high-power amplitude-modulated (AM) and/or sideband amplifiers, digital operators propose to provide direct local coverage, such as with COFDM (coded orthogonal frequency division multiplexed) signals—digital signals using data blocks transmitted simultaneously at multiple, narrowly-spaced frequencies according to a highly robust scheme. Suitable modulators form the signals according to at least one published standard (refer to ETSI ES 201 980, latest edition, for encoding algorithms), while conventional and more advanced transmitters can broadcast the signals. Depending on the level of fidelity, resistance to loss, and extra features desired, channels as narrow as 2 kHz or as wide as 32 kHz can be used for digital transmissions. Cobroadcasting of conventional analog signals can allow both digital and conventional radios to pick up programs on the same channel, albeit with differences in quality and features.
The availability of line-of-sight broadcasting in the 26 MHz band potentially enables broadcasters to provide largely local coverage from numerous short-range transmitters. In the major cities of most developed countries, the channels of the FM band are effectively all allocated. In the present era, a broadcaster who desires to establish a new service has had to purchase an allocation from another broadcaster, often at enormous expense. However, since the 26 MHz short wave band is lightly used at present, there exists potential for many new stations to supply local broadcast services, if sky wave propagation can be suppressed.
Unlike an FM-band signal, propagation of which is largely limited to line of sight, a short wave 26 MHz transmission can also propagate by sky wave, and, under certain ionospheric conditions, can produce a strong signal at great distances from the transmitting antenna. For this reason, a 26 MHz short wave antenna intended to broadcast strictly locally must emit a signal that is reduced in strength at those angles that would allow the signal to propagate long distances.
What is needed is a short wave antenna that minimizes the tower space needed for structural support and that, in the same design, minimizes undesirable sky wave propagation.
The foregoing needs are met, to a great extent, by the present invention, wherein an apparatus is provided that in some embodiments provides a short wave broadcast antenna that suppresses sky wave emission while providing gain for low-elevation signals, further providing power handling capability suitable for line-of-sight broadcasting service from ground-mounted transmitting towers.
In accordance with one embodiment of the present invention, a sky wave suppressing broadcast antenna system for short wave radio frequency electromagnetic (RF) signals is presented. The antenna includes a first radiator, configured to emit an RF signal with substantially omnidirectional distribution of energy with respect to azimuth, and a signal directing apparatus configured to direct energy from the first radiator, wherein the energy directed by the signal directing apparatus is energy that would support ionospheric reflective/refractive propagation, wherein the directed energy is so directed as to reinforce line-of-sight propagation.
The above antenna embodiment further includes a reflector positioned further from a mean terrain surface than the first radiator, wherein the substantially cone-shaped reflector surface is formed from a plurality of reflector components.
Another antenna embodiment includes instead a second radiator, configured to couple and reradiate RF energy emitted by the first radiator, wherein RF emission from the second radiator destructively interferes with RF emission from the first radiator in a sky wave direction and constructively interferes with RF emission from the first radiator in a terrestrial wave direction.
In accordance with still another embodiment of the present invention, a sky wave suppressing broadcast antenna system for short wave radio frequency electromagnetic (RF) signals is presented. The antenna includes first means for radiating, configured to emit an RF signal with substantially omnidirectional distribution of energy with respect to azimuth, means for mechanically positioning the first means for radiating in an elevated location, and means for directing signals, configured to direct energy from the first means for radiating, wherein the energy directed by the means for directing signals is energy that would support ionospheric reflective/refractive propagation, wherein the directed energy is so directed as to reinforce line-of-sight propagation below a horizon line as determined with respect to the first means for radiating.
In accordance with yet another embodiment of the present invention, a method for broadcasting short wave radio frequency electromagnetic (RF) signals is presented. The method includes the steps of providing on a broadcast tower a mounting point for an RF signal radiator, wherein the mounting point has sufficient height above mean terrain to permit line-of-sight transmission of high-band short wave RF signals over a specified area, emitting a vertically-polarized RF signal from a first radiator having broadly omnidirectional distribution of energy with respect to azimuth, wherein the first radiator is affixed to the broadcast tower mounting point, and directing the RF signal energy both to suppress sky wave propagation and to reinforce line-of-sight propagation over the specified area.
There have thus been outlined, rather broadly, the more important features of the invention in order that the detailed description thereof that follows may be better understood, and in order that the present contribution to the art may be better appreciated. There are, of course, additional features of the invention that will be described below and which will form the subject matter of the claims appended hereto.
In this respect, before explaining at least one embodiment of the invention in detail, it is to be understood that the invention is not limited in its application to the details of construction and to the arrangements of the components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments, and of being practiced and carried out in various ways. It is also to be understood that the phraseology and terminology employed herein, as well as the abstract, are for the purpose of description, and should not be regarded as limiting.
As such, those skilled in the art will appreciate that the conception upon which this disclosure is based may readily be utilized as a basis for the designing of other structures, methods, and systems for carrying out the several purposes of the present invention. It is important, therefore, that the claims be regarded as including such equivalent constructions insofar as they do not depart from the spirit and scope of the present invention.
The invention will now be described with reference to the drawing figures, in which like reference numerals refer to like parts throughout. The present invention provides an apparatus and method that in some embodiments provides an antenna that suppresses sky wave emission while broadcasting short wave signals by line of sight.
For illustration purposes, a single bay arrangement without sky wave suppression is considered first. With four radiating elements 14, all driven in phase, such an antenna 10 produces a vertically-polarized signal with a radiation pattern in the horizontal plane that is substantially omnidirectional with azimuth, and the influence of the triangular, open-structure conductive mast 12 may be largely neglected. Mast 12 shape, the number and type of radiating elements 14, the distance 16 from the mast 12 to the respective elements 14, as well as embodiment options such as phasing excitation of the elements 14 at 90 degree intervals rather than in phase, affect azimuth uniformity and other characteristics of the antenna 10, but are substantially independent of the inventive characteristic of the instant invention. If the number of radiators 14 is reduced to three (uniformly distributed), a generally omnidirectional characteristic can be maintained, albeit with reduced azimuth uniformity; it is generally understood in the art that an antenna 10 having only two radiators 14 per bay may produce excessive nulls in at least some embodiments, in which case it is no longer seen as omnidirectional. An antenna 10 having a single radiator 14 mounted alongside the mast can achieve a propagation pattern that may be acceptable for at least some applications, provided the relative null where the mast 12 shadows the radiator 14 can be tolerated. For some applications, such a null may be useful, such as to transmit to a nonsymmetrical population area, to transmit from an edge of a town, to avoid interference with a nearby transmitter, and the like.
Length: 0.66 wavelengths to 1.0 wavelength
Angle: 30 degrees to 60 degrees below horizon
The embodiment of
In configuring the antenna 30 embodiment shown in
Near-optimum solutions for angle and diameter can be identified by fairly rapid cut-and-try development, such as by using a wavelength of the center frequency as a first estimate of wire length and selecting a few slope angles between 30 degrees and 60 degrees below the horizon for analysis. Varying wire length L and angle A will indicate trends. Reflector 32 mounting provisions may be developed readily in view of a specific mast cross section and surface arrangement. For particular radiator designs, reflector mounting height with respect to the radiators may require further stepwise analysis. The process may point toward a single optimal solution or may point to families or classes of solutions, wherein each length L of reflector, for example, may have an optimum angle A and position with regard to the achieved combination of signal strength near the horizon and attenuation of the sky wave.
Similarly, shield reflectivity is a function of coverage, with a single solid conductor being most effective, but typically heavy and susceptible to wind loading, while sparse or thin wires become increasingly RF transparent and ultimately fragile in the presence of wind, birds, ice, and the like. Woven mesh, pierced or expanded metal, metal-clad fiber-reinforced plastics, or the like may be an effective reflective component in some applications. Refinement to a final product involves trading off material cost and manufacturability, durability, RF performance, installation considerations, and other issues.
It is to be understood that a reflector 32 added to an existing antenna 10 (
It is to be understood that use of a suitably large vertical aperture—that is, a tall RF radiator, such as one composed of a capacitively-coupled monopole string or an array of dipoles—can cause the elevation radiation pattern of a short wave antenna to be narrowed to substantially any desired extent. However, in the 26 MHz band, in which the wavelength is about 38 feet (11.6 meters), such an aperture might occupy 40 feet to 160 feet (11 meters to 45 meters) or more of vertical space on a supporting tower. Since it is frequently desirable that an antenna be mounted as high as possible on a tower, for example to maximize its line-of-sight range of transmission, it may be impractical for both technical and financial reasons to provide such an aperture on an existing or newly-built tall tower.
For example, in a two-bay 62 arrangement of vertically-polarized, active radiators 64, if the bays 62 have a vertical center-to-center separation V of one wavelength and are driven in phase, then there will be strong gain in the horizontal plane, with significant energy above the horizon, potentially capable of propagating long distances as a sky wave. If beam tilt is applied—i.e., the physical spacing V is less than a wavelength for a bottom-fed array—then less of the signal will propagate upward. This antenna occupies roughly one and a half wavelengths of vertical height, nearly 60 feet (over 17 meters), without allowance for gaps above and below.
Still other configurations and larger numbers of driven radiators 64 can be used to achieve further improvement, at cost of significant increases in overall antenna size and in complexity for power splitting and interconnection. For example, in the embodiment of
It is to be understood that a typical broadcast antenna tower is a conductive and largely unitary assembly built up from multiple tubes, channels, angles, plates, and the like, variously bolted and welded together, having individual segments of varying effective length. Such a tower may have an RF profile that is not configured to be specifically compatible with a given antenna design. As a consequence, the tower may present a variety of reflections with measurable effects on propagation characteristics of the antenna. Where desired, the effect of a particular tower design on far field signal can be simulated in the “method of moments” software previously referred to, modeling the construction of the tower and computing the effect of the presence of a specific tower on overall antenna gain versus elevation for every azimuth. Such a process may yield a tower reflectivity plot having a least squares centroid of reflection not coincident with the structural centroid, that is, the center of moments of the tower structure. A reflection axis identified for the tower may provide a useful term of reference. For typical antennas, the reflection axis or reflection centroid is likely to be a minor factor in overall broadcast performance; nonetheless, it may affect operation and may need to be determined for at least some applications.
Towers may be guyed or free standing. Where guyed, the guy wires are typically configured as multiple segments joined by insulators, with the lengths of the segments typically chosen for minimal interaction with radiated signals. Modeling and test of guy wires as well as the tower structure may be desired for some embodiments.
The multiple-radiator embodiments described above may be modified by adjusting the number and location of the radiating elements located around the tower to provide a pattern that is directional in the azimuth plane. This can be useful, for example, in circumstances wherein the transmitting tower is not located in the center of the target area, so that it is desirable or necessary to minimize radiation in unwanted azimuthal directions while maximizing radiation in directions intended to be served. For example, the embodiments shown in
It may be helpful analytically for some embodiments to assign a vertical mechanical reference axis of the antenna system that broadly coincides with a mechanical centroid of the tower, wherein the tower is understood to provide means for structurally positioning the antenna system. In at least some instances, assignment may be made of a vertical axis that reasonably approximates a centroid of RF signal reflectivity (i.e., a locus of mean squared reflectivity) of the tower, by azimuth, for impinging RF signals, where an applied signal wavelength approximates a median transmission wavelength of the antenna. Further, each impingement angle may be stipulated with respect to a line through the structural reference axis of the antenna system, with the centroid of reflection having been established by calculation, test, or product history. If an embodiment requires that distribution of RF signal energy emitted by the antenna system differ from being omnidirectional with azimuth, then the reference system described allows models to be developed for analysis of effects due to varying the placement of multiple radiators, varying applied power to individual radiators, and varying phase of the signal applied to each radiator. Such a process can realize a particular energy distribution with respect to the reference system, such as by modifying a system and analyzing the effect of such modification. Values such as the centroid may be developed by analysis, by testing on prototypes or production units, by accumulated data from history of multiple products, and the like.
Dipoles 72 and 92 in respective bays 94 and 96 are each roughly a half wavelength in physical length. Because the lower bay 96 is close to the upper, with just over a half wavelength between centers, the lower dipole 92 has reverse phase with respect to the upper dipole 72. By analysis and test, it can be shown that the lower dipole 92 can have instantaneous radiative signal strength on the order of 80% of that of the upper dipole 72 for some embodiments. For an optimized spacing P between the bays 94 and 96, again developed using the abovementioned “method of moments” antenna design software, then validated by prototype testing, a null can be developed in the range of +10 degrees to +30 degrees with respect to the horizon, which is the range most likely to be reflected and/or refracted to form a sky wave. This can leave a lobe roughly 10 dB below the main lobe, tilted up to about +45 degrees, which is generally not susceptible to ionospheric redirection and thus may represent an acceptable radiative energy loss without causing appreciable interference to distant radio services.
The two-bay embodiment of
Extension beyond the two-bay embodiment shown using more parasitic radiators, such as a third bay, is feasible, and may provide further refinement of beam shape in exchange for increased material cost and tower space. Positioning the active and parasitic bays a full wavelength apart (plus the beam tilt dimension) has both benefits, such as somewhat increased gain, and drawbacks, such as substantially the same increase in tower space as adding a third dipole. In some such embodiments, the two dipoles may be in phase rather than having opposite phase. Placing the active dipole 72 below the parasitic 92 produces slight variations in performance. As noted, interbay spacing with the active dipole 72 below must be slightly less than an integer number of half wavelengths instead of slightly greater. This may in turn require adjustment to dipole length and termination, affecting efficiency.
The embodiment of
The structure of
The many features and advantages of the invention are apparent from the detailed specification, and, thus, it is intended by the appended claims to cover all such features and advantages of the invention which fall within the true spirit and scope of the invention. Further, since numerous modifications and variations will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation illustrated and described, and, accordingly, all suitable modifications and equivalents may be resorted to that fall within the scope of the invention.
This application claims priority to U.S. Provisional Application entitled, “Description of Antenna for Transmitting Short Wave Signals in the 26 MHz Band”, filed Jul. 22, 2005, having Ser. No. 60/701,511, which is hereby incorporated by reference in its entirety.
Number | Date | Country | |
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60701511 | Jul 2005 | US |