Abstract:
Each of a pair of antennas for broadcasting has multiple elements arranged vertically on the same tower. The antennas transmit circularly polarized signals of opposite polarization. The opposite circular polarization of the radiated signals increases their mutual isolation and permits broadcast of conventional FM-band signals and digital FM at the same frequency. The polarization technique allows the elements of the two antennas to share an aperture without degradation of function.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates generally to radio frequency electromagnetic wave (RF) transmission equipment. More particularly, the present invention relates to an apparatus and method for broadcasting two FM radio signals at the same frequency using the same aperture space.  
         BACKGROUND OF THE INVENTION  
         [0002]    FM radio is in wide use in the field of radio broadcast. The term FM includes, for example, any of the Frequency Modulation methodologies used or developed for signal broadcasting in a frequency band assigned by the U.S. Federal Communications Commission (FCC), nominally in the transmission range 88 MHz to 108 MHz, which is near the middle of the Very-High-Frequency (VHF) television broadcast band. These Frequency Modulation technologies include both analog FM and digital FM.  
           [0003]    The radio industry and the FCC have at present standardized on the iBiquity® IBOC (In-Band-On-Channel) hybrid analog-digital transmission system. This system permits FM stations in the U.S. to broadcast analog and digital signals simultaneously on their currently allocated channel frequency, if they use a single antenna to perform the simulcast.  
           [0004]    At present, all U.S. FM radio transmission channels are 200 KHz wide, with standard analog FM broadcast modulation occupying only the center 100 KHz of the channel and with the IBOC signal using the outer 50 KHz on each side of the analog part of the channel. This characteristic of the IBOC signal imposes a need for sharp-cutoff filters to maintain signal separation, both between adjacent channels and between the analog and digital portions of the transmission on a single channel.  
           [0005]    As an additional consideration, the FCC stipulates that the transmitted digital signal is to be 20 dB lower in signal strength than the analog signal. This may intrinsically place the digital transmitting antenna in a field as much as 10 times stronger than its own transmission.  
           [0006]    One method of achieving an IBOC simulcast is to use two separate transmission systems feeding into two separate antennas on a single tower. Since the vertical position at which an antenna is mounted on a tower directly affects the antenna&#39;s achieved coverage, it would be desirable to collocate the analog and digital antennas not only on the same tower, but also at the same height above the ground. Further, since the azimuth pattern of an FM antenna is highly dependent on the interaction between the radiating device and the cross section of the tower structure, it would be desirable to mount both the analog and digital antennas in the same orientation to the tower.  
           [0007]    When adding digital FM coverage to towers already in use for analog FM, a concern arises because many towers are full—that is, the towers have no additional aperture space available—so that some FM broadcasters may be required to interleave a second antenna within the aperture of their existing antenna. This introduces a challenge, because the analog and digital signals occupy the same segment of the frequency spectrum, yet are required to be isolated from each other. The current requirement for isolation between the IBOC digital signal and the analog signal is on the order of 35 dB. If the IBOC and analog antennas are to share the aperture, it is desirable to provide satisfactory isolation so that filtering requirements are kept within desirable ranges.  
           [0008]    Accordingly, there is a need in the art for a method and apparatus to achieve isolation between separate in-channel FM antennas sharing common aperture space.  
         SUMMARY OF THE INVENTION  
         [0009]    Preferred embodiments of the method and apparatus achieve isolation at least to some degree between separate in-channel FM antennas sharing common aperture space, employing two antennas that are circularly polarized with opposite orientations.  
           [0010]    In a first aspect, an enhanced-isolation shared-aperture digital and analog FM antenna pair is comprised of two independent circularly-polarized FM transmitting antennas on a tower. In another aspect, each of the two antennas has at least one element, where each element of each antenna can radiate a circularly-polarized RF broadcast signal. In still another aspect, each of the two antennas has a plurality of substantially identical, independently-mounted, individually driven elements spaced vertically along the tower. In yet another aspect, elements of one of the antennas are symmetrical and opposite to the elements of the other antenna, so that the elements of one of the antennas, when driven, radiate a left-hand circularly polarized signal, and the elements of the other antenna, when driven, radiate a right-hand circularly polarized signal. In another aspect, the locations of the elements comprising the first antenna are interleaved with the locations of the elements comprising the second antenna.  
           [0011]    In another aspect, an apparatus for transmitting digital and analog FM radio signals from a common aperture space comprises means for radiating a first FM signal with a first circular polarization and means for radiating a second FM signal with a second circular polarization opposite to that of the first signal. Such an apparatus may be further comprised of means for accepting a first broadcast-level signal from a transmission line and means for distributing the energy of the first broadcast-level signal among multiple transmitting elements with signal-level balance and phase relationships required to create a first circularly-polarized transmission, as well as means for accepting a second broadcast-level signal from a transmission line and means for distributing the energy of the second broadcast-level signal among multiple transmitting elements with the signal-level balance and phase relationships required to create a second circularly-polarized transmission with polarization opposite to that of the first signal.  
           [0012]    In yet another aspect, a method for simulcasting analog and digital FM broadcasts from a single aperture space comprises the steps of driving a first antenna with a first circularly-polarized signal at a particular channel frequency and driving a second antenna with a second circularly-polarized signal at the same channel frequency, where one of the signals is an analog transmission and the other is a digital transmission, and where the polarizations of the two signals are opposite.  
           [0013]    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.  
           [0014]    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.  
           [0015]    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. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]    [0016]FIG. 1 is a schematic diagram of a transmission system combining analog and digital FM radio broadcast signals in an IBOC environment.  
         [0017]    [0017]FIG. 2 is a more detailed view of the two antennas and the associated tower-top apparatus used for a combined IBOC dual broadcast system.  
         [0018]    [0018]FIG. 3 is a diagram of a single circularly polarized multi-element antenna for use in an analog-only or a digital-only (non-IBOC) environment.  
         [0019]    [0019]FIG. 4 is a diagram of an interleaved pair of circularly polarized multi-element antennas configured for opposite-polarization transmission in an IBOC environment. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0020]    Preferred embodiments of the invention provide a method and apparatus for achieving isolation at least to some extent between separate in-channel FM antennas sharing common aperture space. Preferred embodiments of the invention will be described with reference to the figures, in which like reference numerals refer to like elements throughout.  
         [0021]    [0021]FIG. 1 shows an FM radio transmission system including a single content source feeding two complete signal paths. A digital programming source  10  provides a digital signal stream  12 . The digital signal stream  12  feeds a digital transmitter  20  directly. The output of the digital transmitter  20  feeds a circulator  22  with an associated dummy load  24 .  
         [0022]    After processing of the digital signal stream  12  with digital-to-analog conversion  26  (D/A), the analog signal feeds an analog transmitter  32 . The full-power analog signal may drive its antenna  46  without a circulator, since its signal level is far higher than the digital signal level under current FCC regulations and the added isolation is superfluous.  
         [0023]    The digital transmitter  20  and analog transmitter  32  outputs can send their respective signals independently up a tower  38  using a digital signal coax  40  and an analog signal coax  42 . Once the digital and analog signals are present near the digital and analog transmitting antennas  44  and  46 , they may be fed into a passive digital power divider  48  and a passive analog power divider  50 , respectively, in a configuration known in the art as branch or corporate feed. The outputs of the digital power divider  48  are distributed, using individual digital feed lines  52  that are preferably equal in length, to the respective digital antenna elements  54 . Similarly, the outputs of the analog power divider  50  are distributed, using individual analog feed lines  56  that are preferably equal in length, to the respective analog antenna elements  58 .  
         [0024]    A power divider, as the term is used here, is for example a passive device that divides an input into a series of lower-energy duplicates of the original signal, in phase with each other but delayed by the intrinsic propagation time of the device. The exact timing of each of the divided signals may be adjusted with respect to the others by precise control of the length of the feed coax from the power divider to the individual radiating elements. Making the delays to the individual radiating elements unequal can adjust the beam tilt—the energy distribution as a function of the angle to the horizontal—of the radiated signal, and thereby affect the signal&#39;s reception range.  
         [0025]    A circularly polarized signal transmitted as described above is detectable either by a suitable circularly polarized receiving antenna, namely one with the same handedness as the transmitting antenna, or by a linearly polarized receiving antenna, which has less gain with respect to the signal than does a same-handed circularly polarized antenna, but far higher gain with respect to the signal than does an oppositely-handed circularly polarized receiving antenna.  
         [0026]    [0026]FIG. 2 provides a more detailed view of the items located at the top of the tower  38 . A feed from the digital signal power divider  48  via digital signal coaxial feed lines  52  energizes digital radiating elements  54 . Similarly, a feed from the analog power divider  50  via analog signal coaxial feed lines  56  energizes analog signal radiating elements  58 .  
         [0027]    The signal energy may also be distributed directly up the tower  38  with tee junctions, a configuration known in the art as series feed, illustrated in FIG. 3, which shows a single, non-IBOC antenna. FIG. 4 adds a second radiating arrangement of opposite polarization to form an IBOC-compliant combination. FIG. 4 shows on the lower of the two digital elements  54  a fitting that attaches the lower digital element  54  to the tower  38  while passing around and making no electrical contact with the analog coaxial line  56 . FIG. 5 shows the same elements enlarged, with the antenna coupling fitting  66  coupling the analog coax  56  to an analog antenna element  58  and the bypass fitting  68  allowing a digital antenna element  54  to be mounted in its preferred location without electrical contact to the analog coax. The digital elements  54  in FIG. 4 are fed by separate coaxial lines within the figure; whether their feed is series or branch is not shown. Series feed causes each of the elements to be excited with a signal delayed by one cycle from the previous element, a characteristic that can have no appreciable effect on the received FM radio signal. The difference shown in FIGS. 3 and 4 in the relative size of the analog coaxial line  56  and the digital coaxial lines  52  illustrates the tenfold greater power that can be present in an IBOC-compliant system&#39;s analog signal. This power differential can permit a preferred embodiment for the digital signal to incorporate a smaller, lower-cost coaxial line with reduced wind loading, fewer joints, and easier installation, yet meet system requirements.  
         [0028]    Where the elements  58  of the analog antenna are spaced one wavelength apart as shown in FIG. 3, the analog output comprises a single circularly polarized transmission with acceptable uniformity around the tower  38  despite the presence of the conductive tower structure. Polarization may be a function of antenna element  58  design, so that similar antenna elements of opposite handedness will radiate circularly polarized right-handed or left-handed signals.  
         [0029]    Variations in vertical spacing between elements  58  can determine in part the characteristics of the beam pattern generated. Elements  58  spaced uniformly at one wavelength increments can produce a pattern at right angles to the tower, while elements  58  with spacing other than one wavelength, such as {fraction (9/10)}, ⅘, ¾, and the like, can be used to reduce excessive downward radiation.  
         [0030]    [0030]FIG. 4 illustrates the interleaving of digital antenna elements  54  at one-half-wavelength spacing with respect to the analog elements  58 , which establishes one-wavelength spacing between the digital antenna elements  54  themselves. This places the center of the aperture for the digital antenna within the aperture of the analog antenna, and nearly coincident with the center of the analog aperture. If the digital antenna elements  54  are designed to radiate a circularly polarized signal of opposite polarity to the corresponding analog apparatus, then there can be an intrinsic improvement, for example on the order of 10 dB, in the isolation between the digital and analog transmissions when compared to using two antennas of like placement but with the same polarization as each other. This represents a significant portion of the isolation required for collocated transmitting antennas at the same frequency, and can help reduce the filter and circulator hardware size and cost that would otherwise be required in implementing an IBOC system.  
         [0031]    Spacing the digital antenna elements  54  equidistant between the proximate analog antenna elements  58  shown in FIG. 4 can minimize coupling of the analog signal to the digital line, which can in turn minimize the size of the apparatus needed in order to remove the signals coupled thereto.  
         [0032]    In the example in FIGS. 3 and 4, two elements of each of the digital antenna  44  and the analog antenna  46  of FIG. 1 are shown. Each element operating alone can create a circularly polarized signal, while adding more elements can increase range by increasing total radiated power capability and by increasing the directivity of the radiation pattern. Using a larger number of elements, for example up to about twelve in each antenna, is useful in some environments and will typically produce improved performance. Using large numbers of elements may incur greater complexity and necessarily takes up more physical height, the latter of which translates to a greater share of the typically limited aperture space within the confined environment of a transmission tower  38 .  
         [0033]    Alternative embodiments of the invention may use only one element per antenna. In such embodiments, the apertures by definition do not overlap.  
         [0034]    Achievement of the full 35 dB of isolation between the analog and digital transmissions in an IBOC system may require that the intrinsic 12 dB isolation of the two signals and the added 10 dB gained through use of oppositely polarized antennas be: augmented by the use of a circulator or equivalent function in the digital transmitter signal path.  
         [0035]    Circulators, such as the digital signal path component  22  in FIG. 1, are passive devices that can allow RF signals to advance one node around a directional multi-port fitting with acceptable power losses. Following the digital signal path in FIG. 1, outgoing RF from the digital transmitter  20  is allowed by the circulator  22  to advance from that circulator&#39;s first port  60  to its second port  62 , which leads to the digital-signal transmission line  40 . The digital-signal transmission line  40  in turn leads to the digital-signal antenna  44 . Coupled energy from the analog antenna  46 , as well as returning RF from other sources, such as reflections from connectors, antenna mismatches, and the like can travel in the direction opposite to the transmitted signal in the digital-signal transmission line  40 . Such energy reenters the circulator at its second port  62  and advances to its third port  64 , having been deflected by the circulator  22  from the digital-signal transmitter  20 . The third circulator port  64  feeds to a dummy load  24 , which transforms the unwanted energy to heat.  
         [0036]    Since the digital signal may be 20 dB lower in signal strength than the analog signal, and the 12 dB intrinsic isolation and 10 dB added isolation of the invention may further attenuate digital signal energy coupled to the analog path, a circulator placed in the analog signal path may not be needed for a preferred embodiment.  
         [0037]    Numerous styles of antenna elements can intrinsically radiate circularly polarized signals and are thus suitable for simulcasting an analog and a digital signal in a single aperture. Still other styles that do not intrinsically radiate circularly polarized signals can be forced to create such signals when driven by properly configured signals. Any pairs of antennas composed of a plurality of elements per antenna, capable of being configured to radiate oppositely circularly polarized signals, and further capable of being interleaved on a tower with their electrical centers located within +/−2 meters of each other, can potentially be incorporated into a system as described in the present invention.  
         [0038]    A preferred embodiment of the invention uses ring-style antennas. In this embodiment, the helical direction in which the dipoles comprising the separate circularly polarized ring-style antenna elements are wound is opposite between the digital and analog antennas, effectively interleaving right-hand and left-hand polarized antennas in the same aperture. This achieves the required high level of isolation between the antennas collocated in the aperture.  
         [0039]    Unlike the situation for broadcast television, current FCC regulations on FM radio transmission (e.g. 47 CFR 73.316) do not distinguish between right-hand and left-hand circular polarization. While horizontal polarization is standard, either right-hand or left-hand circular polarization is an acceptable alternative under current FCC regulations, as long as the total effective radiated power remains within the licensed limit. Further, it can be demonstrated that a right-hand circularly polarized antenna will exhibit significant rejection of any left-hand polarized signal and vice versa. This observation leads to an approach to increasing isolation.  
         [0040]    An inherent advantage to increasing the isolation between the antennas is a reduction in mutual coupling. When a high level of isolation exists, the second antenna can be placed in the aperture of an existing antenna with minimal effect on the match of the existing antenna, thus potentially reducing field adjustment after installation. Since field adjustment may require repeatedly climbing the tower, energizing and deenergizing the transmitters, and painstakingly adjusting the apparatus, the process may be time consuming and costly. As such, it should be avoided if such avoidance is practical.  
         [0041]    In comparison to more conventional techniques, interleaving oppositely-circularly-polarized antennas within an aperture can, in some embodiments, achieve an extra 10 dB of isolation.  
         [0042]    Although the preferred embodiment is described for use with FM radio, application of the invention to other frequency bands and other modulation methodologies is possible.  
         [0043]    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 that 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, falling within the scope of the invention.