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Broadcasting, IEEE Transactions on

Issue 2 • Date Jun 1989

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Displaying Results 1 - 21 of 21
  • Digital AM transmitters

    Page(s): 131 - 133
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    Digital amplitude modulation (AM) generates a high-quality AM signal with an overall efficiency of greater than 80%. Digital AM was introduced in March 1987 with a 10-kW solid-state transmitter. Also completed are 25-kW and 50-kW transmitters, and plans exist for transmitters up to 2000 kW. The digital AM system utilizes a high-speed 12-bit analog-to-digital converter (A/D), a digital modulation encoder, and a power-multiplying digital-to-analog converter. The latter reconverts the digital information back to analog to form an AM signal. Use of this system results in an AM transmitter with outstanding efficiency, audio performance, and reliability View full abstract»

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  • Relationship between the electrical height of medium-wave antenna and its mechanical dimensions

    Page(s): 241 - 247
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    The design is studied of the mechanical element implementing the wanted height, once the optimum electrical height of an antenna for medium-wave transmission is determined. The problem of how to determine the velocity factor is complex and not yet completely solved in mathematical terms, particularly as regards nonresonant antennas and having a height near to the half wave. The mean factor of velocity is obtained, taking into account both the termination effect caused by top charges on the terminal section of the antenna and the mean phase velocity for a distribution different from the sinusoidal one, but of the real current. The mean factor of velocity, calculated in this way, has a value that not only depends on the height-to-radius-ratio of the tower, but also varies when the antenna height or the frequency varies, namely as a decrease toward resonance. Thus, for a fixed electrical height, the tower should be shortened proportionately to how low this ratio is and the nearer to the resonance the height of the antenna is View full abstract»

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  • Prudent frequency management through accurate prediction of skywave field strengths

    Page(s): 208 - 217
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (748 KB)  

    A nonexhaustive chronological summary of the advances made in MF skywave propagation studies is presented with emphasis on progress made in the United States. A qualitative analysis of the documented methods for calculating skywave field strengths and a quantitative comparison of selected methods with measured data are also included. Methods compared include the FCC clear-channel curves (the method for ITU Region 2), the Cairo North-South curve (the official method for the Asian part of Region 3), and the J.C.H. Wang (1985) method (the proposed modified FCC method). Field-strength values calculated by using these methods are compared with data collected in different parts of the world View full abstract»

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  • Dynamic carrier control, DCC, a valuable method to save input power of medium wave transmitters

    Page(s): 134 - 138
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    A description is presented of the method of dynamic carrier control (DCC), which is characterized by a high margin of safety against adjacent-channel and common-channel interferences, together with remarkable savings in consumption. The special characteristics of this method are presented, along with the technical solutions for PSM and class B modulated transmitters. Examples of recent modifications of older transmitters to DCC are described as well. Experience gathered during various tests on a test platform and at different users' sites is reported View full abstract»

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  • MF sky-wave propagation: a comparison of measurements made at sunset and sunrise

    Page(s): 200 - 203
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    The strength of MF skywave signals is greatest at night and weakest during the day. The increase in field strength which occurs at sunset has been extensively studied but less is known about the decrease which occurs at sunrise. A description is given of a series of measurements made by members of the European Broadcasting Union (EBU), which cover both the sunrise and sunset periods. The results show that during the sunset period the measurements agree closely with the existing CCIR curve but that the sunrise measurements indicate that the existing CCIR curve is inaccurate. A modified curve for MF use is proposed View full abstract»

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  • Directional MF antennas using self-supporting towers with driven wire cages

    Page(s): 193 - 199
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    An examination is made of the synthesis of directional high-power MF antennas that comprise grounded self-supporting steel towers with driven wire cages as radiating elements. Advantages of this radiator concept are discussed. Computer models of such radiator for the method of moments (MM) analysis of synthesis for an antenna of n radiators is achieved by (1) MM analysis of the antenna for n independent modes, and (2) applying an optimization procedure to determine n complex coefficients for superposition of the modes to obtain an optimal approximation to a specified pattern. Aspects for selecting the modes are discussed. Two examples, namely, antennas with two and four towers, and the theoretical models of their radiators are presented. The patterns measured by helicopter show very close agreement to the theoretical patterns View full abstract»

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  • Medium-frequency skywave propagation at high latitudes: results of a five-year study

    Page(s): 218 - 222
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    Skywave transmission from selected standard AM broadcasting stations in the northern tier of the contiguous United States, Alaska and Canada were monitored near Fairbanks, Alaska from late 1981 to 1987. The objective of this work was to better understand, among other things, the effect of latitude on MF skywave propagation. A modern stable and selective receiver was electronically scanned continuously through 12 preselected frequencies in a five-minute cycle including a calibration. Frequent aural monitoring insured that the correct frequencies were being monitored. The main antenna used during this program was a 106-ft (35-m) vertical with modified top-hat and a 30-radial ground system. The daily, seasonal, and magnetic storm-time behavior of signal strength from selected stations was monitored over one-half of sunspot cycle 21. The latitudinal location of the auroral oval in the E-region in respect to the ionospheric reflection points on the propagation paths created profound variations of signal strength. There was also a very wide variation in signal strengths on long north-south paths as a function of sunspot cycle. The results are briefly discussed View full abstract»

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  • Impedance stability and bandwidth considerations for elevated-radial antenna systems

    Page(s): 167 - 171
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    An examination is made of the variations in feed-point impedance that occur in an elevated antenna system due to changes in the electrical characteristics of the underlying soil. The bandwidth of such antennas is also investigated by looking at deviations in the input impedance as the operating frequency is varied about its nominal value. In all cases, results for the elevated antenna are compared with those of the conventional ground-mounted system. The computer program used for this work was NEC (Numerical Electromagnetics Code), a method-of-moments software package View full abstract»

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  • Relative tower currents and fields in an AM directional array

    Page(s): 176 - 184
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (640 KB)  

    With the help of method-of-moments techniques, the antenna monitor parameters that give the correct pattern can be calculated from the field parameters. The base voltages can be computed from the linear relationship between the relative fields and the base voltages. The computed voltages can then be used as sources for MININEC III. The base currents computed by MININEC III for those base voltages are used to determine the antenna monitor parameters that result in the correct directional antenna (DA) pattern. Calculating the antenna monitor parameters that give the correct DA pattern means that trial and error field adjustment is not necessary in those situations where reradiating objects are not present. Thin reradiating objects, such as AC power transmission towers, can be included in the computation when they can be accurately located and described. Detuning impedances from the floated towers to ground are easily calculated from the output of the MININEC III program. Examples of the use of this technique are presented View full abstract»

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  • Implementation of amplitude modulation companding in the BBC MF national networks

    Page(s): 139 - 146
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (684 KB)  

    The design and development of a modulation-controlled system known as amplitude modulation companding (AMC) is described. In this method both carrier and sideband levels are progressively reduced as the modulation level is increased, this effect being compensated in the receiver by the normal action of the automatic gain control circuit. Extensive trials of a fully developed system for 50-kW MW transmitters have achieved input power savings of between 20% and 40%, depending on program content. This is achieved with imperceptible loss of coverage, and further savings may be possible View full abstract»

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  • Analysis of a sectionalized tower as an element in a medium wave phased array using the method of moments

    Page(s): 185 - 189
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    The use of method-of-moments techniques to obtain operating base parameters for a medium wave directional array is described. The array incorporates a 130' tall antenna element that is sectionalized 75' above the base. A matrix of impedance measurements is used to obtain input data for the MININEC analysis of the array. The MININEC output provides base operating parameters for the appropriate field values required to produce the desired pattern. When these values were used, the resulting measured antenna pattern was in close agreement with the original theoretical pattern which had been calculated using traditional assumptions View full abstract»

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  • Broadcasting with megawatts of power: the modern era of efficient powerful transmitters in the Middle East

    Page(s): 121 - 130
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (880 KB)  

    A description is given of the 1- and 2-MW transmitters developed, built, and installed in the Middle East to satisfy requirements that provide for coverage of vast areas that can best be reached with high-power AM transmissions. The discussion also covers: early high-power developments; the Doherty high-efficiency screen-modulated amplifier; the overall power amplifier circuit and its operation; the modulator circuit; antennas; ground systems; and transmission lines. Of of the nine transmitters installed, four have already seen ten years of service and the remaining five are close to reaching a decade of dependable operation View full abstract»

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  • Short low loss AM antenna

    Page(s): 237 - 240
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (296 KB)  

    An exploration is made of ways to improve the performance of a relatively small transmitting or receiving AM antenna system. Short vertical towers can be top-loaded to increase the radiation resistance and lower the base input capacitive reactance. An insulated counterpoise is connected to the coupling unit and to ground through an inductance which is tuned so as to maximize the field strength radiated in the far field View full abstract»

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  • Solar activity and MF skywave propagation

    Page(s): 204 - 207
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (240 KB)  

    An examination is made of the effect of solar activity on MF skywave propagation, and quantitative expressions describing the effect are presented. It is shown that these equations are valid for all latitudes and for the two commonly used reference hours, two and six hours after sunset. Until recently, it has been assumed that the effect of solar activity was more or less the same for all hours of the day. However, the studies reported show that the effect is the greatest during the hours from sunrise to two hours after sunset. During these hours, the reduction of field strength is a function of sunspot number, geomagnetic latitude, path length and frequency. Beginning at two hours after sunset, the effect diminishes with time, reaching a minimum at about six hours after sunset. At six hours after sunset, frequency ceases to be a factor View full abstract»

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  • A proposal of a traffic information signalling compatible with AM-MF broadcasting

    Page(s): 156 - 166
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (716 KB)  

    The possibilities for information signaling through two additional tones, coherent with the usual AM stereo pilot tone used in the present USA AM stereo systems, are investigated. Two criteria are defined: an objective one, based on US Federal Communications Commission rules, and a subjective one, based on average human audio perception levels. On the basis of these criteria, the width of the injection gap (to inject desired two tones) is evaluated. The signaling method includes two signals besides the pilot tone: one for program identification and the other for message-in-progress. The primary intention of the signaling is coupled to the traffic radio information service, but without any modification the proposed system could be used for other purposes (paging, alert signaling, exact time distribution, etc.) View full abstract»

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  • A high power solid-state medium wave broadcast transmitter

    Page(s): 118 - 120
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    The factors leading to the choice of an all-solid-state 50-kW medium-wave broadcast transmitter are examined, and its advantages over vacuum-tube-type transmitters of the same power are discussed. The installation and operation of the transmitter are described. The transmitter has demonstrated high levels of performance and reliability. Energy savings and reduction of maintenance times have been achieved View full abstract»

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  • Matrix method for relating base current ratios to field ratios of AM directional stations

    Page(s): 172 - 175
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (216 KB)  

    A matrix method for relating the field ratios to the voltage sources is presented. As a moment-method program is running, the field and base current for each tower is recorded. Matrices that relate base currents to field ratios, can be extracted from this data or by inverting a matrix. This method is an exact solution and requires only one run of the moment-method program. For this reason, it is superior to other iterative processes View full abstract»

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  • Moment method predicted impedances compared to actual measured impedances of directional arrays

    Page(s): 190 - 192
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    A comparison is made of the self and mutual impedances predicted by a moment method program to show how it derives the Z matrix. A discussion of the theory of these impedances if offered as a foundation to analyze the results. The comparison shows that the field measurements support the data generated by the program View full abstract»

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  • Performance measurement and test techniques for modern AM broadcast transmitters

    Page(s): 147 - 155
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    The numerous tests required in evaluating the performance of a modern AM transmitter are discussed. Both laboratory tests of novel products and environmental tests are covered. The latest attainable performance measurements, environmental testing requirements, and field condition simulation tests are examined View full abstract»

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  • Computer analysis of vertical radiation patterns of dipole panel array antennas

    Page(s): 227 - 236
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    A GW BASIC computer program has been developed to analyze any azimuth vertical radiation pattern of a dipole panel array antenna, fixed on a square tower structure, under different conditions of horizontal progressive quadrature current phase shifting, mechanical panel displacement, beam tilt, feeding current coefficients (null filling) and asymmetric arrangements with and without panel centering. The final effects are singled out by comparing vertical radiation pattern diagrams corresponding to each condition. The result obtained in regard to simultaneous horizontal progressive current phase and mechanical shift show appreciable variations in the vertical radiation patterns, corresponding to arrays of one or two dipole per tower face (omnidirectional arrangement), while for more elements there is a negligible effect on the main beam, but noticeable changes on the sidelobes. On the other hand, for any omnidirectional arrangements under the forementioned conditions of current phase and mechanical shift, almost no vertical radiation is produced by an array of eight dipoles at 0° along the tower (compensation effect). For an array with binomial current coefficients, almost no changes on the beam pattern has been observed View full abstract»

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  • Medium wave skywave beams

    Page(s): 223 - 226
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    The author discusses the advantages and drawbacks of skywaves and why they are avoided by domestic medium-wave broadcaster. He reports some results of a computer investigation of problems encountered with medium-wave antennas coupled to a skywave propagation medium. Experimental results are given showing skywave patterns for broadcasts from Alaska, one using a horizontal and the other a vertical antenna. It is concluded for these two examples that remarkable medium-wave coverage could be obtained using available frequencies View full abstract»

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IEEE Transactions on Broadcasting covers the field of broadcast technology, including the production, distribution, transmission, and propagation aspects of broadcasting.

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Yiyan Wu
Communications Research Ctr Canada