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ELECTRONICS. RADIO ENGINEERING
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PASS-THROUGH FREQUENCY RESPONSE OF THE "TRANSMITTER-RECEIVER" PATH IN LONG WAVE DIGITAL BROADCASTING
Vladimir B. Belyansky,
associate professor of the Moscow Technical University of Communications and Informatics, Moscow, Russia, [email protected]
Keywords: Digital broadcasting, LW range, antenna matching unit, SWR, band quality factor, Chu-Harrington limit, antenna matching unit width losses, effect of adding capacity in the air.
Evgeniya D. Pronina,
Graduate of the Moscow Technical University
of Communications and Informaticss, Moscow, Russia,
The complex frequency characteristics of the "transmitter-receiver" signal path of a digital long wave spectrum broadcast determine the transmission quality, energy conversion efficiency and electromagnetic compatibility (EMC) parameters of the system. Qualitative indicators of the path are primarily determined by the heights of the antennas being used, with a high quality (Q) factor being inversely proportional to the cubic degree of antenna heights. Therefore, a significant factor in determining the quality of the path is the use of a well-designed antenna mathing unit (AMU). An alternative to the use of AMU with accurate impedance matching may be the adjustment of the amplitude-frequency and phase-frequency characteristics of the signal, especially since modern broadcast transmitters have a frequency characteristic correction unit in the modulator. Reducing the antenna height, which significantly affects the cost of the entire system, is also possible by using antennas which overcome the Chu-Harrington limit and by implementing the effect of adding capacity in the air. Thus, the use of separate transmitters and antennas for each group of sub-carriers in a system with OFDM allows the reduction of antenna heights by almost half. We emphasize the importance of adequately normalizing path parameters by taking into account the qualities of specific transmitters, as this can significantly optimize technical and economic factors.
Для цитирования:
Белянский В.Б., Пронина Е.Д. О сквозной частотной характеристике тракта "передатчик-приемник" цифрового радиовещания длинноволнового диапазона // T-Comm: Телекоммуникации и транспорт. 2017. Том 1 1. №2. С. 63-67.
For citation:
Belyansky V.B., Pronina E.D. (2017). Pass-through frequency response of the "transmitter-receiver" path in long wave digital broadcasting. T-Comm, vol. 1 1, no.2, рр. 63-67.
1. General review
Digital LW range broadcasting is an important factor in civil society because of its unique features. The degree of broadcasting effectiveness exemplified by the history of MW radio band introduction in the last century has been very significant in cases of high-quality reception, but negligible when listeners switched to different wavelengths due to poor reception.
The complex frequency characteristics of the "transmitter-receiver" signal path of a broadcasting system determine the transmission quality, energy conversion efficiency and electromagnetic compatibility (EMC) parameters of that system. Qualitative, technical and economic indicators of the system are primarily determined by the heights of the antennas, which are very bulky and expensive in the LW range, as well as by the parameters of the antenna feed system and characteristics of the transmitting device. Therefore, for the development of a broadcasting system in the LW range, it is important to address the systemic issues which determine the complexity of the various system components; the utilization of a particular mode of the transmitter can lead to undue complexity in the antenna system, and vice versa.
In this respect, publication [11] is typical, featuring very strict requirements for the SWR of the system at the operating frequency band (SWR=1.05) and the control frequency band (SWR-1.1). Typically, the realization ofSWR=I.05 will require the use of calibrated elements in an impedance matching device with implementation precision of a fraction of a percent. Furthermore. high SWR requirements in the control frequency band in some cases are excessive, as will be shown below. The redundancy of this type also leads to a sharp increase in the cost of the antenna system, as in reference [8], in which the control band is twice the size of the operating band. Therefore, addressing the systemic issues that determine the requirements for the various elements of the path is an important task. Next we will consider some particularities of the "transmitter-antenna" path of digital LW broadcasting.
2. Antenna systems
The minimum possible Q factor of antennas in the LW range is constrained by the fundamental Obi-Harrington limit [ 1; 2; 3]. This quality factor is inversely proportional to the cube of the antenna's height. Due to the high cost of very tall antennas, the desire to reduce the antenna's height is natural [4; 5; 9; 10J. This can be done using a high efficiency AMU, or by considering the design features of the antenna. In [9J it is shown that by using impedance matching distributed over the volume occupied by an antenna, it is possible to create a small-sized antenna system that approaches or overcomes the Chu-Harrington limit. Fig. la shows an example of this type of antenna: a volumetric log-periodic antenna with a height of about 120-150 meters and an operating frequency bandwidth of 10 kHz on the lowest frequency of the LW spectrum (fc = 150 kHz). Fig. lb shows another variant of a small-sized antenna: a top-fed antenna. With current technology, it is relatively easy to make a top-fed antenna with an effective eapacitive load by using ultra-light, retractable, flexible profiles and thin, heavy-duty thread. Reducing the height of the antenna system is also possible by implementing effect of adding capacity in the air.
Thus, the use of separate transmitters and antennas for each group of sub-carriers in a system with OFDM allows the reduc-
tion of antenna heights by almost half. Fig. 2 shows a diagram of an antenna system consisting of eight individual antennas.
30 M
d
K
1
\
Fig. la. A log-periodic antenna with reduced dimensions 60.00
'///////'
/////////
30,00
Fig. lb. Top-fed antenna with Sliukhov Tower-1 ike support
2
Fig. 2. An antenna array: I - technical unit; 2 - antennas; 3 - transmitter; 4 - feeder
ELECTRONICS. RADIO ENGINEERING
k A
3. matching units
In [8J it is shown, that one can considerably improve the matching of an antenna with a feeder in a wide band of frequencies while introducing only a relatively small loss in the ATU. Lossless AMU are considered in [6; 7]. It should be noted that, as long as certain simple conditions are observed, it is theoretically possible to get perfect matching in unlimited frequencies. Indeed, for the L-shaped AMU illustrated in Fig. 3, the input impedance Zm from terminals 2-2 will be equal to
2 =R when -ao <<a<oo,
if
Z2 =
R
■R.
(1)
Therefore, for this system the implementation of different SWR values in the working and control frequency bands [8, 11] is redundant, since if condition (1) is followed, we can get SWR = 1 in unlimited frequencies. Moreover, slight deviations from this condition worsen SWR but do not substantially impact the quadrupole efficiency, while AMU efficiency depends essentially on the Q-factor of the Z|+Ra chain. Efficiency values of an AMU at Q = 10 and Q = 20 are shown in Fig, 4. The figure shows that the depicted AMU practically does not change the "AMU-antenna" system passband, while improving the matching between the antenna and transmitter. This may be an important factor when accounting for the specific conditions of a particular type of transmitter. In any case, when using the depicted AMUs, one must take steps to correct the frequency characteristics of the signal path.
antenna's input impedance. As is well-known, for electrically small antennas, this function Is quadratic. Furthermore, when the band Q factor is
a=y~>\> (2)
A/c
where Af is the operating frequency band of a path,and Af0 is an antenna's own frequency band at a value of 0.707,the AMU's energy conversion efficiency is sharply reduced.
It is possible to take into account all of the abovementioned factors of impedance matching by further developing the bipole system Z] and implementing bipole Z2. Fig. 5 shows a diagram of the ATU; the relation between its frequency and energy conversion efficiency is shown in Fig. 6. More information on the investigated lossy ATU will be presented in subsequent publications. Here we note that our investigation of lossy two-unit ATUs has not revealed any significant advantages over the one-unit ATU.
Fig. 3. L-shaped ATU
We can implement this by setting additional conditions on the characteristics of the Z] impedance (see Fig. 3) or by using a unit correcting the frequency characteristics in the transmitter, as proposed in [5], particularly since in modern transmitters such a correction is provided in the modulator unit. It is clear that if the value of module impedance Z| is small in the operating frequency band (see Fig.3), then the AMU will have a high transfer ratio in the operating frequency band, ensuring, moreover, good impedance matching in a fairly wide band of frequencies. In addition to this problem, one must also take into account other factors that complicate the implementation of the AMU. One such factor is the frequency function of the active component of the
100
30
r
r|(f,2o)
-c
wö
165
170 f
175
130
Fig. 4. The relation between efficiency and frequency for Q = 10 and O = 20
zi
Fig. 5. ATU with complicated Zl and Z2
T)(f)
nHf)
/•"-J ' r
/ S \
\
\
\ \
y
Fig. 6. The relation between efficiency and frequency for diagrams at figure 3 -ri(f) and figure 5 -T|l(f)
Conclusions
1. During the development of elements of the LW digital broadcasting system, it is important to adequately normalize path parameters by taking into account the qualities of specific transmitters, as this can significantly optimize technical and economic factors.
2. A one-unit lossy AMU provides accurate impedance matching in a wide band of frequencies, but usually requires adjusting the frequency characteristics of a path.
3. Using complicated bipoles in an AMU expands the path passband and increases "transmitter-antenna" efficiency.
4. Using effect of power addition on the air in the OFDM system may lead to a substantial increase in the values of the system's technical and economic indicators.
5. Modem technologies allow for the realization of LW range antennas with improved technical and economic indicators.
References
]. Fano R.M. Theoretical limitations of the band matching of an arbitrary impedance. Soviet Radio. M.: 1965, p. 70.
2. Chu L.J. Physical limitations of omni-directionaJ antennas // Journal of Applied Physics. №19, December, 1948.
3. Belyansky V.B. Is it possible to overcome the Chu-Harrington limit? // T-Comm. No. 8, 2013, pp. 24-27.
4. Belyansky V.B.. Proshin A.B.. Hudaykov K.N. LW, MW and SW spectrum digital audio broadcasting antennas of reduced dimensions // T-Comm. 2013. No. 8, pp. 28-29.
5. Belyansky V.B.. Harmush A.H., Proshin A.B.. Pronina ED. The equivalent Q factor of antennas operating at the first spherical harmonic and in keeping with Fano // T-Comm, 2015. Vol. 9, No,12. pp. 21-26.
6. Gainutdinov T.A., Garanina N.I., Kocherzhevsky V.G. Two-unit matching arrangement of long-wave radio antennas // T-Comm, 2015. Vol 9. No.6, pp. 48-56,
7. Gainutdinov T.A., Garanina N.I., Kocherzhevsky KG., Guseva A.S. Simple broadband matching arrangements of long-wave radio antennas // T-Comm, 2014. No J 1. pp. 33-39.
8. O.V. Varlamov, V.D. Goreglyad Bandwidth extension of LW transmitting, broadcasting antenna systems for operating in DRM mode //T-Comm, 2013. No. 1, pp. 18-22.
9. Belyanskiy KB., Pronina E.D. Small-sized transmitting antennas of the digital long wave broadcasting standard. // Proceedings of the North Caucasian branch of the Moscow Technical University of Communications and Informatics Materials Vlll-th International Youth Scientific and Practical Conference, 2015. pp. 226-229.
10. Pronina E.D. Modernization of LW spectrum broadcasting stations based oil the DRM standard // Proceedings of the North Caucasian branch of the Moscow Technical University of Communications and Informatics, 2014. No. 1. pp. 307-311.
i 1. Jochen Huber. DRM on MF and LF, coverage and technical requirements. EBU-DRM Conference. 26 Nov 2009 / Geneva (CH). URL, viewed 14 December 2016, http://tech.ebu.ch/docs/events/drm09/ prese n tat i on s/e bu_d rm09_huber.pdf.
T-Comm Tom 11. #2-2017
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ELECTRONICS. RADIO ENGINEERING
О СКВОЗНОЙ ЧАСТОТНОЙ ХАРАКТЕРИСТИКЕ ТРАКТА "ПЕРЕДАТЧИК-ПРИЕМНИК" ЦИФРОВОГО РАДИОВЕЩАНИЯ ДЛИННОВОЛНОВОГО ДИАПАЗОНА
Белянский Владимир Борисович, доцент МТУСИ, Москва, Россия, [email protected] Пронина Евгения Дмитриевна, аспирантка МТУСИ, Москва, Россия, [email protected]
Аннотация
Комплексная частотная характеристика тракта "передатчик-приемник" цифрового радиовещания длинноволнового диапазона определяет качество передачи, энергетическую эффективность передатчика и параметры электромагнитной совместимости (ЭМС) системы. Качественные показатели тракта в первую очередь определяются высотой используемых в настоящее время антенн, сравнительно высокая добротность которых обратно пропорциональна кубической степени высоты. Поэтому, существенным фактором, определяющим качество работы тракта, является использование правильно сконструированного согласующего устройства антенны (СУ). Так как расчет ^B СУ с резонансными цепями без потерь является некорректно поставленной задачей, в цепи СУ должны быть внесены виртуальные и реальные потери. Приводятся результаты исследования однозвенных и двухзвенных СУ с потерями (СУСП). Показано, что приближение полосовой добротности цепей однозвенного СУСП к единице приводит к резкому увеличению ^B или снижению КПД устройства. Увеличение числа звеньев СУ повышает требования к точности реализации элементов, поэтому использование числа звеньев больше двух при высоких требованиях к ^B (^B< 1,2) нецелесообразно. Альтернативой использования СУСП с высоким согласованием может явиться корректировка амплитудно-частотной и фазо-частотной характеристик сигнала, тем более что современные передатчики имеют в модуляторе блок корректировки частотной характеристики. Снизить высоту антенны, которая существенно влияет на стоимость всей системы, возможно также использованием антенн преодолевающих предел Чу-Харрингтона и реализацией эффекта сложения мощности в эфире. Предлагаемые антенны уменьшенных размеров могут быть не только более экономичными, но и характерны тем, что они упрощают задачу подавления внеполосных и побочных излучений. Уменьшение высоты антенн можно осуществить, используя антенны с верхней емкостной нагрузкой, которая в настоящее время можно достаточно эффективно реализовать, используя современные материалы - тонких пленок и разворачиваемых гибких профилей. Так, использование отдельных передатчиков и антенн в системе с OFDM для каждой группы поднесущих позволяет уменьшить высоту антенн почти в два раза. Отмечается важность адекватного нормирования параметров тракта с учетом особенностей конкретных передатчиков, что может существенно оптимизировать технико-экономические характеристики. Предлагается схема активного согласующего устройства, расширяющая рабочую полосу частот в два раза.
Ключевые слова: Цифровое радиовещание, ДВ диапазон, согласующее устройство, КСВ, полосовая добротность, предел Чу-Харрингтона, согласующее устройство с потерями.
Литература
1. Фано Р.М. Теоретические ограничения полосы согласования произвольных импедансов. М.: Сов. Радио, I965. 68 с.
2. L.J. Chu. Physical limitations of omni-directional antennas // Journal of Applied Physics. №I9, December, I948.
3. Белянский В.Б. Bозможно ли преодолеть предел Чу-Харрингтона? // T-Comm: Телекоммуникации и транспорт, 2CI 3. №8. С. 24-27.
4. Белянский В.Б., Прошин А.Б., Худяков К.Н. Антенны ДB, CB и ^ диапазонов цифрового звукового вещания уменьшенных габаритов // T-Comm: Телекоммуникации и транспорт, 2CI 3. №8. С. 28-29.
5. Белянский В.Б., Хармуш А.Х., Прошин А.Б., Пронина Е.Д. Эквивалентная добротность антенны, работающей на первой сферической гармонике и согласованной по Фано // T-Comm: Телекоммуникации и транспорт, 2CI5. Том 9. №I2. С. 2I-26.
6. Гайнутдинов Т.А., Гаранкина Н.И., Кочержевский В.Г. Двухзвенное согласующее устройство длинноволновых радиовещательных антенн // T-Comm: Телекоммуникации и транспорт, 2CI5. Том 9. №6. С. 48-56.
7. Гайнутдинов Т.А., Гаранкина Н.И., Кочержевский В.Г., Гусева А.С. Простые широкополосные согласующие устройства длинноволновых радиовещательных антенн // T-Comm: Телекоммуникации и транспорт, 2CI4. №II. С. 33-39.
8. Варламов О.В. Расширение полосы согласования передающих вещательных антенных систем диапазона ДB для работы в режиме DRM // T-Comm: Телекоммуникации и транспорт, 2CI3. № I. С. I8-22.
9. Белянский В.Б., Пронина Е.Д. Малогабаритные передающие антенны длинноволнового диапазона цифрового стандарта радиовещания // Труды Северо-Кавказского филиала Московского технического университета связи и информатики Материалы VIII-ой Международной молодежной научно-практической конференции, 2CI5. С. 226-229.
IC. Пронина Е.Д. Модернизация радиовещательных станций ДB диапазона с учетом стандарта DRM // Труды Северо-Кавказского филиала Московского технического университета связи и информатики, 2CI4. № I. С. 3C7-3I I.
II. Jochen Huber. DRM on MF and LF, coverage and technical requirements. EBU-DRM Conference. 26 Nov 2CC9 / Geneva (CH). URL, viewed I4 December 2CI6, http://tech.ebu.ch/docs/events/drmC9/presentations/ebu_drmC9_huber.pdf.
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