FM wide band panel dipole antenna - Broadcasting ... - IEEE Xplore

IEEE TRANSACTIONS ON BROADCASTING, VOL. 48, NO. 4, DECEMBER 2002

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FM Wide Band Panel Dipole Antenna Valentín Trainotti, Senior Member, IEEE and Norberto Dalmas Di Giovanni, Member, IEEE

Abstract—It is very common that when a broadcaster needs to install an FM transmitting antenna within a large metropolitan area he places it on the tallest structure or building available. When the rooftop is already occupied by a large number of other types of transmitting and receiving antennas, the panel dipole antenna should be chosen. This antenna is secured to the side walls of the upper floors with the panel oriented to obtain full coverage of the most desirable areas of the city. For the Buenos Aires area this orientation avoids radiating toward Uruguay and specifically toward Montevideo, some 140 miles away. A wide band antenna operation permits placing the station on the air and at the same time allows future stations to share it without the installation of new antennas. Details of model and full model impedance and radiation pattern measurements during the antenna development are presented in order to show its technical characteristics. The radiation patterns were measured on a scale model in an anechoic chamber. The full scaled model was measured in an outdoor antenna range. Both E and H plane radiation patterns were measured along the FM band in order to observe pattern variations on both planes. Practically no difference in a panel radiation beamwidth from 88 to 108 MHz was observed and at the same time good input impedance was maintained. A really wide band antenna in pattern and VSWR is obtained. Power division for the antenna system is obtained designing an eight port power divider using the microstrip line technique. In this case, however due to high power operation the ground plane and strip are contained in a sealed metallic box and are separated by high pressure dry air like into the 3″ feeding coaxial line.

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Index Terms—Antenna arrays, antennas, broadcasting, dipole arrays, reflector antennas, transmitting antennas, VHF antennas.

I. INTRODUCTION

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HE REQUIREMENT to cover the large metropolitan area of Buenos Aires with an FM station is solved utilizing the tallest building of the city. Fig. 1 shows the city area and some views from the building rooftop. Numerous antennas occupying the building rooftop require a panel dipole antenna to be selected and placed on the side walls of one of the building corners close to the roof. Fig. 2 shows the rooftop TV, FM and other antennas. This type of installation permits control of the antenna system radiation pattern to cover the city’s most populated areas and at the same time avoids a large amount of radiation toward Montevideo (Uruguay) some 140 miles away, where another FM station is sharing the same frequency. At the same time mutual coupling and shielding on the existing antennas is completely avoided. Manuscript received January 30, 2001; revised September 12, 2002. This work was supported by CITEFA (Research Institute of the Argentine Armed Forces). The authors are with CITEFA, Antenna and Propagation Division, Buenos Aires, Argentina (e-mail: [email protected]; [email protected]). Digital Object Identifier 10.1109/TBC.2002.806196

(b) Fig. 1.

(a) Coverage area. (b) Coverage area view toward W.

In order to cover the full FM band a wide band panel dipole antenna was designed with optimum radiation and impedance characteristics, to take into account possible future frequency changes or the possibility of sharing it with other FM stations to be installed in the future on the same antenna. During the preliminary antenna development a lot of possibilities were analyzed and different dipole shapes were measured on a scaled model in order to optimize mechanical and electrical characteristics. All these measurements were performed in an anechoic chamber to avoid testing delays due to weather problems in the outdoor antenna range. The small size of the model permits finding the optimum characteristics in the fastest way. This same technique was used in order to obtain microstrip power dividers to feed two or eight panel dipole antennas. Once the optimum model was achieved, a full size dipole was constructed to determine its impedance and radiation characteristics within the FM band.

0018-9316/02$17.00 © 2002 IEEE

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(a)

Fig. 2. Installation building.

(b) Fig. 4. Panel dipole antenna model radiation patterns. (a) vertical polarization. (b) horizontal polarization.

Fig. 3.

Panel dipole antenna model.

The optimum model obtained was a flat shape dipole entirely grounded and supported by a symmetric transmission line with short circuits on both ends. The dipole is made with two flat identical structures with a sinusoidal shape and with an angle close to 45 degrees with respect to the ground plane, which permits obtaining an input impedance very close to 50 ohms [5]. This shape has very good feed point impedance characteristics and radiation patterns with approximately the same beamwidth in both E and H planes. II. PANEL ANTENNA MODEL Unidirectional radiation can be achieved with a dipole antenna in front of a metallic sheet or metallic panel. A modified dipole must be used in order to increase the antenna input impedance bandwidth. In general, bandwidth depends on the

antenna input impedance. Bandwidth can be defined as the maximum to minimum frequency ratio where the input voltage standing wave ratio (VSWR) is below 1.2. This could be a difficult task to obtain when covering the full FM band. On the contrary antenna radiation properties are of slow frequency variation and specifications are easily obtained. This optimum model was constructed with brass sheets as a flat dipole and with brass tubes for the supporting transmission line. The panel was built with an aluminum sheet and the scale was one eighth of the actual size. This scale was considered adequate because the model is not very small, making it easier to analyze its mechanical and electrical characteristics and at the same time facilitating its measurements in the anechoic chamber. Fig. 3 shows the antenna model. Fig. 4 shows some of the antenna model radiation patterns in both the E and H planes through the scaled FM band. In it, the radiation pattern consistency within the measured band and at the same time similar beamwidths on both E and H planes, can be seen. These results are given in Table I. Fig. 5 shows the antenna model input impedance plotted on a Smith chart where the wide band characteristics are evident.

TRAINOTTI et al.: FM WIDE BAND PANEL DIPOLE ANTENNA

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TABLE I PANEL DIPOLE ANTENNA MODEL MEASURED 3 AND 10 dB BEAMWIDTHS

Antenna maximum directivity is defined as “the ratio of the maximum radiation intensity to the radiation intensity of the reference antenna.” Fig. 5. Panel diploe antenna model input impedance.

The reference antenna usually is taken to be an isotropic source. Its directivity is unitary or 0 dbi. Isotropic antenna is ideal and its efficiency is 100%. In this definition no antenna losses are considered. When an antenna is unidirectional an approximated formula can be used to calculate its directivity [1], knowing the radiation pattern beamwidth in the E and H plane, or:

TABLE II PANEL DIPOLE ANTENNA MODEL DIRECTIVITY AND GAIN

and are the 3 dB beamwidth in the E and H plane, respectively, in degrees. times Antenna maximum gain is defined as “the ratio of the maximum radiation intensity to the net power accepted by the antenna from a connected transmitter.” In the gain concept antenna power loss is involved. For this reason Directivity and Gain for the same antenna are different numbers. Only ideal antenna (lossless antenna) has the same directivity and gain and its efficiency is unitary. Gain and directivity are related by the antenna efficiency: where is the antenna efficiency. Directivity and gain are power ratio and for this reason they can be expressed in dBi (decibels with respect to isotropic reference). In this work having a directional antenna, directivity is calculated using the radiation pattern beamwidth approximated formula. Gain is determined by the three antenna method, where a power balance is determined from three link power measurements. Making these three measurements, the three antenna gains referred to the isotropic source are obtained. This measurement method with directional antennas gives very good results because reflection errors are minimized. On the contrary it is very difficult making measurements with a half wave dipole due to its wide beamwidth radiation pattern. This is the reason why reference half wave dipole is seldom used in antenna

measurements. Once directivity and gain in dBi are obtained, directivity and gain referring to a theoretical half wave dipole are easily obtained subtracting 2.15 dB at the previous results. Directivity and gain in dBd is generally obtained this way. Table II gives the antenna directivity, gain and efficiency as a function of frequency. For a panel, the antenna gain is obtained using the three antenna method and it is basically a power balance to achieve true gain. Directivity can be obtained by pattern integration or as in this case where a good directivity is evident, it can be obtained by simple beamwidth expression with very good approximations [1]. 75% efficiency was obtained within the band. In order to increase the antenna gain, four panel bays are used and for this reason an array model was constructed. Array model radiation patterns were measured on both planes like in the single panel case and Table III shows the radiation pattern beamwidths as a function of frequency. Fig. 6 shows the antenna array and the array into the anechoic chamber during the radiation pattern measurements. Fig. 7

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TABLE III PANEL DIPOLE ANTENNA MODEL ARRAY 4 ELEMENTS 3dB BEAMWIDTHS

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(b) Fig. 7. Panel dipole antenna array model radiation patterns. (a) Vertical polarization. (b) Horizontal polarization.

Fig. 6. Panel dipole antenna model array into the anechoic chamber.

TABLE IV PANEL DIPOLE ANTENNA MODEL 4 ELEMENTS ARRAY DIRECTIVITY AND GAIN

shows the typical radiation patterns measured across the operational band. In Table IV the array directivity, gain and efficiency values can be seen, where 70% overall efficiency is obtained. Fig. 8 shows one bay with two elements at a right angle to each other radiation patterns in order to observe the area coverage in both polarizations across the scaled FM band. Table V shows the 10 dB beamwidth for both polarizations where good constancy in area coverage can be seen. III. ACTUAL PANEL DIPOLE ANTENNA With the antenna and array model measurement results, the actual panel antenna was constructed. In this case the reflector was made with an aluminum structure to achieve mechanical strength and lightness. Wide band flat dipole and the supporting line were constructed with brass tubes of different sizes. Fig. 9 shows the

actual size antenna where the dipole can be seen as supported by the balanced line and with two isolators on the dipole tips to


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Fig. 9. Actual size panel dipole antennas at right angle each other, during outdoor antenna range measurements (vertical polarization). (a)

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(b) Fig. 8. Two panel antenna models at a right angle to each other, radiation patterns. (a) Vertical polarization. (b) Horizontal polarization.

TABLE V TWO PANEL DIPOLE ANTENNA MODELS AT 90 DEGREES 10 dB BEAMWIDTHS

give a good mechanical strength to the assembly and to avoid any kind of structure vibrations caused by the wind action.

(b) Fig. 10. Actual size panel dipole antenna, radiation patterns. (a) Vertical polarization. (b) Horizontal polarization.

A very light antenna structure was obtained with a total weight of approximately 15 kilograms (about 30 lbs). The brass structure was galvanized to avoid electrolysis and corrosion problems. Radiation patterns were measured for the full size antenna panel in an outdoor antenna range and similar results as in the antenna model are shown in Fig. 10. To fulfill the coverage area each bay has two antenna elements placed at right angles to each other so to cover approximately 180 degrees in azimuth. These radiation patterns can be seen in Fig. 11. Actual antenna element impedance was optimized at a frequency of 94 MHz but the wide band characteristics give a 10 MHz coverage with a VSWR of less than 1.1 and full band coverage with a VSWR of less than 1.2. Smith chart impedance representation can be seen in Fig. 12, as obtained from an HP 8410A network analyzer measurements.

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Fig. 12.

Actual size panel dipole antenna input impedance.

Fig. 13.

Two way power divider.

Fig. 14.

Eight port power divider.

(b) Fig. 11. Actual size panel dipole antennas at right angles to each other, radiation patterns. (a) Vertical polarization. (b) Horizontal polarization.

IV. POWER DIVIDER The antenna system elements are fed in phase and a power divider is necessary to complete this task. Before designing the high power eight port divider, a low power two port divider was designed using the microstrip technique but without the solid substrate between the ground plane and the high potential conductor. The assembly is contained in a metallic box (galvanized brass) and the experimental unit has a size of 0.25 in length, 0.083 in width, and 0.042 in height. Good impedance match was achieved after optimization utilizing an HP 8410A network analyzer. This optimization was obtained changing the line separation over the ground plane or the strip width due to the metallic box interaction. Based on these good results the high power eight port power divider was designed using a size of 0.5 in length, 0.133 in width, and 0.1 in height.

In this latter case, four ports on both ends are placed and the feeding power is injected in the middle through an EIA 3 1/8″ connector. Output connectors are eight EIA 7/8″.


Good impedance match is obtained on the operational frequency but the full band has a relatively high VSWR on the edges due to only one quarter wavelength transformer on both sides, whose reflection coefficient characteristics are too sharp near the optimum frequencies. This power divider works perfectly well for 10 MHz bandwidth but it would be redesigned in case wider band operation is needed including at least two quarter wave or two exponential transformers on each sides. Figs. 13 and 14 show the low and high power divider. V. CONCLUSIONS A wide band panel dipole antenna has been designed and measured with good impedance and radiation characteristics within the FM band. Service area is covered by two panels at right angles to each other and for gain increase, four identical bays are used to put the maximum radiated power on the horizon. A 3 dB beamwidth in the vertical plane was obtained at an average of 19.4 degrees across the band to achieve full coverage from 700 meters to the horizon within the main lobe, taking into account the transmitting antenna height. Of course shorter distances are covered due to a lower free space loss. Polarization of the radiated wave is vertical in order to get the maximum field strength at street level for mobile listeners who are in the more stringent conditions due to the city’s tall buildings wave diffractions and reflections. The original design was the installation of the transmitting antenna on the building walls but, due to the electromagnetic radiation prevention on the working personnel in the last floor, the final installation of the panels was completed in one of the building roof corners over a self supporting metallic structure. ACKNOWLEDGMENT The authors would like to express their gratitude to the late J. Garcia Poza for the mechanical work, and to D. Schweitzer

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and J. Skora for impedance and radiation measurements. They would also like to thank B. Kolundzija and A. Djordjevic for their permanent support. REFERENCES [1] J. D. Kraus, Antennas. New York: McGraw Hill, 1950, 2nd ed., 1988. [2] E. Wolff, Antenna Analysis. New York: John Wiley & Sons, 1966. [3] H. Jasik, Antenna Engineering Handbook. New York: McGraw Hill, 1961, 1984. [4] H. Howe, Jr., Stripline Circuit Design. Boston: Artech House Inc., 1974. [5] V. Trainotti, “Half wave ‘V’ dipole antenna,” in Proc. 1990 Antenna Applications Symp.. Allerton Park, IL: U.S. Force-University of Illinois, Sept. 1990, TR #91-156, ADA #237056, 237 057. [6] R. Collin, Foundation for Microwave Engineering. New York: McGraw Hill. [7] Branko, Kolundzija, and Alt, WIPL-D. Norwood, MA: Artech House, 1995.

Valentín Trainotti received his Electronic Engineering Degree from the Technological National University, Buenos Aires, Argentina in 1963. His post-graduate coursework on antenna measurements and geometric theory of diffraction was completed at California State University in 1981 and Ohio State University in 1985. He has worked from 1963 to present at CITEFA as the Antenna & Propagation Division Chief Engineer. His work also includes being on the Engineering Faculty at the University of Buenos Aires as a part-time Full Professor of Electromagnetic Radiation and Radiating Systems for graduate students. He is an IEEE Senior Member, the IEEE BTS Argentina Chapter Chair, the URSI Commission B Argentina Chair, and the 1993 IEEE Region 9 Eminent Engineer. He has worked more than thirty years developing and measuring antenna systems for several applications from LF to SHF.

Norberto Dalmas Di Giovanni received his Electronic Engineering Degree from the Technological National University, Buenos Aires, Argentina in 1987. He is an Antenna & Propagation Division Project Engineer at CITEFA. He is also part of the Engineering Faculty at the University of Buenos Aires, as well as at the Army University where he serves as a part-time professor of Antennas for undergraduate students. He is also an IEEE Member. He has been working since 1979 developing and measuring antenna systems for several applications from LF to SHF.