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Grounded Medium Frequency Monopole

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Grounded Medium Frequency Monopole GROUNDED MEDIUM FREQUENCY MONOPOLE Valentino Trainotti, Walter G. Fano, L¶azaroJastreblansky. University of Buenos Aires, Argentina ABSTRACT Medium frequency (MF) band isolated monopoles have been used for standard amplitude modulation (AM) broadcast applica- tions for long time, since Stuart Ballantine vertical radiator per- formance study carried out during the twenties decade. Nowaday, they are still doing a good job to medium frequency broadcast stations. Nevertheless, new services are needed at higher frequencies and for them the antenna height is paramount. A medium frequency transmitting mast whose height is in the order of hundreds of me- ters could be a logical option if several services could share the same structure. In order to overcome the high medium frequency voltage in the antenna base, a simple solution is putting the mast base at ground potential and changing the medium frequency tech- niques to feed it. 1 Getting these requirements, a project was carried out during November 2004 in order to modify the existing transmitting mast of the LU22 Radio Olavarria Station located at Olavarria, Ar- gentina (AM 1160 kHz). This project gave good results and the possibilities of sharing this mast for the frequency modulation (FM) transmission and a Studio to Transmitting Plant Link (STL) with the normal medium frequency (MF) broadcast transmission was at hands. Some concerns have been arisen because this installation was operating with 10 kW AM MF transmitter without problems for more than thirty years. Nevertheless during a week end of December 2004, the antenna modi¯cation was carried out and the performance of the new an- tenna was similar to the old one and interactions with the new services sharing this mast were not observed. Input impedance calculations and measurements as well ¯eld strength measurements are presented in order to show the perfor- mance of the new system. Measured antenna bandwidth was ful¯lling the requirements for a medium frequency (MF) amplitude modulated (AM) broadcast transmitting system and future hybrid digital transmissions like IBOC* and Digital Radio Mondiale (DRM)**. * Simultaneous amplitude modulation and digital transmissions by IBIQUITY (www:ibiquity:com) **(www:drm:org) 2 1 INTRODUCTION Standard isolated monopole has been used in medium frequency band for broadcast application since long time, especially after the thorough study made by Stuart Ballantine on Vertical Radiating Mast in the twenties [2, 4, 3]. These kind of radiators have been made a signi¯cant contribution to the broadcast service due to a high e±cient surface wave radiation when a standard 120 buried metallic radials as an arti¯cial ground plane was used [6, 9, 12, 15]. An optimun radiator has been obtained from the radiation properties point of view, especially when the optimum height is used according to the operation frequency and ground physical constants [3, 10]. In this case, this ground plane was adopted in order to get the best antenna e±ciency in the original isolated monopole design [6, 9]. Nevertheless, nowadays when the height of tall metallic mast, like this kind of antennas are using, are necessarily intended to be used, at the same time, supporting several VHF, UHF and Microwave antennas. In the case of one VHF or UHF antenna to be installed on the mast top, a quarter wave insulator could be used, but if several antenna are necessary to be installed, this problem is facing a di±cult solution. A simple solution to this problem is modifying the existing isolated mast to a grounded monopole. This approach permits the installation of several 3 Figure 1: Old Installation Sketch 4 Figure 2: New Installation Sketch 5 antennas close to the mast top for several services and at the same time, an e±cient operation in the medium frequency (MF) band without interaction problems can be obtained. An isolated MF radiator has been modi¯ed in order to be used at the same time for frequency modulation (FM) transmission and a studio to transmitter link (STL) as well as the normal MF amplitud modulated (AM) service. The normal MF AM broadcast service is carried out by mean of 10 kW transmitter and a spare one of 5 kW output power. These transmitters and the antenna have been in service for more than thirty years without any problem, and the logical concerns were arisen about the antenna modi¯cation. In ¯gure 1 the medium frequency (MF) amplitude modulation (AM) transmitting station and isolated monopole antenna sketch can be seen. Project was carried out during November 2004 and the antenna modi¯ca- tion during a week end in December 2004 in order not to disturb very much the normal AM transmissions of the LU22 Radio Olavarr¶³a,Argentina. These modi¯cations consist installing a metallic skirt to the existing mast and the coaxial lines. At the same time the matching unit was modi¯ed in order to match the antenna input impedance to the transmission line characteristic impedance. Transmission line is six wire quasi-coaxial line installed between the tun- 6 ing unit at the base mast and the transmitting building around 200 m away and its characteristic impedance is around 220 ohm. FM and STL equipment were installed inside the tuning unit shelter. This shelter has been provided by a Faraday Shield in order to avoid interac- tions with the MF radiation and the static electricity e®ects during stormy weather. In ¯gure 2 the new transmitting system sketch is shown. 2 Antenna Models Simulations of the old and new radiating system was carried out using WIPL- D software [14] in order to determine the input impedance and the radiated ¯elds. In ¯gures 3 the old isolated monopole antenna model can be seen. The Isolated Monopole Gain, Electric and Magnetic near Fields, as well the wave impedance close to the antenna have been calculated by means of a WIPL-D software and these results can be seen in ¯gures 4, 5, 6 and 7. Near electric and Magnetic Field have been measured before making the antenna modi¯cations by means of a calibrated ¯eld strength meter and these results are plotted in the near electric and magnetic ¯eld ¯gures (5, 6). Good agreement between calculated and measured values can be seen. Field strength meter Singer NM25 uses a calibrated small loop as electric ¯eld sensor. In order to measure the magnetic ¯eld intensity an antenna factor of the loaded loop was obtained as can be seen in the Appendix A. 7 Figure 3: Isolated Monopole Model Sketch ANTENNA GAIN 10 G[dBi] 5 0 −5 −10 −15 −20 −25 −30 0 10 20 30 40 50 60 70 80 90 α [degrees] Figure 4: Isolated Monopole Gain as a function of elevation angle ®. 8 E [dBµ V/m] z 180 calculated measured 170 160 150 140 130 120 110 0 1 2 3 10 10 10 R [m] 10 Figure 5: Isolated Monopole Electric Field as a function of distance. H [dBµ A/m] y 120 calculated measured 110 100 90 80 70 60 0 1 2 3 10 10 10 R [m] 10 Figure 6: Isolated Monopole Magnetic Field as a function of distance. 9 Z [Ω] 600 0 550 500 450 400 377 350 300 250 1 2 3 10 10 R [m] 10 Figure 7: Isolated Monopole Wave Impedance Magnitude as a function of distance In the far ¯eld region, the electric and magnetic ¯elds are related through » the free space impedance Z00 = 377 ­, but this is not true in the near ¯eld region, so separated ¯eld measurements are necessary. From the wave impedance calculations it can be seen that the far ¯eld condition is obtained at a distance of approximately one wavelength or 250 meters were the impedance phase is close to zero degrees and its magnitude is approaching 377 ohms. It can be seen from calculations and measurements, the di®erent electric and magnetic ¯eld variation as a function of distance close to the antenna base. In ¯gure 9 sketch of grounded monopole model can be seen. 10 θ [°] 50 40 30 20 10 0 1 2 3 10 10 R [m] 10 Figure 8: Isolated Monopole Wave Impedance Phase as a function of distance. Figure 9: Grounded Monopole Sketch 11 R [Ω] 1400 a 1200 1000 800 600 60 m 400 50 m 200 40 m 0 1 1.1 1.2 1.3 1.4 1.5 f [MHz] Figure 10: Grounded Monopole Resistance for Hs = 40 m, Hs = 50 m y Hs = 60 m as a function of frequency. 3 INPUT IMPEDANCE Grounded monopole input impedance was analyzed as a function of wire skirt dimensions. Metallic skirt is made up of six wires installed symmetrically all around the supporting tower by means of booms attached to the tower legs. In order to avoid the wire vibrations due to the wind action, plastic insulators were installed along the supporting tower. These insulators were installed with a separation of 10 meters approximately between them. According to the upper skirt short circuit position the antenna input im- pedance has di®erent variations as a function of frequency, but the radiation characteristics are maintained because they depend on the antenna physical dimensions or mast height [11]. 12 X [Ω] 800 a 50 m 40 m 600 60 m 400 200 0 −200 −400 −600 1 1.1 1.2 1.3 1.4 1.5 f [MHz] Figure 11: Grounded Monopole Reactance for Hs = 40 m, Hs = 50 m y Hs = 60 m as a function of frequency These variations can be seen in ¯gure 10 and 11. In this case a low impedance variation is to be chosen and at the same time a minimum input voltage would be important.
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