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 modification 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 field strength measurements are presented in order to show the perfor- mance of the new system. Measured antenna bandwidth was fulfilling 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 significant contribution to the broadcast service due to a high efficient surface wave radiation when a standard 120 buried metallic radials as an artificial 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 efficiency 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 difficult 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 efficient operation in the medium frequency (MF) band without interaction problems can be obtained.
An isolated MF radiator has been modified 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 modification. In figure 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 modifica- tion during a week end in December 2004 in order not to disturb very much the normal AM transmissions of the LU22 Radio Olavarr´ıa,Argentina.
These modifications consist installing a metallic skirt to the existing mast and the coaxial lines. At the same time the matching unit was modified 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 effects during stormy weather.
In figure 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 fields. In figures 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 figures 4, 5, 6 and 7. Near electric and Magnetic Field have been measured before making the antenna modifications by means of a calibrated field strength meter and these results are plotted in the near electric and magnetic field figures (5, 6). Good agreement between calculated and measured values can be seen. Field strength meter Singer NM25 uses a calibrated small loop as electric field sensor. In order to measure the magnetic field 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 α.
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 field region, the electric and magnetic fields are related through ∼ the free space impedance Z00 = 377 Ω, but this is not true in the near field region, so separated field measurements are necessary. From the wave impedance calculations it can be seen that the far field 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 different electric and magnetic field variation as a function of distance close to the antenna base. In figure 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 different 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 figure 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. This statement can assure a good antenna bandwidth suitable for a high fidelity amplitude modulate transmission and at the same time for future digital transmissions like IBOC or DRM. According to the input impedance variation a short circuit skirt height of
Hs = 45 m was chosen assuring a smooth impedance variation and a conve- nient value to be match to the transmission line characteristic impedance. In Figure 12 and 13 the input impedance as a function of frequency can be seen as well the measured values by means of a DELTA BRIDGE at the antenna input terminals.
13 R [Ω] a 300 calculated measured 250
200
150
100
50
0 1 1.05 1.1 1.15 1.2 1.25 1.3 1.35 1.4 f [MHz]
Figure 12: Grounded Monopole Resistance as a function of frequency
X [Ω] a 600 calculated 550 measured
500
450
400
350
300
250
200
150
100 1 1.05 1.1 1.15 1.2 1.25 1.3 1.35 1.4 f [MHz]
Figure 13: Grounded Monopole Reactance as a function of frequency
14 4 ANTENNA MATCHING
Knowing the antenna input impedance the matching system has been calcu- lated from the standard circuit theory. T, π or L networks can be chosen for this purpose [8]. L network has been chosen due to its simplicity after having the antenna resonance by means of a proper reactance. This value has been included later in the matching system. Antenna input impedance at 1160 kHz is inductive or given by
Za = 64 + j255 Ω.
Resonance is obtained by means a capacitive reactance of Xa = −255 Ω and L network is used to match the resistive 64 Ω to 220 Ω of the transmission line characteristic impedance. This can be seen in Appendix B. As a result the L network to match and tune the antenna has two capaci- tors, one in series with the antenna impedance and the other in parallel with the transmission line output terminals.
The capacitance of both capacitors have been found to be Cs = 855 pF and Cp = 973 pF. Two 1500 pF high voltage variable vacuum capacitors were used and adjusted by means of a DELTA BRIDGE (Appendix C) to the transmission line characteristic impedance value at the carried frequency. After that, the impedance value was measured as a function of frequency. A radio frequency choke has been connected in parallel to the antenna terminals in order to permit the continuous static discharge of the antenna structure. Its impedance value is around ten times the antenna impedance so it does not modify the circuit condition. In figure 14 the calculated standing wave ratio (VSWR) is presented from the calculated antenna input impedance. Also, the measured VSWR ratio
15 Figure 14: Measured VSWR at the matching unit input as a function of frequency
16 Table 1: INPUT IMPEDANCE CALCULATED, MEASURED AND VSWR
− CALCULATED − MEASURED −
Frequency Zin VSWR/220 Zin VSWR/220
kHz Ω − Ω −
1140 168+j 32 1.372 165+j 25 1.371
1145 176+j 28 1.301 180+j 15 1.239
1150 191+j 22 1.197 190+j 8 1.166
1155 208+j 13 1.087 210+j 5 1.050
1160 220+j 0 1.001 220+j 0 1.000
1165 232-j 14 1.086 230-j 10 1.065
1170 240-j 31 1.177 235-j 40 1.206
1175 248-j 49 1.272 240-j 55 1.289
1180 252-j 72 1.393 245-j 65 1.348 is presented from the measured antenna input impedance. Both values are included in Table 1. It can be observed a good agreement between the calculated and measured values. Connecting the matching unit to the transmission line, the input im- pedance is measured at the transmitter side by means of the DELTA BRIDGE and using the 5 kW transmitter as generator. The transmission line input impedance was found to be Zin = 220 + j2.5 Ω at the carried frequency or a VSWR = 1.022.
17 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 15: Grounded Monopole Near Electric Field as a function of distance
H [dBµ A/m] y 130 calculated measured 120
110
100
90
80
70
60 0 1 2 3 10 10 10 R [m] 10
Figure 16: Grounded Monopole Near Magnetic Field as a function of distance
18 5 Near Field
Near electric and magnetic fields have been calculated using WIPL-D and measured by means of Singer NM-25 field strength meter with an electric field calibrated loop. In figure 15 and 16 the near electric and magnetic fields can be seen as a function of distance between 5 and 800 meters. Good agreement can be appreciated between calculated and measured fields. Grounded Monopole wave impedance has been calculated as a function of distance using the calculated near electric and magnetic fields. This im- pedance can be seen in figures 17 and 18.
6 Far field
Far field determination is important in order to know the medium frequency (MF) amplitude modulated (AM) station service area. This area depends on the environment where the listener are located, for this reason, more field strength is needed in urban areas, where the noise level is higher, due to man electric activity. In this case 88 dBµV/m (25mV/m) of minimum electric field strength is necessary and for residential areas this value can be lowers to 74 dBµV/m (5 mV/m). For rural areas a minimum level of 54 dBµV/m (0.5 mV/m) can do a rea- sonable service in the medium frequency AM band in moderated atmospheric noise areas.
19 Z [Ω] 800 0
700
600
500
400 377
300 0 1 2 10 10 10 R [m]
Figure 17: Grounded Monopole Wave Impedance Magnitude as a function of distance
θ [º] 50
40
30
20
10
0 0 1 2 3 10 10 10 R [m] 10
Figure 18: Grounded Monopole Wave Impedance Phase as a function of distance
20 Far field of the surface wave (Esu) has been calculated as a function of distance for 10 kW of radiated power and for different soil conditions. This task is obtained using the Sommerfeld - Norton theory for planar earth and introducing the shadow or diffraction factor taking into account the spherical earth [1, 5, 10, 11, 13]. Isolated Monopole far field strength measurements were carried out in November 2004, with some scatter values as a function of distance and in order to get them as a comparison with the field strength produced by the modified antenna. Grounded monopole far field strength measurements were carried out in December 2004, after the antenna modification and more values have been measured as a function of distance in this occasion. Figure 19 shows the electric field values as a function of distance, calcu- lated and measured in November 2004 and in December 2004. It can be seen from this figure that the measured value are practically the same for isolated and grounded monopole and they fit very well the field strength corresponding to wet soil, like it is the soil of the Pampa in the Province of Buenos Aires, Argentina (conductivity σ = 0.03 S/m, relative permittivity ²r = 20).
It is important to indicate where are located the practical limits of each area after the far field strength has been measured. These areas are found to be:
[A] Urban area up to 25 km. [B] Residential area up to 80 km.
21 [C] Rural area up to 200 km.
With these field strength results it can be seen the service areas can fulfill the requirements for this broadcast station in medium frequency.
7 Conclusion
After this work was completed, the measured results of the modified antenna field strength can assure a good service area for the LU22 medium frequency station as was determined by measurements and from the listener point of view by means of a car receiver along the countryside routes and with levels similar to the old transmitting system.
22 E [dBµV/m]
120
110
100 1
90 2 URBAN
80 RESIDENTIAL
70 3
60 RURAL
50
40 0 1 2 10 10 10 R [km]
Figure 19: Far electric field as a function of distance. 1. Wet ground,
σ = 0.03 S/m, ²r = 20 2. Average ground, σ = 0.01 S/m, ²r = 10
3. Dry ground, σ = 0.001 S/m, ²r = 4, Isolated Monopole, ✽ Grounded Monopole
23 8 APPENDIX A
8.1 Magnetic Field Loop Antenna Factor
From Maxwell equation for harmonic fields in free space:
∇ × E = − j ω µ0 H (1)
Integrating on both terms over the N turn loop surface and applying Stokes Theorem [16]:
Z 2 E · dL = − j ω µ0 (N π r ) H (2) L When the loop is oriented for the maximum induced voltage, and its area is Nπr2, as shown in figure 20, the effective voltage is given by:
Vef = 4.44 µ0 f N A H (3)
For a frequency f = 1.16 MHz, N = 3, and loop diameter D = 0.25 m, the effective voltage is given by:
Vef = 0.9531 H (4)
Taking into account the 50 Ω loop load, and the input voltage
Vin ef = Vef /2 in the strength meter, the magnetic field is given by:
H = 2.0984 Vin ef (5)
24 Figure 20: a) Loop geometry. b) Three turn loaded loop. c) Simple equivalent circuit.
25 Figure 21: Theoretical L Network for Rin > Ra 9 APPENDIX B
9.1 L Matching Network
The input resistance (Rin) of a resonant antenna impedance Ra, when
Rin > Ra, according to figure 21 is given by:
−j Ra Xp + Xs Xp Rin = (6) Ra + j Xs − j Xp Operating:
r Ra Xp = ±Rin (7) Rin − Ra
p Xs = ∓ Ra (Rin − Ra) (8)
26 Figure 22: DELTA BRIDGE BASIC CIRCUIT
10 APPENDIX C
10.1 DELTA BRIDGE
Impedance measurements have been made by means of DELTA BRIDGE, permitting high power in the antenna circuit in order to avoid the interference from powerful MF AM station within the operating band and having accurate measurements at the Bridge balance. Figure 22 shows a sketch of the DELTA BRIDGE from DELTA ELEC- TRONICS.
27 11 Acknowledgments
We would like to appreciate the kind support of Mr. Daniel Panarace, Di- rector of LU32 1160 AM Radio Olavarr´ıaand the technical staff, during the antenna modification and field strength measurements.
References
[1] A. Sommerfeld The Propagation of Waves in Wireless Telegraphy. An- nalen der Physik, Vol.28, pp. 665-736, 1909.
[2] S. Ballantine On the Optimum Transmitting Wavelengths for a Vertical Antenna at Wavelengths below the fundamental. P.I.R.E. Vol.12, N◦12, Dec. 1924.
[3] H.E. Gihring, G.H. BrownGeneral Considerations of Towers for Broad- cast Use. P.I.R.E. Vol.23, N◦4, Apr. 1935.
[4] A.V. Chamberlain, W.B. Lodge The Broadcast Antenna. P.I.R.E. Vol.24, N◦1, Jan. 1936.
[5] K.A. Norton The Propagation of Radio-wave over the Surface of the Earth and in the Upper Atmosphere. Part.1, P.I.R.E. Vol.24, N◦10, Oct. 1936. Part. 2 P.I.R.E. Vol.25, N◦10, Oct. 1937.
[6] G.H. Brown, R.F.Lewis, and J.Epstein Ground System as a Factor in Antenna Efficiency. P.I.R.E. Vol.25, N◦6, June 1937.
[7] K.A. Norton The Calculations of Ground Wave Field Intensity over Finitely Conductive Earth. P.I.R.E. Vol.29, N◦12, Dec. 1941.
28 [8] Frederick E. Terman Radio Engineering. Mc Graw Hill Books, NY, 1947.
[9] F. Abbott Design of Optimum Buried Conductor RF Ground System. P.I.R.E. Vol.40, N◦7, July 1952.
[10] V. Trainotti On the Service Area of MF AM Broadcast Stations, the Op- timum MF AM Broadcast Antenna. Proc. of the 1987 Antenna Applica- tions Symposium. Robert Allerton Park, Univ. of Illinois, Sept 23-24-25, 1987.
[11] V. Trainotti Simplified Calculation of Coverage Area for MF AM Broad- cast Station. IEEE AP Magazine, Vol. 32, N◦3, June 1990.
[12] V. Trainotti Asymmetric Vertical Antenna for MF AM Transmitting. IEEE AP Magazine, Vol. 35, N◦3, June 1993.
[13] V. Trainotti Near and Far Field of MF and HF Antennas. Proc. of the 1996 Antennas Applications Symposium. Robert Allerton Park, Univ. of Illinois, Sept 18-19-20, 1996.
[14] B. M. Kolundzija, J. S. Ognjanovic, T. K. Sarkar Electromagnetic Mod- eling of Composite Metallic and Dielectric Structures. Artech House, Boston, 1999.
[15] V. Trainotti, L.Dorado Short Low and Medium Frequency Antenna Per- formance. 54th. IEEE BTS Symposium, Washington DC, Oct. 2004, reprint in QEX May-June 2005.
[16] V. Trainotti, W.G.Fano, L.Dorado Ingenier´ıaElectromagn´etica. Vol.1 and Vol.2, Nueva Librer´ıa,Buenos Aires, Argentina, 2005.
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