GROUNDED MEDIUM

FREQUENCY MONOPOLE

Valentino Trainotti, Walter G. Fano, Lazaro Jastreblansky.

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 Olavarrıa, 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

0 10 20 30 40 50 60 70 80 90−30

−25

−20

−15

−10

−5

0

5

10ANTENNA GAIN

α [degrees]

G[dBi]

Figure 4: Isolated Monopole Gain as a function of elevation angle α.

8

100

101

102

103

110

120

130

140

150

160

170

180

R [m]

Ez[dBµ V/m]

calculatedmeasured

Figure 5: Isolated Monopole Electric Field as a function of distance.

100

101

102

103

60

70

80

90

100

110

120

R [m]

Hy[dBµ A/m]

calculatedmeasured

Figure 6: Isolated Monopole Magnetic Field as a function of distance.

9

101

102

103

250

300

350

400

450

500

550

600

R [m]

Z0[Ω]

377

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

101

102

103

0

10

20

30

40

50

R [m]

θ [°]

Figure 8: Isolated Monopole Wave Impedance Phase as a function of distance.

Figure 9: Grounded Monopole Sketch

11

1 1.1 1.2 1.3 1.4 1.50

200

400

600

800

1000

1200

1400

f [MHz]

Ra[Ω]

40 m

50 m

60 m

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

1 1.1 1.2 1.3 1.4 1.5−600

−400

−200

0

200

400

600

800

f [MHz]

Xa[Ω]

60 m

50 m 40 m

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

1 1.05 1.1 1.15 1.2 1.25 1.3 1.35 1.40

50

100

150

200

250

300

f [MHz]

Ra [Ω]

calculatedmeasured

Figure 12: Grounded Monopole Resistance as a function of frequency

1 1.05 1.1 1.15 1.2 1.25 1.3 1.35 1.4100

150

200

250

300

350

400

450

500

550

600

f [MHz]

Xa [Ω]

calculatedmeasured

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

100

101

102

103

110

120

130

140

150

160

170

180

R [m]

Ez [dBµ V/m]

calculatedmeasured

Figure 15: Grounded Monopole Near Electric Field as a function of distance

100

101

102

103

60

70

80

90

100

110

120

130

R [m]

Hy[dBµ A/m]

calculatedmeasured

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

100

101

102

300

400

500

600

700

800

R [m]

Z0[Ω]

377

Figure 17: Grounded Monopole Wave Impedance Magnitude as a function

of distance

100

101

102

103

0

10

20

30

40

50

R [m]

θ [º]

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 εr = 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

100

101

102

40

50

60

70

80

90

100

110

120

R [km]

E [dBµV/m]

1

2

3

URBAN

RESIDENTIAL

RURAL

Figure 19: Far electric field as a function of distance. 1. Wet ground,

σ = 0.03 S/m, εr = 20 2. Average ground, σ = 0.01 S/m, εr = 10

3. Dry ground, σ = 0.001 S/m, εr = 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]:

∫

L

E ·dL = − j ω µ0 (N π r2) H (2)

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:

Rin =−j Ra Xp + Xs Xp

Ra + j Xs − j Xp

(6)

Operating:

Xp = ±Rin

√Ra

Rin − Ra

(7)

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 Olavarrıa and the technical staff, during the

antenna modification and field strength measurements.

References

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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, N12,

Dec. 1924.

[3] H.E. Gihring, G.H. BrownGeneral Considerations of Towers for Broad-

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[4] A.V. Chamberlain, W.B. Lodge The Broadcast Antenna. P.I.R.E. Vol.24,

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[5] K.A. Norton The Propagation of Radio-wave over the Surface of the

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1936. Part. 2 P.I.R.E. Vol.25, N10, 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, N6, June 1937.

[7] K.A. Norton The Calculations of Ground Wave Field Intensity over

Finitely Conductive Earth. P.I.R.E. Vol.29, N12, 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, N7, 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-

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1987.

[11] V. Trainotti Simplified Calculation of Coverage Area for MF AM Broad-

cast Station. IEEE AP Magazine, Vol. 32, N3, June 1990.

[12] V. Trainotti Asymmetric Vertical Antenna for MF AM Transmitting.

IEEE AP Magazine, Vol. 35, N3, 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-

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Boston, 1999.

[15] V. Trainotti, L.Dorado Short Low and Medium Frequency Antenna Per-

formance. 54th. IEEE BTS Symposium, Washington DC, Oct. 2004,

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29