Jeffrey D. Wilson and Gregory A. Zimmerli

Glenn Research Center, Cleveland, Ohio

Model of the Radio Frequency (RF) Excitation 

Response from Monopole and Dipole Antennas 

in a Large Scale Tank

NASA/TM—2012-217267

January 2012
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Jeffrey D. Wilson and Gregory A. Zimmerli

Glenn Research Center, Cleveland, Ohio

Model of the Radio Frequency (RF) Excitation 

Response from Monopole and Dipole Antennas 

in a Large Scale Tank

NASA/TM—2012-217267

January 2012

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135
Available from

NASA Center for Aerospace Information

7115 Standard Drive

Hanover, MD 21076–1320

National Technical Information Service

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Level of Review: This material has been technically reviewed by technical management. 
NASA/TM—2012-217267 1 

Model of the Radio Frequency (RF) Excitation Response From 

Monopole and Dipole Antennas in a Large Scale Tank 

Jeffrey D. Wilson and Gregory A. Zimmerli 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Summary 

Good antenna-mode coupling is needed for determining the amount of propellant in a tank through 

the method of radio frequency mass gauging (RFMG). The antenna configuration and position in a tank 

are important factors in coupling the antenna to the natural electromagnetic modes. In this study, different 

monopole and dipole antenna mounting configurations and positions were modeled and responses 

simulated in a full-scale tank model with the transient solver of CST Microwave Studio (CST Computer 

Simulation Technology of America, Inc.). The study was undertaken to qualitatively understand the effect 

of antenna design and placement within a tank on the resulting radio frequency (RF) tank spectrum. 

Introduction 

In order to determine the amount of liquid remaining in a propellant tank using the radio frequency 

mass gauge (RFMG) method, it is necessary to mount an antenna inside the tank, excite the 

electromagnetic modes, and measure the eigenmode frequencies (Ref. 1). An important aspect of this 

procedure is the design and position of the antenna inside the tank. Ideally, the antenna should be 

mounted in a position to strongly excite as many of the lower eigenmodes as possible. In this 

investigation, different monopole and dipole antenna mounting configurations and positions were 

modeled and responses simulated in a tank model using the transient solver of CST Microwave Studio 

(CST Computer Simulation Technology of America, Inc.). Three different monopole mounting 

configurations were investigated: top-mounted on the center of the tank lid, side-mounted on the side wall 

of the tank, and center-mounted on a central metal rod running from the top to the bottom of the tank. 

Dipole antenna simulations were performed in the center-mounted configuration with both parallel and 

transverse mountings of the antenna with respect to the long axis of the tank. 

Analysis 

The tank, in the shape of a cylinder with rounded edges (Fig. 1), was modeled with the transient 

solver of Microwave Studio. This solver calculates the time-varying field distribution that results 

from excitation with a Gaussian pulse at the input port. When a steady state solution is reached, the 

S-parameters defining the transmission characteristics as a function of frequency are calculated with a 

Discrete Fourier Transform (Ref. 2).

The tank has a length L = 472.9 cm and a radius of 91.0 cm, and corresponds to actual dimension of a 

tank being used for antenna-mode coupling tests of the RFMG method. The results of those tests will be 

reported in a separate publication. The surfaces of the tank were modeled as perfect conductors. After the 

tank and antenna geometries were modeled as described below, the solver was used to compute the steady 

state response of the return loss spectrum in the frequency range 100 to 200 MHz. 
NASA/TM—2012-217267 2 

Figure 1.—Outer view of the tank model. Figure 2.—Inner view of the tank model with 

off-center mounting rod.

Figure 3.—Mesh at monopole antenna in center-mounted configuration. The base 

of the monopole is flush with the outer surface of the center mounting rod. 

Monopole Antenna Configurations 

For the center-mounted monopole configuration only, a perfect conducting rod of 3.8 cm diameter 

runs along the long axis of the tank (Fig. 2). Figure 3 shows the geometry of the monopole antenna for the 

center-mounted configuration with the Microwave Studio mesh with a 25 cm monopole. The position of 

the antenna is halfway down from the top of the tank. 

A close-up of the antenna excitation port is shown in Figure 4. This is designated in Microwave 

Studio as an impedance element discrete port (S-parameter type) and is used as the RF feed point source 

for the antenna. It is modeled as a perfect conducting wire inside a coaxial cavity and provides an input 

power of one watt (Ref. 2). This type of feed port is also used in the dipole antenna simulations. 
NASA/TM—2012-217267 3 

Figure 4.—Close-up of monopole antenna excitation port. 

Figure 5.—Top-mounted monopole antenna configuration. 

In the side-mounted monopole configuration, the same 25 cm antenna design is used, with the base of 

the monopole flush with the inner surface of the side wall of the tank, and the center rod is not included in 

this tank model. Two mounting positions for the antenna were simulated, one at y = L/2, and another at 

the y = L/3 position. 

In the top-mounted configuration, the center rod is also removed from the tank model and a 25 cm 

monopole antenna is inserted through the center of the top of the tank as shown in Figure 5. A local 

coordinate system u-v-w was defined in Microwave Studio overlaying the global coordinate system x-y-z 

to position the antenna. 
NASA/TM—2012-217267 4 

Figure 6.—Dipole antenna, transverse orientation. Figure 7.—Dipole antenna, parallel orientation. 

Dipole Antenna Configurations 

The response from a dipole antenna was simulated using the same tank and rod, but with the rod 

displaced 11.4 cm in the negative z-direction from the center axis. Two antenna orientations were 

simulated: Figure 6 shows the transverse orientation and Figure 7 shows the parallel orientation of the 

dipole antenna. The dipole length is 40.6 cm and it is positioned 15.2 cm in the positive z-direction from 

the rod. 

Results 

Top-Mounted Monopole Antenna Configuration 

The simulated return loss from the top-mounted configuration is shown in Figure 8. (The responses 

are plotted from 100 to 200 MHz to match the region of experimental interest.) Only the TM modes are 

excited in this frequency range as can be seen by the corresponding electric field and magnetic field mode 

amplitudes in Appendix A. 

Side-Mounted Monopole Antenna Configurations 

The simulated return loss for the two side-mounted monopole antenna configurations are shown 

in Figure 9 (antenna at y = L/3) and Figure 10 (antenna at y = L/2). The model predicts fewer mode 

excitations and a weaker amplitude in the return-loss spectrum than in the top-mounted case. 

Experimentally, we observe a much better return-loss spectrum than that predicted by the side-mounted 

model. The source of the discrepancy is not clear, but indicates that the antenna-tank coupling model can 

only be used to make qualitative predictions. 
NASA/TM—2012-217267 5 

Figure 8.—Return loss with top-mounted monopole 

antenna configuration. 

Figure 9.—Return loss with side-mounted monopole 

antenna configuration mounted one third from the 

top (y = L/3). 

Figure 10.—Return loss with side-mounted monopole 

antenna configuration mounted one half from the top 

(y = L/2). 

Center-Mounted Monopole Antenna Configurations 

The center-mounted configuration was further investigated by varying the length of the monopole 

from 5 to 25 cm. It was found that return loss peaks at the same frequencies for each monopole length, but 

the strength of the response generally increases as the monopole length increases. The position of the 

25 cm monopole antenna within the tank was also investigated. The results for y = L/16 to L/2 are shown 

in Figures 11 to 18. 

From Figures 11 to 18, the model predicts the strongest responses to occur at eigenfrequencies of 

126.4, 156.1, 160.3, and 187.2 MHz. As expected, it is seen that the center-mounted configuration has 

different eigenfrequencies than the top- and side-mounted configurations, due to the introduction of the 

center bar into the tank. There is also a wide variation in response strength depending on antenna position, 

with the 7/16 position being the only one to have a predicted return loss of at least –5 dB for each of these 

eigenfrequencies. The electric and magnetic field amplitudes of the corresponding eigenmodes for the 

7/16 case are shown at a cross section of the tank in Appendix B. 
NASA/TM—2012-217267 6 

Figure 11.—Return loss with center-mounted 

configuration, monopole height = 25 cm, 

mounted at y = L/16. 

Figure 13.—Return loss with center-mounted 

configuration, monopole height = 25 cm, 

mounted at y = 3L/16. 

Figure 15.—Return loss with center-mounted 

configuration, monopole height = 25 cm, 

mounted at y = 5L/16. 

Figure 12.—Return loss with center-mounted 

configuration, monopole height = 25 cm, 

mounted at y = L/8. 

Figure 14.—Return loss with center-mounted 

configuration, monopole height = 25 cm, 

mounted at y = L/4. 

Figure 16.—Return loss with center-mounted 

configuration, monopole height = 25 cm, 

mounted at y = 3L/8. 
NASA/TM—2012-217267 7 

Figure 17.—Return loss with center-mounted 

configuration, monopole height = 25 cm, 

mounted at y = 7L/16. 

Figure 19.—Return loss with parallel (blue) and 

transverse (red) dipole antenna orientations. 

Figure 18.—Return loss with center-mounted 

configuration, monopole height = 25 cm, 

mounted at y = L/2. 

Figure 20.—Return loss for parallel dipole antenna 

with no baffles (blue) and three baffles (green). 

Center-Mounted Dipole Antenna Configurations 

The return losses for the transverse and parallel dipole configurations are compared in Figure 19. In 

general, there are significantly stronger responses predicted for the parallel orientation compared to the 

transverse orientation in this frequency range. The electric and magnetic fields for the eigenmodes excited 

by the parallel dipole antenna corresponding to frequencies 132.4, 136.1, 150.9, 157.7, 167.9, and 

186.4 MHz are shown in Appendix C. 

The effect of conducting baffle rings (slosh baffle plates) in the tank was also simulated. The baffles 

extended from an inner radius of 70.6 cm to the outer tank wall at a radius of 91.0 cm. The thickness in 

the y-direction is 0.3 cm. The top, center, and bottom locations are at 443.2, 236.5, and 29.8 cm from the 

bottom, respectively. The return loss is shown in Figure 20 for the parallel dipole configuration with and 

without baffles. Perhaps not surprisingly, the model predicts that the baffles change the amplitude of the 

response and also cause small shifts in frequency. 
NASA/TM—2012-217267 8 

Conclusions 

In this investigation, different monopole and dipole antenna mounting configurations and positions 

were modeled and responses simulated in a tank model with the transient solver of CST Microwave 

Studio over the frequency range 100 to 200 MHz. Three different mounting configurations were 

investigated for monopole antennas: top-mounted through the center of the tank lid, side-mounted on the 

side of the tank, and center-mounted on a central metal rod running from the top to the bottom of the tank. 

The simulations predict that mounting the antenna at the tank lid (top-mounted) will excite the transverse 

magnetic modes with moderate strength. Antennas mounted on the side of the tank are not predicted to 

strongly excite any modes in this frequency range, but this is contradictory to measurements which show 

a very good response. The center-mounted configuration in which the antenna was mounted on a central 

rod is predicted to be effective in exciting the lower frequency modes. Positioning the antenna at different 

locations along the central rod had a significant effect on the strength of excitation of the individual 

modes. A mounting height of 7/16 from the top was predicted to be especially effective. The responses of 

dipole antennas were modeled with both parallel and transverse mountings relative to a bar running along, 

but offset from, the long axis of the tank. The simulation results predict that a parallel mounting of the 

dipole is generally more effective at exciting the low frequency modes. The effects of baffles in the tank 

were also simulated. This had the effect of changing the amplitude of mode responses and slightly 

shifting the frequencies for several modes. Work is currently underway to experimentally test several of 

the configurations simulated here, and will be reported in a future publication. 
NASA/TM—2012-217267 9 

Appendix A.—Sample Eigenmodes Excited by Top-Mounted Configuration 

The electric field and magnetic field amplitudes of several eigenmodes for the top-mounted monopole 

antenna case are shown at a cross section of the tank: 

Figure A1.—Magnitude of electric field (left) and magnetic field (right) at cross section 

of the tank for eigenmode at 132.6 MHz for top-mounted monopole antenna. 

Figure A2.—Magnitude of (left) electric field and (right) magnetic field at cross section 

of the tank for eigenmode at 144.6 MHz for top-mounted monopole antenna. 
NASA/TM—2012-217267 10 

Figure A3.—Magnitude of (left) electric field and (right) magnetic field at cross section 

of the tank for eigenmode at 162.3 MHz for top-mounted monopole antenna. 

 Figure A4.—Magnitude of (left) electric field and (right) magnetic field at cross section of 

the tank for eigenmode at 184.0 MHz for top-mounted monopole antenna.
NASA/TM—2012-217267 11 

Appendix B.—Sample Eigenmodes Excited by Center-Mounted Configuration 

The electric field and magnetic field amplitudes of representative eigenmodes for the center-mounted 

antenna positioned 7/16 from the top are shown at a cross section of the tank. (The red arrow labeled “1” 

in each figure represents the antenna excitation port.) 

 

Figure B1.—Magnitude of electric field (left) and magnetic field (right) at cross section 

of the tank for eigenmode at 126.4 MHz for center mounted monopole mounted 7/16 

from the top. 

 

Figure B2.—Magnitude of electric field (left) and magnetic field (right) at cross section of the 

tank for eigenmode at 156.1 MHz for center mounted monopole mounted 7/16 from the top. 
NASA/TM—2012-217267 12 

 

Figure B3.—Magnitude of electric field (left) and magnetic field (right) at cross section of the 

tank for eigenmode at 160.3 MHz for center mounted monopole mounted 7/16 from the top. 

 

Figure B4.—Magnitude of electric field (left) and magnetic field (right) at cross section of the 

tank for eigenmode at 187.2 MHz for center mounted monopole mounted 7/16 from the top. 
NASA/TM—2012-217267 13 

Appendix C.—Eigenmodes Excited by Parallel Dipole Orientation 

The electric field and magnetic field amplitudes of several eigenmodes for the parallel dipole 

orientation are shown at a cross section of the tank: 

Figure C1.—Magnitude of electric field (left) and magnetic field (right) at cross section 

of the tank for eigenmode at 132.4 MHz for parallel dipole antenna orientation. 

 

Figure C2.—Magnitude of electric field (left) and magnetic field (right) at cross section 

of the tank for eigenmode at 136.1 MHz for parallel dipole antenna orientation. 

 
NASA/TM—2012-217267 14 

 

Figure C3.—Magnitude of electric field (left) and magnetic field (right) at cross section 

of the tank for eigenmode at 150.9 MHz for parallel dipole antenna orientation. 

 

Figure C4.—Magnitude of electric field (left) and magnetic field (right) at cross section 

of the tank for eigenmode at 157.7 MHz for parallel dipole antenna orientation. 

 
NASA/TM—2012-217267 15 

Figure C5.—Magnitude of electric field (left) and magnetic field (right) at cross section 

of the tank for eigenmode at 167.9 MHz for parallel dipole antenna orientation. 

Figure C6.—Magnitude of electric field (left) and magnetic field (right) at cross section 

of the tank for eigenmode at 186.4 MHz for parallel dipole antenna orientation. 

NASA/TM—2012-217267 17 

References 

1. G.A. Zimmerli, M. Asipauskas, J.D. Wagner, and J.C. Follo, “Propellant Quantity Gauging Using the 

Radio Frequency Mass Gauge,” 49th AIAA Aerospace Sciences Meeting, 4–7 January 2011, Orlando, 

Florida. 

2. CST Microwave Studio Workflow & Solver Overview, Computer Simulation Technology AG, 1998–

2010. 
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Model of the Radio Frequency (RF) Excitation Response from Monopole and Dipole 

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14. ABSTRACT

Good antenna-mode coupling is needed for determining the amount of propellant in a tank through the method of radio frequency mass 

gauging (RFMG). The antenna configuration and position in a tank are important factors in coupling the antenna to the natural 

electromagnetic modes. In this study, different monopole and dipole antenna mounting configurations and positions were modeled and 

responses simulated in a full-scale tank model with the transient solver of CST Microwave Studio (CST Computer Simulation Technology 

of America, Inc.). The study was undertaken to qualitatively understand the effect of antenna design and placement within a tank on the 

resulting radio frequency (RF) tank spectrum. 

15. SUBJECT TERMS

Antenna design; Dipole antennas; Monopole antennas; Microwaves; Propellant tanks; Radio frequencies 

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