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  • The Balcony Mounted Magnetic Loop Antenna

    A NEC Model Comparative Analysis of Physical Orientation and Performance

    by Dr. Carol F. Milazzo, KP4MD (posted 12 June 2015)
    E-mail: kp4md@arrl.net

    INTRODUCTION

    The small magnetic loop is a useful compromise antenna for limited space and portability.  For this reason, the magnetic loop antenna is a practical high frequency antenna solution for the restricted space of apartment dwellers.  Little has been written on the effect of antenna orientation on performance in this scenario.  This study presents a comparative 4nec21 model analysis on the effect of orientation of magnetic loop antennas mounted on an apartment building window or balcony with regard to antenna radiation pattern and efficiency.

    Question: Which orientation offers optimal
                    performance for a magnetic loop antenna mounted on a
                    balcony on this building?
    1. High rise apartment building
    4nec2 model geometry. The 138 foot x 138 foot
                    (42.1m x 42.1m) surface patch represents the
                    reflective surface of the building. The magnetic
                    loop antenna is positioned over the center of the
                    surface at 69 feet (21.05m) height. Distances from
                    the reflecting surface. Tuning capacitor 3.8 feet
                    (1.16m); Feed point 6.2 feet (1.89m).
    2. 4nec2 model geometry
    The magnetic loop antenna is positioned in the
                    horizontal plane. Tuning capacitor (blue); Feed
                    point (red).
    3. Horizontal orientation
    The magnetic loop antenna is positioned in the
                    vertical plane. Tuning capacitor (blue); Feed point
                    (red).
    4. Vertical orientation
    The Balcony mounted magnetic loop antenna is in
                    the red circle, showing its low profile.
    5. The magnetic loop antenna in the red circle,
    showing its low profile.
    Balcony mounted magnetic loop antenna. Vertical
                    orientation. Northwest view towards the Atlantic
                    Ocean and North America.
    6. Magnetic loop antenna. Vertical orientation.
    Northwest view towards North America.

    MODEL DESIGN

    The problem was to compare the performance of a Chameleon CHA F-Loop Antenna,  a 0.74 m (2.44 feet) diameter radiator loop of DX Engineering DXE-400 MAX brand of LMR-400 coaxial cable, mounted either horizontally or vertically on a balcony railing near the center of the high rise apartment building in Figure 1.  This situation is similar to a right angle corner reflector antenna with the ground and the building wall as the reflectors.  A validated NEC model for this antenna2 was positioned precisely over the center of a 20 20 square surface patch vertical reflector measuring 40m 40m (Figure 2).  This placed the antenna at precisely 0.5 wavelength above ground on 7 MHz, one wavelength at 14 MHz, etc.  The ground type was specified as "Average".  The antenna was positioned with its center 5 feet (1.52m) away from the reflector with the capacitor side closest to the reflector (Figures 3 and 4).  Moving the antenna closer to the reflecting wall resulted in Average Gain Test failure due to limitations of the NEC code.  The test was run with the antenna resonated at 7.1, 14.2, 21.2 and 28.4 MHz.  This represents 0.5, 1, 1.5 and 2 wavelengths above ground respectively. To observe the effect of lower antenna heights, the test was then repeated for the 7.1 MHz resonated antenna at 0.1, 0.2, 0.3 and 0.4 wavelengths above ground.

    The predicted gain will vary from the true gain as the model does not reproduce the antenna mounted directly on a balcony railing, nor does it account for the complex reflectivity of the building wall nor for other reflective surfaces and obstacles within the vicinity.  The NEC surface patch offers slightly less accuracy than a wire grid reflector but was chosen for its fourfold faster execution.3  In any case, these models should offer useful comparative information on the expected efficiency and radiation pattern for each antenna orientation.

    DATA

    The data are tabulated below for each tested frequency in the following order:

    1. Main screen (horizontal orientation);
    2. Main screen (vertical orientation);
    3. Azimuth radiation pattern for maximum gain in horizontal orientation;
    4. Azimuth radiation pattern for maximum gain in vertical orientation;
    5. Elevation radiation pattern;
    6. 3 dimensional radiation pattern in horizontal orientation;
    7. 3 dimensional radiation pattern in vertical orientation.

    In each of the two dimensional radiation patterns, the red trace is for horizontal and the blue trace is for vertical orientation.

    0.5 to 2 Wavelengths Above Ground

    7.1 MHz (0.5 λ) 14.2 MHz (1 λ) 21.2 MHz (1.5 λ) 28.4 MHz (2 λ)
    7.1 MHz Magnetic Loop Antenna Parameters -
                    Horizontal orientation
    7.1 MHz Magnetic Loop Antenna Parameters - Horizontal orientation
    14.2 MHz Magnetic Loop Antenna Parameters -
                    Horizontal orientation
    14.2 MHz Magnetic Loop Antenna Parameters - Horizontal orientation
    21.2 MHz Magnetic Loop Antenna Parameters -
                    Horizontal orientation
    21.2 MHz Magnetic Loop Antenna Parameters - Horizontal orientation
    28.4 MHz Magnetic Loop Antenna Parameters -
                    Horizontal orientation
    28.4 MHz Magnetic Loop Antenna Parameters - Horizontal orientation
    7.1 MHz Magnetic Loop Antenna Parameters -
                    Vertical orientation
    7.1 MHz Magnetic Loop Antenna Parameters - Vertical orientation
    14.2 MHz Magnetic Loop Antenna Parameters -
                    Vertical orientation
    14.2 MHz Magnetic Loop Antenna Parameters - Vertical orientation
    21.2 MHz Magnetic Loop Antenna Parameters -
                    Vertical orientation
    21.2 MHz Magnetic Loop Antenna Parameters - Vertical orientation
    28.4 MHz Magnetic Loop Antenna Parameters -
                    Vertical orientation
    28.4 MHz Magnetic Loop Antenna Parameters - Vertical orientation
    7.1 MHz Magnetic Loop Antenna - Azimuth
                    radiation pattern at 25 elevation
    7.1 MHz Magnetic Loop Antenna - Azimuth radiation pattern at 25 elevation
    14.2 MHz Magnetic Loop Antenna - Azimuth
                    radiation pattern at 15 elevation
    14.2 MHz Magnetic Loop Antenna - Azimuth radiation pattern at 15 elevation
    21.2 MHz Magnetic Loop Antenna - Azimuth
                    radiation pattern at 10 elevation
    21.2 MHz Magnetic Loop Antenna - Azimuth radiation pattern at 10 elevation
    28.4 MHz Magnetic Loop Antenna - Azimuth
                    radiation pattern at 20 elevation
    28.4 MHz Magnetic Loop Antenna - Azimuth radiation pattern at 20 elevation
    7.1 MHz Magnetic Loop Antenna - Azimuth
                    radiation pattern at 55 elevation
    7.1 MHz Magnetic Loop Antenna - Azimuth radiation pattern at 55 elevation
    14.2 MHz Magnetic Loop Antenna - Azimuth
                    radiation pattern at 30 elevation
    14.2 MHz Magnetic Loop Antenna - Azimuth radiation pattern at 30 elevation
    21.2 MHz Magnetic Loop Antenna - Azimuth
                    radiation pattern at 40 elevation
    21.2 MHz Magnetic Loop Antenna - Azimuth radiation pattern at 40 elevation
    28.4 MHz Magnetic Loop Antenna - Azimuth
                    radiation pattern at 50 elevation
    28.4 MHz Magnetic Loop Antenna - Azimuth radiation pattern at 50 elevation
    7.1 MHz Magnetic Loop Antenna - Elevation
                    radiation pattern
    7.1 MHz Magnetic Loop Antenna - Elevation radiation pattern
    14.2 MHz Magnetic Loop Antenna - Elevation
                    radiation pattern
    14.2 MHz Magnetic Loop Antenna - Elevation radiation pattern
    21.2 MHz Magnetic Loop Antenna - Elevation
                    radiation pattern
    21.2 MHz Magnetic Loop Antenna - Elevation radiation pattern
    28.4 MHz Magnetic Loop Antenna - Elevation
                    radiation pattern
    28.4 MHz Magnetic Loop Antenna - Elevation radiation pattern
    7.1 MHz Magnetic Loop Antenna Parameters -
                    Horizontal orientation
    7.1 MHz Magnetic Loop Antenna Radiation Pattern - Horizontal orientation
    14.2 MHz Magnetic Loop Antenna Parameters -
                    Horizontal orientation
    14.2 MHz Magnetic Loop Antenna Radiation Pattern - Horizontal orientation
    21.2 MHz Magnetic Loop Antenna Parameters -
                    Horizontal orientation
    21.2 MHz Magnetic Loop Antenna Radiation Pattern - Horizontal orientation
    28.4 MHz Magnetic Loop Antenna Parameters -
                    Horizontal orientation
    28.4 MHz Magnetic Loop Antenna Radiation Pattern - Horizontal orientation
    7.1 MHz Magnetic Loop Antenna Parameters -
                    Vertical orientation
    7.1 MHz Magnetic Loop Antenna Radiation Pattern - Vertical orientation
    14.2 MHz Magnetic Loop Antenna Parameters -
                    Vertical orientation
    14.2 MHz Magnetic Loop Antenna Radiation Pattern - Vertical orientation
    21.2 MHz Magnetic Loop Antenna Parameters -
                    Vertical orientation
    21.2 MHz Magnetic Loop Antenna Radiation Pattern - Vertical orientation
    28.4 MHz Magnetic Loop Antenna Parameters -
                    Vertical orientation
    28.4 MHz Magnetic Loop Antenna Radiation Pattern - Vertical orientation

    0.1 to 0.4 Wavelengths Above Ground

    7.1 MHz (0.1 λ) 7.1 MHz (0.2 λ) 7.1 MHz (0.3 λ) 7.1 MHz (0.4 λ)
    7.1 MHz Magnetic Loop Antenna Parameters -
                    Horizontal orientation at 4m (0.1 λ)
    7.1 MHz Magnetic Loop Antenna Parameters - Horizontal orientation at 4m (0.1 λ)
    7.1 MHz Magnetic Loop Antenna Parameters -
                    Horizontal orientation at 8m (0.2 λ)
    7.1 MHz Magnetic Loop Antenna Parameters - Horizontal orientation at 8m (0.2 λ)
    7.1 MHz Magnetic Loop Antenna Parameters -
                    Horizontal orientation at 12m (0.3 λ)
    7.1 MHz Magnetic Loop Antenna Parameters - Horizontal orientation at 12m (0.3 λ)
    7.1 MHz Magnetic Loop Antenna Parameters -
                    Horizontal orientation at 16m (0.4 λ)
    7.1 MHz Magnetic Loop Antenna Parameters - Horizontal orientation at 16m (0.4 λ)
    7.1 MHz Magnetic Loop Antenna Parameters -
                    Vertical orientation at 4m (0.1 λ)
    7.1 MHz Magnetic Loop Antenna Parameters - Vertical orientation at 4m (0.1 λ)
    7.1 MHz Magnetic Loop Antenna Parameters -
                    Vertical orientation at 8m (0.2 λ)
    7.1 MHz Magnetic Loop Antenna Parameters - Vertical orientation at 8m (0.2 λ)
    7.1 MHz Magnetic Loop Antenna Parameters -
                    Vertical orientation at 12m (0.3 λ)
    7.1 MHz Magnetic Loop Antenna Parameters - Vertical orientation at 12m (0.3 λ)
    7.1 MHz Magnetic Loop Antenna Parameters -
                    Vertical orientation at 16m (0.4 λ)
    7.1 MHz Magnetic Loop Antenna Parameters - Vertical orientation at 16m (0.4 λ)
    7.1 MHz Magnetic Loop Antenna at 4m (0.1 λ) -
                    Azimuth radiation pattern at 40 elevation
    7.1 MHz Magnetic Loop Antenna at 4m (0.1 λ) - Azimuth radiation pattern at 40 elevation
    7.1 MHz Magnetic Loop Antenna at 8m (0.2 λ) -
                    Azimuth radiation pattern at 35 elevation
    7.1 MHz Magnetic Loop Antenna at 8m (0.2 λ) - Azimuth radiation pattern at 35 elevation
    7.1 MHz Magnetic Loop Antenna at 12m (0.3 λ) -
                    Azimuth radiation pattern at 20 elevation
    7.1 MHz Magnetic Loop Antenna at 12m (0.3 λ) - Azimuth radiation pattern at 20 elevation
    7.1 MHz Magnetic Loop Antenna at 16m (0.4 λ) -
                    Azimuth radiation pattern at 15 elevation
    7.1 MHz Magnetic Loop Antenna at 16m (0.4 λ) - Azimuth radiation pattern at 15 elevation
    7.1 MHz Magnetic Loop Antenna at 4m (0.1 λ) -
                    Azimuth radiation pattern at 40 elevation
    7.1 MHz Magnetic Loop Antenna at 4m (0.1 λ) - Azimuth radiation pattern at 40 elevation
    7.1 MHz Magnetic Loop Antenna at 8m (0.2 λ) -
                    Azimuth radiation pattern at 35 elevation
    7.1 MHz Magnetic Loop Antenna at 8m (0.2 λ) - Azimuth radiation pattern at 35 elevation
    7.1 MHz Magnetic Loop Antenna at 12m (0.3 λ) -
                    Azimuth radiation pattern at 30 elevation
    7.1 MHz Magnetic Loop Antenna at 12m (0.3 λ) - Azimuth radiation pattern at 30 elevation
    7.1 MHz Magnetic Loop Antenna at 16m (0.4 λ) -
                    Azimuth radiation pattern at 30 elevation
    7.1 MHz Magnetic Loop Antenna at 16m (0.4 λ) - Azimuth radiation pattern at 30 elevation
    7.1 MHz Magnetic Loop Antenna at 4m (0.1 λ) -
                    Elevation radiation pattern
    7.1 MHz Magnetic Loop Antenna at 4m (0.1 λ) - Elevation radiation pattern
    7.1 MHz Magnetic Loop Antenna at 8m (0.2 λ) -
                    Elevation radiation pattern
    7.1 MHz Magnetic Loop Antenna at 8m (0.2 λ) - Elevation radiation pattern
    7.1 MHz Magnetic Loop Antenna at 12m (0.3 λ) -
                    Elevation radiation pattern
    7.1 MHz Magnetic Loop Antenna at 12m (0.3 λ) - Elevation radiation pattern
    7.1 MHz Magnetic Loop Antenna at 16m (0.4 λ) -
                    Elevation radiation pattern
    7.1 MHz Magnetic Loop Antenna at 16m (0.4 λ) - Elevation radiation pattern
    7.1 MHz Magnetic Loop Antenna Parameters -
                    Horizontal orientation at 4m (0.1 λ)
    7.1 MHz Magnetic Loop Antenna Pattern - Horizontal orientation at 4m (0.1 λ)
    7.1 MHz Magnetic Loop Antenna Parameters -
                    Horizontal orientation at 8m (0.2 λ)
    7.1 MHz Magnetic Loop Antenna Pattern - Horizontal orientation at 8m (0.2 λ)
    7.1 MHz Magnetic Loop Antenna Parameters -
                    Horizontal orientation at 12m (0.3 λ)
    7.1 MHz Magnetic Loop Antenna Pattern - Horizontal orientation at 12m (0.3 λ)
    7.1 MHz Magnetic Loop Antenna Parameters -
                    Horizontal orientation at 16m (0.4 λ)
    7.1 MHz Magnetic Loop Antenna Pattern - Horizontal orientation at 16m (0.4 λ)
    7.1 MHz Magnetic Loop Antenna Parameters -
                    Vertical orientation at 4m (0.1 λ)
    7.1 MHz Magnetic Loop Antenna Pattern - Vertical orientation at 4m (0.1 λ)
    7.1 MHz Magnetic Loop Antenna Parameters -
                    Vertical orientation at 8m (0.2 λ)
    7.1 MHz Magnetic Loop Antenna Pattern - Vertical orientation at 8m (0.2 λ)
    7.1 MHz Magnetic Loop Antenna Parameters -
                    Vertical orientation at 12m (0.3 λ)
    7.1 MHz Magnetic Loop Antenna Pattern - Vertical orientation at 12m (0.3 λ)
    7.1 MHz Magnetic Loop Antenna Parameters -
                    Vertical orientation at 16m (0.4 λ)
    7.1 MHz Magnetic Loop Antenna Pattern - Vertical orientation at 16m (0.4 λ)

    DISCUSSION

    Predicted Antenna Parameters

    Parameter Orientation 7.1 MHz
    @ 0.5 λ
    14.2 MHz
    @ 1 λ
    21.2 MHz
    @ 1.5 λ
    28.4 MHz
    @ 2 λ

    Parameter Orientation 7 MHz
    @ 0.1 λ
    7 MHz
    @ 0.2 λ
    7 MHz
    @ 0.3 λ
    7 MHz
    @ 0.4 λ
    Radiation efficiency Horizontal 1.113% 6.205% 12.63% 18.94%
    Radiation efficiency Horizontal 0.216% 0.608% 1.04% 1.12%
    Vertical 0.695% 4.352% 8.972% 13.67%
    Vertical 0.85% 0.484% 0.9% 0.433%
    Major lobe elevation Horizontal 25 15 10 20
    Major lobe elevation Horizontal 40 35 30 30
    Vertical 55 30 40 50
    Vertical 40 35 20 15
    Major lobe gain dB Horizontal -8.6 0 3.22 3.51
    Major lobe gain dB Horizontal -17 -12 -10 -9.1
    Vertical -11 -3.1 0.2 2.65
    Vertical -11 -13 -14 -14

    The models predict a uniformly higher radiation efficiency and major lobe gain in the horizontal orientation for antenna heights of 0.2 wavelengths and higher. The higher efficiency is expected because in the vertical orientation half of the total radiation is directed towards the ground and therefore subject to ground absorption. At higher frequencies, the radiation pattern exhibits increasing numbers of lobes and nulls. At antenna heights of 0.5 wavelength and above, the horizontal orientation uniformly yields lower angle maximum radiation lobes that would favor long distance ionospheric propagation. At these antenna heights, the vertical orientation yields higher angle radiation lobes, offering a 7 dB signal increase over the horizontal orientation at 55 elevation for short distance near vertical incidence skywave (NVIS) propagation on 7 MHz.

    CONCLUSIONS

    This NEC model comparison suggests that the magnetic loop antenna is preferably oriented in the
    horizontal plane for long distance ionospheric propagation and in the vertical plane for short distance
    NVIS propagation.  In practice, the optimal orientation of the antenna may also depend on the
    polarization of local radio noise.  The vertical orientation would also be preferred for operation
    through vertically polarized 28 MHz FM repeaters.

    REFERENCES

    1. 4nec2 Antenna Modeling Program, Voors, A
    2. Chameleon CHA F-Loop Antenna Parameters, Milazzo, C, KP4MD
    3. Planar Reflectors: Wire Grid vs. SM Patches, Cebik, L, W4RNL
    4. A Universal HF Magnetic Loop Model, Milazzo, C, KP4MD

    PHOTO ALBUMS

    1. Balcony Mounted Magnetic Loop Antenna, Milazzo, C, KP4MD
    2. Balcony Rail Mount for Magnetic Loop Antenna, Milazzo, C, KP4MD
    3. KP4MD/P Amateur Radio Station in Puerto Rico, Milazzo, C, KP4MD

    NEC Model Parameters2

    Height above ground 69 feet (21.05 m)
    Simulated ground type
    Average
    Loop diameter 2.44 feet (0.74 m)
    Loop circumference
    92 inches (2.34 m)
    Loop NEC model segments 18
    DXE-400 cable outer diameter
    0.405 inches (10.3 mm)
    Outer Jacket material
    PVC
    Outer braid diameter
    0.32 inches (8.13 mm)
    DXE-400 braid conductivity
    4500000 mhos/m
    Capacitor Q=Xc/Rc* 118-1800*
    *In switch position A: Rc = 0.032 + 5800/(FMHzCpF) + 0.0039√FMHz
    In switch position B: Rc = 2 (5800/(FMHzCpF) + 0.0039√FMHz)

    4nec2 Model Files

    Antenna Orientation
    Horizontal
    Vertical
    7.1 MHz 7.1 MHz
    14.2 MHz 14.2 MHz
    21.2 MHz 21.2 MHz
    28.4 MHz 28.4 MHz
    Antenna Orientation
    Horizontal
    Vertical
    7 MHz @ 0.1 λ 7 MHz @ 0.1 λ
    7 MHz @ 0.2 λ 7 MHz @ 0.2 λ
    7 MHz @ 0.3 λ 7 MHz @ 0.3 λ
    7 MHz @ 0.4 λ 7 MHz @ 0.4 λ
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