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: [email protected]



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.



1. High rise apartment building



2. 4nec2 model geometry



3. Horizontal orientation



4. Vertical orientation



5. The magnetic loop antenna in the red circle,

showing its low profile.

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:

Main screen (horizontal orientation); Main screen (vertical orientation); Azimuth radiation pattern for maximum gain in horizontal orientation; Azimuth radiation pattern for maximum gain in vertical orientation; Elevation radiation pattern; 3 dimensional radiation pattern in horizontal orientation; 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



14.2 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



7.1 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



28.4 MHz Magnetic Loop Antenna Parameters - Vertical orientation



7.1 MHz Magnetic Loop Antenna - Azimuth radiation pattern at 25° 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



28.4 MHz Magnetic Loop Antenna - Azimuth radiation pattern at 20° 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



21.2 MHz Magnetic Loop Antenna - Azimuth radiation pattern at 40° elevation



28.4 MHz Magnetic Loop Antenna - Azimuth radiation pattern at 50° elevation



7.1 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



28.4 MHz Magnetic Loop Antenna - Elevation radiation pattern



7.1 MHz Magnetic Loop Antenna Radiation Pattern - Horizontal orientation



14.2 MHz Magnetic Loop Antenna Radiation Pattern - Horizontal orientation



21.2 MHz Magnetic Loop Antenna Radiation Pattern - Horizontal orientation



28.4 MHz Magnetic Loop Antenna Radiation Pattern - Horizontal orientation



7.1 MHz Magnetic Loop Antenna Radiation Pattern - Vertical orientation



14.2 MHz Magnetic Loop Antenna Radiation Pattern - Vertical orientation



21.2 MHz Magnetic Loop Antenna Radiation Pattern - 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 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 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 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 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 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 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 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 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 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 16m (0.4 λ) - Elevation radiation pattern



7.1 MHz Magnetic Loop Antenna Pattern - Horizontal orientation at 4m (0.1 λ)



7.1 MHz Magnetic Loop Antenna Pattern - Horizontal orientation at 8m (0.2 λ)



7.1 MHz Magnetic Loop Antenna Pattern - Horizontal orientation at 12m (0.3 λ)



7.1 MHz Magnetic Loop Antenna Pattern - Horizontal orientation at 16m (0.4 λ)



7.1 MHz Magnetic Loop Antenna Pattern - Vertical orientation at 4m (0.1 λ)



7.1 MHz Magnetic Loop Antenna Pattern - Vertical orientation at 8m (0.2 λ)



7.1 MHz Magnetic Loop Antenna Pattern - Vertical orientation at 12m (0.3 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 25º 15º 10º 20º

Major lobe elevation Horizontal 40º 35º 30º 30º Vertical 55º 30º 40º 50º

Vertical 40º 35º 20º 15º 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 55º elevation for short distance near vertical incidence skywave (NVIS) propagation on 7 MHz.