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.

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:

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

---

0.1 to 0.4 Wavelengths Above Ground

---

DISCUSSION

Predicted Antenna Parameters

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 and at least 0.5 λ above ground for long distance ionospheric propagation and in the vertical plane for short distance NVIS propagation.  In practice, the optimal orientation of the antenna will also depend on the polarization of local radio noise and the height above ground.  The vertical orientation would also be preferred for operation through vertically polarized 28 MHz FM repeaters. REFERENCES 4nec2 Antenna Modeling Program, Voors, A Chameleon CHA F-Loop Antenna Parameters, Milazzo, C, KP4MD Planar Reflectors: Wire Grid vs. SM Patches, Cebik, L, W4RNL A Universal HF Magnetic Loop Model, Milazzo, C, KP4MD PHOTO ALBUMS Balcony Mounted Magnetic Loop Antenna, Milazzo, C, KP4MD Balcony Rail Mount for Magnetic Loop Antenna, Milazzo, C, KP4MD 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/(FMHz�CpF�) + 0.0039�√FMHz In switch position B: Rc = 2 � (5800/(FMHz�CpF�) + 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