The Quadrifilar Helix (QFH) antenna, also referred to as the QHA, stands as an excellent choice for signal reception from the satellites of the National Oceanic and Atmospheric Administration (NOAA). This antenna configuration boasts omnidirectional capabilities and is composed of four helically wound wires, intertwining to create a helix—an intricate geometric arrangement that imparts distinctive properties to this antenna.

The design depicted in the image below showcases a configuration with a diameter of approximately 0.14 times the wavelength (λ) and helix lengths of 0.4λ. A resonant frequency of 137.5 MHz ensures its optimal functionality, accompanied by a bandwidth spanning 6%, defined by a standing wave ratio (SWR) below 2. This composition serves a dual purpose: enabling efficient signal reception from NOAA satellites while effectively mitigating external interference.

A noteworthy observation emerges when evaluating the antenna’s radiation pattern under varying conditions. In a vacuum environment, the radiation pattern orients downward when the coaxial cable, simulated as a voltage source in simulations, connects to the antenna’s apex. This phenomenon is depicted in the leftward radiation pattern. However, when the QFH antenna is positioned in real-world scenarios, a substantial transformation occurs. Placing the antenna at an elevation of 5 meters and introducing a real ground plane yields an entirely distinct radiation pattern.

The resultant pattern in this real-world configuration displays omnidirectional properties within the azimuth plane. Notably, this pattern showcases multiple lobes concerning elevation angles. The practical application of this radiation pattern is invaluable for effectively capturing signals from a variety of satellite orientations. AN-SOF allows us to accurately model the behavior of these intricate helices, courtesy of the implementation of the Conformal Method of Moments, which utilizes curved segments to faithfully represent the contour of the helices.

In summary, the Quadrifilar Helix (QFH) antenna stands as an ingeniously designed configuration, adeptly capturing signals from NOAA satellites. The configuration of four helically wound wires offers omnidirectional capabilities and robust interference suppression.

Small helical antennas for the 433 MHz ISM band are a good example where we need advanced software that has the ability to model: 

• Curved wires with exact description of the helix contour.
• Very close wires, spaced a fraction of the wire radius.
• Horizontal wires almost touching the ground plane.
• Bent wires at right angles or less.
• Short segments, below 0.001 wavelength.
• Thick wires by means of Exact Kernel instead of thin-wire approximation.

AN-SOF includes all these features.

In this example, a perfect ground plane is used to get the input impedance: 48 Ohm, very close to the 50 Ohm of the antenna datasheet. We must subtract 3 dBi from the gain to obtain the gain in free space. It is a simple model, but it coincides very well with reality.

Download >