This Quadrifilar Helix (QFH) antenna, also known as QHA, can be used for signal reception from NOAA satellites. These are omnidirectional antennas made up of 4 wires with a helical shape. This design is about 0.14λ in diameter and the length of each helix is 0.4λ. It resonates at 137.5 MHz and has a bandwidth of 6% (SWR < 2). The purpose of the helices is to block surrounding interference.

In free space, the radiation pattern actually points downward when the coaxial cable (the source in the simulation) is connected to the top of the antenna. However, when a real ground plane is added and the antenna is placed 5 meters high, multiple lobes and a more omnidirectional pattern are obtained, as expected.

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Blog > Antennas and Beyond!

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.

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Blog > Antennas and Beyond!