{"id":3834,"date":"2022-11-22T03:38:40","date_gmt":"2022-11-22T03:38:40","guid":{"rendered":"https:\/\/antennasimulator.com\/?p=3834"},"modified":"2026-02-26T18:21:08","modified_gmt":"2026-02-26T18:21:08","slug":"qfh-antenna-for-noaa-satellite-reception","status":"publish","type":"epkb_post_type_1","link":"https:\/\/antennasimulator.com\/index.php\/knowledge-base\/qfh-antenna-for-noaa-satellite-reception\/","title":{"rendered":"Efficient NOAA Satellite Signal Reception with the Quadrifilar Helix Antenna"},"content":{"rendered":"\n

The Quadrifilar Helix antenna (QFH or QHA), with its unique design and circular polarization, ensures efficient NOAA satellite signal reception. This article explores the history, key characteristics, and practical modeling of QFH antennas using AN-SOF, providing valuable insights for RF engineers and enthusiasts.<\/p>\n\n\n\n

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Introduction<\/h3>\n\n\n\n
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The Quadrifilar Helix (QFH)<\/strong> antenna, also known as the QHA<\/strong>, is an excellent choice for receiving signals from the National Oceanic and Atmospheric Administration (NOAA) satellites<\/strong>. This antenna configuration consists of four helically wound monofilar wires that intertwine to form a quadrifilar helix (Fig. 1). This intricate geometric arrangement gives the antenna unique properties in a compact size:<\/strong> a directional radiation pattern, circular polarization, and self-resonance with a feedpoint impedance close to 50 Ohms<\/strong>. While the QFH antenna’s bandwidth is narrower compared to a traveling-wave axial mode helix, its compact form factor makes up for this limitation.<\/p>\n\n\n\n

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Fig. 1: Quadrifilar Helix Antennas (QFH or QHA) (images licensed under Creative Commons).<\/em><\/figcaption><\/figure>\n<\/div>\n\n\n
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Axial Mode Helical Antennas: History and Fundamentals<\/h3>\n\n\n\n
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The axial mode helical antenna<\/strong> was invented in 1946 by John Kraus. Inspiration can come when we least expect it:<\/p>\n\n\n\n

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I attended an afternoon lecture on traveling-wave tubes by a famous scientist… In these tubes an electron beam is fired down the inside of a long wire helix for amplification of waves traveling along the helix. The helix is only a small fraction of a wavelength in diameter and acts as a guiding structure. … I asked the visitor if he thought a helix could be used as an antenna. ‘No,’ he replied, ‘I’ve tried it and it doesn’t work.’ The finality of his answer set me thinking. If the helix were larger in diameter than in a traveling-wave tube, I felt that it would have to radiate in some way, but how, I did not know. I determined to find out.<\/p>\nDr. John D. Kraus, in “Antennas,” 2nd Ed. McGraw-Hill, 1988.<\/cite><\/blockquote>\n\n\n\n

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Today, helical antennas can be easily simulated (Fig. 2). Being wire antennas<\/strong>, the most efficient way to simulate them is through the Method of Moments (MoM)<\/a><\/strong>. However, a poor convergence rate is often obtained for the input impedance when the helix is approximated by straight segments, as is customary. Using curved segments<\/a><\/strong> that exactly follow the helix contour overcomes this problem.<\/p>\n\n\n\n

The axial mode helical antenna is one of the most widely used antennas for UHF and microwave communications. Its robust design makes it ideal for both space and ground applications, contributing significantly to the explosive growth of satellite-based services.<\/p>\n\n\n\n

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Fig. 2: Axial Mode Helical Antenna. The antenna radiates a beam along its axis. The feedpoint is located between the helix base and the ground plane.<\/em><\/figcaption><\/figure>\n<\/div>\n\n\n
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Traveling-Wave Antennas and Axial-Mode Helix:<\/strong> Traveling-wave antennas are a class of antennas where the radio frequency (RF) energy travels along a structure, such as a wire, in a continuous wave. The axial mode helix is a specific type of traveling-wave antenna designed to operate in the axial mode. For a helix to operate in this mode, the circumference of the helix should be close to one wavelength<\/strong> of the operating frequency. Additionally, a metallic ground plane<\/strong>, often of circular shape, is added to the helix base, as shown in Fig. 2. These conditions allow the antenna to produce a directional radiation pattern with a main beam along the axis of the helix.<\/p>\n\n\n\n

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Circular Polarization:<\/strong> The axial-mode helical antenna exhibits circular polarization, which can be either right-hand circular polarization (RHCP) or left-hand circular polarization (LHCP), depending on the winding sense of the helix. Circular polarization is achieved when the electric field of the wave rotates in a circular motion as it propagates. The sense of rotation\u2014RHCP or LHCP\u2014is determined by the direction in which the helix is wound. RHCP helical antennas are typically used for transmitting WEFAX pictures due to their reliable performance in space communications.<\/p>\n\n\n\n

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Importance of Circular Polarization:<\/strong> Circular polarization is crucial in both space communications and terrestrial mobile applications. In space communications, circular polarization helps to mitigate the effects of Faraday rotation<\/strong><\/a>, an unpredictable phenomenon caused by the ionosphere, which can alter the polarization of the signal. By using circular polarization, satellite communications can maintain consistent signal quality despite these changes. In terrestrial mobile applications, circular polarization helps to reduce signal degradation caused by multipath interference, where signals bounce off various obstacles before reaching the receiver.<\/p>\n\n\n\n

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Key Characteristics of Quadrifilar Helical Antennas<\/h3>\n\n\n\n
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The axial-mode helix antenna exhibits a consistent input impedance over a wide bandwidth, thanks to its nature as a traveling-wave antenna. For optimal performance, a ground plane with a diameter of about half to one wavelength is required. This antenna type can achieve a gain of 10-17 dBi over a 60% fractional bandwidth. However, with feedpoint impedances ranging from 150 to 300 Ohms, impedance matching is necessary for efficient operation in 50 Ohm systems.<\/p>\n\n\n\n

By adding extra windings<\/strong>, the radiation pattern of the axial-mode helix can be tightened, and sidelobes can be reduced compared to a monofilar helix. The Quadrifilar Helix (QFH)<\/strong> antenna comprises four windings of equal torsion<\/strong>. Unlike long traveling-wave antennas, the QFH can be shortened to sizes commensurate with half a wavelength and operate as a resonant antenna<\/strong>, similar to a resonant dipole or loop antenna.<\/p>\n\n\n\n

As a resonant antenna, the QFH has a narrow bandwidth<\/strong>, requiring careful attention to its dimensions and construction details. Its compactness and ease of integration with mobile systems make the short resonant QFH ideal for portable applications.<\/p>\n\n\n\n

Each component helix of a QFH is excited in a 90-degree progression<\/strong>, either clockwise or counterclockwise, depending on the desired polarization and lobe direction. QFHs can operate in either endfire<\/strong> or backfire<\/strong> modes, producing a hemispherical directional pattern (Fig. 3). For long quadrifilar antennas, a quadrature feeding network is necessary to generate the 90\u00b0 phase progressions. This can be achieved using quadrature hybrids and power splitters. For the small resonant helix, using two co-wound half helices with slightly different dimensions can induce quadrature excitation, similar to the “nearly square” method of generating circular polarization in a microstrip patch antenna.<\/p>\n\n\n\n

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Fig. 3: Endfire (left) and backfire (right) modes of operation of QFH antennas.<\/em><\/figcaption><\/figure>\n<\/div>\n\n\n
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The feed phasing sense relative to the QFH winding sense determines the radiation mode:<\/strong> if they match, the antenna will be backfire; if they oppose, the antenna will be endfire. The circular polarization sense of the radiation is always opposite to the helix winding sense, regardless of the feed phasing sense. However, if a backfire helix antenna is used with a reflector or ground plane at the feed, the sense of the circular polarization is reversed, making the antenna endfire with the polarization sense corresponding to the helix winding.<\/p>\n\n\n\n

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Summary:<\/strong><\/p>\n\n\n\n