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A helical antenna is an antenna consisting of a conducting wire wound in the form of a helix. In most cases, helical antennas are mounted over a ground plane. The feed line is connected between the bottom of the helix and the ground plane. Helical antennas can operate in one of two principal modes — normal mode or axial mode. In the ''normal mode'' or ''broadside'' helix, the dimensions of the helix (the diameter and the pitch) are small compared with the wavelength. The antenna acts similarly to an electrically short dipole or monopole, and the radiation pattern, similar to these antennas is omnidirectional, with maximum radiation at right angles to the helix axis. The radiation is linearly polarised parallel to the helix axis. These are used for compact antennas for portable and mobile two-way radios, and for UHF television broadcasting antennas. In the ''axial mode'' or ''end-fire'' helix, the dimensions of the helix are comparable to a wavelength. The antenna functions as a directional antenna radiating a beam off the ends of the helix, along the antenna's axis. It radiates circularly polarised radio waves. These are used for satellite communication. ==Normal-mode helical== If the diameter and ''pitch'' (axial distance between successive turns) of the helix are significantly less than a wavelength, the antenna is called a ''normal-mode'' helix. The antenna acts similar to a monopole antenna, with an omnidirectional radiation pattern, radiating equal power in all directions perpendicular to the antenna. However, because of the inductance added by the helical shape, the antenna acts like a ''inductively loaded'' monopole; at its resonant frequency it is shorter than a quarter-wavelength long. Therefore normal-mode helices can be used as electrically short monopoles, an alternative to center- or base-loaded whip antennas, in applications where a full sized quarter-wave monopole would be too big. As with other electrically short antennas, the gain, and thus the communication range, of the helix will be less than that of a full sized antenna. Their compact size makes "helicals" useful as antennas for mobile and portable communications equipment on the HF, VHF, and UHF bands. An effect of using a helical conductor rather than a straight one is that the matching impedance is changed from the nominal 50 ohms to between 25 to 35 ohms base impedance. This does not seem to be adverse to operation or matching with a normal 50 ohm transmission line, provided the connecting feed is the electrical equivalent of a 1/2 wavelength at the frequency of operation. Another example of the type as used in mobile communications is "spaced constant turn" in which two or more different linear windings are wound on a single former and spaced so as to provide an efficient balance between capacitance and inductance for the radiating element at a particular resonant frequency. Many examples of this type have been used extensively for 27 MHz CB radio with a wide variety of designs originating in the US and Australia in the late 1960s. Multi-frequency versions with plug-in taps have become the mainstay for multi-band Single-sideband modulation (SSB) HF communications. Most examples were wound with copper wire using a fiberglass rod as a former. This flexible radiator is then covered with heat-shrink tubing which provides a resilient and rugged waterproof covering for the finished mobile antenna. These popular designs are still in common use and have been universally adapted as standard FM receiving antennas for many factory produced motor vehicles as well as the existing basic style of aftermarket HF and VHF mobile helical. The most common use for broadside helixes is in the "rubber ducky antenna" found on most portable VHF and UHF radios. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Helical antenna」の詳細全文を読む スポンサード リンク
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