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The fundamental resonance of a thin linear conductor occurs at a frequency whose free-space wavelength is twice the wire's length; i.e. where the conductor is 1 / 2 wavelength long. Dipole antennas are frequently used at around that frequency and thus termed half-wave dipole antennas. This important case is dealt with in the next section.
The two-wire version is often described as a "squashed loop antenna", since the total length of wire is one wavelength, and efficiency / radiation resistance of the folded dipole is very high: 4× that of a single dipole, [citation needed] analogous to the high efficiency of large loops. Any number of similar parallel wires may be added, with ...
The dipole elements are 15.55 metres (51.0 ft) and the impedance-matching symmetric feedline (ladder-line or twin-lead) can be either 300 Ω (8.84 metres or 29.0 feet) or 450 Ω (10.36 metres or 34.0 feet). [7]
In physics, a dipole (from Ancient Greek δίς (dís) 'twice' and πόλος (pólos) 'axis') [1] [2] [3] is an electromagnetic phenomenon which occurs in two ways: An electric dipole deals with the separation of the positive and negative electric charges found in any electromagnetic system. A simple example of this system is a pair of charges ...
In general terms, at any given frequency the log-periodic design operates somewhat similar to a three-element Yagi antenna; the dipole element closest to resonant at the operating frequency acts as a driven element, with the two adjacent elements on either side as director and reflector to increase the gain, the shorter element in front acting ...
The angle of the slope is usually between 45°–60° and the lower end of the wire is at least 1 ⁄ 6 wavelength above the electrical ground. [3] A sloper is typically fed with a coaxial cable in the center, at the top of the center support mast. At least 1 ⁄ 4 of the wavelength of feedline must be at 90° angle to the antenna. [3]
Furthermore, the constant of proportionality is the same irrespective of the nature of the antenna, and so must be the same for all antennas. Analysis of a particular antenna (such as a Hertzian dipole), shows that this constant is , where is the free-space wavelength. Hence, for any antenna the gain and the effective aperture are related by
A turnstile antenna, or crossed-dipole antenna, [1] is a radio antenna consisting of a set of two identical dipole antennas mounted at right angles to each other and fed in phase quadrature; the two currents applied to the dipoles are 90° out of phase. [2] [3] The name reflects the notion the antenna looks like a turnstile when mounted ...