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A half-wave dipole antenna consists of two quarter-wavelength conductors placed end to end for a total length of approximately ℓ = 1 / 2 λ. The current distribution is that of a standing wave , approximately sinusoidal along the length of the dipole, with a node at each end and an antinode (peak current) at the center (feedpoint ...
The monopole antenna is essentially one half of the half-wave dipole, a single 1 / 4 wavelength element with the other side connected to ground or an equivalent ground plane (or counterpoise). Monopoles, which are one-half the size of a dipole, are common for long-wavelength radio signals where a dipole would be impractically large.
Hence, the monopole antenna has a radiation pattern identical to the top half of the pattern of a similar dipole antenna, and a radiation resistance a bit less than half of a dipole. Since all of the equivalent dipole's radiation is concentrated in a half-space, the antenna has twice the gain (+3 dB) of a similar dipole, neglecting power lost ...
The driven element of the antenna is usually a half-wave dipole, its length half a wavelength of the radio waves used. The parasitic elements are of two types. A "reflector" is slightly longer (around 5%) than a half-wavelength. It serves to reflect the radio waves in the opposite direction.
Two element dipole array in front of a one wavelength square reflector used as gain standard. The gain of practical array antennas is limited to about 25–30 dB. Two half wave elements spaced a half wave apart and a quarter wave from a reflecting screen have been used as a standard gain antenna with about 9.8 dBi at its design frequency. [4]
Small antennas around one wavelength in size, such as quarter-wave monopoles and half-wave dipoles, don't have much directivity ; they are omnidirectional antennas which radiate radio waves over a wide angle. To create a directional antenna (high gain antenna), which radiates radio waves in a narrow beam, two general techniques can be used:
An antenna array consisting of two such antennas, one above the other and driven in phase has a gain of up to 17 dBi. Being log-periodic, the antenna's main characteristics ( radiation pattern , gain, driving point impedance ) are almost constant over its entire frequency range, with the match to a 300 Ω feed line achieving a standing wave ...
Note that at zero spacing we obtain the self-impedance of a half-wave dipole, 73 + j43 Ω. When a passive radiator is placed close (less than a quarter wavelength distance) to the driven dipole, it interacts with the near field, in which the phase-to-distance relation is not governed by propagation delay, as would be the case in the far field ...
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