Beverage antenna

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Yagi-Uda antenna signal-gathering action compared to other end-fire, backfire and traveling-wave types.

The Beverage Antenna is an inexpensive long wire receiving antenna used by amateur radio, shortwave listening, and longwave radio DXers and military applications. First used in the 1920s and named for its inventor Harold Beverage, it is not a beverage can or beer can antenna. The Beverage antenna was first used (perhaps unknowingly) in the late 1910s and very large Beverage antennas were implemented for commercial and military longwave receiving stations in the 1920s.

While these antennas provide good radio-frequency directivity, a large amount of space is required. Beverage antennas are highly directional and physically far too large to be practically rotated so installations may use multiple antennas to provide a choice of azimuthal coverage..

A wire of one or two wavelengths in length (hundreds of feet at HF to a few kilometres for longwave reception) typically suspended about two meters above the ground. A 470 ohm resistive termination is usually selected for the far end of the antenna, regardless of length, (unless the line is much closer than a two meters above the ground) and is connected to ground; a 50 or 75 ohm coaxial transmission is connected to the opposite end through an impedance-matching transformer. A 470 ohm non-inductive resistor provides excellent results for most soils; however, an impedance bridge may be used to determine the exact termination to be used for a specific installed antenna. Some antenna variants use a two-wire design or a sloped design where the center of the antenna is further above ground than the ends.

1 Technical Description
2 Implementation
3 See also
4 Patents
5 External links

[edit] Technical Description

The Beverage antenna exploits a means of turning an otherwise nearly bidirectional wire antenna into a unidirectional antenna by placing a long wire close to the lossy earth and by terminating one end of the wire with a non-inductive resistor with an resistance approximately matched to the surge impedance of the antenna.

The Beverage Antenna relies on "wave tilt" for its directive properties. At low and medium frequencies, a vertically polarised radio frequency electromagnetic wave travelling close to the surface of the earth with finite ground conductivity sustains a loss that produces an electric field component parallel to the earth's surface. If a wire is placed close to the earth and approximately at a right angle to the wave front, the incident wave generates RF currents travelling along the wire, propagating from the near end of the wire to the far end of the wire. The RF currents travelling along the wire add in phase and amplitude throughout the length of the wire, producing maximum signal strength at the far end of the antenna where a receiver is typically connected. RF signals arriving from the receiver-end of the wire also increase in strength as they travel to end of the antenna terminated in a resistor, where most of the energy propagating in that direction is absorbed.

Radio waves propagate by the ionosphere at medium or high frequencies (MF or HF) typically arrive at the earth's surface with wave tilts of approximately 5 to 45 degrees. Ionospheric wave tilt allows the directivity inducing mechanism described above to produce excellent directivity in Beverage antennas operated at MF or HF.

While Beverage antennas have excellent directivity, because they are close to lossy earth they antennas do not produce absolute gain (typically about -20dBi to -10dBi). This is rarely a problem, because the antenna is used at frequencies where there are high levels of atmospheric radio noise. The antenna has very low radiation resistance (less than one ohm) and will rarely be utilised for transmitting. The Beverage antenna is a popular receiving antenna because it offers excellent directivity over a broad bandwidth, albeit with relatively large size.

Directivity is a function of the length of the antenna. While directivity begins to develop at a length of only 0.25 wavelength, directivity becomes more significant at one wavelength and improves steadily until the antenna length reaches a length of about two wavelengths. Its generally accepted among Beverage antenna experts that directivity no longer improves when the antenna is longer than two wavelengths. Beverages longer than two wavelengths suffer from the phase incoherency of the incoming waves over distances of several wavelengths, resulting in phase incoherency of the currents induced in the antenna that degrades the directivity of the antenna.

The Beverage antenna is most frequently deployed as a single wire. A dual wire variant is sometimes utilised for rearward null steering or for bidirectional switching. The antenna can also be implemented as an array of two to 128 or more elements in broadside, endfire, and staggered configurations offering significantly improved directivity otherwise very difficult to attain at these frequencies. Very large phased Beverage arrays have been implemented for receiving antennas for Over-the-horizon radar systems.

[edit] Implementation

A single wire Beverage Antenna is typically a single straight copper wire, between one and two wavelengths long, running parallel to the earth's surface from the receiver towards the direction of the desired signal. At the frequencies of interest, one wavelength is a substantial length: 3km at 100kHz, 300m at 1MHz, and 30m at 10MHz. The wire is suspended by insulated supports at a height of approximately 2 meters above the ground. A 470 ohm non-inductive resistor is installed from the far end of the wire to a ground rod, although this value is not critical.

An impedance matching transformer (typically a 9:1 transformer to match the antenna to a 50 ohm transmission line) should be used between the transmission line to the receiver and the antenna feedpoint. As an expediency, the transmission line can be connected directly to the end of the antenna and a ground rod usually with satisfactory results.