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 a type of long wire antenna designed for amateur radio usage, shortwave listening, and longwave radio applications. First used in the 1920s and named for its inventor Harold Beverage, it is not a beverage can or beer can antenna.

The antenna type is used in the military and within the hobbyist communities of amateur radio and broadcast DXers. The antenna type was first used (perhaps unknowingly) in the late 1910s and was widely implemented commercially in the 1920s.

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

A wire of at least one or two wavelengths in length (which can easily be as much as a few kilometres for longwave reception) is suspended several feet above ground. A resistive termination at one end is selected to match the (600-ohm typical) characteristic impedance of the antenna and is connected to ground; the radio feed is connected to the opposite end through an impedance-matching transformer. An SWR meter may be used to determine the exact termination to be used for one specific installed antenna.

Many variants use a two-wire design or a sloped design where the centre is further above ground than the endpoints.

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 largely bidirectional antenna into a unidirectional antenna by terminating the antenna with an impedance conjugate to the surge impedance of the antenna (most usually approximated by a non-inductive resistance).

The antenna relies upon the "wave tilt" phenomenon whereby a vertically polarised radio frequency electromagnetic wave travelling parallel to the surface of the earth with finite ground conductivity sustains a loss which is reflected as an electric field component parallel to the surface. In the presence of a conducting wire close to the surface of the earth, an incident wave will generate RF current components travelling in both directions along the wire away from the point of incidence. The generated component travelling along the wire in the same direction as the incident wave will largely add in phase and amplitude throughout the length of the wire, leading to a maximum response in that direction. At higher frequencies, the angle of arrival can also contribute to the horizontal component particularly where the antenna forezone is of very high ground conductivity such as sea water, leading to very low losses at low angles of arrival.

While these antennas do not provide very high absolute gain in terms of dBi (typically about -20dBi to -10dBi), this is not a particular problem, as the antenna is used at frequencies where there is a high level of atmospheric and other radio noise. The antenna has exceedingly low radiation resistance and will rarely be utilised for transmitting. The Beverage antenna is popular because it offers simple directional tuning, albeit with a large size.

Directivity depends upon the electrical length of the antenna. Directivity is present at .25 wavelength, significant by .5 wavelength and increases steadily to perhaps 2 wavelengths. While the source of much anecdotal debate, it is generally accepted that greater lengths generally suffer from polarisation tumbling effects of the incoming wave and phase delays along the antenna itself, such that gain and directivity both suffer.

The Beverage antenna is most frequently deployed as a single wire single element antenna. A dual wire variant is utilised by some aficionados for rearward null steering. The antenna can be implemented as an array of individual elements in broadside, endfire, and staggered configurations offering reception otherwise unattainable at these frequencies.

[edit] Implementation

A single wire single element antenna is typically a single straight copper wire, of between one-half and one wavelength, running parallel to the earth's surface away from the receiver in the direction of the desired transmitter. At the frequencies of interest, one wavelength is a substantial distance: 3km at 100kHz, 300m at 1MHz, and 30m at 10MHz. The wire is suspended by insulated supports at a height typically 1m-3m above the ground. A terminating resistance is often provided at the far end of the antenna; this may not be required with longer antennas, due to incurred signal losses in the double traverse of a reflected signal along the wire.

In theory, an impedance matching transformer should be used between the receiver and the antenna feedpoint, however with sensitive receivers this is often not necessary as signal to noise is determined by external noise.