thumb|The [[AT&T Corporation|AT&T receiving Beverage antenna (left) and radio receiver (right) at Houlton, Maine, used for transatlantic telephone calls, from a 1920s magazine]]
The Beverage antenna, a very early type of wave antenna or traveling wave antenna, is a long-wire receiving antenna mainly used in the low frequency and medium frequency radio bands, invented by H.H. Beverage in 1921. It is used by amateur radio operators, shortwave listeners, longwave radio DXers, and for military applications.
A Beverage antenna consists of a horizontal wire from one-half to several wavelengths long (tens to hundreds of meters / yards for shortwaves; up to several kilometres / miles for longwaves) suspended above the ground, with the feedline to the receiver attached to one end, and the other end of the wire terminated through a resistor to ground. The antenna has a unidirectional radiation pattern with the main lobe of the pattern at a shallow angle into the sky off the resistor-terminated end, making it ideal for reception of long distance skywave (skip) transmissions from stations over the horizon which reflect off the ionosphere. However the antenna must be built so the wire points in the direction of the transmitter(s) to be received. He discovered in 1920 that an otherwise nearly bidirectional long-wire antenna becomes unidirectional by placing it close to the lossy earth and by terminating one end of the wire with a resistor. In 1921, Beverage was granted a patent for his antenna. That year, Beverage long-wave receiving antennas up to long had been installed at RCA's Riverhead, New York, Belfast, Maine, Belmar, New Jersey, and Chatham, Massachusetts receiver stations for transatlantic radiotelegraph traffic. Perhaps the largest Beverage antenna – an array of four phased Beverages long and wide – was built by AT&T in Houlton, Maine, for the first transatlantic telephone system, opened in 1927.
Description
thumb|upright=1.8|Animation showing how the antenna works. Due to ground resistance the [[electric field of the radio wave (<span style="color:red;"> long, tilted red arrows</span>) is inclined at an angle to the vertical, creating a small horizontal component parallel to the antenna wire (<span style="color:red;">short, horizontal red arrows</span>). The horizontal, smaller component of the electric field, <span style="color:red;"></span> creates a traveling wave of oscillating current (<span style="color:blue;"> blue, wavy curve</span>) along the wire. All along the length of the wire, the electric field, <span style="color:red;"></span> continues to build up even more current. The current's amplitude, <span style="color:blue;"></span> is at its maximum when it reaches the (left) feedpoint; from there passes out of the antenna, into the feed line, and off to the receiver. Any radio waves arriving from the opposite direction, moving toward the terminated end, also create traveling waves, but those are eliminated by being absorbed through the terminating resistor, making the antenna's receiving pattern one-directional.
Operation
Unlike other wire antennas such as dipole or monopole antennas which are typically used on their resonant frequencies, with the radio currents traveling in both directions along the element, bouncing back and forth between the ends as standing waves, the Beverage antenna is a traveling wave antenna; the radio frequency current travels in one direction along the wire, in the same direction as the radio waves. The lack of resonance gives it a wider bandwidth than resonant antennas. It receives vertically polarized radio waves, but unlike other vertically polarized antennas it is suspended horizontally and close to the ground, and requires some resistance in the ground to work.
The Beverage antenna relies on "wave tilt" for its operation. At low and medium frequencies, a vertically polarized radio frequency electromagnetic wave traveling close to the surface of the earth with finite ground conductivity sustains losses that are greater nearer the ground; the reduction near the ground causes the net wavefront to "tilt over" at a small angle.
Directivity increases with the length of the antenna. Useful directivity begins to develop at a length of only it becomes more significant at one wavelength and improves steadily until the antenna reaches a length of about two wavelengths, depending on the soil and the antenna height. For Beverages longer than two wavelengths, its directivity no longer improves, since the slightly slower electrical waves in the antenna wire cannot remain in phase with the slightly faster radio waves in the air. A non-inductive resistor approximately equal to the characteristic impedance of the antenna wire, about 400~600 Ohms, is connected from the far end of the wire to a ground rod. The other end of the wire is connected to the feedline to the receiver.
A dual-wire variant is sometimes utilized for rearward null steering or for bidirectional switching. The antenna can also be implemented as an array of elements in broadside, endfire, and staggered configurations, offering significantly improved directivity otherwise very difficult to attain at these frequencies. A four-element broadside / staggered Beverage array was used by AT&T at their longwave telephone receiver site in Houlton, Maine. Very large phased Beverage arrays of 64 elements or more have been implemented for receiving antennas for over-the-horizon radar systems.
thumb|Feed point of multi-element Beverage receiving system at amateur radio station in Poland.
The driving impedance of the antenna is equal to the characteristic impedance of the wire with respect to ground, somewhere between 400~800 Ohms, depending on the height of the wire and its thickness. On the opposite end of the wire, a matching transformer is typically used to join the high-impedance antenna wire to a low-impedance feedline to the receiver,