Beverage antenna

Lore & Background

Harold Beverage experimented with receiving antennas similar to the Beverage antenna in 1919 at the Otter Cliffs Radio Station. In 1920 he discovered that an otherwise nearly bidirectional long-wire antenna becomes unidirectional by placing it close to the lossy earth and terminating one end with a resistor. He was granted a patent for his antenna in 1921. That year, Beverage long-wave receiving antennas up to 14 km (9 miles) 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 5 km (3 miles) long and 3 km (2 miles) wide – was built by AT&T in Houlton, Maine, for the first transatlantic telephone system, opened in 1927.

The Beverage antenna consists of a horizontal wire one-half to several wavelengths long, suspended close to the ground, pointed in the direction of the signal source. At the end toward the signal source, the wire is shorted to ground through a resistor whose electrical resistance is close to the characteristic impedance of the antenna wire (typically 400~800 Ohms). At the other end, the antenna is connected to the receiver through a transformer (balun) that matches the antenna's impedance to the feed line's impedance, conventionally 50 or 75 Ohms. The 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. It receives vertically polarized radio waves, but 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 wave traveling close to the earth's surface with finite ground conductivity sustains losses greater nearer the ground, causing the wavefront to tilt over at a small angle. The horizontal wire is suspended close to the earth and approximately parallel to the wave's direction, and the small horizontal electric field generates a horizontal wave of RF current in the wire, propagating in the same direction as the external radio waves. The RF current traveling along the wire adds in phase and amplitude throughout the length, producing maximum signal strength at the far end where the antenna connects to the matching transformer.

Reader's Guide

The Beverage antenna's significance lies in its unique combination of directivity and bandwidth for low and medium frequency reception. Its unidirectional radiation pattern, with the main lobe at a shallow angle into the sky off the resistor-terminated end, makes it ideal for receiving long distance skywave (skip) transmissions from stations over the horizon that reflect off the ionosphere. The antenna must be built so the wire points in the direction of the transmitter(s) to be received. Its advantages include excellent directivity, wider bandwidth than conventional resonant antennas, and the ability to clearly receive distant and overseas transmitters. Disadvantages are its very long physical size, requiring considerable land area, and being unfeasible to rotate to different reception directions. As a work-around, installations often use multiple Beverage antennas to provide wide azimuth coverage. While Beverage antennas have excellent directivity, they do not produce absolute gain (typically −20 to −10 dBi), but this is rarely a problem because the antenna is used at frequencies where there are high levels of atmospheric radio noise. In long- and medium-waves, natural atmospheric noise is the limiting factor for signal-to-noise ratio, so an inefficient antenna such as a Beverage can be used for receiving, and its excellent directivity becomes the deciding factor for good SNR. Directivity increases with length, becoming significant at one wavelength and improving steadily until about two wavelengths, after which it no longer improves. Although excellent receiving antennas, Beverage antennas are rarely used to transmit, since doing so would waste a large amount of transmitter power as heat in the terminating resistor.

Did You Know?

• The Beverage antenna was invented by H.H. Beverage in 1921.
• It relies on 'wave tilt' caused by losses near the earth's surface for its operation.
• The largest known Beverage antenna was an array of four phased Beverages 5 km long and 3 km wide, built by AT&T in Houlton, Maine.
• Beverage antennas are rarely used for transmitting because most transmitter power would be wasted as heat in the terminating resistor.