Radome

• 1. Introduction

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  In 1948, at the Cornell Aeronautical Laboratory, Walter Bird built the world's first pneumatic radome. Bird's dome was held rigid by air pressure alone, not by metal struts. It was the first pneumatic construction of any kind in history. The word 'radome' is a portmanteau, fusing 'radar' and 'dome.' It names a weatherproof enclosure designed to protect radar antennas while remaining transparent to radio waves. Radome use dates back to 1941, predating Bird's inflatable approach by several years. Shapes range from spherical to geodesic to planar, depending on the application. What exactly does ice do to a radar antenna, and how does the engineered fix sometimes cause its own cascade of problems? When a military aircraft needs to watch the entire sky at once, how does radome design make that possible? And what connects a radome built in France in 1962 to a satellite television signal relayed from across the Atlantic?

• 2. The Ice Problem

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  World War II created pressing requirements for radome enclosures, and materials like fiberglass and PTFE-coated fabric proved ideal for the task. But the engineering challenge radomes address is more specific than simply keeping rain off a dish. When excessive ice accumulates on a stationary antenna, it de-tunes the antenna. The impedance at the input frequency rises drastically. As impedance rises, the voltage standing wave ratio, or VSWR, climbs alongside it. That reflected power returns to the transmitter, where it can cause overheating.

  A device called a foldback circuit can intercept that reflected power. But the protection comes at a cost: the foldback circuit forces the station's output power to drop dramatically, cutting effective range. A radome sidesteps the cascade by keeping ice and debris off the antenna's exposed surfaces from the start. Radomes also reduce wind load substantially, both in normal conditions and when ice forms. Many tower sites require or prefer them for exactly that reason. Spinning radar installations gain an additional benefit: the enclosure smooths rotational irregularities caused by wind pressure.

  Direct current powers the electric antenna heaters that can substitute for a radome where placing a visible dome would look unsightly. Those heaters do not interfere with the alternating current of the radio transmission. For large radar dishes, a single ball-shaped dome shelters the rotational mechanism and the sensitive electronics. In colder climates, those large domes are also heated to prevent icing. Beyond ice and wind, radomes also shield nearby personnel from being struck by a quickly rotating antenna.

• 3. Airborne Design

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  The American E-3 Sentry carries a discus-shaped radome on top of its fuselage, built to rotate during flight. That rotating dome, called a rotodome, provides 360-degree scanning coverage. The E-3 Sentry is an airborne early warning and control aircraft. These platforms require the ability to observe the full surrounding sky at once. Not every AEW&C design uses a rotating dome. The Chinese KJ-2000 and India's DRDO AEW&C aircraft both house three 120-degree phased array modules inside a stationary radome instead.

  On fixed-wing aircraft equipped with forward-looking radar for object or weather detection, the nose cone itself typically serves as the radome. That nose cone does double duty: it protects the radar antenna and acts as a fairing, streamlining the antenna assembly to reduce drag. On fixed-wing and rotary-wing aircraft using microwave satellite links for beyond-line-of-sight communication, the radomes appear as bulged blisters on the fuselage. That same neutrality, concealing an antenna's purpose from outside view, would find equally important uses on the ground.

• 4. Cold War Stations

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  RAF Menwith Hill contains more than 30 radomes within its perimeter. The electronic surveillance base is widely believed to regularly intercept satellite communications. At Menwith Hill, the domes serve a purpose beyond weather protection: they prevent outside observers from seeing which direction the antennas are pointing. Knowing the direction of an antenna reveals which satellite it is targeting, so the enclosures deny that intelligence to anyone outside. ECHELON facilities follow the same principle, using radomes to prevent observation of the antennas in their network.

  The United States Air Force Aerospace Defense Command maintained dozens of air defense radar stations during the Cold War. Those stations spread across the contiguous United States and into Alaska. Most of the radars at those ground stations were protected by rigid or inflatable radomes, typically at least 50 ft in diameter. Each radome was attached to a standardized radar tower building that housed the transmitter, receiver, and antenna together. Some radomes were built not for military surveillance but to receive live television signals from space.

• 5. Satellites and Ships

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  In 1962, a radome at Pleumeur-Bodou in France protected the PB1 antenna. That antenna was designed to receive mondovision television transmissions from the Telstar satellite, relaying data across the Atlantic from the United States. Today, the Pleumeur-Bodou radome has become a museum. Its American twin was dismantled along with the antenna it once protected.

  Large cruise ships and oil tankers may carry radomes over 3 m in diameter. Those domes cover antennas that track fixed satellites continuously while the ship pitches, rolls, and yaws in open water. The transmissions they handle include television, voice, data, and the internet. The SES Broadband for Maritime system delivers comparable services using an 85 cm motorised dish. At 26 cm in diameter, a radome small enough for a private yacht delivers voice and low-speed data from space.