Radome

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Radomes at the Misawa Security Operations Center, Misawa, Japan

A radome (a portmanteau of radar and dome) is a structural, weatherproof enclosure used to protect an antenna. What distinguishes a radome structure from other structures is that the material used in building the radome allows a relatively unattenuated electromagnetic signal between the antenna inside the radome and outside equipment. Using conventional building materials (i.e. steel, aluminum, bricks, etc.) would block most if not all of the antenna signal. Radomes are used to protect the surfaces of the antenna from the effects of environmental exposure (i.e. wind, rain, sand, UV, ice, etc.) and/or conceal antenna electronic equipment from public view. They also protect personnel who work nearby from being accidentally struck by a fast-moving antenna.

Radomes can be constructed in several shapes (spherical, geodesic, planar, etc.) depending upon the particular application using various construction materials (fiberglass, Teflon® coated fabric, etc.). When used on UAVs or other aircraft, in addition to such protection, the radome also streamlines the antenna system, thus reducing drag.

[edit] Purpose

A radome is often used to prevent ice and freezing rain from accumulating directly onto the metal surface of the antenna. In the case of a spinning radar dish antenna, the radome also protects from debris and rotational irregularities due to wind.

For stationary antennas, excessive amounts of ice can de-tune the antenna to the point where its impedance at the input frequency rises drastically, causing voltage standing wave ratio (VSWR) to rise as well. This reflected power goes back to the transmitter, where it can cause overheating. A foldback circuit activates to prevent this. However, it causes the station's output power to drop dramatically, reducing its range.

One of the first radomes: the radome (top) covering the H2S radar system rotating antenna (bottom) on a Halifax bomber

A radome prevents this by covering the antenna's exposed parts with a sturdy, weatherproof material, typically fiberglass, which keeps debris or ice away from the antenna to prevent any serious issues. It is interesting to note that one of the main driving forces behind the development of fiberglass as a structural material was the need during World War II for radomes.^[1] A radome does, however, add to the wind load and the ice load, in addition to its own weight, and so must be planned for when considering overall structural load.

For this reason, and the fact that radomes may be unsightly if near the ground, heaters are often used instead. Usually running on DC, the heaters do not interfere physically or electrically with the AC of the radio transmission.

For radar dishes, a single, large, ball-shaped dome (usually geodesic) also protects the rotational mechanism and the sensitive electronics, and is heated in colder climates to prevent icing.

The Menwith Hill spy base, which includes over 30 radomes, is widely believed to regularly intercept satellite communications. At Menwith Hill, the radome enclosures have a further use in preventing observers from deducing the direction of the antennae, and therefore which satellites are being targeted. The same point was also made with respect to the radomes of the ECHELON facilities. (After all, it is still not entirely clear on whom the Echelon antennae spy.)

For maritime satellite communications service, radomes are widely used to protect dish antennas which are continually tracking fixed satellites while the ship's deck experiences pitch, roll and yaw movements. Large cruise ships and oil tankers may have radomes over 3 m in diameter to suit broadband transmissions for television, voice, data and Internet. Small private yachts may use radomes as small as 26 cm for voice and low-speed data.

[edit] Alternatives

An Active Electronically Scanned Array is a form of radar installation that has no moving parts as such and in ground based installations a radome is not necessary. An example of this is the "tourist attraction" golfball-style radome installations at RAF Fylingdales.