A spiral antenna is used to receive or transmit a wide range of electromagnetic signals. They can be used by themselves or as a part of an array. Telecommunications devices often make use of these types of antennas.
The simplest version of a spiral antenna consists of a tight coil that circles around in either a clockwise or counter-clockwise direction. The coils can be encased in a solid metal fixture. The antenna can be relatively flat, or it can consist of a spiral ribbon of metal that may be as deep as the diameter of the largest circle in the spiral. More complex versions of spiral antennas are constructed out of two complementary spirals, one inside the other, which are wound in opposite directions.
In order to achieve the best results, the spiral form and the empty space within it must be in equal proportion. Depending on the composition and size of the coil, the spiral antenna is able to pick up a wide range of electromagnetic waves. It is considered a broad range antenna, as the wave length of the shortest wave it can pick up is generally nine or more times smaller than the wave length of the longest wave in its range. This makes it an ideal antenna to use for communications, where many frequencies are monitored at once.
It is possible to both receive and transmit electromagnetic information with a spiral antenna. All spiral antennas are capable of receiving linear signals of any polarization, so long as these signals fall within their frequency range. The waves that a spiral antenna emits have a circular polarization, which means that the waves spiral back in on themselves, much like the wire of a spring. It is not possible for all types of antennas to pick up electromagnetic waves of this shape, though other spiral antennas can pick them up. In order for a spiral antenna to pick up a signal with circular polarization, the wave must have the opposite polarization of the antenna.
One spiral antenna may be used alone, or a number of them may be hooked together into an array. Two complementary antennas are frequently used in order to receive signals of opposite circular polarization. Using more antennas also increases the gain, which allows the antennas to pick up signals that are weak.