What is a Slot Antenna?

C.B. Fox

A slot antenna is a simple type of antenna that consists of a plate of metal and one or more slots through the middle. The layout and size of these slots determines the frequencies that this type of antenna can pick up. The most common uses for the slot antenna are in radar and cell phone towers. They work best with short wavelengths in the ultra-high frequency (UHF) range.

Slot antennas are commonly used on cell phone towers.
Slot antennas are commonly used on cell phone towers.

The main component of a slot antenna is a large, rigid sheet of metal. It cannot be flimsy or flop over in one direction or another. In most slot antennas, the metal sheet is rectangular and the length of the slot or slots cut into the metal runs parallel to the longer side of the rectangle.

A slot antenna can have one slot in the middle but usually has a series of slots staggered on either side of the center. Each slot is half the length of the wavelength that the antenna is designed to receive and is much less wide than it is long. Constructing a slot antenna that has multiple slots requires precise calculation of the spacing between the slots and from the edges of the antenna.

A dipole antenna, which is simply a metal pole, functions much like the slot antenna. It would be possible to cut a slot out of a sheet of metal, creating a slot antenna, and then to use the piece that was cut out as a dipole antenna, optimized to receive the same frequency. The difference between the two types of antennas is in the polarization of the electromagnetic signal.

When an electromagnetic signal is received by a slot antenna, it excites the antenna, creating both an electrical and a magnetic field as the signal moves through the antenna and into the receiver attached to it. Current travels throughout the entire sheet of metal and is not confined to the slot. The electrical field is vertical while the magnetic field is horizontal. This is opposite from the effect on a dipole antenna and means that the polarization of the electromagnetic signal will be vertical in a horizontal slot and horizontal in a vertical slot.

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