Slot Antenna: The Ins And Outs You Need To Know

As the demand for wide bandwidth, high efficiency, and high gain antenna is increasing in the field of wireless communication systems, different types of antennas are inventing. A rectangular waveguide slot antenna is one of them.

This antenna is small but has higher aperture efficiency compared to a reflector antenna. Plus, the slot antenna offers high gain and sharp directivity. 

But what is it actually? How does it work? What are radiation patterns, structures, and applications? Read on to get your answers.

What is Slot Antenna?

An aperture antenna is an example of a slot antenna. This antenna is built with a flat metal surface like a plate. Like a dipole antenna, a slot antenna radiates electromagnetic waves when radio frequency current drives the plate as an antenna.

Slot antenna usually operates at SHF, UHF, and microwave frequencies. The operating bandwidth of the slot antenna is between 300 MHz to 25 GHz frequency range. However, the frequency wavelength is small; hence, the slot and the ground plate are small too.

Moreover, a waveguide, which consists of slots, conducts the radio wave frequencies. Multiple slots can release a narrow fan-shaped microwave beam and work like a directive array antenna.

Basics Of Slot Antenna

A slot antenna was invented by an English electronic engineer, Alan Blumlein, in 1938. He was known as the most significant and innovative engineer throughout England for his inventions in sound recording, telecommunication, stereophonic sound, radar, and television.

Alan Blumlein invented a slot antenna for VHF television broadcasting with an omnidirectional horizontal and narrow vertical pattern and horizontal polarization.

Why is Slot Antenna Important?

Slot antennas can transmit and receive super high frequency (SHF) and ultra-high frequency (UHF). Plus, this type of antenna is small and low profile. 

Hence, it’s generally used where the space for installing an antenna is narrow. In cell phone base stations, this antenna is utilized as a sector antenna and radar antenna.

Structure Of Slot Antenna

Structure Of Slot Antenna
Slot Antenna

The slot antenna design structure determines its working principle. Suppose an infinite rectangular conducting sheet has a horizontal slot cut of length λ/2 and width much less than λ/2.

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This will create a half-wavelength slot that simulates a half-wave dipole in terms of maximum gain and radiation. Although the radiation pattern is the same for slots and dipoles, the electric field is polarized horizontally for dipoles and vertically for slots.

Slot antennas generally follow Babinet’s principle of optical. This principle states that two complementary screens produce the same diffraction pattern.

According to this principle, the slot antenna structure and half-wave dipole complement each other. Here the complementary dipole antenna acts like a slot’s complementary antenna.

The external feed is transferred to the slot center through two-wire transmission lines. And the feed point location determines the slot antenna’s impedance. It’s because the impedance may reduce when the feed point travels from the center to the edge.

Types Of Slot Antenna

Two types of antennas are widely used: longitudinal slotted waveguide antenna and transverse slotted waveguide antenna. 

Here are details about these antennas:

  1. Longitudinal Slotted Waveguide Antenna

The antenna axis resembles the waveguide’s axis. Its radiation pattern is mounted vertically, which is parallel to collinear dipole antennas.

The radiation pattern is narrow in the vertical plane. But in the horizontal plane, it’s nearly omnidirectional, upright to the antenna before the slot above the 180° azimuth.

This slot antenna is utilized at UHF television stations as a vertical omnidirectional transmitting antenna. Sometimes a semicircular or cylindrical waveguide is utilized with multiple slots cut columns for broadcasting. 

  1. Transverse Slotted Waveguide Antenna

In the waveguide axis, the slots are nearly upright. However, the slots, including alternate slots, are inclined in opposite directions. This slot antenna emits a very sharp beam and dipole pattern in the antenna plane.

These antennas are widely used for microwave marine radar. The antenna is ascended above the mechanical drive, making it rotate around the vertical axis.

The vertical fan-shaped beam of the antenna scans 360° surrounding the surface of the water with every revolution around the ship to the horizon.

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Operation Of Slot Antenna

The operating principle of slot antennas is energy will emit whenever the metallic sheet in the slot senses a high-frequency field.

This is why it emits electromagnetic waves when the conductive plate’s surface cuts a slot while reinforcing the slot. Therefore, it works like an antenna with the presence of slots.

Hence, it can be said that a conductive surface, including specific dimensions of rectangular slots, is known as the slot antenna.

A large slot antenna, including 24 slots, offers a maximum antenna gain capacity as high as 16 dB, which is quite amazing in the case of an omnidirectional microwave antenna. However, it’s still a lower gain antenna compared to a large horn or a small dish antenna.

Radiation Of Slot Antenna

The radiation pattern of the slot antenna is determined by the slot’s driving frequency and size. However, this antenna mostly follows omnidirectional radio patterns like half-wave dipole antennas.

Feeding Methods In Slot Antenna

Generally, the feeding process of this antenna is completed through a two-wire line. The feeding point determines the impedance of the antenna, like dipole antennas. The impedance decreases gradually when the feed location shifts to the edges from the center.

Advantages Of Slot Antenna

Here are the advantages of slot antenna:

  • Simple Design

The slot antenna design is simple and small, yet it can emit high power levels. Therefore, a slot antenna is popular in weather radar and navigation system applications. Again, these are equivalent to a dipole antenna.

  • Low Wind Load

Unlike other antennas, this antenna has a low wind load facility. In the case of outdoor antennae or aircraft, a slot antenna can be a highly functional antenna due to this benefit, thanks to its compact size.

  • Mass Production

With the help of any technology, mass production is easy for slot antennas. This antenna can easily cut, regardless of its mounting surface. Hence, this antenna is very convenient for a project requiring multiple antennas.

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Disadvantages Of Slot Antenna

Here are the disadvantages of slot antenna:

  • Low Radiation Efficiency

Although this antenna operates at UHF, it still has low radiation efficiency. Plus, they are heavier compared to their complementary dipole antenna.

Applications Of Slot Antenna

Some of the most common applications of slot antennae are as follows:

  • This antenna is commonly used as a radar antenna, particularly navigation radar. 
  • Used as a sector antenna at cellular base stations.
  • For study and research in standard laboratory microwave source.
  • Utilized as an ultra-high frequency TV transmitting antenna.
  • Other common fields like aircraft, marine, and missile use slot antennas.
slot antenna


What is the difference between a slot antenna and its complementary dipole antenna?

Polarization is the main difference between the slot antenna and its complementary dipole antenna. 

How to increase slot antenna bandwidth?

The proposed wide-slot antenna’s impedance of the slot antenna can be enhanced by adding rotated square slot diagonal points at the strip line’s center.

What are the features of slot antennas?

This versatile and compact antenna includes a metal surface, a narrow slit in the ground plane, and slots cut out. It’s compatible with coplanar waveguides, microstrip feeding schemes, coaxial, or slot lines with modification.

Others antenna

Final Words

You now have ideas about what a slot antenna is, how it works, its structure, radiation patterns, and common applications. This antenna is widely picked for the outdoor antenna as it has low wind load, compact size, and mass production.

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