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The Pros and Cons of Slot Antennas

Tamara Wilhite is a technical writer, industrial engineer, mother of two, and published sci-fi and horror author.

What Is a Slot Antenna?

Slot antennas typically consist of a metal surface like a flat plate with one or more “slots” cut out. This makes slot antennas a type of aperture antenna. These slots may be holes instead of rectangular slots.

The size and shape of the slot and the driving frequency determine the radiation pattern of the slot antenna. Slot antennas are roughly half of a wavelength long, while the width should be a small fraction of the wavelength.

The slot antenna behaves in accordance with Babinet’s principle as a resonant radiator. Babinet’s principle relates the slot antenna to a dipole, though the polarization of the slot antenna is rotated compared to that of a dipole antenna. (A vertical dipole has a vertical field, whereas it is horizontal for a vertical slot antenna.) You can approximate the performance of the slot antenna to a dipole antenna with the dimensions of the slot. That theoretical dipole antenna is called a “complimentary dipole”.

Closeup of slot antenna.

Closeup of slot antenna.

The Advantages of Slot Antennas

One benefit of the slot antenna is its sheer simplicity. Widen the slot, and it is equivalent to a thicker dipole. This is equivalent to increasing the bandwidth.

Slotted antennas can transmit high power levels. This is why they are popular in applications like navigation systems and weather radar.

When you’re selecting an outdoor antenna, the low wind load of the slot antenna is a point in its favor.

You can hid a slot antenna within a metallic object like a fake air vent, making them ideal for covert communications. They can also be manufactured to conform to the surface on which they are mounted like an aircraft’s skin.

Slot antennas are easy to mass produce with any technology. The slot antenna can be cut out of whatever surface the antenna is mounted on. You tune the antenna performance by altering the slot size, shape and the design of the cavity behind it.

Slot antenna radiation patterns are broadly directional. However, they are not omni-directional like wheel antennas.

More current is required to produce a given power output with a dipole antenna than is achieved with a slot antenna.

Slot antennas are more efficient than a comparably sized dish antenna. This makes slot antennas an ideal choice for radar dishes in the nose cone of an aircraft, since you can make the slot antenna smaller where a few more inches dramatically improves the aerodynamics.

The Disadvantages of Slot Antennas

Waveguide slot antennas are heavy compared to their dipole equivalents.

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Slot antennas have low radiation efficiency.

Slot antennas have high cross-polarization levels.

Observations about Slot Antennas

Slot antennas are sometimes called slot radiators. A slot antenna is not the same thing as a slot periodic antenna.

A single narrow slot antenna can work on frequencies plus or minus ten percent of their resonant frequency. This is difficult to achieve with array antennas.

The longer the slotted antenna, the narrower the beam. However, this is often resolved by arranging slotted waveguides in parallel.

A vertical slot antenna has a horizontal electric field. A horizontal slot antenna has a vertical electric field. In contrast, a vertical dipole antenna has a vertical electrical field.

Slot antenna from an aircraft nose cone.

Slot antenna from an aircraft nose cone.

Common Applications of Slot Antennas

Slot antennas are regularly used for microwave and UHF frequencies, though they were originally invented for VHF television broadcasting. Most slot antennas are used for frequencies between 300 MHz and 25 GHz.

Slotted pole antennas or pylon antennas are narrowband antennas used for broadband applications. Radial line slot antennas are periodically used for DBS reception. One of the most common uses for slot antennas is as radar antennas, especially navigation radar. They are sometimes used as sector antennas for cell phone base stations. They can be used as an array when fed by a waveguide.

Slotted antennas can be made using the same technology as printed circuit board antennas like these UWB and Vivaldis.

Slotted antennas can be made using the same technology as printed circuit board antennas like these UWB and Vivaldis.

© 2017 Tamara Wilhite


SALVAONEGIANNAOLCOM from south and west of canada,north of ohio on November 20, 2017:

That's a SLOT of Antennas lol!

Haider Jahan on November 19, 2017:

Awesome information.

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