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U- SLOTS ANTENNA DESIGN Four band linear polarization asymmetrical U slot patch antenna’s geometry is shown in figure 1.In ground plane under the patch, a square copper plate was used along its edge having length L=50 mm. Air is used as substrate and coaxial probe is connected directly to the patch. Inner and outer. Implementation and Developments of Single Feed Design using Multiple U- Slotted Patch Antenna for Wireless Applications - written by Vikram Thakur, Sanjeev Kashyap published on 2015/03/10 download full article with reference data and citations.

Slot antennas are used typically at frequencies between 300 MHz and 24 GHz. The slot antenna is popular because they can be cut out of whatever surface they are to be mounted on, and have radiation patterns that are roughly omnidirectional (similar to a linear wire antenna, as we'll see). The polarization of the slot antenna is linear. The slot size, shape and what is behind it (the cavity) offer design variables that can be used to tune performance.

Consider an infinite conducting sheet, with a rectangular slot cut out of dimensions a and b, as shown in Figure 1. If we can excite some reasonable fields in the slot (often called the aperture), we have a slot antenna.

Figure 1. Rectangular Slot antenna with dimensions a and b.

To gain an intuition about slot antennas, first we'll learn Babinet's principle (put into antenna terms by H. G. Booker in 1946).This principle relates the radiated fields and impedance of an aperture or slot antenna to that of the field of its dual antenna.The dual of a slot antenna would be if the conductive material and air were interchanged - that is, the slot antenna became a metal slab in space. An example of dual antennas is shown in Figure 2:

Figure 2. Dual antennas - (left) the slot antenna, (right) the dipole antenna.

Note that a voltage source is applied across the short end of the slot antenna. This induces an E-field distribution within the slot, and currents that travel around the slot perimeter, both contributed to radiation. The dual antenna is similar to a dipole antenna. The voltage source is applied at the center of the dipole, so that the voltage source is rotated.

Babinet's principle relates these two antennas. The first result states that the impedance of the slot antenna() is related to the impedance of its dual antenna () by the relation:

In the above, is the intrinsic impedanceof free space. The second major result of Babinet's/Booker's principle is that the fields of the dual antenna are almost the same as the slot antenna (the fields components are interchanged, and called 'duals'). That is, the fields of the slot antenna (given with a subscript S) are related to the fields of it's complement (given with a subscript C) by:

Hence, if we know the fields from one antenna we know the fields of the other antenna. Hence, since it is easy to visualize the fields from a dipole antenna, the fields and impedance from a slot antenna can become intuitive if Babinet's principle is understood.

Note that the polarization of the two antennas are reversed. That is, since the dipole antenna on the right in Figure 2 is vertically polarized, the slot antenna on the left will be horizontally polarized.

Duality Example

As an example, consider a dipole similar to the one shown on the right in Figure 2. Suppose the length of the dipole is 14.4 centimeters and the width is 2 centimeters, and that the impedance at 1 GHz is 65+j15 Ohms. The fields from the dipole antenna are given by:

What are the fields from a slot at 1 GHz, with the same dimensions as the dipole?

Using Babinet's principle, the impedance can be easily found:

The impedance of the slot for this case is much larger, and while the dipole's impedance is inductive (positive imaginary part), the slot's impedance is capacitive (negative imaginary part). The E-fields for the slot can be easily found:

We see that the E-fields only contain a phi (azimuth) component; the slot antenna is therefore horizontally polarized.

Video: Analysis of the Slot Antenna

To see this material presented another way, here is a video on the analysis of the slot antenna. Some of the information below will be complimentary to the above analysis. Hence, if you enjoy short lectures this video may be of interest to you.

Next: Cavity-Backed Slot AntennasThis page on the slot antenna is copyrighted. It can be referenced without permission form the author, so long as the original source is cited. Copyright antenna-theory.com, 2010-2021, slot antennas.

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Slot Antenna is an example of Aperture antenna. A rectangular slot is made on the conducting sheet. These slot antennas can be formed by simply making a cut on the surface, where they are mounted on.

Frequency Range

The frequency range used for the application of Slot antenna is 300 MHz to 30 GHz. It works in UHF and SHF frequency ranges.

Construction & Working of Slot Antennas

The use of slot antennas is well understood through its working principle. Let us have a look at the structure of a slot antenna.

When an infinite conducting sheet is made a rectangular cut and the fields are excited in the aperture (which is called as a slot), it is termed as Slot antenna. This can be understood by observing the image of a slot antenna. The following image shows the model of a Slot antenna.

The working of Slot Antenna can be easily understood through Babinet’s principle of optics. This concept gives an introduction to the slot antennas.

Babinet’s Principle

Babinet’s principle states that- “When the field behind a screen with an opening is added to the field of a complementary structure, the sum is equal to the field when there is no screen”.

The above images clearly explain the principle. In all the regions, which are non-collinear with the beam, the above two screens, in figures 1 & 2, produce the same diffraction pattern.

Case 1 − Consider a light source and a conducting plane (field) with an aperture before a screen. The light does not pass through the opaque area, but passes through the aperture.

Case 2 − Consider the light source and a conducting plane of the size of the aperture in the previous case, being held against the screen. The light does not pass through the plane but through the remaining portion.

Case 3 − Combine these two conducting planes of both the cases and put before the light source. The screen is not placed to observe the resultant combination. The effect of screen gets nullified.

Working of Slot Antenna

This principle of optics is applied to electromagnetic waves for the wave to get radiated. It is true that when a HF field exists across a narrow slot in a conducting plane, the energy is radiated.

The image shows a slot antenna, which explains well about its working.

Consider an infinite plane conducting screen is taken and pierced with apertures of desired shape and size and this will be the screen of slot antenna. Another screen is considered interchanging the places of aperture and screen area which is the complementary screen.

These two screens are said to be complementary as they result in complete infinte metal screen. Now, this becomes the slot antenna. The terminal impedance is quite desirable for the radiation.

Radiation Pattern

The radiation pattern of the Slot antenna is Omni-directional, just like a half-wave dipole antenna. Take a look at the following illustration. It shows the radiation pattern of Slot antenna drawn in Horizontal and Vertical planes respectively

Advantages

The following are the advantages of Slot antenna −

It can be fabricated and concealed within metallic objects
It can provide covert communications with a small transmitter

Disadvantages

The f