The Jampro JFVD is a Dual Vertical Dipole Panel Antenna. Each panel consists of balun fed dual dipoles, featuring high gain and low downward radiation. Slot Loaded Square Microstrip Patch Antenna for Dual Band Operation. Faria Jaheen1,. Analyzed using the duality relationship between the dipole and the slot. The Babinet’s principle 33 of optics is used to. Slot Loaded Square Microstrip Patch Antenna for Dual Band Operation.
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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.
Slot And Dipoles As Dual Antennas For Sale
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 following are the disadvantages of Slot antenna −
- Higher cross-polarization levels
- Lower radiation efficiency
Applications
The following are the applications of Slot antenna −
- Usually for radar navigational purposes
- Used as an array fed by a wave guide
In this section, the dipole antenna with a very thin radius is considered. The dipole antennais similar to the short dipoleexcept it is not required to be small compared to the wavelength (at the frequency the antenna is operating at). For a dipole antenna of length L oriented along the z-axis and centered at z=0, the current flows in the z-directionwith amplitude which closely follows the following function: Note that this current is also oscillating in time sinusoidally at frequency f.The current distributions for the quarter-wavelength (left) and full-wavelength (right) dipole antennas are given in Figure 1. Note that the peak value of the current is not reached along the dipole unless the length is greater than half a wavelength. Figure 1. Current distributions on finite-length dipole antennas. Before examining the fields radiated by a dipole antenna, consider the input impedance of a dipole as a function of its length, plotted in Figure 2 below. Note that theinput impedance is specified as Z=R + jX, where R is the resistance and X is the reactance. Figure 2. Input impedance as a function of the length (L) of a dipole antenna. Note that for very small dipole antennas, the input impedance is capacitive, which means the impedance is dominated by a negative reactance value (and a relatively small real impedance or resistance). As the dipole gets larger, the input resistance increases, along with the reactance. At slightly less than 0.5 the antenna has zero imaginary component to the impedance (reactance X=0), and the antenna is said to be resonant. If the dipole antenna's length becomes close to one wavelength, the input impedance becomes infinite. This wild change in input impedance can be understood by studying high frequency transmission line theory. As a simpler explanation, consider the one wavelength dipole shown in Figure 1. If a voltage is applied to the terminals on the right antenna in Figure 1, the current distribution will be as shown. Since the current at the terminals is zero, the input impedance (given by Z=V/I) will necessarily be infinite. Consequently, infinite impedance occurs whenever the dipole antennais an integer multiple of a wavelength. In the next section, we'll consider the radiation pattern of dipole antennas. Radiation Patterns for Dipole AntennasThe far-fields from a dipole antenna of length L are given by: The normalized radiation patterns for dipole antennas of various lengths are shown in Figure 3. Figure 3. Normalized radiation patterns for dipole antennas of specified length. The full-wavelength dipole antenna is more directional than the shorter quarter-wavelength dipole antenna. This is a typical result in antenna theory: it takes a larger antenna in general to increase directivity. However, the results are not always obvious. The 1.5-wavelength dipole pattern is also plotted in Figure 3. Note that this pattern is maximum at approximately +45 and -45 degrees. The dipole antenna is symmetric when viewed azimuthally (around the long axis of the dipole); as a result the radiation pattern is not a function of the azimuthal angle . Hence, the dipole antenna is an example of an omnidirectional antenna. Further, the E-field only has one vector component and consequently the fields are linearly polarized. When viewed in the x-y plane (for a dipole oriented along the z-axis), the E-field is in the -y direction, and consequently the dipole antenna is vertically polarized. The 3D pattern for the 1-wavelength dipole antenna is shown in Figure 4. This pattern is similar to the pattern for the quarter- and half-wave dipole antenna. Figure 4. Normalized 3d radiation pattern for the 1-wavelength dipole antenna. The 3D radiation pattern for the 1.5-wavelength dipole antenna is significantly different, and is shown in Figure 5. Figure 5. Normalized 3d radiation pattern for the 1.5-wavelength dipole antenna. The (peak) directivityof the dipole antenna varies as shown in Figure 6. Figure 6. Dipole Antenna directivity as a function of dipole length. Figure 6 indicates that up until approximately L=1.25 the directivity increases with length. However, for longer lengths the directivity has an upward trend but is no longer monotonic. In the next section, we'll look at the most common dipole antenna, the half-wave dipole antenna. Next: Half-Wave Dipole Antennas Up: Dipole Antennas Antenna Theory .com |