Antenna Theory - Aperture
An Antenna with an aperture at the end can be termed as an Aperture antenna. Waveguide is an example of aperture antenna. The edge of a transmission line when terminated with an opening, radiates energy. This opening which is an aperture, makes it an Aperture antenna.
The main types of aperture antennas are −
- Wave guide antenna
- Horn antenna
- Slot antenna
Let us now have a look at these types of aperture antennas.
Waveguide Antenna
A Waveguide is capable of radiating energy when excited at one end and opened at the other end. The radiation in wave guide is greater than a two-wire transmission line.
Frequency Range
The operational frequency range of a wave guide is around 300MHz to 300GHz. This antenna works in UHF and EHF frequency ranges. The following image shows a waveguide.
This waveguide with terminated end, acts as an antenna. But only a small portion of the energy is radiated while a large portion of it gets reflected back in the open circuit. It means VSWR (voltage standing wave ratio, discussed in basic parameters chapter) value increases. The diffraction around the waveguide provides poor radiation and non-directive radiation pattern.
Radiation Pattern
The radiation of waveguide antenna is poor and the pattern is non-directive, which means omni-directional. An omni-directional pattern is the one which has no certain directivity but radiates in all directions, hence it is called as non-directive radiation pattern.
The above figure shows a top section view of an omni-directional pattern, which is also called as non-directional pattern. The two-dimensional view is a figure-of-eight pattern, as we already know.
Advantages
The following are the advantages of Aperture antenna −
- Radiation is greater than two-wire transmission line
- Radiation is Omni-directional
Disadvantages
The following are the disadvantages of Aperture antenna −
- VSWR increases
- Poor radiation
Applications
The following are the applications of Aperture antenna −
- Micro wave applications
- Surface search radar applications
The waveguide antenna has to be further modified to achieve better performance, which results in the formation of Horn antenna.
