03-12-2012, 06:09 PM
HORN ANTENNA
HORN ANTENNA.pptx (Size: 1.42 MB / Downloads: 66)
What is Horn Antennas?
It is basically a flared out waveguide.
Produces a uniform phase front with a larger aperture than that of the waveguide and of greater directivity.
Horns provide high gain, low VSWR (with waveguide feeds), relatively wide bandwidth, and they are not difficult to make.
The horns can be also flared exponentially. This provides better matching in a broad frequency band, but is technologically more difficult and expensive.
Basic Types of Horn Antennas:
Rectangular Horn Antennas
Circular Horn Antenna
Horn Antenna
When rectangular waveguide is energized with a TE10 mode wave electric field the horn is flared out in a plane perpendicular to E, this is the plain of magnetic field H & this type of Horn is called H-plane horn.
When horn is flared out in the plane of electric field E, this type of horn is called as E plane sectoral horn.
Rectangular horn with flared in both plane E & H, then it is called as pyramidal horn.
A horn in the shape of a cone, with a circular cross section. They are used with cylindrical waveguides is called Conical Horn.
Why Horn Antenna ?
If a simple open-ended waveguide is used as an antenna, without the horn, the sudden end of the conductive walls causes an abrupt impedance change at the aperture.
When radio waves travelling through the waveguide hit the opening, this impedance-step reflects a significant fraction of the wave energy back down the guide toward the source, so that not all of the power is radiated.
The reflected waves cause standing waves in the waveguide, increasing the SWR, wasting energy and possibly overheating the transmitter. In addition, the small aperture of the waveguide (less than one wavelength) causes significant diffraction of the waves issuing from it, resulting in a wide radiation pattern without much directivity.
How it operates?
The ends of the waveguide are flared out to form a horn. The taper of the horn changes the impedance gradually along the horn's length. This acts like an impedance matching transformer, allowing most of the wave energy to radiate out the end of the horn into space, with minimal reflection.
The wide aperture of the horn projects the waves in a narrow beam.
The horn shape that gives minimum reflected power is an exponential taper. Exponential horns are used in special applications that require minimum signal loss, such as satellite antennas and radio telescopes.
However conical and pyramidal horns are most widely used, because they have straight sides and are easier to design and fabricate.