Optimum Horn
For a given frequency and horn length, there is some flare angle that gives minimum reflection and maximum gain. The internal reflections in straight-sided horns come from the two locations along the wave path where the impedance changes abruptly; the mouth or aperture of the horn, and the throat where the sides begin to flare out. The amount of reflection at these two sites varies with the flare angle of the horn (the angle the sides make with the axis). In narrow horns with small flare angles most of the reflection occurs at the mouth of the horn. The gain of the antenna is low because the small mouth approximates an open-ended waveguide. As the angle is increased, the reflection at the mouth decreases rapidly and the antenna's gain increases. In contrast, in wide horns with flare angles approaching 90° most of the reflection is at the throat. The horn's gain is again low because the throat approximates an open-ended waveguide. As the angle is decreased, the amount of reflection at this site drops, and the horn's gain again increases.
This discussion shows that there is some flare angle between 0° and 90° which gives maximum gain and minimum reflection. This is called the optimum horn. Most practical horn antennas are designed as optimum horns. In a pyramidal horn, the dimensions that give an optimum horn are:
For a conical horn, the dimensions that give an optimum horn are:
where
- aE is the width of the aperture in the E-field direction
- aH is the width of the aperture in the H-field direction
- LE is the slant length of the side in the E-field direction
- LH is the slant length of the side in the H-field direction.
- d is the diameter of the cylindrical horn aperture
- L is the slant length of the cone from the apex.
- λ is the wavelength
An optimum horn does not give maximum gain for a given aperture size; this is achieved by a very long horn (an aperture limited horn). It gives the maximum gain for a given horn length. Tables showing dimensions for optimum horns for various frequencies are given in microwave handbooks.
Read more about this topic: Horn Antenna
Famous quotes containing the words optimum and/or horn:
“Everywhere—all over Africa and South America ... you see these suburbs springing up. They represent the optimum of what people want. There’s a certain sort of logic leading towards these immaculate suburbs. And they’re terrifying, because they are the death of the soul.... This is the prison this planet is being turned into.”
—J.G. (James Graham)
“The hounding of a dog pursuing a fox or other animal in the horizon may have first suggested the notes of the hunting-horn to alternate with and relieve the lungs of the dog. This natural bugle long resounded in the woods of the ancient world before the horn was invented.”
—Henry David Thoreau (1817–1862)