What is the most appropriate length of an omnidirectional antenna of a radio system that operates at 900 MHz?
What is the most appropriate length of an omnidirectional antenna of a radio system that operates at 900 MHz (c = 300e6 m / s)?
Please include references of books or web sites
TELECOMUNICATIONS
I have five options:
a) 0.33 cm
b) 3.3 cm
c) 33cm
d) 3.3m
e) 33 m
The question says that I have to find the approximate answer.
I thought there was a formula, so am I wrong?
One Response
Numbat
27 Nov 2009
Almost any conductor will radiate electromagnetic energy. In general, we like to use resonant antennae for our transmissions but a non-resonant antenna will work just as well. A non-resonant antenna could be any length. The problem with non-resonant antennae is in feeding them.
An omni-directional antenna will almost certainly be a vertical (any horizontal antenna does not radiate off the ends so is not omni-directional). Omni-directional in this sense means it will radiate in any horizontal direction (it still will not radiate of the ends but we ignore that case since that direction is straight up).
The shortest length that will give us a reasonable impedance to allow the transmission line to couple to the antenna is usually accepted as being one quarter wavelength or 8.328cm. In practice, slightly shorter than this would be used. You will also need a ground plane. This antenna would be vertically polarized.
Another length that would be suitable would be a dipole which would be one half wavelength or 16.65cm. No ground plane would be required and the transmission line could feed the center of the dipole (still vertical).
Many other lengths and feed systems are possible.