Omni-directional / Dipole antennas
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Omni-directional antennas
- Omnidirectional antennas radiate RF signal in all directions.
- A perfect omnidirectional antenna would radiate RF signal like the theoretical isotropic radiator.
- An easy way to explain the radiation pattern of a typical omnidirectional antenna is to hold your index finger straight up (this represents the antenna) and place a bagel on it as if it were a ring (this represents the RF signal).
- If you were to slice the bagel in half horizontally, as if you were planning to spread butter on it, the cut surface of the bagel would represent the azimuth chart, or H-plane,
- If you took another bagel and sliced it vertically instead, essentially cutting the hole that you are looking through in half, the cut surface of the bagel would represent the elevation, or E-plane, of the omnidirectional antenna.
- Omnis are typically higher-gain (>2.14dBi) than a dipole
- With higher-gain omnidirectional antennas, vertical signal is decreased and the horizontal power is increased.
- The figure below shows the elevation view of three theoretical antennas. Notice that the signal of the higher-gain antennas is elongated, or more focused horizontally.
The horizontal beamwidth of omnidirectional antennas is always 360 degrees, and the vertical beamwidth ranges from 7 to 80 degrees, depending on the particular antenna. - Indoor installations typically use low-gain omnidirectional antennas with gain of about 2.14 dBi.
- Omnidirectional antennas are typically used in point-to-multipoint environments.
- The closest thing to an isotropic radiator is the omnidirectional dipole antenna.
- High-gain omnidirectional antennas can also be used outdoors to connect multiple buildings together in a point-to-multipoint configuration.
Dipole antennas
- A dipole antenna most commonly refers to a half-wavelength
- Dipoles are typically low-gain 2.14 dBi
- This is a simple antenna that radiates its energy out toward the horizon
- The small, rubber-coated dipole antenna, often referred to as a rubber duck antenna, is the classic example of an omnidirectional antenna and is the default antenna of many access points
- Antennas are most effective when the length of the element is an even fraction (such as 1/4 or 1/2) or a multiple of the wavelength (λ).
Reference:
Coleman, David D.,Westcott, David A. CWNA: Certified Wireless Network Administrator Official Study Guide: Exam CWNA-106 Wiley.
https://www.cisco.com/c/en/us/products/collateral/wireless/aironet-antennas-accessories/prod_white_paper0900aecd806a1a3e.html
Coleman, David D.,Westcott, David A. CWNA: Certified Wireless Network Administrator Official Study Guide: Exam CWNA-106 Wiley.
https://www.cisco.com/c/en/us/products/collateral/wireless/aironet-antennas-accessories/prod_white_paper0900aecd806a1a3e.html
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