Gain: Difference between revisions
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==What is Gain?== | ==What is Gain?== | ||
=== Gain compared to a half wave dipole - dBD=== | |||
The gain of an antenna is the relative increase in radiation at the maximum point expressed as a value in dB above a standard. The standard to which other antennas are compared is usually a ½-wavelength dipole. The standard antenna is given a reference gain of 0dBD (zero decibel referenced to dipole). This comes from: | The gain of an antenna is the relative increase in radiation at the maximum point expressed as a value in dB above a standard. The standard to which other antennas are compared is usually a ½-wavelength dipole. The standard antenna is given a reference gain of 0dBD (zero decibel referenced to dipole). This comes from: | ||
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<math> 10 \times \log \left( \frac {3}{1} \right) = 4.77\mbox{dBD} </math> | <math> 10 \times \log \left( \frac {3}{1} \right) = 4.77\mbox{dBD} </math> | ||
===Gain compared to an isotropic readiator = dBi === | |||
An isotropic radiator is a theoretical "point source" antenna that radiates the same amount of energy in all directions. Although an isotropic antenna cannot be constructed, it is sometimes useful to compare actual antennas to it. | |||
=== Comparison of dBD and dBi === | |||
Graphically, the radiation patterns of isotropic and dipole antennas - for equal power inputs - can be represented thus: | |||
[[image:Vk4yeh_dbi_vs_dbd.jpg |400px]] | |||
The area between the isotropic radiator circle and the half wave circle radiator circle represents a gain difference of approximately 2.15dB | |||
hence, <math> dBi = dBD + 2.15 </math> | |||
Revision as of 01:08, 7 August 2008
Related wiki pages Antennas
What is Gain?
Gain compared to a half wave dipole - dBD
The gain of an antenna is the relative increase in radiation at the maximum point expressed as a value in dB above a standard. The standard to which other antennas are compared is usually a ½-wavelength dipole. The standard antenna is given a reference gain of 0dBD (zero decibel referenced to dipole). This comes from:
<math> \mbox{gain} = 10 \times \log \left( \frac {Pe}{Pi} \right) </math>
Where Pe is effective radiated power and Pi is input power
An antenna with the effective radiated power of three times the input power would therefore have a gain of:
Pe = 3, Pi = 1
<math> 10 \times \log \left( \frac {3}{1} \right) = 4.77\mbox{dBD} </math>
Gain compared to an isotropic readiator = dBi
An isotropic radiator is a theoretical "point source" antenna that radiates the same amount of energy in all directions. Although an isotropic antenna cannot be constructed, it is sometimes useful to compare actual antennas to it.
Comparison of dBD and dBi
Graphically, the radiation patterns of isotropic and dipole antennas - for equal power inputs - can be represented thus:
The area between the isotropic radiator circle and the half wave circle radiator circle represents a gain difference of approximately 2.15dB
hence, <math> dBi = dBD + 2.15 </math>