Last Week's Question
The directivity of an antenna, D(θ,φ), is the ratio of the power density radiated in a particular direction to the,
- Total power radiated
- Power density from an isotropic source radiating the same total power at the same distance
- Power density from a dipole antenna radiating the same total power at the same distance
- Power density from another identical antenna radiating the same total power at the same distance
Answer
The correct answer is "b". The directivity of an antenna is defined as the ratio of the radiated power density in a given direction from the antenna to the power density averaged over all directions. An isotropic source would radiate the same in all directions, so in any direction it would be radiating the average power density.
Note that isotropic sources would have a directivity equal to one in all directions. Since isotropic sources of EM fields do not exist, all real antennas have a maximum directivity greater than one. Electrically small antennas generally have a maximum directivity of about 1.5, while resonant half-wave dipoles have a maximum directivity of 1.64.
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