Isotropic radiator antenna?
Has there ever been an antenna design that's been devised that radiates equally (as good as possible) in all directions, with 0 dBi?
Radiating equally in all directions is quite a trick, and of course there's imperfections in the pattern. The question isn't about perfection, but "damn close"
An antenna that fit the definition of an isotropic radiator would have a radiation pattern like that of the sun. (Somebody might say only the sun's top hemisphere.)
A low, straight 1/2λ dipole has most of its radiation going straight up, and there's little radiation at low angles or directly off the ends.
The 3-D folded loop has a nearly isotropic radiation pattern.
Brian Beezley published one design oh, 15 years ago in CQ.
Originally Posted by KE7HQY
We often work with the concept of an antenna that radiates power equally in all directions (Directivity=1).
When you talk about a practical isotropic antenna, do you expect that a pair of them would exhibit path loss given by Friis's formula over a free space path. That would imply that they was no cross polarisation loss for any random orientation of the two 'isotropic' antennas.
What is the application of the real isotropic antenna? Perhaps the thought is to prove that it cannot be done, and to use that to argue that the concept of isotropic antennas, or the use of them as a gain reference is invalid.
If you are looking at a practical antenna that is isotropic, in a hemisphere alone, or in a sphere, you need to state your polarisation requirement.
Or perhaps you are looking for an omni-directional antenna, for example an omni vertical would have constant gain at all azimuth angles, though gain at different elevation angles may be different.
An Isotropic antenna does not have a mast or tower to hold it, no coax feeding it, and there is no earth below it. An isotropic antenna is a radiator in free space on a computer display to show the actual performance of the antenna with no obstructions in any direction to affect it's radiating pattern. This increases it's gain factor, which is why dB/i figures are always higher than dB/d figures. Usually by about 2.2dB. A mast and earth below the antenna changes the antenna's pattern drastically.
Isotropic displays should only be used for comparisons of different antenna designs by manufacturers. In the real world, we have earth, and obstructions to consider.
Not quite, a half wave dipole in free space has a gain of 2.14dBi, and it is due to its directivity, not the influence of masts, ground etc as you say.
Originally Posted by K3DAV
A half wave dipole in proximity of the ground might well have a gain of upwards of 6dBi, due in part the the effect of the ground.
Thank you Mike. This proved my point.
Originally Posted by W0BTU