Dear Rik, LF group,
At 14:36 10/07/2002 +0200, you wrote:
What about small loops ?
I found several descriptions of small transmitting loops close to ground
and there loops were fed at different places (center horizontal side, at
the edges). Polarization seems to be vertical in all cases.
And what about a small loop is free space ? Current distribution is
uniform over the entire loop, regardless of the feeding point. So one
would expect always the same polarization (but which one ?). Unless also
the voltage distribution plays a role.
In the far field, the E field vector of a small loop is always parallel to
the plane of the loop and at right angles to the direction of propagation -
so an observer standing on the ground the E-field is always vertical for a
vertical loop, irrespective of where the loop is being fed. The assumption
made for a small loop is that the current is constant throughout the loop,
and since it is the current that is responsible for the production of the
radiated waves, it makes no difference if the loop is rotated around its
axis, which is effectively the same thing as changing the feed point.
If there is no ground plane, ie. in free space there is no meaningful way
of distinguishing horizontal and vertical, so one just talks of "linear"
polarization, with an angle relative to some convenient set of coordinates
- bur the E field will still always be at right angles to the direction of
propagation and parallel to the plane of the loop.
BTW, does anybody know what the directive gain of a small, vertical loop
above a ground plane is? The text books always talk about loops in free
space - for a loop in free space it is 1.5, same as for an infinitesimal
dipole. But I would imagine that when very close to an ideal ground plane
the directive gain would increase by 3dB to 3, by analogy with a short
monopole over a ground plane. This would also increase the radiation
resistance of the loop by a factor of 2, as in the case of a monopole
versus a free-space dipole of the same length. These factors would make a
significant difference in the calculation of the relative efficiency of
loop vs, vertical TX antennas.
Cheers, Jim Moritz
73 de M0BMU
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