[Lf] [Fwd: LF: polarization of small loops]

[Andre Kesteloot]

James Moritz wrote:

> 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