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

[Andre Kesteloot]

Rik Strobbe wrote:

> Dear Jim, LF group,
>
> >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.
> OK, that makes sense. I received a similar explanation from Claudio Gerardi
> (IN3OTD), thanks to both.
>
> >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.
> I agree on the relativity of horizontal/vertical in free space. But since
> the polarization of a small loop is parallel to the plane of the loop one
> could state that when the loop is assumed to be vertical (wherever the loop
> is) polarization will also be vertical.
>
> >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.
> That would have been my next question ...
> And to make things more complex, what about a loop close to a
> not-so-perfect ground plane ?
> Either the loop is not (or little affected) by the ground. That means
> little or no groundloss (as is claimed by some) but at the same time it
> would mean that the loop will also not benefit from any "ground gain".
> On the other hand, if the loop benifits form significantly from
> "groundgain" then I would expect that the loop would also suffer from the
> groundlosses, as a vertical monopole does.
>
> Any clues ?
>
> 73, Rik  ON7YD
>
> PS : A Dutch proverb says "One fool can ask more questions than a hundred
> wise men can answer"