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"
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