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Date: Fri, 12 Jul 2002 11:58:05 +0200
To: rsgb_lf_group@blacksheep.org
From: "Rik Strobbe" <rik.strobbe@fys.kuleuven.ac.be>
Subject: Re: LF: polarization of small loops
In-reply-to: <5.1.0.14.0.20020711180215.00abbba8@gemini.herts.ac.uk>
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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"