[Fwd: LF: Looping the Loop and all that.]

[André Kesteloot]

Prof R. Jennison wrote:

> I had just drafted the following comment and was about to send it when I
> read Mike Dennison's latest email.  It must be telepathy! :-
>
>   I have been extremely interested in all of the correspondence about the
> effect of the ground on aerial systems.
>
>   Those of you who heard my talk at the 1997 HF Convention may recall that
> I stressed the importance of earth losses.  At my own QTH I have extremely
> poor earth (six inches of soil over chalk) and, what makes it worse, I
> cannot erect a very tall aerial system.  That is why I chose to use a loop.
>
>   With a loop aerial, the emission is primarily in the magnetic component
> and the electrical countrerpart is formed in the far field.  The earth
> losses are still important but they are secondary to the operation of the
> system.
>
>   A Marconi aerial is a hybrid  (ie. it is capable, in principle, of
> generating both the E and the H components), and it is very well suited to
> high frequency transmitters operating at a wavelength of the order of ten
> times its dimensions .  It should produce both the electric and magnetic
> components at the outset if the grounded side is reasonably lossless.  As
> we have seen, this is seldom the case with the miniature versions that we
> use on 73 and 136 kHz.
>
>   As I mentioned earlier, a two turn loop is essentially a magnetic system
> and the electric components are formed in flight, so that the losses
> thereafter are the usual propagation losses over land or sea  and are
> relatively independent of the launching site.
>
>   The loop aerial is the complete opposite to a toroidal aerial.  A
> toroidal aerial is entirely an electric radiating system which forms the
> magnetic field system in flight.  My early experiments with a toroid on
> 73kHz showed that the ground soaked up the whole of the electric field so
> that there was virtually nothing to produce the orthogonal magnetic
> component.   A few years earlier I had to use a short radial a few feet
> above the ground to get a decent signal from a tiny toroid on top band.
> Earthing the radial reduced the signal by over 10 dB!
>
>   As I said at the Convention, I recommend amateurs who are not at sea, who
> have little space and also restricted available height, to try a two turn
> loop, - but do not directly earth the ground side of the loop, that
> includes extra earth losses in the system and the radiated power goes down.
>
>   Incidentally, I use a two turn loop with a set of series/parallel
> capacitors across it as the tank circuit of my transmitter.  The feed point
> for matching to a typical valve P.A. is the half way point.  One needs no
> separate matching network.  For a solid state P.A., one should be able to
> use a series feed connection.
>
>   73,
>   Roger (G2AJV)