[Fwd: LF: Looping the Loop and all that.]
Wed, 10 Jun 1998 22:49:21 -0400
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
> 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.
> Roger (G2AJV)