LF: Thoughts about Loops

[MarkusVester@aol.com]

Dear Rik and group,

just a few thoughts about your loop project:

- ground reflection
 Jim mentioned a while ago the doubling of radiation resistance above ground. I think this is always the case, no matter how lossy the ground is, as long as the skin-depth of the shielding currents is much less than a wavelength / 2pi. - Even with 1mS/m conductivity, the skindepth is only 43m.

- voltage
 I also believe that there is the possibility of E-field losses with loops, especially in the parts of the wire that are close to the capacitor. The potential will drop linearly along the wire from there to the voltage minimum at the opposite side. It may be well worthwile to place the "hot" ends somewhere high up in the clear and have the minimum and the coupling structure close to the ground.

- fine tuning
 As the main capacitor will then be fairly inaccessible, you could still do the inband-tuning at the transmitter side of the coupler, as the reactive power for this correction would only have to be a few percent of that in the main capacitor and thus the additional losses would be minimal.

- feeding
 An alternative to tight transformer-coupling might be loose inductve coupling employing a parallel-resonant tank circuit, directly fed by 50 ohm. It could have a few turns around an an area of perhaps one square meter and be placed in proximity to the loop wire. By shifting or rotating it, you could adjust the coupling to achieve any desired impedance matching.

- calculating eddy current losses
 There is a somewhat surprising theorem claiming that eddycurrents induced in a conducting halfspace (ie. earth) will only have components parallel to the surface, no matter what the orientation of the loop is; a vertical loop above ground will induce horizontal figure-eight eddy patterns underneath. I'm not sure, but this seems to imply that the powerloss density in the ground could be calculated simply using the free-space vector potential of the horizontal loop currents alone, without having to solve for the charge-buildup at the surface. By numerical integration of these losses one could then find the optimum height of the bottom wire, at a given top height and length of the loop. 

73 and wishing success

Markus, DF6NM