[Elecraft] Small WIre Loops

[James R. Duffey]

The radiation resistance and the far field strength of an antenna depends on
the current distribution on the antenna, which does not have to be
sinusoidal. I believe that George is correct that the voltage and current
must be 180 degress out of phase for efficient power transfer. But it is the
current distribution that drives the antenna radiation resistance and hence
the efficiency, not the phase relationship between the voltage and current.

I must confess that I do not understand your statement:

"There are certain areas around the circumference of a mag loop where low
resistance does not seem important from the point of view of radiating into
free space. I suspect, but do not have the tools to prove, this is where
voltage and current are more in phase than elsewhere in the circle diameter,
and the current is relatively low."

For a circular loop, the radiation resistance will be the same no matter
where the antenna is fed along he circumference. There is no unique
orientation for a loop in free space. For example, in free space hotizontal
and vertical have no meaning as there is no reference datum to define these
points from. 

Likewise, the circular loop is a series circuit, and no matter where it is
fed the Ohmic resistance appears in series with the feedpoint and will
dissipate the same amount of power.

When you add the capacitors to the loop you are changing the current
distribution to make it more constant, that is less sinusoidal. This makes
the radiation resistance and the far field field strength go up. It has
nothing to do with the phase between the current and the voltage.

You have empirically discovered one of the drawbacks of small loops; even
low powers can generate high voltages and hefty components are usually
required. - Dr. Megacycle KK6MC/5
______________________
James R. Duffey KK6MC/5
Cedar Crest NM 87009 DM65