At 12:00 13/05/01 +0200, PA0SE wrote:

So my conclusion is that the difference in current must be due to radiation
from the coil.  The late Klaas Spaargaren, PA0KSB, once told me that in
an article on mobile antennas it was stated that a coil of L metre length
produced a radiation equal to a straight wire of that same length.
As reported before that would in my case produce a current difference of
only 1.55%, as shown by computer modelling; much less than the measured
difference.
Perhaps the statement in the article is not true, or not applicable for the
sort of coils we
use on LF?
Could that explain the succes of the helical antenna?

Hello Dick,

as I see it a (short) vertical wire acts mainly as a capacitance
distributed over the wire and has to be brought to resonance by an
inductance (loading coil). 'Below' the loading coil the antenna voltage is
low and the antenna current remains alomost constant, 'above' the coil the
antenna voltage is high and the antenna current has a linear decrease (to
be 0 at the end of the vertical wire).
With a helical antenna both the inductance and capacitance are distributed
over the entire antenna. If the helical is the right size no additional
inductance is needed, the antenna has a 'natural resonance' on 136kHz. In
that case antenna voltage and current will increase (voltage) and decresae
(current) sinusoidal.
Due to the better current distribution a helical antenna will have a 1.54
times higher radiation resistance that a vertical wire of the same length
(with the loading coil at ground level), this equals a gain of 1.9dB.
I believe that the main problem with a helical antenna on 136kHz is
'mechanical'. You have to make a 10 to 15m high construction that will
withstand wind and rain. The only ham tht used a helical with succes (as
far as I know) was Tony, HB9ASB, but even his antenna came down in a gale.

73, Rik  ON7YD