Thank you Stefan for this interesting experiment
and report. Here are a couple of thoughts I have
discussed with Stefan, and which I would like to share with the
group.
Regarding the decrease of the resonance frequency
close to earth: The wire is a line with distributed capacitance and
inductance. If you would bring a halfwave line in close proximity to a
perfectly conducting plane, the capacitance would be increased. But at the same
time the inductance would decrease, as the magnetic flux would be confined
by the smaller gap between the wire and the ground plane. So the wave
impedance would go down but the resonance frequency would remain
nearly unchanged.
A couple of meters above real ground, the
capacitance would still be increased as long as its impedance is
high compared to that of the soil. However the magnetic fields
will penetrate the soil to some depth, so the effect on the inductance is
much less. Thus the resonant frequency will go down at low height.
However when the wire is actually laid on the
ground, the capacitance becomes so large that its impedance falls below
the ohmic ground resistance. So then you will end up with a nonresonant
nearly real impedance - just what Stefan observed.
Regarding the radiation pattern: An low
dipole, oriented eg. in the E-W direction, would be expected to have
three distinct modes of radiation:
- a high angle skywave ("Near Vertical
Incidence Skywave"), with more or less omnidirectional pattern, and the E-field
vector polarized E-W,
- a horizontally polarized low angle radiation,
broadside to N and S. However this would be heavily attenuated by the ground
acting as a short circuit, and would not be able to develop into a
propagating ground wave,
- a vertically polarized groundwave to the E and W,
generated by an earth loop formed by the wire and the subsurface
return current. This is the same mechanism as in the VLF earth-antenna
experiments, only with the galvanic end contacts replaced by capacitive
coupling. I believe that this mode enabled the daytime QSO between Stefan and
myself, 180 km due east of him. This axial radiation would of course not be
present for a free space dipole.
Best 73,
Markus (DF6NM).
Sent: Thursday, August 02, 2012 5:51 PM
Subject: Re: LF: MF: 630m band dipole tests today
