[Fwd: LF: Comment on dielectric loading]
Mon, 07 Dec 1998 18:03:27 -0500
> Mike Dennison wrote:
> > Interesting phenomenon during the weekend. The mornings were
> > extremely cold and frosty. I found the vertical antenna was
> > resonant some 3kHz above the band and needed retuning. During
> > the day as the sun shone the resonant frequency returned to
> > normal. There are two trees very close to the vertical so (1) is the
> > sap rising during the day and detuning the ant? (2) is the soil
> > conductivity getting better when it thaws, thus increasing the
> > capacitance with the antenna? or (3) is the loading coil (or
> > matching toroid) changing inductance with the cold? Any thoughts?
> > Mike Dennison, G3XDV
> The dielectric constant of water is 81. Ice is similar. Wet or icy
> antenna wire concentrates electric flux terminating on the wire surface,
> so increases the capacitance compared to dry wire. Plastic coated wire
> is another form of "dielectric loading" but the dielectric constant of
> plastic is usually a lot less than water.
> Water dielectric loading also happens at other frequencies. Yagi
> antennas can be detuned by water droplets if the elements are cut too
> close to the high gain point (too long).
> Also soil moisture under the LF antenna varies with rainfall, and
> seasonally, and this influences the value of capacitance and resistance
> "seen" at the antenna feedpoint. Most of the aeronautical beacon
> antennas in New Zealand are fitted with automatic tuners, to cater for
> the short and long term variations, with unattended operation. Most of
> the ZL amateur LF experimenters use the "Armstrong method" with a
> variometer as part of antenna tuning, and they do get regularly tweaked
> to maintain resonance.
> I understand that sap rising in trees is also a reality, but this is
> seasonal and not daily.
> Bob ZL2CA