James MB0BMU and others
I agree with your comments that impedance bridges are up against it for
directly measuring electrically short antennas, and that prior resonating
with a loading coil (and correcting for coil loss) is a better way.
I don't know why rain has so much effect, but my favorite theory for
the major cause of loss resistance at the moment is that it is
caused mainly by dielectric losses in the ground, where the electric
field of the antenna penetrates to some depth at LF. This is
contrary to the conventional view that the major losses are due to
the resistance of the ground system. I don't think there is really a
contradiction, just that amateur antennas have relatively high
dielectric losses because they are smaller than conventional LF
antennas. A bit of thought shows that a predominance of dielectric
loss would explain lower loss resistance at higher frequency, and
G3AQC's "footprint" effects, among other things.
My view on "moisture loss" is that water is a very lossy material at RF.
The dielectric constant of water or ice is about 80, so rain drops or any
surface moisture on an antenna significantly influence electric flux
terminating on the wire. Natural wavelength in a media varies as the square
root of dielectric constant, so at first it would seem a good idea to make a
vertical in a plastic pipe filled with water, but that does not account for
what additional loss arises from the water jacket. I do not have
dissipation figures for water at LF so I can not attempt a back of the
envelope loss estimate, but I intuitively know it is quite high. As an
aside, water entering the braid of coaxial cable plays merry hell with loss
and SWR, and that is caused by a combination of dielectric loading and loss
from water molecules. Does any reader have some loss data for water, and
how it varies with frequency?
I believe there is merit in the "environment dielectric loss" idea for
amateur LF antennas. Some time back I carried out an experiment by
measuring the base impedance (R and jX) for all combinations of a top loaded
vertical, by having vertical part alone, one side top loading only, then the
other, then combined, also for short and long top loading. I have a number
of bare copper buried ground radials. The resulting impedance figures made
little sense at all until I converted them to parallel components. The
parallel (admittance) data showed a clear trend that connecting top loading
was like connecting parallel lossy capacitors. What was even more striking
was that the power factor of each added part was very similar, irrespective
of wire length (a normalised parallel RC figure per metre wire length could
be assigned for a given environment?). It follows from this that the bigger
the top loading, the lower should be the net resistive component of base
impedance.
Re: Insulators - Although better insulators, ie big glass or ceramic
ones would help prevent the antenna catching fire or falling down,
they would not actually stop corona discharge from taking place.
This is a function of the field gradient around the antenna wires,
and so needs attention to the conductors more than the insulators,
hence the usefulness of corona rings. Prevention of corona is also
a good idea from the QRM point of view.
Agreed. Always use good insulators. Avoid "sharp corners" on wires and be
aware of deploying corona rings for higher powers. The ZL LF band 165 - 190
kHz is less demanding than the EU 136 kHz band for "loose caloric" in the
antenna department.
I can confess to melting some fairly thick monofilament nylon when I moved
to a higher power transmitter. Braided nylon rope can act like fusewire
when it is damp (another indication of dielectric loss in water?)
73, Bob ZL2CA
|
