Dear LF group,
Re- antenna loss resistance: A basic difficulty with direct
impedance measurement of an LF antenna is that the loss
resistance is much smaller than the magnitude of the reactance.
For example, an antenna with R(loss) = 50ohm, C = 500pF, has a
capacitive reactance of 2.3405kohm. The magnitude of the
combined impedance is 2.341kohm, and the phase angle
88.8degrees, so quite close to that of the pure capacitance. A
bridge will produce an accurate measurement of the capacitance
component, but much larger errors will occur in measuring the
resistive component, which is only a few percent of the total
impedance. Even a very good bridge will struggle to get anything
more than a rough figure for the resistance.
A better way to measure loss resistance is to cancel out the
capacitive reactance of the antenna with a series loading coil (ie.
tune it to resonance), and then measure the remaining resistance,
as done by PA0SE.
A seperate measurement is then required to determine the loss
resistance of the loading coil. This can be done by resonating the
loading coil with a low-loss capacitor in place of the antenna, and
measuring the resistance again.
Re: antenna losses - I use my "Scopematch" tuning aid (see LF
Handbook and errata) to continuously monitor the antenna
resistance while transmitting. I also see the resistance go up (and
the capacitance, to a smaller extent) when it is raining. The change
occurs practically instantly when the rain starts, and is usually
about 10 - 20%. I don't think wet insulators can be responsible -
estimating the additional power loss at a couple of hundred watts in
my case, they should dry themselves out quite fast!
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.
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.
Cheers, Jim Moritz
73 de M0BMU
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