Thanks to Dick PA0SE for doing further
tests. Replacing the antenna with a
increased capacitance from a vacuum variable has lower losses than the "antenna
circuit" but it is a reasonable approximation for gathering measured
My earlier bulletins assumed that the current going
in to the cold end of the coil could be considered as I+Q components. I
now think this is an over-simplification, as it is not the case that a myriad of
stray capacitance to ground can be replaced by a single shunt capacitor at the
feedpoint (or anywhere else), and then assume a perfect inductor. The real
situation involves distributed capacitance, along the coil. I'm no longer
keen on an I+Q current model, as if all stray capacitance was a single
capacitor at the feedpoint.
Test data appears to support that there is only one
system resonance, and the input impedance is resistive at this resonant
point. This is reasonably consistent with the expected performance of a
series resonant circuit (traditional theory of LF and MF vertical
antennas). The distributed capacitance lowers the resonant frequency
compared to a perfect inductor.
If any simple model is to be considered, it
could be that the RF current divides between the capacitance of the antenna
system proper (current going in to the antenna wire) and the net stray
capacitance of the coil to ground. PA0SE found this apportionment to be
about 95/5. Data from a ZL advised privately to me, for a loading coil in
a metal box, shows a ratio of close to 90/10, and this is likely because the
metal box definitely has higher shunt capacitance, but also likely to be low
loss "flux catching".
A practical result is that it is generally safest
to place an RF ammeter in series with the cold end of the loading coil, and
maximum current (at loading coil resonance) does concur with maximum
radiation. The trap is to claim that this indicated current is the antenna
current, but as recent bulletins have underscored, the current in the antenna
wire (from the coil hot side) is less, by some 5 to 10%.
73, Bob ZL2CA