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Spurred on by Steve, GW4ALG, I also measured the current above and
below my aerial loading/tuning coil.
The aerial is my multiband one for 10 - 160 m; consisting of a 2 x
20 m dipole, centre fed by 11m open line feeder. For 136 kHz the feeder wires
are strapped together in the attic shack . There the loading coil is
located; its bottom end is connected to a radiator of the central
heating system. The current from the bottom end of the coil flows from
that radiator to the boiler, which is adjacent to the shack, and from there via
its gas pipe to the gas main which is the only earth
connection. So the gas pipe actually forms part of the radiating system,
the vertical part of which is about 18 m in total (7 m gas pipe + 11 m strapped
feeder line). The earth resistance varies a bit with the season but usually
is around 30 ohms.
The coil is 56 cm high of which the upper 12
cm is shortcircuited. I have found out earlier that it makes no difference in
aerial current whether the upper unused turns are left open or shortcircuited.
Fine tuning is by a vacuum capacitor in parallel with the coil. Because this
causes an extra current in the coil, increasing its loss, I take care to make
the coil so large that only a small capacitance is required for
resonance.
I adjusted the transmitter for 2A aerial current flowing into the
strapped feeder wires.
Current in the connection from the coil to the central heating
radiator was then 2.1A.
I used the same thermo couple instrument for both
measurements.
73, Dick, PA0SE
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