Dear LF Group,
Dick's experiments looking at V and I on a 'scope simply
demonstrate that the antenna was resonant - voltage and current
in phase being one way of defining resonance in an LC circuit. This
does not mean that significant currents are not flowing in the
distributed capacitance of the coil or that they are in or out of
phase with the applied voltage, just that the overall resultant
voltage and current at the feed point are in phase. At my station, I
use what is effectively the same method as Dick's experiment to
tune my antenna during normal use - see the article in the LF
handbook. It has the advantage that you can see at a glance if the
antenna is resonant, and if the load resistance is correct.
Any 2 conductors anywhere will have some capacitance between
them. The distributed capacitance between a loading coil and
ground can be quite substantial - several 10s of pF - because the
coil has a big diameter compared to the antenna wire, and is close
to the ground. A current must flow in the distributed capacitance,
because there is a voltage between the coil and surroundings. The
loading coil terminals and ground effectively form a 3 terminal
impedance matching network containing distributed elements. For
a small antenna with the resulting big loading coil, the current at
each end of the coil will be significantly different due to the current
flowing in the distributed capacitance. Conversely, a big antenna
requiring a small loading coil with distributed capacitance much
smaller than the antenna capacitance will have little difference in
current at it's ends
The distributed capacitance won't significantly reduce the overall
efficiency of an antenna, provided the loss in the distributed
capacitance is small compared to the overall antenna losses. It just
means that a greater current is required at the cold end of the coil
to get a particular antenna current. To minimise additional losses,
much the same considerations should apply to siting the coil as the
rest of the antenna, ie. as far as possible from lossy dielectric
materials. Since the ground is a very lossy dielectric, there is a lot
to be said for raising the coil off the ground as far as possible. The
altenative is a low - loss electrostatic screen, to prevent the
electric field of the high voltage parts of the coil reaching the lossy
dielectrics. G3YXM seems to have done this succesfully,
completely surrounding the coil. I once tried a similar experiment to
GW4ALG's, covering a 3m x 3m area of ground around the loading
coil with wire mesh, but it made no observable difference to the
loss resistance. But then, my coil is already about 1.5m off the
ground. There was only a few percent difference in current
between the hot and cold ends of the coil, but my antenna is
considerably bigger than Steve's, with about 50m of wire in total
(305pF capacitance). When I had an indoor loading coil (with a
similar antenna), the difference in current was considerably higher,
about 10%, presumably due to higher distributed capacity
Since most amateur LF antennas are high impedance devices (ie,
high voltages, relatively low currents), I would guess that the
dielectric losses will normally dominate the magnetic ones
(hysteresis, eddy currents). The magnetic field of a coil falls off
rapidly with distance - an inverse cube law, I believe, so keeping it
a reasonable distance away from conducting materials is probably
adequate to minimise the magnetic losses. However, a poorly
designed electrostatic shield around a loading coil might increase
the eddy current losses drasticaly.
Any kind of experiment that involves measurement of V or I at the
"hot" end of the coil using an oscilloscope is going to be very
difficult to obtain meaningful results from - any connecting leads will
considerably alter the capacitances, and the high electric and
magnetic fields will induce signals in leads and probes that lead to
erroneous results. Phase measurements using a 'scope are
notoriously inaccurate, and small changes in phase will be hard to
detect reliably.
Cheers, Jim Moritz
73 de M0BMU
|
