Dick PA0SE and others,
I believe a good way to analyse why more current goes in the bottom of the
coil than comes out the top is to resolve the current in to in-phase (I) and
quadrature (Q) components. RF ammeters or other sampling current meters
display the resultant (R) current (which equals the square root of [I
squared plus Q squared]). The I current eventually does some radiating
(from the whole antenna system) and the Q current feeds the parasitic
capacitance (and is a low loss even though it can not be eliminated). The
actual mechanism is lots of small Q currents distributed up the coil, some
between turns and some to the environment, so the above simplifies these
currents to a single equivalent value of Q at the bottom of the coil.
Using 10% as a difference between I and R currents, the Q current calculates
as being some 46% of I or some 42% of R. The values are quite sensitive to
the observed ratio of bottom to top resultant currents, and the value of Q
is quite a significant current.
What I'm not so sure about is how to tune in practice for best far field
radiation from the antenna, and whether that condition gives a resistive
input at the bottom end of the coil. If my above above suggestion is a
valid way of working, then it would seem that a secondary matching network
could be needed to cater for the Q current?
73, Bob ZL2CA
To All from PA0SE
Several amateurs have found that the current at the bottom end of the
loading coil is higher than at the top (aerial side) of the coil.
In my station the difference is of the order of 10%.
William, PA0WFO, has a large coil of 8 mH and a 23 m long wire as aerial.
He measures 1.5 A at the bottom of the coil en 0.6 A at the top.
My theory is that the "lost current" flows via the capacitance of the
coil
to its surrounding (even a metal object in free space has capacitance).
The current at the bottom of the bottom of the coil divides between the
capacitances of coil and aerial.
I suggested to William he measure the capacitance of the coil and of the
aerial. For the coil he found 150 - 200 pF, depending upon the position of
the coil and for the aerial 210 pF.
But these values do not explain the large difference in current at bottom
and top of the coil.
In a transmitting aerial the current increases going from the end of the
radiator towards the coil.
Now to my question: does this increase in current also occur in the
winding of the coil? My feeling is that the current at the beginning and
end of a coil should be the same; apart from the current that flows via
its
capacitance to the surrounding.
I also have read that the coil should be considered as an aerial with a
length equal to the length of the coil. But on 2 km
that would be an extremely small aerial, reckoned in wavelength. So
radiation by the coil must be negligible.
There are certainly experts on the reflector who know the answers. I
welcome their views.
73, Dick, PA0SE
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