Can it be that for years we have all looked at the aerial current
distribution diagrams in the handbooks which show a constant for the
loading coil and a sine-wave portion for the "whip" and nodded without ever
demanding that it be a sloping line for the coil?
G3GVB
----- Original Message -----
From: Vernall <[email protected]>
To: <[email protected]>
Sent: Tuesday, March 01, 2005 7:57 PM
Subject: LF: Re: Current "lost" in loading coil
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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