Hi Dick It doesn't make any sense to me at all that the current would be
the same at the top of the coil as it is at the bottom. If it were you
should be able to replace the coil by a wire.
If the impedance of
the coil is at the bottom is different than the impedance at the top the
current should be different . Perhaps the coil losses can mask this
effect but it still
should be true.
Thinking of it another way
if the bottom of the coil is grounded we can match the tx to the
antenna( assuming resonance ) by moving the point where we connect
the tx up and down the coil
73 de John VE1ZJ
Dick Rollema wrote:
To All from PA0SE A.
I have modelled my aerial using Antenna Optimizer by K6STI.This
also provides the current distribution on the aerial.The coil is
modelled as a lumped inductance.As expected the current
above and below the coil is shown to be the same.Now the books have
it that a coil with a length of L metres radiates like a straight wire
of the same length. To find the radiation from my 58cm high coil
I looked at the current distribution on the first 58cm of aerial wire immediately
above the coil, where the current that enters at the bottom end is the
same as in the coil . AO indicates that the current at the top end of that
piece of wire is 1.55% lower than at the bottom end. So that is current
lost by radiation.I measured a difference of 10% between currents
at the top and bottom of my coil. So most of the current lost must be due
to capacitance to surrounding objects. B. I also modelled
Steve's 12m high vertical. The program assumes that the aerial is over
perfect earth. The real earth may be some distance below the surface so
the actual aerial may be longer than 12 m. (Remember Jim's aerial on a
hill at Puckeridge that radiated better than expected from its physical
length?)The program shows that a coil of 12.87mH will be needed
for resonance.For the difference in current between 2.2A at the
bottom and 1.8A at the top of the coil to be entirely caused by radiation
the coil must be 1.85m high. Looking at the picture at Steve's
website this is not the case. So some current must be escaping through
capacitance to the surrounding. C. I entirely agree with
those who state that a difference in current between the two ends of a
coil can only be caused by radiation and capacitance to surrounding objects.Any
other effects, like loss in the coil, distributed capacitance etc. remain
internal to the coil and cannot cause a difference between current flowing
in and out. 73, Dick, PA0SE
|
|
