> Well, if that were the case, how is it that the voltage rises so much from
> the bottom of the loading coil to the top? If the current is the same,
that
> would mean that power is being created in the coil.
The coil primarily cancels reactance or corrects power factor.
The very short antenna being almost a pure capactance, has voltage and
current nearly 90 degrees out-of-phase. The coil brings current and voltage
into phase by adding an opposite sign reactance.
As the signal moves along the coil from top to bottom, phase difference
between voltage and current is progressively brought closer to in phase.
>Sounds like the beginning
> of a perpetual motion machine.
Every reactive system in the world works that way and always has. That why
we have VAR (volt-amperes- reactive) power or power factor.
That's why the voltage across a capacitor in a small transmitting loop
antenna with only 100 watts applied is ten-of-thousands-of-volts, while the
current through the capacitor is dozens of amperes. The current peak, in the
reactive part of the system, does not occur at the same time as the voltage
peak and so the reactive power is much much higher than real power, hence a
power factor.
In your mobile antenna, the real power only appears at the feedpoint. Above
the power factor correction, the VAR power can be hundreds of kilowatts.
That why you get corona and heat (from current) at the same time near the
coil.
> So, Tom, I think we are singing from the same hymnal, but the verse about
> *non-radiating* device is giving us trouble. The loading coil is indeed a
> radiating device, but the majority of it's field is doing nothing but
causing
> heat.
It is a two-terminal device Barry. Kirchhoff's laws require it have the same
current into one terminal as out the other. The only exception to this is if
the physical area of the coil is so large it has significant radiation to
the outside world compared to the rest of the antenna.
This is why a long helical antenna radiates quite well, while a loading coil
does not. Coil losses, even those related to magnetic energy storage fields
or shunting capacitances, are contained within the coil area and do not
cause a current taper in the coil. Only true radiation or true capacitance
shunting to the outside world allows the current to taper, and that taper is
normally inconsequential in a typical mobile loading coil of reasonable form
factor.
Coil loss does not affect the current taper. Coil size does.
Another way to look at it is as radiation resistance of the coil. It has to
approach the radiation resistance of the antenna, or the antenna is the
significant source. That requires the coil to have significant height
compared to the antenna, since even at half-height it has 1/4 the radiation
resistance. Unless it radiates, the current stays in the coil.
73 Tom
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