At 07:22 PM 6/21/2005, Tom Rauch wrote:
>Reactance doesn't matter....
>
> > Just ballparking the capacitive coupling.. 4x4ft, 1/2"
>away... call it 1.5
> > m2, 0.01 m spacing, about 1300 pF. At 4 MHz, about 30
>ohms reactance
> >
> > Comparing the car, say 6x12 ft, 6" away, about 394 pF., at
>4MHz, about 120
> > ohms reactance.
> >
> > The flat plate on the ground has a low enough impedance to
>the ground that
> > it's going to couple pretty strongly, compared to the
>truck.
> >
> > Neither is going to be particularly wonderful, of course.
>
>since it isn't dissipative and simply causes the antenna to
>"shift frequency".
>
>The resistance is what causes the loss.
True.. but if you had a very huge series reactance to ground, then not much
current is going to flow through that path, so the actual loss (for that
part) will be low. Of course, it may well be that the current now flows
through something else which is even lossier.
One model might be to consider the base as a resistor (the car body) and a
capacitor and resistor in series (the coupling to the ground and the ground
loss).
Just that plates a fraction of an inch off the ground are coupled much more
strongly than a car body, and the effect isn't negligible.
>73 Tom
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