"Dave" wrote in message
...
Cecil Moore wrote:

Here's a question overflowing from eHam.net and it's
not a trick question.

Assume that the radiating portion of a 40m vertical
is made out of 33 feet of RG-213 and the braid is
the radiator. The center conductor of the coax is
left floating at both ends. How much RF voltage
and/or current will be induced in that center wire
when using the outside braid as the radiator for
100 watt operation? How much of an EM field can
exist inside the coax braid?

From my EM311 days:

If I integrate the fields inside a short conducting cylinder, since there
is no charge within the surface integral, there is no field within the
cylinder.

So the volume integral, ?E.dL = 0 = No voltage. But, this isn't the case!

Now, if the cylinder is a 1/4 wavelength with distributed L and C ... ???

Since the cylinder is long compared to a wavelength, the distributed
capacitance will couple a voltage to the inner conductor. The terminal
impedance is open circuited [High Z] so no current flows.

ah, but it isn't! remember, 1/4 wave long (more or less) makes the open at
one end look like a short at the other end. there is also that distributed
capacitance all along the length between the inner side of the shield and
the center conductor... and also, the driving voltage at the ends are about
90 degrees out of phase, so there could be some non-trivial currents in the
center conductor.



Conclusion: a standing wave exists on the inner conductor. It is caused by
the distributed capacitance and the magnitude of the standing wave on the
cylinder.

Been away from EM for almost 50 years. I've probably forgotten too much.