[TowerTalk] Transmission Line Theory and Practice

[Tom Rauch]

> Given: (1) a length of hardling (1/4 wave, 1/2 wave, odd
length, open-ended
> (2) from a 50-Ohm source, feed just the center conductor
of the hardline,
> leaving the outer jacket unconnected to the source ground
>
> Questions: (1) What's expected to happen to the r.f.
inside the coaxial
> section --
> (a) it's my understanding that the SWR should be
infinite -- will the r.f.
> couple to the INSIDE of the jacket, OUTSIDE, or both?
Neither?
> (b) will this property be exhibited in a resonant 1/4-wave
or 1/2-wave
> length? In other words, does it matter how (practially)
long the hardline is?
>
> Purpose: to create a radiator from hardline by a means
other than attaching
> to the outer (aluminum) jacket.

Bert,
If you look at
http://www.w8ji.com/coaxial%20currents/coaxial_line_and_shielded_wires.htm

you'll see a diagram of what happens when the center is fed
and the shield remains open.

The effect of feeding the center is the shield does all the
radiating. This is true in "shielded loops", snake antennas,
that silly "shielded ground lead" idea, and any other system
with common mode currents.

As long as the shield is several skin depths thick, we can
consider the following as the true behavior:

When the shield and center conductor are connected in
parallel, ALL of the RF current flows on the outside of the
shield.

When the center floats and the shield is fed, all of the
current in on the outside of the shield.

When the center is fed and the shield floats on one or both
ends, the current loops like the diagram shows on that link.
The effect of this is the loss resistance of the cable goes
up and the series impedance increases compared to a "feed
the shield alone" system. The outside of shield still does
ALL of the radiating, nothing radiates through the shield.

73 Tom