Hi Eric,
I'm glad to see someone answered David. I agree with everything you said,
but I'd like to expand one point you made.
> What usually causes a transmission line to radiate is:
>
> 1. Using the wrong type of line for the load (unbalanced line,
> balanced load, etc.). Note that this is the typical case for
> a center fed dipole or Vee antenna fed with coaxial feedline.
Of course this also includes using a UNbalanced line to feed a
less-than-perfectly-UNbalanced antenna. For example, a coaxial line
feeding a conventional four radial groundplane will radiate! That's because
the electric field also extends below the radials, and the feedline does
not leave the antenna at a neutral point in the electric field. Drooping
the radials down helps, and if you form the radials into a solid cone that
surrounds the feedline it virtually eliminates the problem. AEA did that in
a VHF antenna.
Another way to picture this is this. The radials have high voltage at the
ends and the voltage difference between the ends of the radials causes
current to drives the line, with the return path through the electric field
back to the radials. (Actually the field is maximum at the end, with the
entire radial length contributing to current flow on the outside of the
feedline.) If the groundplane forms a cone around the feedline, the
electric field induced current is confined to the inside of the cone, and
the line will not have much or any unwanted shield current outside the
cone.
The same is true for a vertical with a small elevated radial system, or a
less than perfect radial system of any kind. The better (more dense) the
radial system, the less the problem. Failure to use a choke balun on an
elevated radial system vertical decreases FS a measurable amount (about 1-2
dB in my measurements).
Another less common problem is feeding an unbalanced antenna with balanced
line, like a open wire line feeding a vertical. Some books and articles
incorrectly suggest a vertical can be properly fed with balanced line if a
balun is used at the transmitter, but such systems actually require a balun
at each end.
Line radiation is caused by any improperly treated balanced to unbalanced
junction or excitation of the line by induction or radiation fields. As
Eric points out, SWR hasn't got a thing to do with it.
73 Tom
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