Yes, it distorts the pattern, radiating or not.
How much depends upon the situation,
frequencies, spacing, size of feedline.
It is a conductive surface so it will distort pattern.
Side mounted on a tower changes pattern quite a lot
depending upon spacing from the tower, you can tune
the pattern that way. dB Products used to have a good
Catalogue that showed this.
I would not expect much pattern distortion from a feedline
very small, but would from a tower.



"Richard Fry" wrote in message
...
Post below from May 20, 2004

Wouldn't a "non-radiating" feedline in the field of a radiator also
distort
the patterns of that radiator? Any conductor can do that, even if it is
not
a feedline. NEC-2 models of FM broadcast transmit elements (and test
range
patterns) show this clearly.

Paper 6 at http://rfry.org shows the free space patterns that the element
arms of a rototiller FM broadcast transmit antenna develop if they could
be
driven from internal power sources -- and then the effects of adding the
element stem, mounts, feedline, and some nearby tower structure. The
patterns can get very skewed, even though the only radiators getting power
via a metallic path from the tx are the element arms themselves.

RF
____________

"Roy Lewallen" wrote
With a typical ground plane antenna, the feedline can radiate
significantly, distorting the pattern. This effect could easily be
different for the different antennas. Modeling indicates that two baluns
are often needed to suppress the current on the outside of the feedline.
A model which includes the feedline might give some insights as to why
the antennas behave so differently.