Lee,
Chokes at the feedpoint of dipoles and other simple antennas reduce the
level of common mode currents that a. get into gear or on their surfaces
and b. radiate from the feedline, thus distiring to one or antoher degree
the intended antenna pattern. Transmission line transformers and ferrite
cores surrounding the outside of caox--as well as coax coils, have been
the most common means of effecting the choke.
Roy Lewallen did a study (I think it is in one of the Antenna Compendiums)
on the effectiveness of chokes, making actual measurements of current
amplitude on the outside of the coax line he used in the test set-up. The
benefits are well-worth studying.
In the study of the phenomena, there have been experiential reports
ranging from extreme to extreme. At one end has been the experience that
a very good 90 degree angle of coax to dipole yields no troubles, even
without a choke. At the other is the report that current flows on the
outside of the braid as a matter of course. However, even without
physically detectable currents (meaning, no shocks from the panels, etc.),
the choke is good practice for antennas that are well matched to their
feedlines--if for no other reason than the ounce of prevention, pound of
cure principle.
These notes do not apply to multi-band doublets using parallel feedline,
etc. Nor are they a replacement for the literature on the subject.
Hopefully, they might point in the right directions for further reading.
-73-
LB, W4RNL
L. B. Cebik, W4RNL /\ /\ * / / / (Off)(423) 974-7215
1434 High Mesa Drive / \/ \/\ ----/\--- (Hm) (423) 938-6335
Knoxville, Tennessee /\ \ \ \ / / || / (FAX)(423) 974-3509
37938-4443 USA / \ \ \ \ || cebik@utk.edu
URL: http://funnelweb.utcc.utk.edu/~cebik/radio.html
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