[Antennas] Interaction Between 40m Attic Dipole and Cable TV Coax

[Linden, Mike (BRC-Hes)]

  Somebody once said that, "Antennas are outdoor creatures by nature". But
what about us poor folks that must place them in the attic due to CC&Rs?
Hmmm...

  I just determined that the 40m portion of my 10/15/20/40m coaxial trap
attic dipole is interacting with a cable TV coax line running along the
floor of the attic. The antenna runs along the ridge of the attic about nine
feet above the coax which also runs almost the full length of the attic. The
dipole has a current balun at the feed point. The interaction is primarily
with 40m -- interaction with other bands is minor enough that I am willing
to ignore it.

  When the cable TV coax IS NOT connected to the splitter in the basement,
my best SWR on 40m is 1.2:1 (not bad for an attic antenna!). However, when
the cable TV coax IS connected to the splitter in the basement my best SWR
moves up to 1.4:1 and the point of resonance (based on SWR dip) moves about
100kHz.

  Articles and books I've read infer that the solution is to "break-up" the
coax with chokes so that it becomes less resonant with respect to 40m (of
course, this could result in it becoming more resonant on the other bands!).
I'm looking for suggestions as to the most simple route to "break-up" the
coax's resonance without cutting into the cable TV coax in the attic.

  My thoughts on possible solutions include two-piece clamp-on ferrites (I'm
not sure these would provide enough common-mode attenuation to make a
difference), Radio Shack style rectangular two-piece split ferrites that
would allow me to wrap the cable TV coax through the ferrite several times
WITHOUT cutting the coax, or wrapping the cable TV coax around a ferrite rod
several times.

  Any thoughts? If legitimate solutions get too complex, it might not be
worth pursuing...

  Thanks, Michael N9BDF


PS  The "Amateur Radio Emergency Communications Consistency Act" (HR 1417)
and the "Amateur Radio Spectrum Protection Act" (HR 713 & S. 537) have been
reintroduced in Congress. Please contact your Congressmen to ask for their
support of these bills! See the following ARRL link for details:
http://www.arrl.org/govrelations/