[TowerTalk] 135 foot dipole, 300 ohm twinlead

[Tom Rauch]

> Buck, something isn't right.  A properly constructed and
tuned balanced
> feed line antenna should be less likely to generate TVI
than a coax fed
> antenna because there should be little or no radiation
from the feed
> line.

There isn't much difference at all if any.  The radiation
comes from common mode currents, and either could have
problems. Actually twinlead radiates slightly more if
everything is perfect, but the amount of radiation from
either is negligable unless something is hosed up.

> If you're tuner uses a ferrite balun, it could be
saturating and
> generating all sorts of spurious radiation.

Ferrites are the materials of choice in baluns. It could be
a VOLTAGE balun, rather than a current balun. It could be a
poor balun design or bad balun constuction. What kind of
tuner and balun is it?


> >I had a 129 foot dipole fed with CATV coax.  It did ok,
but where the
> >aluminum met the copper, it oxidized and I have been
suffering with AM
> >broadcast band bleedover on all frequencies below 14 MHz.
I recently
> >replaced it with a 135 foot dipole fed by 300 ohm TV
twinlead to my tuner on
> >which my IC-706 sits.  The problem I have now is TVI.
When I get the
> >antenna tuned and run more than 30 watts, the TV pictures
turn red, green,
> >or blue.  We are on Cable TV and I cannot recall the last
time I had TVI
> >other than when I operated 6 meters and the rig is
sitting less than two
> >feet from my TV.  (the TVI now is all over the house.)  I
don't know if it
> >is getting into neighbors homes at this time.

If the problem is on all channels, it sounds like common
mode RF. If it is common mode RF, both the TV system and
perhaps your antenna may be at fault. Certainly the TV
shares some or most of the blame, unless you have a heap of
problems.

If the problem is channel specific, like say only on low VHF
channels, you most likely have harmonic issues.

I'd sort out what the problem is Buck by giving enough
information, otherwise anything anyone says is nothing but a
wild guess. My wild guess at the moment is you have an
unbalanced feedline condition resulting in common mode
currents on the chassis of your equipment.

What tuner?
What balun?
How is the twinlead routed? Is it kept away from other
things?
What channels are affected?

73 Tom