On Thu,5/19/2016 6:41 AM, Warren Merkel wrote:
Thanks for the note Jim,
However I've already got multiple passes of the OCF Feedline fed through
a stack of five 2.4" #31 donuts, per your very own, excellent
RFI-Ham.pdf article. Picture on my QRZ page.
https://www.qrz.com/db/KD4Z
But that choke is in the wrong place -- to kill feedline current, it
must be at the FEEDPOINT, up in the air where the feedline connects to
the horizontal part of the antenna. Remember -- this is not DC, it is
RF, and the feedline between the feedpoint and the choke is part of the
antenna. All you have done is make current small at the location of the
choke, but because the feedline is part of the antenna, current will
vary along it following the laws of physics that determine how antennas
work. To understand this, think of an ordinary resonant center-fed
dipole. Current is near zero at the ends (it's an open circuit, with
only capacitive coupling to space), and peaks at the center (because the
center is a quarter wavelength from the end point). The same thing is
happening with your feedline -- common mode current is near zero at the
choke (you've forced that with the choke) and increases along the line
toward the feedpoint.
The problem with antennas fed with open wire line is that it is simply
not practical to choke them at the feedpoint. OCF antennas have the
additional problem that the off-center feed makes them VERY unbalanced,
which creates a lot of common mode current on the feedline. This
doesn't prevent them from working as antennas, but it does put a lot of
RF in the shack and the feedline can also receive a lot of noise (if
there is any around it).
The RG6 has a stack of three donuts with a large number (didn't count)
of turns, located at the demark point to the house. I guess I could add
another set and spread out the Z over frequency a bit more.
Spreading Z over frequency depends on the frequency range where
transmitting causes problems.
I believe I need to look for other entry points to the entertainment
system as a whole, as the DirecTV DVR is network connected and of
course, connected to an AV receiver. Lots of points of entry to squelch.
I don't remember seeing a description of the symptoms of your problem.
I would expect that interference to the DirectTV unit would be indicated
by "breakup" -- a failure to decode, or interruptions in the decoding of
the signal. If that isn't happening, but you're hearing detected audio
or clicks, I'd suspect gear in the entertainment system and work on
choking cables connected to it.
But never rule out the possibility that there can be multiple points of
ingress to that system.
73, Jim K9YC
Warren, KD4Z
On 5/18/2016 8:29 PM, Jim Brown <j...@audiosystemsgroup.com> wrote:
Your antenna, an off-center-fed Windom, is notorious for generating
common mode current on the feedline, and there's no way to choke it
effectively to kill that current. So what you're calling a feedline
Mother Nature calls part of the antenna. THAT'S a primary cause of your
problem.
If you're having issues on 80M, you'll need at least 13 turns of the RG6
through a single #31 toroid to make a dent in the common mode current.
______________________________________________________________
Elecraft mailing list
Home: http://mailman.qth.net/mailman/listinfo/elecraft
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:Elecraft@mailman.qth.net
This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html
Message delivered to j...@audiosystemsgroup.com
______________________________________________________________
Elecraft mailing list
Home: http://mailman.qth.net/mailman/listinfo/elecraft
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:Elecraft@mailman.qth.net
This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html
Message delivered to arch...@mail-archive.com
