[Elecraft] RFI and Direct tv or Dish network

[Jim Brown]

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 <jim at 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.
>
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