On 4/17/20 11:59 AM, Jim Brown wrote:
On 4/17/2020 9:47 AM, K6UFO Mark Aaker wrote:
Brian said: ...several other feeds that are bundled together for 40,
6, and 2 meters
in the same run up the tower...
I have always wondered if there was noise induced by "bundled" coax
cables. If you ever have a chance to check that it would be interesting.
Hi Mark,
Several thoughts.
First, each of those feedlines ought to have an effective choke at the
feedpoint.
If they're on a tower, they ought to be bonded to the tower top and
bottom for lightning protection.
Bundling essentially causes them to be a single conductor in the common
mode circuit, including the tower itself. This causes the total common
mode current to be divided between them.
The potential mechanisms I see for the coupling of noise in this
situation are 1) coax with very poor shielding, hence the mechanism
quantified by the cable's "transfer impedance" (see the RX choke pdf)
and 2) lack of proper grounding and bonding in the shack and the
building that houses it.
I can envision another way in which feedlines could be a problem..
that's by being a conduit or conductor of interference on the outside of
the coax to the vicinity of the antenna.
Say you have a noisy power supply in that sprinkler timer or hot tub
controller radiating RF next to where your coax leaves the house, and
your coax runs above ground 50 feet to the antenna. You've done the
right thing by choking the coax near the feedpoint. Since it's well
nigh impossible to have the coax perfectly symmetric with the dipole,
any interference that couples onto the outside of the coax can ride the
wire up to your antenna.
The same thing could apply if the conductor is something like a
raingutter instead of coax.
Obviously, the *real* solution is to get rid of the RFI source, but that
might not be feasible. In this case, choking along the conductor might
help (big *might* there).
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