[TowerTalk] RFI, BN-86, and Choke Balun

[jimlux]

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).