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

[Jim Brown]

On 4/16/2020 5:20 AM, Brian Rehmer via TowerTalk wrote:
> For sometime I have been struggling with noise (S4-7) on HF bands with my TH-5DX tribander.  I have ~125 feet of RG-8 routed to a BN-86 at the antenna feed.  There are several other feeds that are bundled together for 40, 6, and 2 meters in the same run up the tower.  The cables route to the tower from my second floor radio room inside my house.  So, there are lots of things that can couple to the outside of the feeds.  I am considering adding common mode chokes to reduce/eliminate the noise.  A lot of the time the noise covers the entire 15 M band. I can take a small loop and couple to the feeds at base of tower and other spots along the feeds and see the noise and it varies with position.  It seem this points to common mode as culprit.  So, there are lots of things that can couple to the outside of the feeds.  I am considering adding common mode chokes to reduce/eliminate the noise.

That could be a good thing, but you may also need to chase down and kill 
the source(s) of the noise. http://k9yc.com/KillingReceiveNoise.pdf
> 
>   Question is should I add the choke prior to the BN-86 or replace the BN 86. 

The choke should ALWAYS be placed between the antenna designer's 
matching device and the coax, and it should always be right at where the 
coax connects to it, NOT further down the feedline.

  I expect I may need to add chokes to each of the other lines too, but 
where.

Where they connect to the antennas.

  Should I add in the shack too

No
or just exit from the house to the tower.

No

Study k9yc.com/RFI-Ham.pdf  and
http://k9yc.com/2018Cookbook.pdf

The other thing worth noting is that, at this low point in the sunspot 
cycle, propagation on 15M and above is pretty stinko, and is likely to 
remain so for a couple of years.

73, Jim K9YC