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

[Brian Rehmer]

I think I have messed up the links to my original post (sorry) and thanks to all the responses.  K9YC provided the following response and I would like to continue on this line of thought.  

Thanks,  Brian
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

>>bjr>> I have read the doc and thank you.  I have chased tons down and many more from neighbors that I will not have access to.  My thought is that they are problems to me now because my feeds are just big receive antennas and common mode isolation would reduce how well they receive these fields and conduction routes into the feeds.  Do this first then start hunt and eliminate second.  

>>bjr>> A perfect example of this is now that I am working at home I have my dual monitor setup and I can see there signals well in lab/shack and (this is key) on the feeds at the base of my tower (loop coupled to the feed) which is on the other side of the house.  The feed length from office to tower base is 50-75 feet. The sigs are seen in my receiver in the shack as well.  So, looks like the signals are coupling into the feeds up the tower to the antenna feedpoint on the tribander which has the BN-86 balun.  I think you identify same issue in your nice papers.

>   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.

>> bjr.  I agree, but is the BN-86 redundant, provides poor common mode isolation, and should be replaced.

 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

>>bjr>> Suspicion for putting one at exit of house is to choke off possible conducted noise pickups in the house between shack and exit from the feed going up the tower. But, maybe your point is the one at the feed will do the job.  There are so many cross coupling paths in the bundle of feeds and control cables that getting all of them isolated will be a feat I expect too (ie 2 m feed without an isolator would just couple around the other isolators, ugg.   


 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.

>>bjr>>  I agree on the propagation hit on 15M.  What I am experiencing though is no change in background noise all day and over multiple days.  I would expect some variation over the day. 

 73, Jim K9YC