[TowerTalk] A few notes about the BN-86 topic - my experience

[Jim Brown]

On 4/18/2020 9:38 AM, Tim Duffy wrote:
> I have researched high performance feedline chokes for 35 years. Many years
> ago I ran across an article written by Bill, W0IYH now an SK. This is his
> feedline choke (also known as a very good 1:1 HF balun)

Tim,

I do NOT agree that this is a good choke below 20 MHz because it is 
predominantly inductive, not resistive, in that range, and because it's 
choking impedance falls with decreasing frequency. Even with 100 cores, 
it would be only about 1.1K + j 700 on 20M (depending on which core was 
used). When you and corresponded about this ten years or so ago, I 
advised you that performance could be acceptable on 10M, where it wasn't 
practical to lengthen a feedline to wind a choke. This addressed in 
http://k9yc.com/CoaxChokesPPT.pdf  The discussion of "string of beads" 
chokes begins with slide #55.

Yes, it's important to hold any choke away from a boom or other 
conductive element, because the capacitive coupling to the boom adds 
capacitance to the choke, moving its resonance down in frequency.

I recently developed a method for doing that for a 6M choke. It's shown 
in a new app note about chokes for VHF and UHF. 
http://k9yc.com/ChokesVHF.pdf  I sliced 1.5-in and 2" PVC conduit in 
half lengthwise; when cut like this, they nest nicely together and to 
the boom of the 6M antenna on which it would be used. I then cut pieces 
into lengths to avoid mounting screws for the elements, and taped the 
chokes to the boom, with the nested PVC as a spacer from my long boom M2 
6M yagi.

XE2K recently did some tower work for me, replacing old style chokes 
with newer, and bonding around rotators. He came up with another rigging 
method for my SteppIR, which let the choke hang down below the boom.

73, Jim K9YC