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
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|