Good morning - I am on the digest version of the list and it took a while to
sort all the emails regarding my question out.
Background: I chose the multiband with a 450 ohm feedline and a Johnson
matchbox as the best way to address my particular circumstances (multiband
operation with a single horizontal wire, available antenna attachment points,
location of shack, neighborhood powerlines, spousal boundaries, etc) while
meeting my operating needs (regional nets, casual DX, mostly CW and digital
modes, using less than 100W). I understood the various compromises built into
the design (which I won't belabor) and decided that I could live with them when
operating as I operate. I have been generally right about that.
After a year of using this antenna system, though, it is clear that I have
serious receive noise problems. I addressed a lot of them by removing RF
sources from my operating environment and incorporating the sorts of fixes that
K9YC recommends. But I haven't got them all and I am guessing that my
unbalanced "balanced system" creates a common mode path for noise ingress.
That's why I asked about common mode choking. If my guess is wrong, someone
just tell me that and I will go away quietly. ;)
Here's what I think I've heard or confirmed:
Designing a common mode choke that can withstand the mismatch caused by the
wide variation of antenna impedances at the feedpoint of an multiband antenna
*and* effectively choke RF noise is not impossible but the weight/cost to power
ratio of such a choke is going to be very large.
Designing a common mode choke that works across 3 to 30 MHz requires multiple
chokes in series, with different core materials in each to insure adequate
choke impedance across the frequency range with the consequent high weight/cost
to effectiveness ratio.
So... even if my guess is right (about imbalance creating a common mode path
for noise), designing a choke to solve the problem is very difficult.
Does that sum it up?
BTW, as my wife will tell you, I have spent a lot of the last year with my
noise buried in a stack of books, papers, and website screendumps on antenna
design, impedance transformation and chokes. I am really grateful for all the
work that you all have shared. I won't mention all the call signs because I
would accidently leave someone out and piss them off but trust me, there's a
highlighted and penciled copy of your stuff around here somewhere.
73 de bill K7WXW
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