Date: Fri, 31 Dec 2010 20:05:57 +0000
From: Steve Hunt <firstname.lastname@example.org>
Subject: Re: [TowerTalk] 1/4w matching, Common mode chokes and BAL-UN
I have some modeled examples on my web site which illustrate the effect
Jim [K9YC] is talking about. Take a look at the section "Why reactive chokes
are undesirable" on this page:
The section finishes: "We conclude that a high value Resistive choke is
the safe option for all scenarios. We also conclude that Rules-of-Thumb
which equate the required choke impedance to some multiple of the
differential-mode load impedance are unsound."
## excellent info. My conclusion is... for a monoband yagi set up, use a 1/4
3/4 wave of coax from DE to tower... IF possible.. or close to it. Grnd braid
at top of tower or on mast. Or grnd braid via the remote switch box, if
box located at top of tower, or on mast. OK, then use a balun at the
DE....that is mostly resistive.
IF the balun is mostly resistive, the combo of a phys 1/4 wave coax [x .98] +
= excellent combo. I can pull that off on 20-17-15m. Can't be done on 80-40m.
remote switch box is 1/2 way up the mast, a line isolator will be located at
the Input of the remote
switch box. The line isolator is just 393 wrapped around some type 31
on both ends. It's job is to ...'clean up' any residual CM currents, b4 the
single coax heads down the
tower. Braid of single coax is bonded to base of tower /gnd sytem via
## Coax then routed 75' along the grnd to just inside of basement..and
terminates on a 2nd
lightning arrestor..which is bolted to the SPG [large sheet of 1/4" thick AL
plate]. More coax to LP
filter [also bolted to SPG plate], then more coax to a 2nd line isolator....
then more coax to
wattmeter coupler/amp etc. Both line isolators are inside plastic nema box's,
with type 31 cores
suspended in mid air inside the nema box. [to minimize stray C] . If all
that doesn't do it... I give up.
later... Jim VE7RF
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