[TowerTalk] antenna choices for K4XS
jim at audiosystemsgroup.com
Mon Apr 13 12:58:49 EDT 2015
On Mon,4/13/2015 5:10 AM, Stan Stockton wrote:
> For K9YC.... If, like the C31XR, SkyHawk, etc, the Mosley antennas performed as expected for boom length on 20 and 15, I wonder how they might have performed if they had one of your chokes instead of having the coax directly connected to the driven element?
I don't know enough about the antenna to speculate. Steve, who's
probably installed hundreds of HF antennas, says that Moseley antennas
don't match well unless you do exactly what they say.
The best chokes are resistive in their operating range -- an inductive
choke can resonate with a length of line that is capacitive by virtue of
its length, which increases the common mode current rather than
suppressing it. The "string of beads" chokes so often used as "baluns"
are only resistive if constructed with the Fair-Rite #73 material that
W2DU used. Those beads are small, so they only fit small diameter coax
(RG58 or smaller), and hundreds of them are required to get high choking Z.
I've heard that, based on my recommendation for #31 material for
multi-turn chokes, many are using them for string of beads chokes.
That's bad, because this material is inductive below about 75 MHz unless
many turns are wound through it.
The point of this long-winded reply is that it's hard to speculate, even
with modeling, how that common mode Z would interact with the antenna
when feedline length is a variable, and even more difficult with a
trapped antenna where parts of the antenna (the traps) are unknowns. :)
My general advice for choking any manufacturer's antenna is to first use
the matching method and "balun" they recommend (or provide) and ADD an
effective common mode choke as close as practical on the feedline.
73, Jim K9YC
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