My apologies for confusing two different posters to this thread. Partly a
consequence of threading and me using two different computers and two different
email clients.
I want to specifically address this:
Rick claims that when using a series connection of two different transmission
lines, one coaxial, one window line, changing the length of one (coax) affects
something or the other, while changing the length of the other portion (window
line) has no effect whatsoever.
This is clearly impossible. I suggested earlier that one possibility is
common-mode current on the coax that is confusing the instrumentation. Just
because a choke is applied doesn't mean that it is actually effective.
Rick should not be faulted for assuming that the window line is "low loss" while
the coax is not. I have been trying to debunk this myth since 1993 when, "The
Lure of the Ladder Line" was published in QST. At that time, I had lots of
(snail mail!) correspondence with then Antenna Book editor, Dean Straw, about
this. I pointed out that the chart of line loss vs. frequency that had appeared
in every ARRL Handbook and Antenna Book since antiquity was wrong (easily noted
by inspection). We collaborated on a revision of this chart and I was invited
to write something about balanced line use. See:
http://k6mhe.com/n7ws/Ladder_Line.pdf published in the ARRL Antenna Compendium
Vol 6.
I should mention that during this correspondence I pointed out that tuner and
balun losses should not be ignored because they could be as detrimental as line
loss. I had access to Touchstone, an early professional circuit analysis
program, and gave examples of (IMHO) excessively high tuner losses that resulted
from low Q components and the misadjustment of Tee type tuners. (Regrettably, I
don't have copies of these letters any more.) After doing so, a QST favorite
author, Frank Witt had a two-part article about tuner losses published in the
April and May 1995, QST. (I wasn't invited to write this one) This month's
"Hands-On Radio" column in QST brings up these losses again.
Straw wrote the program TLW, bundled with the ARRL Antenna Book, which (again)
underestimated window line loss. ARRL has just admitted this and offered a new
version that allegedly fixes the problem. (I don't know, I don't have a new
enough version to qualify for the update, and I wouldn't use it anyway. AC6LA's
programs (ac6la.com) are superior and highly recommended.)
Sorry to ramble on this somewhat off-topic thread but I think this is worth
mentioning.
Wes N7WS
On 5/15/2014 12:54 PM, Rick Bates, WA6NHC wrote:
[snip]
The coax portion of the feed should be as SHORT as possible, in my case it is
currently about 10' (2.8 meters). The losses are highest there and the extreme
SWR makes it MUCH worse, keep it short; use the best stuff, not the cheap
stuff. Changing the length of the coax portion has a HUGE impact on where (or
if) the system tunes; the window line, none.
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