[TowerTalk] Open wire
w9ac at arrl.net
Tue Sep 29 06:02:14 PDT 2009
> I'm not sure I would use the same language as Jim, but I *do* think
> there is widespread misunderstanding amongst Hams about the performance
> of "ladderline".
Open wire transmission line can be the "near miracle" form of line if
several electrical parameters are known. All too often, open or ladder line
is deployed without regard to line length from inefficient tuners.
To optimize open wire lines, one should know: (1) the line to load mismatch;
(2) length of transmission line and its velocity factor; and (3) the
impedance at the shack end of the line for any given frequency in order to
minimize coupling losses. For example, it's entirely possible to connect a
random length of open line to a loop antenna, and incur substantial coupling
losses as the tuner tries to efficiently couple power from the transmitter
to a line with a very low Z at the shack end of its terminals.
Experimenting with the W9CF "T-Network Tuner Simulator" provides some
valuable insight regarding tuner design. While it's possible to achieve a
50-ohm VSWR with several iterations of the tuner, many combinations of
adjustments -- and choice in component values result in very high coupling
loss. And, that's not taking into consideration an input or output balun.
For a T network to couple efficiently into low impedance loads down to 160m
without optimizing line length, one needs input and output capacitors *at
least* 3,000 pF in value. That's a factor of about 6X what most commercial
tuners use in their designs. Reasonably good Q of components is also
important. Don't believe it? Try it yourself with the on-line W9CF
simulator and pick a low Z value of say 10 ohms resistive at 1.8 MHz and run
the calculation. With 250pF In/Out caps, that's 2 dB of loss just in the
tuner at the *best* possible matching combination of controls. Now add in a
balun, as well as the additional loss resulting from the antenna line to
load mis-match and you've easily lost half your power in the system-- using
open transmission line. And yet, the rig's SWR meter shows 1:1 and all
along you've been fooled into thinking that just because you're using open
line, it somehow *must* be efficient.
To make best use of open line: (1) the line-to-load mis-match should be
reasonable; (2) The line length optimized for a reasonable Z seen by the
tuner; and (3) good selection of tuner component values and Q. These
factors play an important role in tuner coupling losses, particularly a low
Z seen by the tuner. 1 and 2 above are probably the easiest to manage by
calculation. Item 3 oftentimes requires extensive modification to an
existing tuner, or use of a link-coupled tuner. Link tuners without a
capacitive voltage divider do best when the operator can select parallel or
series feeds, the latter of which do best for low-Z loads (ref. Cebik).
One of the most efficient systems I've seen to manage all of the above is
from W5DXP. He uses switched sections of open line, fed at current maxima
point at the coaxial transition. The variable length of line through the
switch matrix performs the antenna tuning and matching function and results
in a 50-Ohm VSWR of less than 1.5:1 even when the 450-ohm VSWR is 10:1. If
his matching system cannot attain reasonable results, it's an indication to
the operator that something is unreasonable in the design, including antenna
and/or transmission line length. Unlike a T tuner, it does not give the
operator a false sense of security when the SWR bridge reads 1:1.
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