[TowerTalk] Open wire

[Richard Hill]

http://fermi.la.asu.edu/w9cf/
http://www.w5dxp.com/

Rich
NU6T

Martin Ewing wrote:
> References to W9CF and W5DXP info would be helpful.  (Googling fails this
> morning.)
>
> 73 Martin AA6E
>
> On Tue, Sep 29, 2009 at 9:02 AM, Paul Christensen <w9ac at arrl.net> wrote:
>
>   
>>> 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.
>>
>> Paul, W9AC
>>
>>
>>
>>
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>>     
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