Anyone has any information on how high Q can we expect from an open
or
a short quater wavelength coaxial stub? How can we achieve
moderately
high (200-300) Q by using a coax stub?

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Program COAXTUNE calculates Q when the line is tuned to resonance by a
capacitor across the open end. For practical purposes this is the same
as Q when off-resonance at the same frequency.

Open-circuit and-short circuit stub lines have the same Q.

As a check on dimensions of the inner and outer conductors the program
calculates line impedance Zo. If you are uncertain of dimensions then
vary them to make Zo equal to what particular type of line you have in
mind.

The velocity factor of solid polyethylene coax is 0.665

The program allows physical line length to be adjusted for resonance
at any frequency, and for any length in wavelengths.

At HF, for typical types and sizes of coax, you will find Q to be very
much lower than 200 or 300. High values of Q are obtainable only at
UHF and above on large diameter lines.

Download program COAXTUNE from website below. It is very simple to
use.

Program TWINTUNE does similar things with balanced pair lines.

For design of a complete impedance matching system using a stub-line,
coax or balanced-twin, between main line and antenna, program STUMATCH
may be of interest.
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Regards from Reg, G4FGQ
For Free Radio Design Software go to
http://www.btinternet.com/~g4fgq.regp
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