How high Q can we expect from a coax?
 

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?

How high Q can we expect from a coax?
 

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?

============================================

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.
----
.................................................. ..........
Regards from Reg, G4FGQ
For Free Radio Design Software go to
http://www.btinternet.com/~g4fgq.regp
.................................................. ..........

How high Q can we expect from a coax?
 

On 13 Dec 2005 02:58:04 -0800, "
wrote:

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?

the uncomplicate answer is "depends on the COAX losses which is
related to inner conductor diameter and insulator used".

Example RG58 is a poorer than UT141 but 1" Hardline will be better.

Tools like coaxtune and others will get you a better ide of what will
achieve your specific goals.

Allison

How high Q can we expect from a coax?
 

I'm sure Reg's programs will give you a very good estimate. For a
quick "rule of thumb" estimate, assuming ~77ohm air-insulated coax with
an inside diameter of the outer conductor of D inches, at frequency f
MHz, Qu will be about 100*D*sqrt(f). So a 1" line at 100MHz will give
you Qu about 1000. This quick estimate may be all you need, and even
quicker than loading a program.

I trust you are aware that the Qu won't tell you how sharp a filter
will result; for that, you need to know the configuration and the
loaded Q--or better, where the poles and zeros are. There are lots of
ways to do that; RFSim99 might be a useful (and free) tool for you. By
appropriately tapping the thru-line down on an appropriate stub, you
can arrange to pass one specific frequency with very little disturbance
while blocking one other specific frequency of your choice (and another
you don't have so much control over, and of course others at multiples
of the lowest freqs). Or you can combine a stub that blocks a
particular frequency with a reactance (capacitor or inductor) that
tunes out the reactance of the stub at the frequency you specifically
want to pass.

For example, put a 77-ohm shorted stub that's a half-wave long at
125MHz across your line, and you notch 125MHz. Put an 18.8pF
capacitance across that, and you will pass 150MHz with little effect,
because the 18.8pF resonates the inductive reactance that the stub
represents at 150MHz. But it will also pass 46.5MHz with little
effect. Put about 27nH across the same stub and you'll notch 125 while
nicely passing 115MHz. Include line and inductor loss in the model to
see the depth of the notch and the attenuation at the "pass" frequency.

Cheers,
Tom

How high Q can we expect from a coax?
 

On 13 Dec 2005 02:58:04 -0800, wrote:
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?

Something like a 2" copper pipe centered in a metal garbage can?

Or, 19" of RG-174?

Jonesy