At 06:49 AM 7/14/2005, W0UN -- John Brosnahan wrote:
>At 11:36 PM 7/13/2005, Al Toothaker wrote:
> >Wouldn't the different electrical lengths indicate that the cable
> >manufacturing process is not consistent? Any spec on a cable has a
> >tolerance and though they may wander in performance, it could still meet
> >their specs.
>
>
>The velocity factor in foam coax is the most difficult thing to control, since
>it is a function of the percentage of air bubbles in the dielectric. Some
>manufacturers specify it for a range of values -- such as 0.78 -- 0.82 VF,
>others just give a nominal value.
Note that the same change in bubble percentage that changes the velocity
factor also changes the characteristic impedance, because they're both
related to the effective permittivity (dielectric constant).
That means that any impedance transforming effects will also
change. Whether it's important is another story entirely.
There's also temperature effects to consider. At HF, you're probably not
going to have enough wavelengths of cable to worry about, but get up into
the microwave area, and it's a real problem.
>Solid dielectric is much more controllable, but I even measure the actual
>phase shift for each delay line of solid dielectric coax, if the
>application is critical.
>
>And with foam coax it is MANDATORY to TUNE (trim) each delay line to
>the correct electrical length.
>
>--John W0UN
There ARE foamed dielectric coaxial cables that are extremely consistent
and stable. For instance, Gore makes coax with foamed PTFE dielectric, and
it's used for network analyzer test port cables.
Most PTFE cables also have a discontinuity in their phase shift/dielectric
constant vs temperature curve, unfortunately, right around 15-20 C (room
temperature).
However, the stable foamed, low loss stuff is wretchedly expensive, and I
can't imagine a ham using it unless they happened on a spool of the stuff
surplus.
Jim, W6RMK
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