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K0CQ > COAX 29.12.05 19:02l 59 Lines 2427 Bytes #999 (0) @ WW
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Read: GUEST DL1LCA OE7FMI
Subj: Coax dielectric constant measurement
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From: K0CQ@W0AK.#CIA.IA.USA.NOAM
To : COAX@WW
Being a collector of data books, I look there first for
dielectric type or the velocity of propagation, then to refine my
quarter wave transformer lines, I measure a quarter wave
resonance. I cheat by owning a grid dip oscillator. I take a line
long about a quarter wave long with an assume velocity of
propagation (solid dielectric close to 66% and foamed around 82%
but more variable). I leave one end open and connect a single
sire to short the other end. I search with grid dipper coupled to
that wire which acts as a small loop for a resonant dip. Then I
know the actual RF dielectric constant and can adjust the length
if need be.
A similar measurement could be made with a coax connector on the
end of the coax and an antenna impedance bridge looking for the
frequency where the input impedance is lowest, but that may be
out of the range of the bridge and complicates the results by
including the connector as part of the line.
Back to the reference books. Sometimes one can measure center
conductor diameter and dielectric outside diameter and compare
those to standard coaxes to get an idea of the DESIGN dielectric
constant. The trouble with that is that makers willing to cut
corners will cut the center conductor size and let the impedance
wander high and won't control the density of a foam well so while
the conductors may match those in a reputable catalog, the actual
impedance and velocity factor (and loss) may be far from the
standard coax.
Its not beyond reason at UHF or VHF to find there is a
periodicity in the foam leading to different velocities (and
impedances) in different sections only a foot or two apart. That
shouldn't happen in quality coax, but consumer grade coax for TV
may not be quality coax.
73, Jerry, K0CQ @ W0AK.#CIA.IA.USA
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