Dick Rollema wrote:
To All from PA0SE
Andy Talbot wrote:
> I find that rather impossible to believe - 300m of thick cable being a
dummy
> load at 137kHz !
>
> Go back to the fundamental equations and calculate properly rather than
rely
> on tables and software used for the wrong purpose .
snip snip
Andy says "Go back to the fundamental equations and calculate properly". A
good advice but Andy did not follow it himself.
snip snip
I could not spot a computational error is in either calculations, but
with a low loss situation the formula could be very sensitive to being
close to "divide by zero" in parts of a fomula. I am strongly of the
view that coaxial cable with the far end either short or open
termination is mostly reactive at LF when "looking" in to the near end
of the cable. The main issue is the loss factor (or Q). Which has the
higher Q, a coil or a shorted end coaxial cable?
From an intuitive point of view, I would think a purpose built coil
would give lowest losses for a given value of inductance needed. The
designer is in full control of countermeasures for skin effect,
proximity effect, voltage gradient and can select the best length to
diameter ratio for a given frequency band. On the other hand, coaxial
cable is widely used at VHF/UHF for "stub matching", however it does not
appear to be technique that has been used on lower bands, where lumped
components are generally preferred, and can be variable (variable
capacitors and variometers) and so are more useful for "tuning up" or
adjusting for a good match to 50 ohms.
Does any reader have a drum of coaxial cable and a low frequency bridge
test set? A practical measurement should sort out a quantitative
result. In the meantime my guesstimate is that a long length of shorted
coaxial cable (up to a quarter wave electrical) would be quite a good
inductor at LF, but the R part of the R+jX value would be higher than
that of a "well made coil" with similar X value.
73, Bob ZL2CA
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