On 9/15/2011 6:58 AM, ZR wrote:
> Some loops have such poor Q that they are wide open to interference
> problems. The hardline I chose has the lowest C per foot and therefore the
> highest Q with the resultant narrow bandwidth and need to retune for even a
> small QSY. The preamp is very helpful and is not run at high gain, just
> enough to give a comfortable signal level.
> Carl
> KM1H
C per foot is a function of characteristic impedance and velocity
factor. 75 ohm foam RG59, as recommended in the article,
has capacitance as low as any hardline or heliax.
In any event, C per foot has nothing to do with Q. That is determined
by the outer diameter of the coax, and its loss properties. Large
diameter aluminum hardline will clearly have a Q potentially several
times higher than the braided shield of RG59. The transformer would
need to be well optimized to actually realize this higher Q. And
as you say, the bandwidth would be quite narrow, at least if the loop
were critically coupled. Even then, it wouldn't provide enough
selectivity for really strong BCB stations, at least at the upper
end of the dial. We have a strong one at 1700 kHz :-(
C per foot is only relevant because it limits the upper tuning range
of the loop.
Rick N6RK
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UR RST IS ... ... ..9 QSB QSB - hw? BK
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