I've now run this experiment a further 3 times with pretty consistent
results: on average, adding the water dropped the velocity factor by
1.8% and increased the matched loss by 0.25db/100ft at 26MHz.
That change in loss is way short of the nearly 2dB/100ft figure reported
by one of the on-line calculators.
Interestingly, the best match for the loss vs frequency characteristics
between dry and wet is achieved simply by changing the dielectric loss
coefficient and leaving the copper loss coefficients the same; that's
consistent with the change in Vf. It also seems logical - it's difficult
to see how the water could change the copper loss, but you can see how
it might affect the dielectric properties.
I'd intended doing this experiment with 450 Ohm (nominal) line, but I
only had short lengths of it.
So I'm left pondering why these results are so different from Wes
Stewart's and the on-line calculators. Possibilities are:
* The wetting agent used by Wes introduced some gross loss effect
* 450 Ohm window behaves very differently from 300 Ohm window (not very
likely I would have thought)
* There was a flaw in Wes' experimental method
* There is a flaw in my experimental method
I'll repeat the experiment with 450 Ohm line when I can get some, and
when my wrist recovers from all the spraying :)
Steve G3TXQ
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