> The whole problem here is the comparison between high
> impedance twin lead and low impedance coax. Is the
> difference of impedance really coax attenuation?
Maybe I'm not understanding the question. In the most general terms, loss
has no direct correlation to impedance because there are other parameters
that affect loss aside from Z, including conductor sizes and dielectric
materials.
> If you kept
> I equal between twin lead and coax who would win?
If you kept I equal, then the Z would be equal too (assuming we're not
changing power).
Which cable (balanced vs unbalanced) would have less attenution depends
primarily on the effective resistance of the conductors as a function of
skin depth, at least at VHF/UHF with the kinds of dielectric we typically
use in the field. Like I said before, you can't compare two cables based
only on their characteristic Z; you have to consider the effective
resistances at the frequency of interest as a function of conductor size and
skin depth, and then added to that, the dielectric losses which contribute
much less to the total attenuation.
> Skin effect has attenuation which increases with the square
> root of the frequency, so at higher frequencies it becomes
> less.
No, the attenution becomes MORE at higher frequencies, not less.
> Dielectric loss is directly proportional to frequency
> as you go up in frequency at some point it becomes the major
> factor of coax loss.
At VHF/UHF, dielectric loss is almost always less of an issue than ohmic
losses except in the case of extremely poor dielectrics, and I don't know of
any dielectric used today that would qualify as being "extremely poor".
Maybe if the dielectric were made out of wood, play-doh, or Cherry Garcia
ice cream the dielectric losses would dominate... :-)
> Dielectric loss is because of the
> capacitor in the cable, not the capacitance.
There's a capacitor hidden in in my coax? Where? I can't find it, and now
I've ruined my cable looking for it :-)
> All capacitors
> have loss, this is the dielectric loss. It has to do with
> the dielectric material in the cable, air being one of the best.
Capacitors also have ohmic (resistive) losses too; it's not all dielectric
loss. Only "ideal" capacitors have neither dielectric nor resistive losses.
All of the capacitors we deal with in the real world have both.
--- Jeff
