On Mon, 04 Apr 2005 17:35:37 -0700, Roy Lewallen took
the words right out of my mouth:

The loss has nothing to do with the speed of travel, except that the
effective dielectric constant has a direct effect on speed and an
indirect effect on loss.

At frequencies from at least HF well into the UHF range or higher, the
loss in transmission lines having decent insulation (e.g., PE or PTFE)
is almost all due to conductor loss rather than dielectric loss. Higher
impedance line has lower loss simply because for a given amount of power
being conveyed, the current is lower. Therefore, the conductor I^2 * R
loss (which is nearly the total loss) is lower.

If you introduce a dielectric material (other than air) between
conductors, the characteristic impedance drops and the velocity factor
increases, due to the same effect. Only in that way are they related in
a ladder line.

In a coax cable, some of the plastic insulation is sometimes replaced by
gas or air to make "foamed" dielectric cable, or by other devices such
as plastic disks or a helically wound plastic string. This reduces the
effective dielectric constant of the cable, which if the dimensions
remained the same, would raise the characteristic impedance. It also
increases the velocity factor. In those cables, the characteristic
impedance is lowered to its nominal value by increasing the diameter of
the center conductor. That is, for a given cable outside diameter and
Z0, a cable with more air and less plastic will have a larger center
conductor. The larger conductor reduces the I^2 * R loss by decreasing
the R. So foam dielectric cable and others having a high velocity factor
have lower loss than solid dielectric cables with the same OD because
the center conductor is larger.

At a frequency of about 1 - 10 GHz or so, dielectric loss begins to
dominate, and different relationships exist.

The equations describing the relationships among dielectric constant,
velocity, impedance, and loss are simple and can be found in a great
number of texts. I'm sure they can also be easily found on the web.

One old wives' tale (*not* attributed to Roy) is that ladderline has
lower loss than coax (given as a blanket statement). Therefore,
laderline is "good" and coax is "bad."

However, compare something like Andrew LDF4-50 to Wireman 554 and you
find that the "lossy" coax has a loss of 0.48 dB/100' @ 50 MHz and the
"low-loss" ladderline has a loss of 0.41 dB under the same conditions.