That makes perfect sense. Just like for power lines - higher voltage means
less loss over the same line. I need to try to use ohms law a little more
often. I've wound impedence matching transformers myself - without even
thinking about the fact that I was also increasing (or decreasing -
depending on the flow) voltage.

thanks

"Roy Lewallen" wrote in message
...
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.

Roy Lewallen, W7EL

Hal Rosser wrote:
I've noticed, (but have not studied), some loose relationships in
transmission line characteristics (and I guess waveguides fit in here).
From an observer's point of view, it seems that a high characteristic
impedence line (like 400-ohm or 600-ohm ladder line) also is usually a
lower-loss line, and has a higher velocity factor.
It also seems that some coax may have a low VF and high loss.

Is there a real cause for the relationship of these 3 characteristics of
transmission lines ? Is it something we can generalize ?
It makes some sense to say that the faster a signal gets through the
line,
the less loss it will have - and that gives some credence to the
relationship in VF and loss being inversely associated.