On 2 May, 15:16, Chris Jewell wrote:
Someone asked in a Technician license class *why* 450 ohm window line
has much lower loss than an equal length of 50 ohm coax does at the
same frequency. The instructor knew that it is true, but could not
say why. I have an idea about the answer, and would like to know how
I'm doing. I'm one of those hams who is NOT a EE, so I'm trying to
work this out based on high school physics, which at least back in
1964, covered AC but not RF.

I observe that at higher Z, the voltage is higher while the current is
lower for a given power level. Ohmic losses are proportional to the
square of the current. This is the same reason that long distance
power transmission is done at high voltage.

For example, 100 W through 50 ohms is 1.4A @ 70V, while 100W through
450 ohms is 0.47A @ 212V. That is, 9 times the impedance results in
1/3 the current, which results in 1/9 the ohmic loss through the
resistance of the transmission line.

I expect that we also need to account for the difference in R
resulting from different conductor diameter and skin effect, and
probably difference in the dialectrics, neither of which I yet know
how to calculate. Apart from those factors, is my explanation based
on the current vs. impendance:

1. Basically correct?

2. On the right track, but oversimplified, and thus not much use?

3. Completely out in left field?

Thank you.

--
73 DE KW6H Chris Jewell Gualala CA USA

I think you did pretty good Chris, Basically it is correct, it is of
use and not out in left field.Tecnician level did you say?
Problem is that you asked what the time is whereas they wanted
you to pose the question how to make a watch.
Art