On Aug 9, 5:13 am, "Jimmie D" wrote:
I need to measure the loss of aproximately 200ft of coax @ a freq of 1Ghz.
The normal procedure for doing this is to inject a signal at one end and
measure the power out at the other. Using available test eqipment this is a
real pain to do. I propose to disconnect the cable at the top of the tower
terminating it in either a short or open and measure the return loss at the
source end. I have done this and measured 6.75 db and I am assuming that 1/2
of this would be the actual loss of the cable. These numbers do fall within
the established norms for this cable. Can you think of a reason thiis method
would not be valid?

Jimmie

It will be valid if the Z0 of the line is uniform, and matches the
calibration of the instrument you use to measure it. If the Z0 is
uniform but different than the impedance to which the instrument is
calibrated, you can easily see that effect by measuring the return
loss with the far end open and with it shorted. You can get the same
info, again assuming a uniform line, and assuming essentially
unchanged attenuation over a 2.5MHz span around your measurement
frequency, by measuring at multiple frequencies (doing a sweep). If
the line is the same impedance the instrument is calibrated to, the
return loss will trace out a circle centered on the middle of a Smith
display (assuming that display is referenced to the instrument's
impedance); in any event, the circle will be centered on the line's
Z0. If the line Z0 is non-uniform, expect the attenuation to vary
with frequency; the Smith display of a sweep likely will be quite non-
circular.

Cheers,
Tom