In article EgEui.4923$MT3.3995@trnddc05, "Jerry Martes"
wrote:


I consider "return loss" to be a ratio related to the mismatch of the load
to the line. A short on the end of a low loss line will have high Return
Loss. You probably did some math that isnt apparent in the statement "I am
assuming that 1/2 (of 6.75 dB) is the actual loss". .


Hello, and you don't have to "consider" what return loss is. At an
interface/boundary it is the ratio of incident power to reflected power.
Mismatch loss is the the ratio of incident power to that dissipated in the
load at the interface/boundary. These losses in terms of VSWR are given
by

RL (dB) = 20*log(S + 1)/(S-1)

ML (dB) = 10*log(S + 1)^2/(4*S)

where S is the VSWR and logarithms are to base 10.

A lossless transmission line fed at one end and ideally short-circuited on
the other end would display a feedpoint impedance that is totally reactive
(no resistive component). If a resistive component is present it must be
due to dissipative loss in the line and since power has to travel to the
load (short) and return to the feedpoint this resistance must be twice the
dissipative loss in the line.

The challenge here is, given a transmission line of certain physical
length, to find a measurable value at the operating frequency(s). An RF
signal source with a surplus (but in proper operating order) General Radio
(Genrad) impedance bridge is good for this type of measurement. Keep in
mind that any coupling from the line to nearby structures will affect the
measurement. Sincerely, and 73s from N4GGO,

John Wood (Code 5550) e-mail:
Naval Research Laboratory
4555 Overlook Avenue, SW
Washington, DC 20375-5337