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

J. B. Wood wrote:

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,

Even more of a challenge might be getting that impedance bridge to work
at 1 GHz...grin

In article , wrote:

J. B. Wood wrote:

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,

Even more of a challenge might be getting that impedance bridge to work
at 1 GHz...grin

Hello, and I must have had a senior moment. Forgot what freq the OP was
interested in. Too many years spent making measurements in the 2-30 MHz
band I guess ;-) Of course now we're looking at a vector network analyzer
to make the measurement (not something most Hams have in the shack). I
wonder if MFJ has anything? Sincerely,

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

Hello, and I must have had a senior moment. Forgot what freq the OP was
interested in. Too many years spent making measurements in the 2-30 MHz
band I guess ;-) Of course now we're looking at a vector network analyzer
to make the measurement (not something most Hams have in the shack). I
wonder if MFJ has anything?
====================================
The MFJ259 antenna analyser can measure coax loss at any frequency
between 1.8 and 170 MHz.


Frank GM0CSZ / KN6WH

On Aug 14, 2:22 pm, (J. B. Wood) wrote:
In article , wrote:
J. B. Wood wrote:

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,

Even more of a challenge might be getting that impedance bridge to work
at 1 GHz...grin

Hello, and I must have had a senior moment. Forgot what freq the OP was
interested in. Too many years spent making measurements in the 2-30 MHz
band I guess ;-) Of course now we're looking at a vector network analyzer
to make the measurement (not something most Hams have in the shack). I
wonder if MFJ has anything? Sincerely,

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

Later on in the thread, the OP said he has an Agilent network
analyzer, presumably a VNA, and an HP power meter. Certainly the VNA,
connected to the feed end with the tower end open or shorted, swept
over a fairly narrow range (since he has 200 feet of line) around
1GHz, should tell him enough to characterize the impedance and the
loss. He indicated that he's happy with that solution, some time back
in the thread.

Cheers,
Tom