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

"Jimmie D" wrote in message
...
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

Hi Jimmie

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". .

How difficult would it be to take a length of some decent RG-6 up the
tower to send the signal down to the *lower end*?

Jerry

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

"Jimmie D" wrote in message
...
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

Half the return loss is a valid method of determining the transmission line
loss.

Frank

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

K7ITM wrote:


It will be valid if the Z0 of the line is uniform, and matches the
calibration of the instrument you use to measure it.

SNIP

It may be worth adding that even when
the line is neither uniform nor matched
to the impedance of the RLB, the
measured return loss will correctly
indicate the sum of losses due to the
mismatch and to the line losses.

When the line impedance is uniform, the
mismatch loss can be simply calculated
and the cable loss can then be found.

73,

Chuck

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