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?

Sounds right...

Send the signal up, have a loss of 3.375 dB, all of it reflects back
from either short or open, another 3.375 dB loss, so the reflected
signal is down 6.75dB.


If you wanted to get real fancy, you could terminate in a known mismatch
too..

But that's getting up towards doing port cals on a VNA.

I assume you're not looking for tenth of a dB precision?

Jim

I assume you're not looking for tenth of a dB precision?

Jim

Actually yes I am..Power must be maintained +- 1db at the antenna.

Jimmie

"Jimmie D" wrote in message
...


I assume you're not looking for tenth of a dB precision?

Jim

Actually yes I am..Power must be maintained +- 1db at the antenna.

Jimmie

Hi Jimmie

What test equipment are you using to record the 6.75 dB?

Jerry

Jimmie D wrote:
I assume you're not looking for tenth of a dB precision?

Jim


Actually yes I am..Power must be maintained +- 1db at the antenna.

You've got a bit of a challenge, then.. although +/- 1 dB (is that a 1
sigma or a 3 sigma or a absoulate max min spec?) might not require a
tenth of a dB precision.


1 dB is 25%
1% is 0.04dB

(measuring power at 1 GHz to 0.1dB absolute is moderately challenging,
especially outdoors) For reference, an Agilent E4418 is specified at
+/-0.6% (25C +/- 10 degrees).. plus you have a linearity spec which can
range from 1% to 4% depending on the relative levels of the reference
and unknown.

A good return loss measurement with a decent PNA (like an E8363) should
get you down in the sub 0.1dB transmission measurement with overall loss
in the 0 to 20dB range, so the measurement is clearly feasible at some
level.

The same piece of gear, measuring reflection coefficient (i.e. the put a
short or open at the other end, and measure mag(rho) and work back to
loss)... you said you have about 6dB return loss, so that's a reflection
coefficient (at the analyzer) of about 0.5, and for 2GHz, the
uncertainty would be about 0.01 (out of the 0.5), or, call it 2%...
again, about a 0.1 dB uncertainty.

OTOH, that's a $50K piece of test gear, sitting in a lab at 25C +/- 1C

There's also the temperature coefficient of the coax to worry about.
Copper has a temperature coefficient of 0.4%/degree C. A 10 degree
change in temperature is a 4% change in resistance (0.2dB), and the
resistance is a big part of the loss (dielectric loss changes
differently, and you'd have to worry about the dimensional changes too).

In any case, measuring the loss by terminating it in a reflection is
probably the easiest way, and potentially the most precise, because you
can have the source and the measurement at the same location. If you
tried to measure it by transmission loss (put the source at one end and
the detector at the other) you have the problem of the stability of the
source. In a bridge type scheme (which the reflection technique is) you
can essentially compare between the unknown (your cable) and a standard,
and adjust the standard until they match, so the variations in the power
level of the source cancel out (or use something that inherently
measures the ratio of the powers).

Something like the LP-100 wattmeter can probably make the measurement.
It's good to 5% typical, and can do ratioed/match measurements to much
better. I don't know if it can go to 1 GHz, though.


Something like the Anritsu SiteMaster (like the S311D) can do this for
sure(after all, it's what it was designed to do.. measure coax on towers)
http://www.us.anritsu.com/downloads/...1410-00419.pdf

If you need to measure loss on the fly, it's a bit trickier, but one way
is to put a deliberate small mismatch at the end (i.e. you put a 10 dB
directional coupler in the line at the antenna end, with the coupled
port terminated into a short). This reflects a known -20dB back down
the line. You look for changes in the amount of reflected power.
Obviously, if the antenna changes it's reflection, you have to separate
that out. There are clever techniques for this too (like having the
coupler terminate in a switch that is either a load or a short). This
kind of thing is pretty common on antenna measurement ranges, where you
need to remove the effects of the feed cable from the measurement.




Jimmie

Jim Lux wrote in news:f9gg4i$7q9$1
@nntp1.jpl.nasa.gov:

....

Jim good points and all noted.

Jimmie hasn't give a lot of detail about the specification he is
apparently trying to meet. Reading between the lines, it might be an
EIRP, and assuming a given antenna gain, he is trying to calculate the
permitted transmitter power output.

Not only is the uncertainty of practical service equipment an issue in
tenth dB accuracy, but no mention has been made of transmission line loss
under mismatch conditions, and mismatch loss.

Jimmie, if you have a plausible story to tell the regulator, then that
might suffice.

If you have assessed the Return Loss of a rho=1 termination, then you
could use that and the measured Forward and Reverse power using say a
Bird 43 at the transmitter end of that known line loss (being half the
return loss) to calculate the power absorbed by the load. The calculator
at http://www.vk1od.net/tl/vswrc.php does just that. The calculator at
http://www.vk1od.net/tl/tllc.php could be used to calculate the expected
RL of the o/c or s/c line section, just specify a load impedance of 1e6
or 1e-6 for each case. For example, at 1GHz, the RL of 200' LDF4-50A with
a 1e-6 load is 8.9dB, and if you got much higher than that, you might
suspect the cable to be faulty.

Tenths of a dB, remember that most service type power meters are probably
good for 6% to 10% of FSD, so I will go with Jim's 1dB accuracy.

BTW, directional wattmeters for the ham market are often not capable of
reasonable accuracy on loads other than the nominal 50 ohm load. There
are a range of tests that such an instrument should satisfy, but for
hams, it is usually considered sufficient if the "reflected" reading is
approximately zero on a 50 ohm load.

Owen

"Owen Duffy" wrote in message
...
Jim Lux wrote in news:f9gg4i$7q9$1
@nntp1.jpl.nasa.gov:

...

Jim good points and all noted.

Jimmie hasn't give a lot of detail about the specification he is
apparently trying to meet. Reading between the lines, it might be an
EIRP, and assuming a given antenna gain, he is trying to calculate the
permitted transmitter power output.

Not only is the uncertainty of practical service equipment an issue in
tenth dB accuracy, but no mention has been made of transmission line loss
under mismatch conditions, and mismatch loss.

Jimmie, if you have a plausible story to tell the regulator, then that
might suffice.

If you have assessed the Return Loss of a rho=1 termination, then you
could use that and the measured Forward and Reverse power using say a
Bird 43 at the transmitter end of that known line loss (being half the
return loss) to calculate the power absorbed by the load. The calculator
at http://www.vk1od.net/tl/vswrc.php does just that. The calculator at
http://www.vk1od.net/tl/tllc.php could be used to calculate the expected
RL of the o/c or s/c line section, just specify a load impedance of 1e6
or 1e-6 for each case. For example, at 1GHz, the RL of 200' LDF4-50A with
a 1e-6 load is 8.9dB, and if you got much higher than that, you might
suspect the cable to be faulty.

Tenths of a dB, remember that most service type power meters are probably
good for 6% to 10% of FSD, so I will go with Jim's 1dB accuracy.

BTW, directional wattmeters for the ham market are often not capable of
reasonable accuracy on loads other than the nominal 50 ohm load. There
are a range of tests that such an instrument should satisfy, but for
hams, it is usually considered sufficient if the "reflected" reading is
approximately zero on a 50 ohm load.

Owen

I think I have given enough info. But I will try yo expess it in another
way.
Power delivered to the antenna but be maintained with in +- 1 db in this
case that power is 100 watts. Power is normally
checked at the TX and recorded after allowing for line loss as "power at
the antenna". Power checks are done on a weekly basis. Once a year the line
loss is measured and this value is used to subtract from the power at the
transmitter for the rest of the year. With this in mind it would be most
prudent to measure the cable loss accurately. to establish the annual
benchmark.

Considering the test equipment I have available to use in a temperature
stablized building an Agilent network analyzer or use an old HP power meter
at the top of the tower I am thinking that measuring rho of the cable while
terminated in a short may be the more accurate way to go.


Jimmie

BTW, directional wattmeters for the ham market are often not capable of
reasonable accuracy on loads other than the nominal 50 ohm load. There
are a range of tests that such an instrument should satisfy, but for
hams, it is usually considered sufficient if the "reflected" reading is
approximately zero on a 50 ohm load.

I should think, though, that one could calibrate such a
reflectometer/directional wattmeter. That is, you could test it with a
suitable variety of source and load impedances and develop a fairly
simple arithmetic correction that would be accurate.

The interesting question might be whether you could unambiguously take a
particular fwd and rev reading and turn that into a true fwd and true
rev, essentially solving for the mismatch.

Down in the lab here at work we have a whole rack of precision
misterminations (1.1:1, 1.2:1, 1.5:1, etc.) that some talented engineer
built and calibrated some decades ago. They're built on the Maury
bluedot N terminations.



Owen

"Jim Lux" wrote in message
...
Jimmie D wrote:
I assume you're not looking for tenth of a dB precision?

Jim


Actually yes I am..Power must be maintained +- 1db at the antenna.

You've got a bit of a challenge, then.. although +/- 1 dB (is that a 1
sigma or a 3 sigma or a absoulate max min spec?) might not require a tenth
of a dB precision.


1 dB is 25%
1% is 0.04dB

(measuring power at 1 GHz to 0.1dB absolute is moderately challenging,
especially outdoors) For reference, an Agilent E4418 is specified at
+/-0.6% (25C +/- 10 degrees).. plus you have a linearity spec which can
range from 1% to 4% depending on the relative levels of the reference and
unknown.

A good return loss measurement with a decent PNA (like an E8363) should
get you down in the sub 0.1dB transmission measurement with overall loss
in the 0 to 20dB range, so the measurement is clearly feasible at some
level.

The same piece of gear, measuring reflection coefficient (i.e. the put a
short or open at the other end, and measure mag(rho) and work back to
loss)... you said you have about 6dB return loss, so that's a reflection
coefficient (at the analyzer) of about 0.5, and for 2GHz, the uncertainty
would be about 0.01 (out of the 0.5), or, call it 2%... again, about a 0.1
dB uncertainty.

OTOH, that's a $50K piece of test gear, sitting in a lab at 25C +/- 1C

There's also the temperature coefficient of the coax to worry about.
Copper has a temperature coefficient of 0.4%/degree C. A 10 degree change
in temperature is a 4% change in resistance (0.2dB), and the resistance is
a big part of the loss (dielectric loss changes differently, and you'd
have to worry about the dimensional changes too).

In any case, measuring the loss by terminating it in a reflection is
probably the easiest way, and potentially the most precise, because you
can have the source and the measurement at the same location. If you
tried to measure it by transmission loss (put the source at one end and
the detector at the other) you have the problem of the stability of the
source. In a bridge type scheme (which the reflection technique is) you
can essentially compare between the unknown (your cable) and a standard,
and adjust the standard until they match, so the variations in the power
level of the source cancel out (or use something that inherently measures
the ratio of the powers).

Something like the LP-100 wattmeter can probably make the measurement.
It's good to 5% typical, and can do ratioed/match measurements to much
better. I don't know if it can go to 1 GHz, though.


Something like the Anritsu SiteMaster (like the S311D) can do this for
sure(after all, it's what it was designed to do.. measure coax on towers)
http://www.us.anritsu.com/downloads/...1410-00419.pdf

If you need to measure loss on the fly, it's a bit trickier, but one way
is to put a deliberate small mismatch at the end (i.e. you put a 10 dB
directional coupler in the line at the antenna end, with the coupled port
terminated into a short). This reflects a known -20dB back down the line.
You look for changes in the amount of reflected power. Obviously, if the
antenna changes it's reflection, you have to separate that out. There are
clever techniques for this too (like having the coupler terminate in a
switch that is either a load or a short). This kind of thing is pretty
common on antenna measurement ranges, where you need to remove the effects
of the feed cable from the measurement.




Jimmie


Sounds like using my network analyser to measure return loss at the TX in an
envoromentally stabalized building is going to be a lot better than taking
my HP power meter up on the antenna in the middle of the night to measure
the power level at the end of the cable.


Jimmie

Sounds like using my network analyser to measure return loss at the TX in an
envoromentally stabalized building is going to be a lot better than taking
my HP power meter up on the antenna in the middle of the night to measure
the power level at the end of the cable.

you betcha..

But you still have the tempco of the cable to agonize about.

On Thu, 9 Aug 2007 08:13:45 -0400, "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

This is way too complicated for me!
My solution would be to build/buy an RF probe and permanently mount it
at the top of the tower. Bring a pair of wires (Coax if you want it to
look really professional) down to the bottom and measure it whenever
or even all the time.

Climb tower once.

John Ferrell W8CCW
"Life is easier if you learn to
plow around the stumps"