RadioBanter - measuring cable loss

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measuring cable loss

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

measuring cable loss

"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

measuring cable loss

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

measuring cable loss

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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measuring cable loss

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

measuring cable loss

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

measuring cable loss

"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

measuring cable loss

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

measuring cable loss

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