"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