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