K7ITM wrote:
On Aug 13, 11:50 am, Jim Lux wrote:

John Ferrell wrote:

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.

Considering he needs sub 1dB accuracy, this is challenging..it would
work if you assume your RF probe never needs calibration and is stable
over the environmental range of interest. Not a trivial thing to do.

A diode and a voltmeter certainly won't do it. (A typical diode detector
might vary 1 dB over a 20 degree C range.. judging from the Krytar 700
series data sheet I have sitting here. Granted that's a microwave
detector (100MHz to 40 GHz), but I'd expect similar from most other
diodes. I've given the link to an Agilent Ap note that describes various
detectors in excruciating detail.

A diode, voltmeter, and temperature sensor might work, though.

useful stuff athttp://rfdesign.com/mag/608RFDF2.pdfhttp://cp.literature.agilent.com/litweb/pdf/5966-0784E.pdf


Seems like modern RF detector ICs offer much better stability than
diodes. An AD8302, for example, has a typical +/- 0.25dB variation
from -40C to +85C, with a -30dBm signal level.
indeed...
The temperature
variation could be calibrated before installation; if necessary, an
especially temperature-stable part could be selected from a batch.
Then knowing the ambient within 20C would be sufficient. You'd need
to arrange sampling at a low level, which could be a well-constructed
90 degree hybrid.

or, even simpler, what about a resistive tap (or a pair of resistive
taps separated by a short length of transmission line). If you're
sending, say, 100W (+50dBm) up the wire, and you want, say, -30dBm out,
you need a 80 dB coupler. Or, something like a 50k resistor into a 50
ohm load will be about 60 dB down, and you could put a 10-20dB pad in
before the detector. Calibration would take care of the coupling ratio,
although, you might want to be careful about the tempco of the resistor.
With two channels in the AD8302, you could even
monitor antenna reflection coefficient (including angle), and be aware
of changes there. Analog Devices and Linear Technology both seem to
be strong in the RF power monitor IC area.

Those are truly nifty parts, and form the basis of some very interesting
ham products over the past couple years (like LP-100 vector wattmeter
and various ham-oriented VNAs). What would be very cool is if AD would
combine something like the 8302 and the A/D so it would have a serial
digital output. Pretty close to a powermeter on a chip.

Functionally, this would be close to what you get with a Rhode+Schwartz
NRP series, a Boonton 52000, an Agilent U2000

Cheers,
Tom