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 at
http://rfdesign.com/mag/608RFDF2.pdf
http://cp.literature.agilent.com/lit...5966-0784E.pdf

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. 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. 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.

Cheers,
Tom

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

On Mon, 13 Aug 2007 13:09:09 -0700, Jim Lux
wrote:

Or, something like a 50k resistor into a 50
ohm load will be about 60 dB down,

Hi Jim,

Unlikely.

With parasitic capacitance at a meager 1pF across the 50K, its Z at
10MHz would compromise the attenuation presenting closer to 50 dB
down. At 1Ghz it would plunge like a rock. This, of course, presumes
a 1/4 watt resistor.

A better solution is to use surface mount resistors where the
parasitics are down at 100aF - but then you will have a frequency
dependant divider unless you can guarantee that the parasitic
capacitance of the 50 Ohm resistor is 100pF (sort of casts us back
into using a 1/4 watt resistor with a padding cap). At 1GHz, it is
not going to look like a trivial 50K load anymore.

A Pi attenuator will do it better.

73's
Richard Clark, KB7QHC

On Aug 13, 1:09 pm, Jim Lux wrote:
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/...

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.
....

The OP said this is at 1GHz. It's really tough to get a reliable
resistive divider at 1GHz, with that sort of ratio. Actually, a
capacitive divider probably stands a better chance of working, though
getting that really right isn't trivial. (We used to worry about
variation in humidity and atmospheric pressure affecting the
dielectric constant of air, in using a capacitive sampler...though
admittedly that was for work to a level well beyond 1dB accuracy.)

I am rather fond of the coupled-line hybrid idea: it can be built in
a way that everything stays ratiometric, so the coupling ratio is very
nearly constant over temperature, and of course the directionality
lets you observe things you can't just from monitoring voltage at a
point. It's possible to build one with low coupling without too much
trouble; -60dB coupling isn't out of the question, for sure. I'm
imagining a design I could make reliably with simple machine tools
that would work well for the OP's application: 100 watts at about
1GHz as I recall in the through line, and coupling on the order of
-60dB to get to about -10dBm coupled power and have negligible effect
on the through line. There's a free fields solver software package
that will accurately predict the coupling, and with the right design
and normal machine shop tolerances the coupling and impedance should
be accurate to a fraction of a dB and better than a percent,
respectively. Perhaps I can run some examples to see if I'm off-base
on that, but that's what my mental calculations tell me at the moment.

Cheers,
Tom

If the speed of the "Drop" is measured in "measurable electricity" - then
what is
cycle time of the measuring device in Question?

So, in other words - if the "measuring Device" is working at the same speed
of
the thing that it is measuring (or faster) then how can it measure what it
is
measuring?

Supposedly the cycle time for low voltage is about 1000 times per second
versus
AC which is 60 times per second ... so is the device you are using "capable"
of
doing the job at all? Just a Question?

Here is my Web Site URL ... it might have some interesting solutions - or
Questions?
http://www3.telus.net/public/quark5/home.html






"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

Jimmie D wrote:

Own, beg, borrow or steal a watt meter. Hook to back of rig, measure
wattage, hookup rig through coax to watt meter, measure wattage--no
conversion math/chart necessary. You'll directly know the loss ...

Regards,
JS