On 11/03/2010 03:22 AM, ashwanthh wrote:
Hi all,


For my project, I want to perform Distance to fault measurement in the
cable connecting the antenna. The principle is that a signal should be
sent to the cable and using the forward and reflected voltage VSWR
should be calculated and should be viewed in a spectrum analyzer. My
question is

1. What transducer is used to detect the transmitted/reflected signals
from the cable?

2. Should the transducer be connected in series or parallel?

Anyone, Please help



Or just use a grid dip meter.

1) Determine if the fault is a short or an open circuit with any tester.

2) Connect a small loop at the input of the cable.

3) Couple the dip meter and read the minimum frequency with a dip.

4) If a short circuit ===
Distance(meters) = 75 * cable velocity factor / frequency (MHz)

5) If an open circuit ===
Distance(meters)= 150 * cable velocity factor / frequency (MHz)
--
Alejandro Lieber LU1FCR
Rosario Argentina

Real-Time F2-Layer Critical Frequency Map foF2:
http://1fcr.com.ar

I have a similar practical real problem.

A vertical 120 ft working 75 ohm coax line hung from the top.

The soft foamed dielectric MAY be crushed under the cable's own weight
10 ft from the top end.

If it is I must change the hanging system.

ASSUMPTIONS
- the dielectric is crushed to 1/2 the original thickness over 2
inches length 10 ft from the far end
- I can put a good quality non-reflecting 75 ohm load at the far end.

QUESTION
Can I expect such a defect to generate a detectable reflection at
2GHz?

"spamhog" wrote in message
...
I have a similar practical real problem.

A vertical 120 ft working 75 ohm coax line hung from the top.

The soft foamed dielectric MAY be crushed under the cable's own weight
10 ft from the top end.

If it is I must change the hanging system.

ASSUMPTIONS
- the dielectric is crushed to 1/2 the original thickness over 2
inches length 10 ft from the far end
- I can put a good quality non-reflecting 75 ohm load at the far end.

QUESTION
Can I expect such a defect to generate a detectable reflection at
2GHz?

Yes. The cable impedance will be lowered in the vicinity of the crush.

A good time-domain reflectometer can see the impedance mismatch of
connectors - even you are using types C, N, TNC or BNC. UHF type
connectors, such as the PL-259, are readily seen. Your section of
crushed dielectric should "stick out like a sore thumb" on a TDR trace.
Termination at the far end is not necessary but it will make the
understanding of the TDR waveform easier.

Impedance mismatches, even though they are short in length, become more
important the higher in frequency as you go.

73, Dr. Barry L. Ornitz WA4VZQ

A good time-domain reflectometer

Thank you Barry! Would TV/FM transmitter and cell BTS installers
normally have such a device? I don't know any ham who has one but
these days I am talking to professional antenna people. And, by the
way, if one thing I am working on pans out, they'll come to install a
BTS right atop my condo. (The challenge will be to convince them to
let me hang HF wires from the same tower :-)

spamhog wrote:

A good time-domain reflectometer

Thank you Barry! Would TV/FM transmitter and cell BTS installers
normally have such a device? I don't know any ham who has one but
these days I am talking to professional antenna people. And, by the
way, if one thing I am working on pans out, they'll come to install a
BTS right atop my condo. (The challenge will be to convince them to
let me hang HF wires from the same tower :-)

If you have an oscilloscope, a pulse generator and a calculator you have
a TDR.

If you don't have a pulse generator it takes about 2 IC's to build one.


--
Jim Pennino

Remove .spam.sux to reply.

wrote in message
...
spamhog wrote:

A good time-domain reflectometer

Thank you Barry! Would TV/FM transmitter and cell BTS installers
normally have such a device? I don't know any ham who has one but
these days I am talking to professional antenna people. And, by the
way, if one thing I am working on pans out, they'll come to install a
BTS right atop my condo. (The challenge will be to convince them to
let me hang HF wires from the same tower :-)

If you have an oscilloscope, a pulse generator and a calculator you
have
a TDR.

If you don't have a pulse generator it takes about 2 IC's to build one.


--
Jim Pennino


You would need an extremely fast rise-time pulse generator and an
exceptionally fast oscilloscope to match a typical commercial TDR. The
Tek 1502 unit I used when at Eastman's Research Labs had a pulse
rise-time of 140 picoseconds, and the internal display was a sampling
scope with a bandwidth of over
3 GHz. I now have a Tek 1S2 plug-in for my Tek 585A oscilloscope at
home. It's fine for amateur use, but don't expect to see the tiny
impedance "burbles" of BNC connectors with it. {And carrying around a
1502 is _FAR_ easier than the 585!}

Quoting from a Tektronix application note on TDR resolution:
“the resolution limit … wherein two discontinuities or changes on the
transmission line … begin to merge together … Per this definition, the
resolution limit is: half the … 10% to 90% risetime or 90% to 10% fall
time (depending on whether the TDR response is calibrated with a short or
open circuit).”
To convert this to distance, you need to know the velocity factor for the
cable you are testing and the speed of light.

Your installers may not always carry a TDR with them, but I would bet
they have access to one.

73, Barry WA4VZQ

Barry wrote:
wrote in message
...
spamhog wrote:

A good time-domain reflectometer

Thank you Barry! Would TV/FM transmitter and cell BTS installers
normally have such a device? I don't know any ham who has one but
these days I am talking to professional antenna people. And, by the
way, if one thing I am working on pans out, they'll come to install a
BTS right atop my condo. (The challenge will be to convince them to
let me hang HF wires from the same tower :-)

If you have an oscilloscope, a pulse generator and a calculator you
have
a TDR.

If you don't have a pulse generator it takes about 2 IC's to build one.


--
Jim Pennino


You would need an extremely fast rise-time pulse generator and an
exceptionally fast oscilloscope to match a typical commercial TDR.

Yeah but in the real world you don't usually need to match a commercial
TDR to find a fault.

Back in the days of ethernet over RG-58 I was able to find lots of poorly
attached connectors and cables crushed by the electricians that installed
them (there were no network engineers and installers in those days) with
a 100 MHz 'scope and a pulse generator made from a 555 timer and a 50 Ohm
line driver.

The accuracy of the measurement for fault finding doesn't need to be much
better than a few feet to be able to find the fault visually once you know
about where it is and faults at the end are immediately obvious.

If the task is to find faults, a 'scope and simple pulse generator works
just fine.

If the task is to certify 6 inches of hard line to GHz to some mil-spec, you
probably want something more sophisticated.


--
Jim Pennino

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On Tue, 9 Nov 2010 22:01:27 -0500, "Barry" wrote:

You would need an extremely fast rise-time pulse generator and an

This can be easily be obtained (provided you can find one) from a
mercury wetted switch. Typical rise times in the sub-nano to
nanoseconds.

exceptionally fast oscilloscope

Which, is something you would have to spend money on - most assuredly.

OR

Use a wide field microscope against the CRT being driven at the
deflection plates (the early design for these applications of
extremely fast pulse measurements) which can be far more affordable.

73's
Richard Clark, KB7QHC

wrote in message
...
Yeah but in the real world you don't usually need to match a commercial
TDR to find a fault.

Back in the days of ethernet over RG-58 I was able to find lots of
poorly
attached connectors and cables crushed by the electricians that
installed
them (there were no network engineers and installers in those days)
with
a 100 MHz 'scope and a pulse generator made from a 555 timer and a 50
Ohm
line driver.

Can you detect where RG-58 was kinked and then straightened out with the
damaged area being less than 1/2 inch? Likewise can you detect an
abraded shield leaving a 1/4 inch hole in the shielding?

An LM555 has a rise and fall time of 100 nanoseconds. I have no idea of
how much your line driver sharpens the edges of the pulse, but a good
line driver should provide rise and fall times of 5 nanoseconds. The
original TEK 1502 at 140 picoseconds can resolve to about 2 centimeters.
With a risetime of 5 nS, your resolution will be about 70 times worse
(140 cm). Add to this the 100 MHz limitation of the scope (an additional
10 nS), and it becomes far worse.

Yes, you can easily detect shorts and open circuits with your setup, but
you will not be able to detect the 1 to 2 inches or so where the center
conductor has migrated in the foamed dielectric from hanging the cable
over a building edge.

The accuracy of the measurement for fault finding doesn't need to be
much
better than a few feet to be able to find the fault visually once you
know
about where it is and faults at the end are immediately obvious.

If the task is to find {gross} faults, a 'scope and simple pulse
generator works
just fine.

If the task is to certify 6 inches of hard line to GHz to some
mil-spec, you
probably want something more sophisticated.

The kind of fault Spamhog originally described, a 75 ohm cable dropping
to less than 50 ohms over two inches and going back to 75 ohms, will not
be detected by your setup.

73, Barry WA4VZQ

Barry wrote:
wrote in message
...
Yeah but in the real world you don't usually need to match a commercial
TDR to find a fault.

Back in the days of ethernet over RG-58 I was able to find lots of
poorly
attached connectors and cables crushed by the electricians that
installed
them (there were no network engineers and installers in those days)
with
a 100 MHz 'scope and a pulse generator made from a 555 timer and a 50
Ohm
line driver.

Can you detect where RG-58 was kinked and then straightened out with the
damaged area being less than 1/2 inch? Likewise can you detect an
abraded shield leaving a 1/4 inch hole in the shielding?

I was able to detect where cables were squashed by overly tight bundle lacing
and bending over a short radius.

It got me close enough that a visual inspection of a few feet found the
"bad" place.

An LM555 has a rise and fall time of 100 nanoseconds. I have no idea of
how much your line driver sharpens the edges of the pulse, but a good
line driver should provide rise and fall times of 5 nanoseconds. The
original TEK 1502 at 140 picoseconds can resolve to about 2 centimeters.
With a risetime of 5 nS, your resolution will be about 70 times worse
(140 cm). Add to this the 100 MHz limitation of the scope (an additional
10 nS), and it becomes far worse.

Like I said, if you have a pair of eyes, all you have to do is get close.

And in any case, the solution for a cable run that is hosed somewhere in
the middle is to replace the entire section. Cutting a section out and
putting in connectors to splice the cable is just asking for more problems.

Yes, you can easily detect shorts and open circuits with your setup, but
you will not be able to detect the 1 to 2 inches or so where the center
conductor has migrated in the foamed dielectric from hanging the cable
over a building edge.

Yet I could, so either I'm a lier or you are overestimating how hard it
is in the real world.

The accuracy of the measurement for fault finding doesn't need to be
much
better than a few feet to be able to find the fault visually once you
know
about where it is and faults at the end are immediately obvious.

If the task is to find {gross} faults, a 'scope and simple pulse
generator works
just fine.

If the task is to certify 6 inches of hard line to GHz to some
mil-spec, you
probably want something more sophisticated.

The kind of fault Spamhog originally described, a 75 ohm cable dropping
to less than 50 ohms over two inches and going back to 75 ohms, will not
be detected by your setup.

It would detect that there was an impedance lump.

It would not tell you it was a 50 Ohm lump.


--
Jim Pennino

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