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

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