P
Peter
Guest
How do I determine the impedance of a length of coax that I have?
I don't imagine an ohmmeter will do it?
P
I don't imagine an ohmmeter will do it?
P
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J
John Popelish
Guest
Peter wrote:
--
John Popelish
Is there any information written along the outside?How do I determine the impedance of a length of coax that I have?
I don't imagine an ohmmeter will do it?
--
John Popelish
P
Peter
Guest
No it's just an old piece about 3 feet long.
"John Popelish" <jpopelish@rica.net> wrote in message
news:3FF3610E.9B9DBE89@rica.net...
"John Popelish" <jpopelish@rica.net> wrote in message
news:3FF3610E.9B9DBE89@rica.net...
Peter wrote:
How do I determine the impedance of a length of coax that I have?
I don't imagine an ohmmeter will do it?
Is there any information written along the outside?
--
John Popelish
J
John Popelish
Guest
Peter wrote:
conductor and the inside diameter of the outer braid or foil, with a
less sensitive dependence on the dielectric material. If you can
measure center conductor diameter, and the overall outer diameter, and
guess the kind of plastic used in the dielectric (teflon,
polyethylene, foam, air with spiral spacer, etc.) you can probably
identify it in a table of common types.
http://www.therfc.com/coax.htm
--
John Popelish
Well, impedance is primarily determined by the diameter of the centerNo it's just an old piece about 3 feet long.
conductor and the inside diameter of the outer braid or foil, with a
less sensitive dependence on the dielectric material. If you can
measure center conductor diameter, and the overall outer diameter, and
guess the kind of plastic used in the dielectric (teflon,
polyethylene, foam, air with spiral spacer, etc.) you can probably
identify it in a table of common types.
http://www.therfc.com/coax.htm
--
John Popelish
K
Keith R. Williams
Guest
In article <G6IIb.237404$ea%.151591
@news01.bloor.is.net.cable.rogers.com>, pierrevachon@rogers.com
says...
TDR (makes infinite - finite), or look up its specs.
--
Keith
@news01.bloor.is.net.cable.rogers.com>, pierrevachon@rogers.com
says...
It will if your hunk is infinitely long! ;-) Otherwise use aHow do I determine the impedance of a length of coax that I have?
I don't imagine an ohmmeter will do it?
TDR (makes infinite - finite), or look up its specs.
--
Keith
J
Joe McElvenney
Guest
Hi,
If you look at the characteristics of different coax
cables with the same impedance and dielectric type but with
various diameters, you will notice that the capacitance per
foot is nearly independent of the diameter. IIRC it is
about 22pF/ft for 50-ohm with a solid polythene inner and I
expect then that it will be just as constant across the
diameters for other types. This leads to the possibility of
identifying the impedance of a length of feeder by using a
simple DVM with a capacitance range. Of course the method
would be more accurate, the longer the test piece.
Cheers - Joe
If you look at the characteristics of different coax
cables with the same impedance and dielectric type but with
various diameters, you will notice that the capacitance per
foot is nearly independent of the diameter. IIRC it is
about 22pF/ft for 50-ohm with a solid polythene inner and I
expect then that it will be just as constant across the
diameters for other types. This leads to the possibility of
identifying the impedance of a length of feeder by using a
simple DVM with a capacitance range. Of course the method
would be more accurate, the longer the test piece.
Cheers - Joe
J
Jerry G.
Guest
To measure a cable's impedance yourself would be fairly complex. Look for
an types of numbers on it to see if you can identify it from catalogue
listings.
The impedance is determined by the type of dielectric material that the
centre conductor is passing through and the shielding is around, and the
distance between the centre conductor and shield. The gauge of the centre
conductor, and outside diameter is also a determining factor.
By doing precise physical measurement, knowing the types of materials
involved, and the type of alloys used in the composition of the conductors,
a mathematical model of the characteristics of the cable can be determined.
This is very complex, and would not be accurate in all practicality for you
to determine.
The best way would be to use a cable network analyser, or to use a sweep
signal generator and oscilloscope, with a RCL matching network to determine
the impedance curve over a range of frequencies. From this the actual
impedance can be determined.
Accurately measuring the characteristics of any type of cable or wire can
turn out to be a complex and expensive ordeal.
If your piece of cable is from a cable TV distributor, or from a video
distribution system, it is most likely a 75 ohm cable. If it is from an
older Ethernet system, it is probably a 150 ohm cable. If it is from the
antenna system used on communications radio, it is most likely a 50 ohm
cable.
--
Greetings,
Jerry Greenberg GLG Technologies GLG
=========================================
WebPage http://www.zoom-one.com
Electronics http://www.zoom-one.com/electron.htm
=========================================
"Peter" <pierrevachon@rogers.com> wrote in message
news:G6IIb.237404$ea%.151591@news01.bloor.is.net.cable.rogers.com...
How do I determine the impedance of a length of coax that I have?
I don't imagine an ohmmeter will do it?
P
an types of numbers on it to see if you can identify it from catalogue
listings.
The impedance is determined by the type of dielectric material that the
centre conductor is passing through and the shielding is around, and the
distance between the centre conductor and shield. The gauge of the centre
conductor, and outside diameter is also a determining factor.
By doing precise physical measurement, knowing the types of materials
involved, and the type of alloys used in the composition of the conductors,
a mathematical model of the characteristics of the cable can be determined.
This is very complex, and would not be accurate in all practicality for you
to determine.
The best way would be to use a cable network analyser, or to use a sweep
signal generator and oscilloscope, with a RCL matching network to determine
the impedance curve over a range of frequencies. From this the actual
impedance can be determined.
Accurately measuring the characteristics of any type of cable or wire can
turn out to be a complex and expensive ordeal.
If your piece of cable is from a cable TV distributor, or from a video
distribution system, it is most likely a 75 ohm cable. If it is from an
older Ethernet system, it is probably a 150 ohm cable. If it is from the
antenna system used on communications radio, it is most likely a 50 ohm
cable.
--
Greetings,
Jerry Greenberg GLG Technologies GLG
=========================================
WebPage http://www.zoom-one.com
Electronics http://www.zoom-one.com/electron.htm
=========================================
"Peter" <pierrevachon@rogers.com> wrote in message
news:G6IIb.237404$ea%.151591@news01.bloor.is.net.cable.rogers.com...
How do I determine the impedance of a length of coax that I have?
I don't imagine an ohmmeter will do it?
P