the "300/75/50 ohms" of TV ribbon, coax
 

Dear antenna pros, I've always taken it for grunted about the
300/75/50 ohms of TV ribbon, coax, etc. But how does one measure it?
My ohmsmeter doesn't budge. Is there some standard formula, like wrap
grandma 100 times, with the far end connected to a cheeseburger in her
mouth, the near end finally displaying the characteristic 300/75/50
whatever ohmses?

The impedance of a transmission line is dependent on the physical
characteristics such as conductor spacing, conductor size and the insulation
characteristics. Impedance is an AC parameter and can't be measured with an
ordinary DC ohmeter. But if you had an infinite length of any transmission
line unterminated and connected an impedance bridge, it would read the
characteristic impedance. The ARRL handbook or any decent antenna books
will have the formulas you want and IIRC there's no factors for grandmas or
cheeseburgers, hi hi.
hank wd5jfr
"Dan Jacobson" wrote in message
...
Dear antenna pros, I've always taken it for grunted about the
300/75/50 ohms of TV ribbon, coax, etc. But how does one measure it?
My ohmsmeter doesn't budge. Is there some standard formula, like wrap
grandma 100 times, with the far end connected to a cheeseburger in her
mouth, the near end finally displaying the characteristic 300/75/50
whatever ohmses?

Dear antenna pros, I've always taken it for grunted about the
300/75/50 ohms of TV ribbon, coax, etc. But how does one measure it?
My ohmsmeter doesn't budge. Is there some standard formula, like wrap
grandma 100 times, with the far end connected to a cheeseburger in her
mouth, the near end finally displaying the characteristic 300/75/50
whatever ohmses?

You don't measure it with most instruments you would have . It is a
calculated value from the size and spacing of the conductors and the
insulation between them. There are ways to measure it but most would not
have them.

Here is a place you can go for an explination.

http://www.epanorama.net/documents/w...impedance.html

I've always taken it for grunted

I have often grunted when taking transmission line measurments, but I try to
keep it to a low volume.
Your ohmmeter will budge if you terminate the line with a short, or some
resistance. The Characteristic Impedance (300/75/50 ohms) is an AC or RF
measurement, and can not be done with a DC VOM. The devices that the average
Ham has on hand include, antenna analyzer, grid dip meter, and rf signal
generator. Transmission line measurements with these devices can been found in
the ARRL books or with the instruction books with the instruments.
73 Gary N4AST

As stated in other responses, it is not measured with simple instruments.

Every wire has self inductance. It can be calculated using commonly
available equations.

Every pair of wires has self capacitance. It can be calculated using
commonly available equations.

So, the 300/75/50 ohm term, characteristic impedance, is the square root
of L/C

W1MCE

Dan Jacobson wrote:

Dear antenna pros, I've always taken it for grunted about the
300/75/50 ohms of TV ribbon, coax, etc. But how does one measure it?
My ohmsmeter doesn't budge. Is there some standard formula, like wrap
grandma 100 times, with the far end connected to a cheeseburger in her
mouth, the near end finally displaying the characteristic 300/75/50
whatever ohmses?

Henry, WD5JFR wrote:
"can`t be measured with an ordinary ohmmeter. But if you had an infinite
length of any transmission line unterminated and connected an impedance
bridge, it would read the chatracteristic impedance."

Reg, G4FGQ observed on these pages long ago that an ordinary ohmmeter
would read Zo if connected to the end of an infinite line. He is right
of course.

Best regards, Richard Harrison, KB5WZI

For you young fellas, it used to be called "surge impedance" just for that
reason.

Steve K/9/d/c/i
"Richard Harrison" wrote in message
...
Henry, WD5JFR wrote:
"can`t be measured with an ordinary ohmmeter. But if you had an infinite
length of any transmission line unterminated and connected an impedance
bridge, it would read the chatracteristic impedance."

Reg, G4FGQ observed on these pages long ago that an ordinary ohmmeter
would read Zo if connected to the end of an infinite line. He is right
of course.

Best regards, Richard Harrison, KB5WZI

Reg, G4FGQ observed on these pages long ago that an ordinary ohmmeter
would read Zo if connected to the end of an infinite line. He is right
of course.

He is right of course. Do you know of anyone that has an infinite length of
transmission line? Or an infinite anything? :-).
73 Gary N4AST

Gary, N4AST wrote:
"Do you know of anyone that has an infinite length of transmission
line?"

I may as soon as the line constructor is finished. Now, I can`t say that
I do, but for some purposes a line of random length which is terminated
in Zo serves as well as an infinite line of the same Zo.

A dissipation line, of course, must have enough length not to overload
the termination resistor at the end of the real line which does have an
end.

Best regards, Richard Harrison, KB5WZI

Get an impedance bridge to measure the coax.. A directional coupler will
help. The other way is calculate the impedance based on wire size, spacing,
dielectric, etc,
al

"Dan Jacobson" wrote in message
...
Dear antenna pros, I've always taken it for grunted about the
300/75/50 ohms of TV ribbon, coax, etc. But how does one measure it?
My ohmsmeter doesn't budge. Is there some standard formula, like wrap
grandma 100 times, with the far end connected to a cheeseburger in her
mouth, the near end finally displaying the characteristic 300/75/50
whatever ohmses?

Howdy,

These numbers are characteristic impedances of the cables. If you had an
infinite length of it all you would have to do is use an impedance meter and it
would yield the correct C. I.

It is a function of the frequency and the distributed R, L, C, and G of the
cable.

The ARRL Handbook and their antenna book can tell you much.

7e de Jack, K9CUN

It's actually Ohms with a capital OH.

:-)

Nope, it's actually ohms with a lower case oh, at least in the US.

According to the metric standard promoted by NIST and most of the world, proper
names are not capitalized, with one exception. The exception is Celsius, but
only because the correct unit is "degree Celsius" not "Celsius".

Contributing to the confusion is the standard that many symbols are capitalized
even when the spelled out unit name is not. For example, W is the symbol for
watt, Pa is the symbol for pascal, J is the symbol for joule, etc.

If this stuff actually interests anyone the reference is:

http://www.physics.nist.gov/Pubs/SP811/contents.html

73,
Gene
W4SZ

JDer8745 wrote:
It's actually Ohms with a capital OH.

:-)

On Tue, 09 Dec 2003 20:46:18 GMT, Gene Fuller
wrote:

Nope, it's actually ohms with a lower case oh, at least in the US.

Not according to the referance you gave at
http://www.physics.nist.gov/Pubs/SP811/contents.html

Quote:
6.1.2 Capitalization
Unit symbols are printed in lower-case letters except that:
(a)
the symbol or the first letter of the symbol is an upper-case letter
when the name of the unit is derived from the name of a person...

End quote.

Ohm is a name of a person.

Danny

Danny,

Sorry, please read more carefully.

Check out section 4. The units have both names and symbols. Names are not
capitalized except at the beginning of a sentence. Symbols are capitalized if
they are derived from a person's name.

Ohm is a person's name, but it is not a unit symbol. The unit symbol for
resistance is capital omega. The correct unit name for resistance is ohm.

73,
Gene
W4SZ


Dan Richardson wrote:
On Tue, 09 Dec 2003 20:46:18 GMT, Gene Fuller
wrote:


Nope, it's actually ohms with a lower case oh, at least in the US.


Not according to the referance you gave at
http://www.physics.nist.gov/Pubs/SP811/contents.html

Quote:
6.1.2 Capitalization
Unit symbols are printed in lower-case letters except that:
(a)
the symbol or the first letter of the symbol is an upper-case letter
when the name of the unit is derived from the name of a person...

End quote.

Ohm is a name of a person.

Danny

Ohm is a name of a person.

=======================

During which era did Mr Inch live?

Someone sed,

"Reg, G4FGQ observed on these pages long ago that an ordinary ohmmeter would
read Zo if connected to the end of an infinite line. He is right of course."

But the Zo of a line varies with frequency. How will the "ordinary ohmmeter"
do the job at, say, 100 kHz?

73 de jack

On Wed, 10 Dec 2003 17:35:08 +0000 (UTC), "Reg Edwards"
wrote:

Ohm is a name of a person.

=======================

During which era did Mr Inch live?


Georg Simon Ohm

Born: 16 March 1789 in Erlangen, Bavaria

"Reg, G4FGQ observed on these pages long ago that an ordinary ohmmeter
would
read Zo if connected to the end of an infinite line. He is right of
course."

But the Zo of a line varies with frequency. How will the "ordinary
ohmmeter"
do the job at, say, 100 kHz?

73 de jack

YOu should get a lot of people calling BS on the Zo changing with frequency.
It does not change at any reasonable frequency for the line. That is at
least anything below 1 ghz for coax.

Zo of ALL real, ordinary, transmission lines changes versus frequency over a
very wide frequency range.

Zo ranges over lots of thousands of ohms at a few cyles of seconds,
thousands of ohms at power frequencies, hundreds of ohms at audio
frequencies, and from tens to a few hundred ohms from 100KHz up to as many
GHz as you like.
---
Reg.


--
.................................................. ..........
Regards from Reg, G4FGQ
For Free Radio Design Software go to
http://www.btinternet.com/~g4fgq.regp
.................................................. ..........
"Ralph Mowery" wrote in message
...

"Reg, G4FGQ observed on these pages long ago that an ordinary ohmmeter
would
read Zo if connected to the end of an infinite line. He is right of
course."

But the Zo of a line varies with frequency. How will the "ordinary
ohmmeter"
do the job at, say, 100 kHz?

73 de jack

YOu should get a lot of people calling BS on the Zo changing with
frequency.
It does not change at any reasonable frequency for the line. That is at
least anything below 1 ghz for coax.

"Reg Edwards" wrote in message
...
Zo of ALL real, ordinary, transmission lines changes versus frequency over
a
very wide frequency range.

Zo ranges over lots of thousands of ohms at a few cyles of seconds,
thousands of ohms at power frequencies, hundreds of ohms at audio
frequencies, and from tens to a few hundred ohms from 100KHz up to as many
GHz as you like.
---
Reg.



Are we talking the same thing for Zo ? That a piece of say rg-8 ( whatever
they want to call it now) that is 50 ohm coax is not 50 ohms over its normal
operating frequency range ?

"Reg Edwards" wrote in message
Zo of ALL real, ordinary, transmission lines changes versus frequency
over
a
very wide frequency range.

Zo ranges over lots of thousands of ohms at a few cyles of seconds,
thousands of ohms at power frequencies, hundreds of ohms at audio
frequencies, and from tens to a few hundred ohms from 100KHz up to as
many
GHz as you like.
---
Reg.



Are we talking the same thing for Zo ? That a piece of say rg-8 (
whatever
they want to call it now) that is 50 ohm coax is not 50 ohms over its
normal
operating frequency range ?

=============================

What is your normal operating frequency range?

Here is Zo typical of cable similar to RG-58 versus frequency.

Nominal Zo = 50 ohms.
Degrees = Angle of Zo.

Freq Zo Degrees
------ ------- -------
10 Hz 3000 -45.0
100 Hz 950 -44.9
1000 Hz 301 -44.1
10 KHz 97 -36.4
100 KHz 54.2 -10.1
1 MHz 50.0 -2.6
10 MHz 48.4 -0.84
100 MHz 48.0 -0.26
1 GHz 47.8 -0.08

Superimposed on the above Zo vs F characteristics are manufacturing
reel-to-reel variations of 2 or 3 percent.
----
Reg, G4FGQ

Someone sed about the characteristic frequency of a transmission line::

"It does not change at any reasonable frequency for the line. That is at
least anything below 1 ghz for coax."

The formula for Zo contains the frequency.

Look it up.

73 de Jack K9CUN

Someone sed:

" Do you know of anyone that has an infinite length of transmission line? Or
an infinite anything? :-)."

===============

Some of these threads are PRETTY LONG, approaching infinity???

73 de Jack, K9CUN

Someone sed:

"So, the 300/75/50 ohm term, characteristic impedance, is the square root of
L/C"

This is an *approximation* that is useful when complex arithmetic is over the
capability of your calculator or if the imaginary components of the formula for
Zo are negligible.

At the usual HF through UHF ham frequencies the imaginary components are
negligible so the approximation suffices.

UNITS: 10-pF capacitor. 10-pF is hypenated when used as a modifier, i. e.,
adjective.

Others: 10-ft pole, 5-cent cigar, 2-dollar pistol, 12-V battery, 500-mile
track.

73 de Jack, K9CUN

JDer8745 wrote:

snip


UNITS: 10-pF capacitor. 10-pF is hypenated when used as a modifier, i. e.,
adjective.

Others: 10-ft pole, 5-cent cigar, 2-dollar pistol, 12-V battery, 500-mile
track.

73 de Jack, K9CUN


Partly correct.

The use of "10-picofarad capacitor" is preferred under the standard rules of
English and the NIST style guide. The use of "10-pF capacitor" should be
replaced by "10 pF capacitor" according to NIST.

See section 7.2 in http://www.physics.nist.gov/Pubs/SP811/contents.html

"Even when the value of a quantity is used in an adjectival sense, a space is
left between the numerical value and the unit symbol. (This rule recognizes that
unit symbols are not like ordinary words or abbreviations but are mathematical
entities, and that the value of a quantity should be expressed in a way that is
as independent of language as possible.)"

"When unit names are spelled out, the normal rules of English apply. Thus, for
example, ‘a roll of 35-millimeter film’ is acceptable."

73,
Gene
W4SZ

When putting up an antenna, do you use different Zo coax for 2 Vs. 80 meters
to account for frequency change?

The Zo is constant for all practical purposes below Giga Hz freqs.

"JDer8745" wrote in message
...
Someone sed,

"Reg, G4FGQ observed on these pages long ago that an ordinary ohmmeter
would
read Zo if connected to the end of an infinite line. He is right of
course."

But the Zo of a line varies with frequency. How will the "ordinary
ohmmeter"
do the job at, say, 100 kHz?

73 de jack

As most of us do not transmit in the audio range, your own data shows a
range of 50 to 47.8 Ohms for normal ham use. Close enough for government
work, at least as far as I am concerned..

"Reg Edwards" wrote in message
...

"Reg Edwards" wrote in message
Zo of ALL real, ordinary, transmission lines changes versus frequency
over
a
very wide frequency range.

Zo ranges over lots of thousands of ohms at a few cyles of seconds,
thousands of ohms at power frequencies, hundreds of ohms at audio
frequencies, and from tens to a few hundred ohms from 100KHz up to as
many
GHz as you like.
---
Reg.



Are we talking the same thing for Zo ? That a piece of say rg-8 (
whatever
they want to call it now) that is 50 ohm coax is not 50 ohms over its
normal
operating frequency range ?

=============================

What is your normal operating frequency range?

Here is Zo typical of cable similar to RG-58 versus frequency.

Nominal Zo = 50 ohms.
Degrees = Angle of Zo.

Freq Zo Degrees
------ ------- -------
10 Hz 3000 -45.0
100 Hz 950 -44.9
1000 Hz 301 -44.1
10 KHz 97 -36.4
100 KHz 54.2 -10.1
1 MHz 50.0 -2.6
10 MHz 48.4 -0.84
100 MHz 48.0 -0.26
1 GHz 47.8 -0.08

Superimposed on the above Zo vs F characteristics are manufacturing
reel-to-reel variations of 2 or 3 percent.
----
Reg, G4FGQ

Jack, with all due respect, you need a hobby...


"JDer8745" wrote in message
...
Someone sed:

"So, the 300/75/50 ohm term, characteristic impedance, is the square root
of
L/C"

This is an *approximation* that is useful when complex arithmetic is over
the
capability of your calculator or if the imaginary components of the
formula for
Zo are negligible.

At the usual HF through UHF ham frequencies the imaginary components are
negligible so the approximation suffices.

UNITS: 10-pF capacitor. 10-pF is hypenated when used as a modifier, i.
e.,
adjective.

Others: 10-ft pole, 5-cent cigar, 2-dollar pistol, 12-V battery, 500-mile
track.

73 de Jack, K9CUN

"Reg Edwards" wrote in message
Zo of ALL real, ordinary, transmission lines changes versus frequency
over
a
very wide frequency range.

Zo ranges over lots of thousands of ohms at a few cyles of seconds,
thousands of ohms at power frequencies, hundreds of ohms at audio
frequencies, and from tens to a few hundred ohms from 100KHz up to as
many
GHz as you like.
---
Reg.



Are we talking the same thing for Zo ? That a piece of say rg-8 (
whatever
they want to call it now) that is 50 ohm coax is not 50 ohms over its
normal
operating frequency range ?

=============================

What is your normal operating frequency range?

Here is Zo typical of cable similar to RG-58 versus frequency.

Nominal Zo = 50 ohms.
Degrees = Angle of Zo.

Freq Zo Degrees
------ ------- -------
10 Hz 3000 -45.0
100 Hz 950 -44.9
1000 Hz 301 -44.1
10 KHz 97 -36.4
100 KHz 54.2 -10.1
1 MHz 50.0 -2.6
10 MHz 48.4 -0.84
100 MHz 48.0 -0.26
1 GHz 47.8 -0.08

Superimposed on the above Zo vs F characteristics are manufacturing
reel-to-reel variations of 2 or 3 percent.
----
Reg, G4FGQ


Think I must have missed the point about REAL transmission lines. Trying to
keep up with too many discussions at one time. I was thinking of the simple
formular where frequency is not mentioned. I do know about the problem of
making the coax exectally the same all the time where even in the same reel
of coax you can get into suckout problems due to slight variations in the
coax.

w4jle wrote:

Jack, with all due respect, you need a hobby...



He was just getting his 2-cents worth in! :-)
--
--------------------------------------
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Beating it with diet and exercise!
297/215/210 (to be revised lower)
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--------------------
Irv Finkleman,
Grampa/Ex-Navy/Old Fart/Ham Radio VE6BP
Calgary, Alberta, Canada

Reg Edwards wrote:

Ohm is a name of a person.

=======================

During which era did Mr Inch live?

Wasn't he at Cambridge with Furlong,
Yard, Chain and a Polish guy called
Rod Perch? I think that was the group
that first discovered length, or was
it distance? ;-)

(Actually derived from Latin, uncia, an ounce.)

In fact the SI units don't have a fixed rule for
capitalisation. When the unit is spelt out it should
not be capitalised - ohm, kelvin, farad - to avoid
confusion with the scientist. The abbreviation or
symbol should be capitalised for all those named
after people and for litre - Hz, L, V. The ohm is
normally written with a capital omega or written
in full as 'ohm'. Ohm at the beginning of a sentence
is capitalised.

See http://www.poynton.com/PDFs/Writing_SI_units_(USL).pdf

vy 73

Andy, M1EBV

Someone sed:

"So, the 300/75/50 ohm term, characteristic impedance, is the square root of
L/C"
==================

Not always!

73 de Jack, K9CUN

To a first approximation ... YES. In the context of providing a simple
explanation, an introductory level explanation, as in the context of the
original question ... YES.

In the interest of more advanced analysis:

Zo = SQRT[[R + jwL]/[G + jwC]]

In a lossless line that converges to SQRT [L/C].

DD

JDer8745 wrote:

Someone sed:

"So, the 300/75/50 ohm term, characteristic impedance, is the square root of
L/C"
==================

Not always!

73 de Jack, K9CUN

"In a lossless line that converges to SQRT [L/C]."

There is no such thing as a lossless line. The formula becomes SQRT(L/C) as
the frequency increases, but the losses don't go away. In fact losses of TLs
increase as frequency increases. Nice graphs of this in the Handbook.

73 de jack, K9CUN