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?