# Characteristic impedance - low vs high frequencies

S

#### [email protected]

Jan 1, 1970
0
Hi

Why characteristic impedance of transmission line isn't important at
low frequencies (1000kHz) and it is one of the main parameters in RF?
At high frequencies if a generator isn't matched with the impedance
of the line the reflection of the power can damage the genarator, why
isn't that the case at low frequencies? Or am I wrong about the
destroying the generator?
(example: If we connect high power audio amp to speaker we don't have
to worry about the characteristic impedance of the cable) Can
somebody explain this?

Thanks

A

#### Andrew Holme

Jan 1, 1970
0
Hi

Why characteristic impedance of transmission line isn't important at
low frequencies (1000kHz) and it is one of the main parameters in RF?
At high frequencies if a generator isn't matched with the impedance
of the line the reflection of the power can damage the genarator, why
isn't that the case at low frequencies? Or am I wrong about the
destroying the generator?
(example: If we connect high power audio amp to speaker we don't have
to worry about the characteristic impedance of the cable) Can
somebody explain this?

Transmission line effects are negligible if the line length is a tiny
fraction of a wavelength.

S

#### Stanislaw Flatto

Jan 1, 1970
0
Hi

Why characteristic impedance of transmission line isn't important at
low frequencies (1000kHz) and it is one of the main parameters in RF?
At high frequencies if a generator isn't matched with the impedance
of the line the reflection of the power can damage the genarator, why
isn't that the case at low frequencies? Or am I wrong about the
destroying the generator?
(example: If we connect high power audio amp to speaker we don't have
to worry about the characteristic impedance of the cable) Can
somebody explain this?

Thanks
Totally wrong.
Just ask a 'serious' electic company engineer on matching generators to
It is the physics that is involved, not beliefs or frequencies.

HTH

Stanislaw

E

#### Eeyore

Jan 1, 1970
0
Hi

Why characteristic impedance of transmission line isn't important at
low frequencies (1000kHz)

Who says it isn't ? That's the upper end of the DSL frequencies on a phone line
and it's very important to use transmissin line thinking for that to work.

Graham

E

#### Eeyore

Jan 1, 1970
0
(example: If we connect high power audio amp to speaker we don't have
to worry about the characteristic impedance of the cable) Can
somebody explain this?

Audio only goes to 20kHz. The wavelength @ 20kHz is 1.5 km. A speaker cable is
maybe 10m. There's no transmission line effect to consider here.

In long-distance telephony, the long length of the cables make transmission line
thinking important even at these audio frequncies.

Graham

S

#### [email protected]

Jan 1, 1970
0
Eeyore je napisao/la:
Who says it isn't ? That's the upper end of the DSL frequencies on a phone line
and it's very important to use transmissin line thinking for that to work.

Graham

Sorry, I meant to say 1000Hz not 1000kHz

P

#### Phil Allison

Jan 1, 1970
0
"Andrew Holme"
Transmission line effects are negligible if the line length is a tiny
fraction of a wavelength.

** WRONG !!

Same old ham radio bollocks trotted out as fact about a different area of
electronics.

........ Phil

P

#### Phil Allison

Jan 1, 1970
0
"Eeysore Fucking Pommy IDIOT "
Audio only goes to 20kHz. The wavelength @ 20kHz is 1.5 km.

** NOT AGAIN !!!

Can't the Graham Stevenson MORON ever get a

FUCKING decimal point right ?????

Or any other point right ??

........ Phil

P

#### Phil Allison

Jan 1, 1970
0
Why characteristic impedance of transmission line isn't important at
low frequencies (1000Hz) and it is one of the main parameters in RF?

** That is not a correct statement - just another myth from the mouths and
keyboards of fools. All signal and power cables have a characteristic
impedance and it can matter at low frequencies.

The most important fact about characteristic impedance is that IF the load
on the end matches the cable's characteristic impedance - the parallel
capacitance & series inductance of the cable no longer has any effect.

Capacitance in a mic cable 100 metres long can have a significant effect on
the high frequency output of a mic.

The inductance of a twin wire speaker cable of only 10 metres long will
cause a drop in level at 20kHz of several dB if the load impedance is 1 ohm.

....... Phil

E

#### Eeyore

Jan 1, 1970
0
Phil said:
"Eeysore Fucking Pommy IDIOT "

** NOT AGAIN !!!

Can't the Graham Stevenson MORON ever get a

FUCKING decimal point right ?????

Or any other point right ??

It's that damn Windows calculator !

Graham

J

Jan 1, 1970
0
J

Jan 1, 1970
0
P

#### Phil Allison

Jan 1, 1970
0
"John Fields"
A poor workman blames his tools.

** However, in this case the man's " tool " is clearly the problem.

........ Phil

J

#### Jasen

Jan 1, 1970
0
Audio only goes to 20kHz. The wavelength @ 20kHz is 1.5 km.

??? how do you get that? I get 10km for free space so probably 5 or 6
km for twisted pair - what propagation rate does a twisted-pair give?

S

#### [email protected]

Jan 1, 1970
0
Can someone recommend me a good book about that topic or a web site!

J

#### John Larkin

Jan 1, 1970
0
??? how do you get that? I get 10km for free space so probably 5 or 6
km for twisted pair - what propagation rate does a twisted-pair give?

I'd vote for 15 km.

Figure twisted pair at roughly 0.7, depending.

John

E

#### Eeyore

Jan 1, 1970
0
Jasen said:
??? how do you get that? I get 10km for free space so probably 5 or 6
km for twisted pair - what propagation rate does a twisted-pair give?

I get it using the MS calculator which I find very easy to accidentally move the
decimal point one place.

Graham

R

#### Rich Grise

Jan 1, 1970
0

Because in Phil's universe, everything is wrong? ;-)

Cheers!
Rich

R

#### Rich Grise

Jan 1, 1970
0
I get it using the MS calculator which I find very easy to accidentally
move the decimal point one place.

It's lame to blame the calculator. Man up, say, "I made a mistake", and

Cheers!
Rich

J

#### John Larkin

Jan 1, 1970
0
Because in Phil's universe, everything is wrong? ;-)

Cheers!
Rich

umm, good point.

John

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