How do Coaxial Cable 50 Ohm represent ?

Now I need to transfer 13.56MHz sinewave power from Class-E Power Amplifier to 50 Ohm Dummy Load through Coaxal Cable.

I studied about Coaxial Cable and I found something don't understand why .

I study about Coaxial Cable has it own impedance (normally 50 Ohm )

Is it represent It's own resistance of 50 Ohm at certain frequency range?

if it is true , A transmission line has resister on it , is it cause power loss on transmission line itself when current pass through it? why don't use near 0 Ohm cable?

If I make Cable by myself, can I Increase or decrease length for vary the resistance in Coaxial Cable?


I go study wiki but I really don't understand the reality about Cable impedance.

You need to understand transmission lines. I am not sure of the best source explaining it - try this **broken link removed**

While a coax will have some resistance and hence some loss, it is a lot less than 50 ohms. The 50 ohms is the "characteristic impedance". It will have a minimal effect on a 50 ohm terminated signal driven from a 50 ohm source. This will apply at all frequencies (within reason - until "real" effects start to cause unwanted side effects) and at any length.

Keith.

Just to add to Keith's excellent reply, the impedance of a cable is set by its physical make up, ratio of inner diameter of the "tube"to outer diameter of the "wire" and the dielectric constant of the insulator. The 50 ohms figure is the ratio of the voltage to the current (V/R = r) that will occur if a sine wave is sent down an infinite length of it.
Frank

In fact characteristic impedance has unit of Ohms but it is not a Ohmic impedance like resistor. His equation is : Zc= SQRT((R+jLw)/(G+jCw)).
SQRT stand for Square Root. R is the effective loss per meter of the wire constituting the line [Ohms/m] , L the inductance per meter of the wire constituting the cable [H/m], C the capacitor per meter between the two wire constituting the cable [F/m], G the conductance per meter of the dielectric between the two wire [S/m] in fact the loss of the dielectric. And finaly, w is the pulsation or 2 pi x frequency. These for parameters are called primary parameters of the line.
If the line is lossless R=G=0, the characteristic impedance is independant of the frequency that is Zc = SQRT(L/C) and it is often the case.

This notion of impedance is general and you can find a similar notion in waveguide (metallic hollow rectangular pipe) where we speak about imlpedance of mode.

These impedance are important when the signal have to travel from one guide to another guide : it's the impedance matching. The signal have to see the same medium to avoid any reflection.

keith1200rs said:

You need to understand transmission lines. I am not sure of the best source explaining it - try this **broken link removed**
While a coax will have some resistance and hence some loss, it is a lot less than 50 ohms. The 50 ohms is the "characteristic impedance". It will have a minimal effect on a 50 ohm terminated signal driven from a 50 ohm source. This will apply at all frequencies (within reason - until "real" effects start to cause unwanted side effects) and at any length.
Keith.

Click to expand...

Excellent response, Keith ...
I think the source you gave is a little bit confusing, so here is the same thing explained almost in layman's terms:
**broken link removed**

:wink:
IanP