>What does it mean that a cable (for networking, or TV, for instance) has an "impedance of 50 ohms", for instance?
>
>I understand that impedance (Z) is calculated as sqrt(R^2 + X^2). Resistance (R) is dependent on the length of the cable, and the cable might be cut to different lengths. Reactance (X), on the other hand, is frequency dependent. So what does the "50 ohms" (or whatever the value stated) actually mean?
>
>TIA, Hilmar.

A transmission cable may come in many different types and for different purposes. We have balanced and unbalanced cables. TV twin lead is an example of balanced cable. Microphones come in balanced and unbalanced versions. Audio often uses 600 ohms as a line impedance value. TV uses different impedance’s such as 300 ohm (balanced) and 72 ohm unbalanced. Transmitters often use 50 ohm unbalanced line. There are some exceptions to these numbers and I am aware of them. Two much detail can be a pain.

Any transmission line (cable having impedance) must be terminated in a restive load equal to its characteristic impedance to provide the maximum transfer of power. If this is not done there will be a reflection of power which can produce heat as an example, with power amplifiers. This can cause breakdown of cable or equipment at worst and reduced efficiency as a minimum.

What becomes more interesting is the effect frequency has upon cable. If we are talking low frequency (audio) things are not too bad. As we go up in frequency cable may act as an antenna. A coaxial cable is said to not radiate energy. That is not true! The radiation of coaxial line with an rf (radio frequency) signal is about 40 db below the total transmitted power level. So engineers came up with “double shielded” coaxial cable. This provides perhaps 90 db loss but the cable is still a radiator or antenna if you will.

One measurement of rf power in a transmission line is called SWR. That stands for Standing Wave Ratio. It is the amount of power radiated back from the load – whatever the cable is terminated into. The ideal transfer of power occurs at an SWR of 1 to 1. That is an indication that the load is said to be “resistive”.

Now what do you do if you have “excessive SWR” in any cable? Such a device will not provide maximum exchange of power from the transmitter to the load. (Amplifier to speaker as an example) and there will be heat involved with the possibility of parts breaking down due to Z, C, and L. (Z = impedance, C = capacitance and L = inductance). As the phase angle of a property exceeds a limit due to mismatched conditions large spikes of voltage can destroy components.

A load does not have to be resistive for the maximum transfer of power to occur. If we are working with a limited range of frequencies we can build an impedance network for a transmission line using inductors and capacitors and this will provide a resistive load. This is used when we have reflections upon a cable. Another limitation of an impedance network is that the length of the cable cannot change once components are selected and the network installed.

There are many other factors to be considered and this is an interesting topic as I have been involved with it as an audio enthusiast, amateur radio operator and electronic engineer. I have also worked at Television and high power radio stations as well as the audio and video industries.

Thank you for your question - it has been a while since I thought of such things. It is so nice to know there is something else in life besides programming and computers. :) Perhaps that is why I play flamenco guitar.

Tom