What's the Difference Between Static and Dynamic Cables?

By Douglas Krantz | Electronics

It's implied, and often just assumed, but electrical signal cables are used in two different ways. The difference between the two is the signal wavelength as compared to the cable length.

Dynamic - Waves Traveling Along the Cable: Compared to the wavelength of the signal being sent, the length of the cable is long. Like waves traveling in the ocean, the signal travels along the wires; the voltage/current, because of the speed-of-electricity in the cable, travels from source end of the cable to the receiving end of the cable in waves.

The voltage/current ratio in the cable is affected by the cable's characteristic impedance. Common values for the characteristic impedance are: 50 ohms, 75 ohms, 100 ohms, 110 ohms, 300 ohms, etc.

An example of this would be RS485 wiring, computer network wiring, radio and television transmission lines, etc.

Static - Long, Thin Capacitor: Compared to the wavelength of the signal being sent, the length of the cable is very short. Because the cable is so short compared to the wavelength of the signal, in the same instant of time, the voltage measured at both ends of the cable is always the same.

Think of a water tank requiring time to fill up or empty, the total capacitance of the wire takes time to fill up or empty.

• The longer the wire, the greater the added-up capacitance of the wire, and the longer it takes for the capacitor to charge-up or discharge.
• The less current provided by the signal source (power supply), the longer it takes for the capacitor to charge-up or discharge.
• The higher the frequency change of the signal source (power supply), the faster the capacitance of the cable has to charge-up or discharge.

An example of this would be RS232 signaling, fire alarm wiring, or the old fashioned, "Plain Old Telephone System (POTS) wiring".