More about the ImpedanceConcept

The concept of impedance is very important in antennas and transmission lines. Remember that impedance is the ratio of voltage to current. Let's look at what happens when we apply a radio frequency voltage to one end. When the voltage is first applied, a certain amount of current will flow. What ever the ratio voltage to current turns out to be is called the characteristic impedance of the transmission line. If we apply 10 volts and find that initially .2 amps flows, then 10/. 2 = 50. The impedance of the line is 50 ohms. If the line was 186,000 miles long we would be able to measure this .2 amp current for about two seconds. After however long it takes the applied voltage and current to reach the end and be reflected this current may change. If the transmission line is connected to an impedance equal to the characteristic impedance, then there will be no reflection and the current remains at its initial value. You will also hear the term surge impedance used interchangeably with characteristic Impedance. Usually you will hear it shortened to just impedance. Most of us will be using coax for transmission line and it will probably have an impedance of 50 ohms or somewhere close to that. Coax is also frequently found with 75-ohm impedance. In order to get as much of your power as possible into the antenna so it can be radiated, you need to have the antenna impedance as close to the transmission line's impedance as possible. If the antennas impedance does not match that if the line, you can use a simple matching network. The main idea is to present a load to the transmission line so there is a minimum mismatch and minimum reflected voltage and current where the line connects to the antenna.
If there is a reflection it causes standing waves to be set up on the transmission line. Usually I like to see the standing wave ratio less than 2:1.
The higher you go in frequency, the more important it is to have a low SWR. We will discuss SWR in more detail at a later date.