Impedance is an important subject in amateur radio so we want to spend a little time discussing it here. Several topics on this site will involve impedance so it’s good to have this basic concept well understood.
In ham radio work we deal with impedance in transmission lines, antennas, transmitter outputs, receiver inputs, microphones, speakers, headphones, and other devices. Impedance matters everywhere a signal couples to something different.
Basic resistance (R) is what opposes current in a DC circuit, and all components have measurable resistance.
But things get more complicated in AC circuits. Capacitors and inductors (coils) oppose change. This includes alternating current, a characteristic of audio, video and radio frequencies. The properties of capacitance and inductance have well-defined opposition to AC which varies by signal frequency.
All components have measurable capacitance and inductance so there is always some reactance (X) in a circuit. There are two flavors of reactance: capacitive and inductive. Interestingly, they respond oppositely to signal frequency. Inductive reactance (XL) goes up with frequency while capacitive reactance (XC) goes down.
When you add the constant resistance in a circuit to the capacitive and inductive reactance, the result is impedance (Z=R+jX). In broad terms, it can be considered “AC resistance”, which is legitimate when we don’t care about the complex phase angle part of the equation. Resistance plus reactance equals impedance (Z).
Like DC resistance, impedance (AC resistance) is measured in ohms.
OK so far? Click on the many hyperlinks in this article for more detail, along with helpful links below. Don’t worry, you only need to grasp the basics here; high-level math is not necessary for a working knowledge of impedance.
Now that you know what impedance is, the next important thing to understand is that when an AC signal interfaces with a new circuit, the impedances should match.
When impedance of a source (ZS) equals the load impedance (ZL), the best possible signal coupling occurs. Conversely, when impedances are not the same, signals couple poorly.
The maximum power-transfer theorem says that to transfer the maximum amount of power from a source to a load, the load impedance should match the source impedance (ZS=ZL).
Good examples of impedance matching are: audio amplifier output to speaker (8Ω); transceiver RF circuits to antenna feed line (50Ω); microphone to audio input (2000Ω).
Impedance matching can be accomplished by circuit design, by using transformers, LC filters, or even by use of antenna feed line.
Again, the main reason for matching impedance is for optimum coupling from one device or circuit to another.
A detailed article on impedance from ham radio school is here. 3 parts; click on subsequent links.
A good textbook reference is found here.
And an interesting video from a ham perspective is here.