What is Impedance? (And Why Radios Care)

The Big Picture

Every part of your radio — the transmitter, the feedline, the antenna — has a property called impedance. Think of it as "how much a circuit resists or stores AC energy."

If impedances don't match between parts of your radio system, power gets reflected back instead of going where you want it. That's why understanding impedance matters — it's the foundation of getting power from your radio to your antenna efficiently.

Resistance vs Reactance vs Impedance

Let's break this down with a water analogy:

Resistance (R) is like a narrow section of pipe — it restricts flow and converts energy to heat. A resistor does this. Measured in ohms (Ω).
Reactance (X) is like a stretchy balloon or a heavy flywheel in the pipe — it stores energy temporarily and releases it. Capacitors and inductors do this. Also measured in ohms.
Impedance (Z) is the total opposition — resistance and reactance combined.

Why this matters on air: Your transmitter expects to "see" 50 Ω of impedance. Your coax is 50 Ω. If your antenna isn't close to 50 Ω, you get reflected power — that's what SWR measures.

The Two Types of Reactance

Inductors (coils of wire) oppose changes in current. Their reactance increases with frequency:

\( X_L = 2\pi f L \)

In plain English: the higher the frequency, the more an inductor blocks the signal. That's why inductors are used in low-pass filters — they let low frequencies through but block high ones.

Capacitors (two plates separated by an insulator) oppose changes in voltage. Their reactance decreases with frequency:

\( X_C = \frac{1}{2\pi f C} \)

The higher the frequency, the easier it passes through a capacitor. That's why a capacitor in series blocks DC but passes RF.

Putting It Together: The Impedance Formula

Engineers write impedance as:

\( Z = R + jX \)

The "j" is just a mathematical way to keep track of the reactive part separately from the resistive part. Think of it as two dimensions:

The resistive part (R) is the "real" part — energy actually consumed
The reactive part (jX) is the "imaginary" part — energy stored and returned

How to Find the Total Impedance

The total impedance is found using Pythagoras (since R and X are at right angles):

\( |Z| = \sqrt{R^2 + X^2} \)

Example: An antenna has R = 50 Ω and X = 25 Ω. The total impedance magnitude is √(2500 + 625) = √3125 ≈ 56 Ω. Not a perfect match to 50 Ω coax, but close enough for an SWR under 1.5:1.