Physics & RF
B11 Impedance & VSWR — Study Guide
This guide covers Impedance & VSWR as part of Physics & RF. Start with the vocabulary, then try to answer each question before revealing the answer. Connect these facts to a real radio link or electrical circuit and practice the subject in the arcade when you are ready.
Acronyms first
| Term | Meaning |
|---|---|
| VSWR | Voltage Standing Wave Ratio |
| VNA | Vector Network Analyzer |
| Z | Impedance |
| X | Reactance |
| ZL | Load impedance |
Key facts
What does Z₀ (ZO) stand for?
Characteristic impedance of the transmission line
Impedance is total opposition to AC/RF, combining…
resistance (R) and reactance (X)
Which part of impedance turns energy into heat, like friction?
Resistance
Which part of impedance temporarily stores energy in electric/magnetic fields and gives it back?
Reactance
Reactance comes from which components?
Capacitors and inductors
Impedance matching ensures…
maximum power transfer from transmitter to antenna with minimal reflections
An impedance mismatch causes…
reflections, standing waves and loss
Which is NOT a result of reflected energy?
Increased transmission range
VSWR is the ratio of…
maximum to minimum voltage of the standing wave
The wave traveling from transmitter toward the load is the…
incident (forward) wave
The wave bouncing back from a mismatched load is the…
reflected wave
The stationary pattern formed by forward and reflected waves:
standing wave
A perfect VSWR is…
1:1
VSWR rating 1.0:1 – 1.3:1:
Excellent
VSWR rating 1.3:1 – 1.5:1:
Good
VSWR rating 1.5:1 – 2.0:1:
Acceptable
VSWR rating 2.0:1 – 3.0:1:
Poor
VSWR rating above 3.0:1:
Dangerous
About how much power is reflected at 1.5:1?
~4%
About how much power is reflected at 2.0:1?
~11%
Scenario: your meter reads 1.4:1 on a critical link. Verdict?
Good — the typical professional goal
Scenario: VSWR reads 2.5:1 and the radio automatically reduces power. This is called…
folding back
Scenario: VSWR reads 3.5:1. Main risk?
Damaging the transmitter's power amplifier from reflected heat
Scenario: VSWR reads 1.8:1 on a ham radio. Verdict?
Acceptable for most commercial/amateur use
To improve VSWR, match…
the line's characteristic impedance (Z₀) to the load (ZL)
Which is a practical fix for high VSWR?
Tune the antenna length; minimize cable length and connectors
Which tool is inserted into the transmission line to measure forward and reflected power?
VSWR (SWR) meter
Which high-precision instrument measures the reflection coefficient across a range of frequencies?
Vector Network Analyzer (VNA)
Inductance and capacitance are unavoidable natural effects in circuits, especially at high frequencies.
True
A higher VSWR number means a better match.
False
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