Physics & RF
B18 Harmonics & Sub-harmonics — Study Guide
This guide covers Harmonics & Sub-harmonics 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 |
|---|---|
| f | The fundamental frequency |
| GUI | Graphical User Interface |
| TX | Transmit |
Key facts
The linear region is where an amplifier's output…
changes in direct proportion to the input
The non-linear region is where output…
no longer follows the input proportionally
Analogy for the linear region:
Steering at normal speed — small turns, predictable response
Analogy for the non-linear region:
Redlining an engine — more throttle, just more noise and heat
What pushes a system non-linear?
Overdriving, poor matching, exceeding design limits
Amplifier maxed out and can't increase output:
Saturation
Unwanted energy spread to nearby frequencies:
Splatter
EW bottom line: in the linear region…
you control the spectrum
Why does non-linear operation raise exploitation risk?
It creates extra emissions adversaries can detect and analyze
Harmonics occur at…
integer multiples of the fundamental
Sub-harmonics occur at…
fractions of the fundamental (f/2, f/3…)
Scenario: fundamental 100 MHz. The 2nd harmonic is…
200 MHz
Scenario: fundamental 100 MHz. The 3rd harmonic is…
300 MHz
Scenario: fundamental 150 MHz. The 4th harmonic is…
600 MHz
Scenario: fundamental 300 MHz. The 2nd sub-harmonic is…
150 MHz
Scenario: fundamental 300 MHz. The 3rd sub-harmonic is…
100 MHz
Scenario: fundamental 300 MHz. The 4th sub-harmonic is…
75 MHz
Scenario: you see peaks at 120, 240 and 360 MHz, each weaker than the last. Most likely?
One emitter at 120 MHz plus its harmonics
Harmonics typically ____ in amplitude as the order goes up.
decrease
Risk of not recognizing harmonics:
You may think there are multiple transmitters and jam the wrong frequency
Quickest and safest fix if YOU are radiating harmonics:
Reduce power or input
Filter that removes out-of-band harmonic energy:
Low-pass or band-pass filter
Sub-harmonics appear because of…
severe non-linearity, instability or faulty equipment (frequency division/bad sampling)
Which is UNDER the operator's control?
Sample rate/filter settings in the GUI, antenna placement, TX power, software reboot
Which is NOT under the operator's control?
Oscillator drift and clock instability
Harmonics go ____; sub-harmonics go ____.
up; down
Which is expected and often strong/obvious?
Harmonics
Which may travel FARTHER than the main signal and is often overlooked?
Sub-harmonics
EW takeaway: harmonics make you ____; sub-harmonics make you ____.
loud; visible from farther away
Both harmonics and sub-harmonics indicate the system has entered the non-linear region.
True
Practice this module
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