Physics & RF
B20 Signal-to-Noise Ratio — Study Guide
This guide covers Signal-to-Noise Ratio 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 |
|---|---|
| SNR | Signal-to-Noise Ratio |
| DF | Direction Finding |
| dB | Decibel |
Key facts
What does EMS stand for here?
Electromagnetic Spectrum
A signal is…
an electrical or EM wave carrying information
Noise is…
unwanted interference that degrades signal quality
Jamming that spreads noise across a wide range of frequencies at once:
Barrage jamming
SNR formula in dB:
SNR = signal (dBm) − noise floor (dBm)
To measure the noise floor, you measure…
with the signal suppressed
Scenario: signal −70 dBm, noise floor −95 dBm. SNR?
25 dB
Scenario: signal −90 dBm, noise floor −100 dBm. SNR?
10 dB
SNR below 0 dB means…
signal is weaker than noise — decoding unlikely
SNR 0–6 dB means…
Detectable — rough bearings, decoding marginal
SNR 6–12 dB means…
Basic decode — simple modes work, voice poor to fair
SNR 12–20 dB means…
Reliable decode — good for most comms and DF
SNR above 20 dB means…
Clean — high-quality audio, high-rate data, strong DF
Acceptable SNR for reliable radio communication generally ranges…
20–30 dB
20 dB SNR equals a power ratio of…
100
DF rule: SNR above 15 dB gives…
a solid DF lock
DF rule: SNR below 10 dB gives…
a wobbling bearing
DF rule: SNR below 6 dB gives…
an unreliable or false bearing
Scenario: your DF bearings jump around. First thing to check?
Your SNR
Receiver sensitivity is…
the minimum signal level a receiver can detect and process
Scenario: a transmitter outputs 100 W through coax with 3 dB loss. Power at the antenna?
About 50 W
Where does the 'missing' power in lossy coax go?
Mostly heat in the cable
Scenario: you're stuck in a multipath null. Quick fix?
Move the receiver a little
Spatial diversity uses…
two antennas separated by some distance
Polarization diversity uses…
antennas with different orientations (vertical + horizontal)
A jammer's goal is to…
push the victim's SNR below its operational threshold
Scenario: the victim needs +10 dB SNR to decode; your jamming drops it to 0 dB. Result?
Success — decoding fails
Scenario: you can't raise signal power. How else can you improve SNR?
Move away from generators, power lines and vehicles
Simple rule: every 3 dB…
doubles your effective signal quality
Scenario: your radio needs −112 dBm; you receive −77 dBm. Link margin?
35 dB
When logging an emitter, always record…
both signal power and noise floor
Interference where signals have the same frequency and synced phase:
Coherent
Noise-like interference that distorts and lowers SNR:
Incoherent
Overlapping signals that amplify each other:
Constructive interference
Overlapping signals that cancel each other:
Destructive interference
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