Physics & RF
B19 Signal Path Loss — Study Guide
This guide covers Signal Path Loss 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 |
|---|---|
| FSPL | Free Space Path Loss |
| LM | Link Margin |
| PRX | Received power |
| dBm | Decibels relative to 1 milliwatt |
| C2 | Command and Control |
| SIGINT | Signals Intelligence |
Key facts
Path loss is…
the weakening of a signal from transmitter to receiver, mainly from spreading
FSPL formula from the slides:
20log₁₀(d) + 20log₁₀(f) + 32.44
As frequency increases, FSPL…
increases (smaller wavelengths are harder to collect)
As distance increases, FSPL…
increases
FSPL is caused by…
the wavefront spreading spherically, spreading the same energy over a larger area
FSPL assumes…
a clear LOS path with no reflections, scattering or absorption
Which is NOT a LOS path loss type listed?
Power factor loss
Ground reflection loss depends on frequency, angle of incidence, ground irregularities and…
electrical conductivity
Which ionospheric layer absorbs low frequencies and causes attenuation?
D-layer
Which layer is most important for long-range propagation?
F-layer
Fresnel zones (pronounced fray-NELL) are…
elliptical (cucumber-shaped) regions between Tx and Rx where RF energy travels
Fresnel zones are used ONLY for…
line-of-sight links
Which Fresnel zone is most critical?
Zone 1
Minimum clearance rule for the first Fresnel zone:
At least 60% clear
Scenario: a hilltop pokes into half of Fresnel zone 1 even though you can see the far antenna. Result?
Degradation — less than 60% clearance
Best fix for an obstructed first Fresnel zone:
Raise antenna height or reposition
Fresnel zones represent the transmitted wave itself.
False
Link margin formula:
LM = P_RX − receiver sensitivity
Scenario: signal detected at −85 dBm; enemy receiver needs −105 dBm. Link margin?
20 dB
Scenario: P_RX −70 dBm, sensitivity −100 dBm. Link margin?
30 dB
+3 dB means…
double the power
−3 dB means…
half the power
+10 dB means…
10× the power
−10 dB means…
1/10 the power
+20 dB means…
100× the power
+6 dB means…
4× the power
Scenario: an amp turns 5 W into 50 W. Gain?
10 dB
Scenario: a cable cuts 8 W down to 4 W. Loss?
3 dB
A negative dBm value is a negative amount of power.
False
Practice this module
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