The Basic lessons · Radio wave propagation · B-007-004

Why HF Signals Fade and How Ionospheric Layers Interact

Learn how the daytime D region absorbs low-frequency signals and why multi-path travel causes fading. Discover practical fixes for signal loss during ionospheric disturbances and how bandwidth affects reception quality.

This lesson is part of SignalPrep Pro. The subsection's 11 exam questions are free to practise: practise B-007-004.

What this subsection asks

• What effect does the D region of the ionosphere have on lower frequency HF waves in the daytime?
• Why can you not hear distant 160-metre and AM broadcast stations during daytime hours?
• A radio transmission may follow two or more different paths during propagation, and this may result in phase differences at the receiver. What is the effect at the receiver?
• While using a 2-metre hand-held transceiver in an urban setting, you notice that moving less than one metre can severely attenuate your received signal. What is the likely cause?
• A transmitted radio signal reaches a receiver by both one-hop and two-hop skip paths. What can small changes in the ionosphere cause?
• What can be done to continue HF communications during a sudden ionospheric disturbance (SID)?
• On the VHF and UHF bands, the polarization of the receiving antenna in relation to the transmitting antenna is very important, yet on HF bands it is relatively unimportant. Why is that so?
• What causes selective fading?
• How does the bandwidth of a transmitted signal affect selective fading?
• What effect do refraction, reflection and Faraday rotation have on a radio wave?
• If a radio transmission follows two or more different paths during propagation, the received signal may degrade due to fading. What other type of degradation can occur?