17.2 Multipath, Fading & Doppler Effects

17.2 Multipath, Fading & Doppler Effects

Quick Answer: Multipath = same signal via multiple routes → phase cancellation/boost → fading. Flat fading hits the whole channel; selective fading warps spectrum inside the channel. Knife-edge diffraction bends energy over ridges into shadow. Ducting = tropospheric over-horizon VHF/UHF. Doppler shifts frequency when TX/RX move (aircraft). Faraday rotation twists linear polarization through the ionosphere.

Section 17.1 described ideal paths. Real commercial links—airport VHF, coastal MF, aircraft HF, shipboard UHF—almost never have a single clean ray. Element 3 and the practice bank emphasize multipath, fading, Doppler, and related propagation impairments that technicians must recognize before they retune or replace hardware.

Multipath propagation

Definition

Multipath occurs when a transmitted wave reaches the receive antenna by two or more paths of different lengths. Common contributors:

Because path lengths differ, arrival times and RF phases differ. At the antenna terminals the phasors add vectorially:

• Constructive interference — nearly in-phase → stronger signal (peaks).
• Destructive interference — nearly opposite phase → deep nulls (fades).

A small geometry change (tide height, aircraft bank angle, truck driving by, ionospheric height change) can swing a deep null into a peak in milliseconds to seconds.

Observable symptoms

Pool wording: multipath is signals arriving via multiple paths due to reflections, causing fading and distortion—not “multiple stations transmitting” and not “automatic diversity gain.”

Fading types

Flat fading

Flat (non-selective) fading means the entire occupied channel rises and falls roughly together. The channel’s frequency response stays relatively flat while overall amplitude changes. Causes include large-scale shadowing or multipath delay spreads that are small compared with the reciprocal of the signal bandwidth.

Effects: SNR swings, possible squelch chopping, but the spectral shape of the emission is not shredded. Narrowband FM voice often survives flat fades better than complex wideband data if the carrier remains above threshold most of the time (with capture effect helping on FM).

Frequency-selective fading

Selective (frequency-selective) fading occurs when multipath delay spread is large enough that different frequencies inside the same emission experience different phase shifts. Part of the sideband structure cancels while another part peaks.

Mitigation concepts (service literacy): narrower bandwidth, frequency diversity (change channel), space diversity (two antennas), adaptive equalizers on digital links, and avoiding multipath-rich geometry when siting antennas.

Slow vs fast fading (time scale)

Aircraft VHF can show both: slow as the jet climbs through pattern geometry, fast as multipath from ground reflections beats against the direct ray.

Knife-edge diffraction

When a sharp obstacle (mountain ridge, building edge, ship island structure) sits near the LOS path, geometric optics predicts a deep shadow. Real RF diffracts over the edge: secondary wavelets from the “knife edge” illuminate the shadow region.

Fresnel-zone clearance rules for microwave links are the engineered version of the same physics: keep the first Fresnel zone clear for reliable LOS; obstructed zones invite diffraction multipath and fades.

Tropospheric ducting and atmospheric bending

Ducting forms when refractive-index gradients in the troposphere (lowest atmosphere) trap VHF/UHF energy in a waveguide-like layer—often associated with stable high-pressure systems, temperature inversions, or over-water humidity ducts.

Contrast again: ducting ≠ F-layer sky wave. Ducting is tropospheric and mainly VHF/UHF; sky wave is ionospheric and mainly HF.

Other tropospheric effects include scattering and mild super-refraction that extend radio horizon slightly beyond geometric LOS even without a full duct.

Doppler shift on moving platforms

Definition (pool)

The Doppler effect is the change in observed frequency caused by relative motion between transmitter and receiver:

• Moving toward each other → observed frequency increases.
• Moving apart → observed frequency decreases.

Approximate radial shift magnitude:

[ \Delta f \approx f_c \times \frac{v_r}{c} ]

where (f_c) is carrier frequency, (v_r) is radial relative speed, and (c) is speed of light. Higher carriers (UHF/microwave) and higher speeds (jets) produce larger hertz shifts.

Aviation and maritime relevance

Service misdiagnosis: a “transmitter off-frequency” complaint from only high-speed aircraft may be Doppler, not a bad crystal—compare with a stationary ground station on the same channel.

Multipath + Doppler

When multipath rays have different radial velocities (direct path vs ground bounce), each ray can have a different Doppler, spreading the spectrum (Doppler spread). That creates time-varying selective fades—hard on digital demodulators and a reason mobile aeronautical channels need robust designs.

Faraday rotation (overview)

Faraday rotation is the progressive rotation of a linearly polarized wave’s electric-field orientation as the wave travels through a magnetized plasma—the ionosphere in Earth’s magnetic field.

You will not derive magnetoionic equations on Element 3, but you should recognize: ionospheric paths can change polarization, so a pure “vertical-to-vertical always matches” assumption fails on long sky-wave and space links.

Putting impairments on the bench and bridge

Troubleshoot in this order before condemning a transceiver:

1. Is the path LOS or HF sky wave? Wrong expectation → wrong fix.
2. Moving antennas a few feet change multipath nulls—classic proof of spatial standing waves.
3. Spectrum analyzer / second receiver — selective fade vs true transmitter splatter (splatter is continuous spectral mess; multipath notches move).
4. Aircraft-only pitch shift — consider Doppler and clarifier/AFC, not only ovenized oscillator failure.
5. Weather-linked VHF DX — ducting/interference, not “new super antenna.”
6. Polarization — try alternate antenna sense on satellite/HF problems that look like deep fade with strong field meters elsewhere.

Exam-day multipath & Doppler checklist

1. Multipath = multiple paths → phase interference → fading/distortion.
2. Flat fading = whole channel level; selective fading = in-band spectral warping.
3. Knife-edge diffraction = energy into geometric shadow over a sharp edge.
4. Ducting = tropospheric VHF/UHF enhancement/interference, weather-driven.
5. Doppler = frequency shift from relative motion (toward = higher f).
6. Faraday rotation = ionosphere rotates linear polarization; mitigate with circular/dual polarization.
7. Do not confuse path physics with spurious emissions or receiver IMD—those are the next section’s radio-practice duties.

When the path is hostile, good engineering practice is to change geometry, frequency, bandwidth, or diversity—not only to turn the power knob to 100%. Next: interference duties, spurious emissions, identification, tolerances, and EMI control.