T3A

Fading, polarization and absorption

Explain why signals fade, why antennas must match polarization, and what absorbs or reflects them.

12 pool questions1 on the exam

Syllabus: Radio wave characteristics: how a radio signal travels, fading, multipath, polarization, wavelength vs absorption; Antenna orientation

Several paths add up, or cancel

A signal often arrives by a direct path and by reflections off buildings, hills or the ionosphere. The paths differ in length, so the waves arrive out of step: they add or cancel.

Schematic: the wavelength is exaggerated so the effect shows within a short move.

Antenna position

Two paths areCancelled

Move the antenna a few feet and the sum swings strong to weak.
Picket fencing: rapid flutter on a moving (mobile) signal.
Fading on ionospheric signals: random combining of paths.
Data: delayed copies corrupt bits, so error rates rise.
Blocked line of sight? Aim at a reflecting path instead.

RememberMultipath: reinforce or cancel, depending on a few feet of position.

Match polarization on line-of-sight paths

Polarization is the direction of the electric field. On a straight path, a transmitting and receiving antenna must be parallel. Cross-polarized (one vertical, one horizontal) reduces signal strength.

Line-of-sight VHF/UHF: the receiving antenna must be parallel to the transmitting one.

Tilt of receiving antenna0°

ResultSame polarization

Path	Polarization
Long-distance CW and SSB on VHF/UHF	Horizontal (FM and repeaters use vertical)
Via the ionosphere	Wave becomes elliptical, so either works

RememberSame orientation at both ends. Weak-signal VHF/UHF: horizontal.

Short waves are soaked up by water and leaves

Schematic: wave height after the barrier shows how much signal survives.

Conditions	Effect
Vegetation, UHF and microwave	Absorbs signals; weak ones suffer
Precipitation, microwave	Decreases range
Fog or rain, 10 m and 6 m	Little effect

RememberThe higher the frequency, the more water and foliage matter.

The ionosphere reflects HF

The ionosphere is a layer of charged air high above the Earth. It bends HF waves back down, so they come down far away. The troposphere (weather layer) and stratosphere do not do this.

Schematic side view. The ionosphere is a high layer of electrically charged air.

RememberHF reflects off the ionosphere.

Loading lesson…

Go deeper

Compendium articles on the ideas in this lesson, beyond what the exam asks.

Check what stuck

Practise the 12 pool questions for this group. Answers are shuffled, and every one comes with a one-line "why".

Practise T3A →

Path

Polarization

Long-distance CW and SSB on VHF/UHF

Horizontal (FM and repeaters use vertical)

Via the ionosphere

Wave becomes elliptical, so either works

Conditions

Effect

Vegetation, UHF and microwave

Absorbs signals; weak ones suffer

Precipitation, microwave

Decreases range

Fog or rain, 10 m and 6 m

Little effect