Multipath propagation

VHF signals often reach a receiving antenna by more than one route — for example a direct path plus reflections from the ground, buildings, water, or layers in the atmosphere. Each path has a slightly different length so the same radio wave can arrive with different phase. When those components add at the receiver they do so vectorially; they can reinforce each other (constructive interference) or cancel each other (destructive interference). A small physical movement of the antenna changes the relative path lengths by a fraction of a wavelength, which can flip the interference from constructive to destructive or vice versa. Because VHF wavelengths are on the order of a meter to a few meters, moving an antenna only a few feet is enough to produce large changes in received signal strength.

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Vegetation absorbs UHF and microwave signals, much like it absorbs light and heat. Water in leaves and branches and the complex structure of foliage cause signal absorption and scattering, so signals passing through or into trees are attenuated. This effect is especially noticeable at higher frequencies and can cause poor reception or block long-range point-to-point links when a path goes through trees or heavy foliage.

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Long-distance VHF and UHF contacts (DX) are usually made using horizontal polarization. Many long‑range agreements use antenna setups oriented horizontally (for example, horizontally mounted Yagi arrays) because horizontal polarization tends to work better for the common long‑distance propagation modes used on VHF/UHF, and matching the other station's polarization gives the strongest signal.

Memory aids:

• Think “horizon‑tal” for over‑the‑horizon (long‑distance) contacts.
• Polarization must match between stations to get the best signal; use the same polarization the other stations are using.

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When two antennas in a line-of-sight VHF or UHF link have different polarizations, the receiving antenna cannot pick up the full transmitted field. Polarization describes the orientation of the electric field of the radio wave; for maximum power transfer the receive antenna must have the same orientation as the transmitted field. If the polarizations are mismatched, only the component of the transmitted field that aligns with the receive antenna is received, so the received signal strength is reduced. In the extreme case of perfectly orthogonal (90°) linear polarizations, the ideal received power would be zero.

Note that the question specifies a line-of-sight path because reflections can change a wave's polarization. If the signal is reflected, its polarization may be altered so the simple receive/transmit polarization relationship can be different. If you experience weak reception, changing antenna orientation (tilting or rotating it) can often improve the signal by better matching polarizations.

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A directional (beam) antenna concentrates radiated energy into a narrower beam, so you can aim the main lobe at a specific direction. If direct line-of-sight to the repeater is blocked by buildings or other obstructions, you can aim the antenna at a nearby surface (roof, wall, water, metal structure) that will reflect the signal toward the repeater. The reflected path can carry enough energy to reach the repeater even though the straight-line path is obstructed. This is effectively using a deliberate multipath reflection to get around obstacles.

Memory aids:

• Think of it like playing pool: if you can't hit the target directly, aim at a cushion at the correct angle so the ball bounces to the target.
• Surfaces that are large, flat, and conductive (metal roofs, large windows, calm water) make good reflectors for VHF/UHF signals.

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“Picket fencing” describes rapid flutter or fading of a received signal as a mobile station moves. The effect is caused by multipath propagation: radio waves reach the receiver by multiple paths (direct and reflected), and as the relative phases of those paths change with movement they alternately add and cancel, producing quick rises and drops in signal strength.

Memory aids:

• Imagine watching a light through a picket fence: it appears and disappears as the gaps and slats pass — the same flicker happens to a signal experiencing multipath.
• Often heard as a rapid flutter while driving with a handheld or mobile radio.

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Higher radio frequencies are more strongly absorbed and scattered by water and hydrometeors (raindrops, snowflakes, fog). At microwave frequencies, precipitation causes significant attenuation of the signal by converting RF energy into heat and by scattering the waves, which reduces range.

Other weather factors listed have little direct effect on microwave propagation (wind might move or misalign antennas but doesn't absorb the signal; barometric pressure and modest temperature changes do not cause the same absorption/attenuation as precipitation).

Memory aids:

• Think of a household microwave oven: it uses a frequency that is readily absorbed by water to heat food. That same tendency for microwaves to be absorbed by water explains why rain, snow, or heavy fog reduce microwave radio range.

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Fading due to random combining of signals arriving via different paths is called multipath fading. It occurs when a signal reaches the receiver by more than one route — for example, direct plus one or more reflections from mountains, buildings, or the ionosphere. Each path has a different length, so the signals arrive with different phase relationships.

When two or more of these signals arrive out of phase (for example, about 180° apart) they can cancel each other and cause the received signal strength to drop (fade). When they are more nearly in phase they can add and cause brief increases in signal strength. If the relative path lengths change with time (due to movement of the transmitter, receiver, or reflecting regions), the combining changes and the fading appears irregular.

Multipath can also cause distortion when the phases are neither fully aligned nor fully opposite, since different parts of the signal can be shifted differently.

Memory aids:

• Think “multipath” = multiple paths = signals arriving at different times and phases, causing constructive and destructive interference.

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Skywave signals (signals reflected or refracted by the ionosphere) can be used to communicate beyond the horizon, at intercontinental distances.

Signals that have been propagated by the ionosphere are elliptically polarized, which means the wave has both vertical and horizontal components. Because of those components, either a vertically polarized antenna or a horizontally polarized antenna can pick up some of the transmitted energy.

If a signal were strictly horizontally polarized, a vertically oriented antenna would receive very little of the signal (and vice versa). Elliptical polarization avoids that severe mismatch by providing both components.

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When the same signal propagates over multiple paths the different paths will generally be slightly different in distance and angle. As a result, the signal arrives at the destination from multiple directions at slightly different times. Even small timing and phase differences cause the arriving signals to combine in ways that distort the original waveform — a phenomenon called multipath distortion. For data signals this distortion corrupts the information carried by the waveform, producing bit errors and therefore increasing error rates.

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The ionosphere is the upper portion of Earth's atmosphere that becomes ionized by solar radiation. That ionization creates a layer of charged particles that can reflect (or refract) high-frequency (HF) radio waves back toward the ground. Because of this property, HF signals can travel beyond the horizon by “bouncing” off the ionosphere and returning to Earth, enabling long-distance skywave propagation.

Memory aids and mnemonic/analogy:

• Think of the ionosphere like a mirror in the sky: HF radio waves hit it and are sent back down, allowing stations far beyond line-of-sight to communicate.
• "Ionosphere = ionized mirror for HF."

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The important principle is that absorption of radio-frequency energy by water (in fog or rain) increases with frequency. At relatively low VHF and HF frequencies, water droplets do not absorb much energy.

Both the 10-meter band (around 28 MHz) and the 6-meter band (around 50 MHz) have wavelengths long enough that fog and light rain do not significantly absorb or attenuate the signals. Therefore, fog and light rain have little effect on signals in those bands.

Memory aids / mnemonics:

• Think of a microwave oven: its frequency is chosen to be strongly absorbed by water so it heats food. 10-meter and 6-meter frequencies are far lower, so they wouldn’t heat your leftovers much — and similarly, fog and light rain don’t absorb them much.

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