T3A: Radio Wave Characteristics

Radio signals do not travel in a simple straight line from transmitter to receiver. They reflect, bend, scatter, and interact with the environment in ways that profoundly affect signal strength and quality. Understanding these behaviors helps you predict why a signal is strong in one location and weak a few feet away, why mobile signals flutter, why certain frequencies cut through obstacles while others do not, and how the atmosphere itself shapes what you can hear and how far your signal reaches.

T3A covers the key physical behaviors of radio waves in transit: multipath propagation and its effects, polarization and why it matters for antenna alignment, how vegetation and precipitation interact with different frequency ranges, how reflected paths can help you work around obstructions, and how the ionosphere influences HF and VHF signals through refraction and multipath fading.

Multipath Propagation and Signal Variability

When a radio signal travels from a transmitter to a receiver, it rarely travels along a single path. In any real environment — a city, a neighborhood, even an open field with nearby structures — signals bounce off buildings, the ground, vehicles, and other surfaces. The receiver picks up not one signal but several copies of the same signal, each having traveled a slightly different path and therefore arriving at a slightly different time and phase.

This is multipath propagation. The key consequence is interference between the copies: if the reflected copies arrive in phase with the direct signal, they reinforce it and the received signal is stronger. If they arrive out of phase, they partially or completely cancel each other and the received signal weakens or disappears. The critical detail for the exam is that this cancellation or reinforcement changes dramatically over very short distances. Moving a VHF antenna just a few feet can shift the phase relationships entirely, turning a dead spot into a strong signal location or vice versa. This is why VHF signal strengths can vary greatly when an antenna is moved only a few feet.

Picket Fencing

When a mobile station moves through an environment with multipath propagation, the received signal alternates rapidly between reinforcement and cancellation as the vehicle passes through successive zones of constructive and destructive interference. The audio result is a rapid, rhythmic flutter — a sound often described as resembling someone running a stick across a picket fence. This phenomenon is called picket fencing, and it is a characteristic artifact of multipath propagation on mobile signals.

Picket fencing is not caused by repeater equipment, antenna damage, frequency drift, or any malfunction. It is the normal physical result of a moving receiver in a multipath environment. Operators familiar with the term can immediately recognize the cause when they hear it.

Ionospheric Fading and Multipath

Signals propagated by the ionosphere also experience multipath effects, but the mechanism is different. The ionosphere is not a smooth reflective surface — it is a region of ionized gas with varying density, and signals can reach the same destination via multiple ionospheric paths of different lengths and angles. When multiple copies of the same signal arrive at a receiver after traveling different ionospheric paths, they combine randomly. The amplitude and phase relationships between these copies change as atmospheric conditions shift, producing the irregular fading characteristic of ionospheric propagation.

This type of fading is not caused by Faraday rotation, thunderstorm interference, or intermodulation. It is the direct result of randomly combining signals that have traveled different ionospheric paths — a signal-level version of the same multipath cancellation that causes VHF dead spots on the ground.

Multipath and Data Transmission Errors

Multipath propagation affects digital data transmissions in a particularly damaging way. When copies of the same signal arrive at slightly different times, they smear into each other — an effect called intersymbol interference. A bit that should be clearly a "1" gets contaminated by a delayed copy of a previous "0," making it harder for the receiving equipment to decode the signal correctly. The result is an increased error rate. Multipath propagation does not require you to increase or decrease transmission rates by a fixed factor, and FM does not immunize digital signals against multipath errors. The practical consequence is simply that error rates are likely to increase in multipath environments.

Polarization and Antenna Alignment

The polarization of a radio wave describes the orientation of its electric field. A vertically polarized wave has its electric field aligned vertically; a horizontally polarized wave has its electric field aligned horizontally. Antennas radiate and receive most efficiently when they match the polarization of the signals they are working with.

For long-distance CW and SSB contacts on VHF and UHF — the weak-signal modes used by operators pushing for maximum range — horizontal polarization is the established standard. This convention has developed because horizontal antennas at both ends of a link produce the best results for this type of operation, and the community-wide consistency ensures that stations can work each other reliably. Circular polarizations (right-hand or left-hand) are used in other applications, particularly satellite work, but horizontal is the norm for terrestrial weak-signal VHF and UHF.

When the antennas at opposite ends of a VHF or UHF line-of-sight link are not using the same polarization — one vertical, one horizontal, for example — the mismatch causes a significant reduction in received signal strength. The signal does not become inverted or echo-distorted; it simply arrives weaker because the receiving antenna is not aligned to efficiently capture the incoming wave's electric field orientation.

Ionospheric Polarization and Antenna Choice

When radio signals pass through the ionosphere, the ionized medium rotates their polarization — an effect called Faraday rotation — by an unpredictable amount that changes with ionospheric conditions. The result is that ionosphere-propagated signals arrive with elliptical polarization: the electric field traces an ellipse rather than a fixed straight line.

The practical consequence is that either a vertically or a horizontally polarized antenna can receive an elliptically polarized signal with reasonable efficiency. You do not need to match the exact polarization of an ionosphere-propagated signal the way you would for a direct line-of-sight link, because the ionosphere has already randomized it. Both transmitting and receiving operators can use whatever antenna orientation they prefer — the ionospheric rotation means polarization matching is not the critical factor it would be on a direct VHF link.

Using Signal Reflection Around Obstructions

When buildings or terrain features block the direct line-of-sight path between your station and a distant repeater, the direct path is not your only option. Radio signals can reflect off buildings, hillsides, water towers, and other structures. By aiming a directional antenna toward a reflective surface that has a clear path to the repeater, you can effectively route your signal around the obstruction. This technique — finding a reflected path — is a practical tool for making contacts in environments where the direct path is blocked.

Changing polarization does not solve a line-of-sight blockage, and increasing SWR is never a productive approach to any RF problem. Reflecting signals off available surfaces is the correct technique when the direct path is obstructed.

Vegetation Absorption at UHF and Microwave

Trees, shrubs, and other vegetation absorb radio signals, and the amount of absorption depends heavily on frequency. At UHF and microwave frequencies, vegetation is a significant obstacle. The water content in leaves and branches interacts with the electromagnetic wave, converting signal energy into heat. A dense tree line between your antenna and a UHF or microwave link can produce substantial signal loss.

This absorption effect is not diffraction, amplification, or polarization rotation — it is simply absorption of signal energy by organic material. Lower frequencies (HF, VHF) pass through vegetation with much less loss because their longer wavelengths interact less efficiently with the sizes of individual leaves and branches. At UHF and above, vegetation becomes a genuine propagation obstacle.

Weather Effects on Different Frequency Ranges

Precipitation — rain, snow, and wet weather — affects radio signals differently depending on frequency. At microwave frequencies (above roughly 10 GHz), precipitation causes significant signal attenuation. Raindrops are comparable in size to microwave wavelengths, making them effective absorbers and scatterers of microwave energy. Operators using microwave frequencies for links or amateur satellite work must account for rain fade in their link budgets.

At lower VHF frequencies — the 10 meter and 6 meter bands — fog and rain have little practical effect on signal propagation. The wavelengths are far too long relative to water droplet size for meaningful absorption to occur. Operators on these bands do not need to worry about weather-related signal attenuation the way microwave operators do.

The Ionosphere as a Refraction Region

The ionosphere is the region of the Earth's upper atmosphere — roughly 60 to 1000 kilometers in altitude — where solar radiation ionizes gas molecules, creating a layer of free electrons and ions. This ionized layer can refract (bend) radio waves that pass through or enter it at appropriate angles. For HF frequencies, the ionosphere is capable of bending signals back toward Earth, enabling long-distance propagation. For lower VHF frequencies, partial refraction is also possible under certain conditions.

The ionosphere is the correct answer for which region of the atmosphere can refract or bend HF and VHF radio waves. The stratosphere, troposphere, and mesosphere also exist and serve atmospheric functions, but radio wave refraction for HF and VHF propagation is a property of the ionosphere specifically. (The troposphere does enable a separate propagation mode — ducting — covered in T3C.)

T3A Practice Questions