Simplex (direct) UHF signals are rarely heard beyond the radio horizon because the ionosphere does not normally reflect them back to Earth. Long-distance HF and some VHF signals can travel far by bouncing off the ionosphere, but the much shorter wavelengths at UHF pass through the ionosphere into space rather than being refracted back down. That is also why UHF frequencies are useful for satellite communication. As a result, when you hear a UHF station directly (not via a repeater or satellite), the transmitter is usually within line-of-sight or only a little beyond the radio horizon due to local propagation enhancements.

Memory aids:

• Higher frequency = straighter path (UHF passes through the ionosphere)
• UHF works well for satellites — because it goes through the ionosphere, not bouncing off it

Last edited by jones0575. Register to edit

Tags: none

HF (high frequency) signals commonly travel long distances by reflecting or refracting from the ionosphere. The HF portion of the spectrum (roughly 3–30 MHz) can be bent back toward Earth by ionized layers in the upper atmosphere, producing skywave or "skip" propagation that can carry signals hundreds or thousands of miles. By contrast, VHF and higher frequencies are mostly line-of-sight and do not reliably use ionospheric hops for long-distance contacts.

Because of the lower frequencies, HF antennas are generally larger, HF signals tend to support narrower bandwidths for a given antenna/system design, and atmospheric and man-made noise is often greater at HF — all of which help explain why the standout characteristic of HF compared with VHF+ is the prevalence of long-distance ionospheric propagation.

Memory aids:

• Think of VHF as very "sharp" waves that tend to pass through the atmosphere — line-of-sight.
• Think of HF as "dull" waves that can bounce off the ionosphere — long-distance.
• "Skip" and "skywave" are common terms used to describe ionospheric hops on HF.

Last edited by jishmukherjee@outlook.com. Register to edit

Tags: none

Auroral backscatter is caused by irregularities in the ionosphere created by energetic particles during auroral activity. Those irregularities rapidly change the path of VHF radio waves, causing rapid amplitude and phase fluctuations and spreading the signal in time and frequency. The result is a distorted received signal that often has a characteristic raspy or fluttering sound as the signal is scattered and Doppler-shifted by moving ionospheric structures. The rock-in-a-pond-and-flashlight analogy helps: the flashlight beam dancing on the ripples is like the radio signal being scattered by the moving auroral ripples.

Memory aids:

• Think of throwing a rock in a pond and shining a flashlight on the ripples to visualize the scattered, fluttering signal
• Auroral-scattered signals often sound raspy or fluttery on VHF
• Rapid changes in amplitude and frequency are typical of auroral backscatter

Last edited by camplate. Register to edit

Tags: none

Sporadic E is the propagation mode that most commonly produces occasional strong signals on the 10-, 6-, and 2-meter bands from beyond the radio horizon. It happens when localized, intensely ionized clouds form in the E region of the ionosphere (typically about 90–120 km altitude). Those ionized patches can refract VHF signals back toward the Earth, allowing signals on these frequencies to travel much farther than line-of-sight for short periods.

Other propagation phenomena do not fit the described behaviour: backscatter tends to scatter energy in many directions rather than produce strong, focused over-the-horizon signals; D-layer absorption attenuates signals rather than refracting them for long-distance VHF paths; and gray-line propagation is a twilight effect that mainly affects lower HF bands and is not the typical cause of the occasional strong VHF openings on 10, 6, and 2 meters.

For more information see: "Sporadic E, Es Propagation" on Electronics-Notes.com: https://www.electronics-notes.com/articles/antennas-propagation/ionospheric/sporadic-e-es.php

Memory aids:

• "Sporadic" = occasional
• 10, 6 and 2 are (E)ven → Sporadic (E)

Last edited by liezlwimmer. Register to edit

Tags: none

Radio waves generally do not pass easily through solid obstacles like rock or earth. When a wave encounters a sharp obstacle or edge, diffraction can occur: the edge acts like a secondary source and the wavefront bends around the obstacle so energy can reach locations that are not in the direct line of sight. This is called knife-edge diffraction and explains how signals can be heard on the far side of a hill or building.

Other phenomena mentioned in the question are unrelated to getting a signal around an obstruction. Faraday rotation affects the polarization of waves (typically in the ionosphere), quantum tunneling is a microscopic effect in certain electronic devices and not a propagation mechanism for macroscopic radio signals, and Doppler shift changes the received frequency when the source or receiver is moving relative to the other — it does not allow signals to go around obstacles.

Memory aids:

• A knife can cut things in the way — think "knife-edge" for diffraction around obstructions.
• "Obstruction, knife-edge" (short cue to recall the effect).

Last edited by vance.mahoney1. Register to edit

Tags: none

Tropospheric ducting occurs when a layer of warm air overrides cooler air near the surface, creating a temperature inversion that traps radio waves between layers of the troposphere and the ground. This creates a "duct" that guides VHF and UHF signals over distances well beyond line-of-sight — commonly out to a few hundred miles (around 300 miles) on a regular basis, especially in summer and autumn.

Other long‑range mechanisms do not match this behavior for VHF/UHF. Tropospheric scatter can also carry signals beyond the horizon but is most effective at higher UHF/microwave frequencies (around 2 GHz) and is more random. Ionospheric refraction (D- or F-region effects) is important for HF skywave propagation but does not reliably refract VHF frequencies back to Earth for routine 300-mile paths. Faraday rotation refers to rotation of a signal's polarization as it passes through the ionosphere and does not extend range.

More information: https://en.wikipedia.org/wiki/Tropospheric_propagation

Memory aids:

• "Ducting = duct = channel between layers"

Last edited by benmacy1. Register to edit

Tags: none

Meteor scatter communication uses reflections from ionized trails left by meteors as they pass through the upper atmosphere. Those short-lived ionized trails are effective reflectors at VHF frequencies around the 6-meter band (about 50 MHz). The 6-meter band has a wavelength and propagation characteristics well matched to typical meteor ionization trails, and it is relatively quiet, making it easier to detect the brief, weak signals from distances of a few hundred to around 1,500 miles. Much longer wavelengths are not efficiently reflected by these small, transient ionized trails, and higher-frequency (shorter-wavelength) bands tend to be noisier or require different conditions, so they are less well suited for routine meteor-scatter contacts.

Memory aids:

• The "6" in 6 meters looks like a meteor with a curved tail

Last edited by cfadams. Register to edit

Tags: none

Tropospheric ducting is an atmospheric effect caused by a layer in the lower atmosphere where temperature changes with height in a way that bends or traps radio waves. A temperature inversion (a layer of warmer air above cooler air) produces a refractive index gradient that can refract radio waves back toward the Earth, creating a "duct" that guides signals over unusually long distances. Signals traveling in such ducts can propagate hundreds of miles — commonly 300 to 500 miles and sometimes as far as 1000 miles — well beyond normal line-of-sight range.

The troposphere is the lowest layer of the atmosphere and is where temperature inversions commonly occur; knowing this connection explains why temperature inversions cause tropospheric ducting.

Further information: http://en.wikipedia.org/wiki/Tropospheric_propagation#Tropospheric_ducting

Memory aids:

• Think of heating "ducts" in a building that carry different-temperature air long distances from a central unit, just like temperature inversions that cause tropospheric ducting.

Last edited by chilty. Register to edit

Tags: none

Long-distance propagation on 10 meters is primarily via refraction from the F region (particularly the F2 layer), which requires sufficient ionization. Sunlight is the main source of that ionization, so the band is generally best during daylight hours — from dawn to shortly after sunset. Higher sunspot activity increases ionization of the F region, improving 10 meter propagation and making long-distance openings more likely and stronger. Near the equator, the ionosphere can be sufficiently ionized for 10 meter propagation even during periods of low solar activity, but in general the best conditions occur during daylight and especially when sunspot activity is high.

Memory aids:

• "10 meters follows the sun" — best in daylight hours
• High sunspot activity ⇒ greater F-region ionization; openings may begin before sunrise and extend into the evening

Last edited by kd7bbc. Register to edit

Tags: none

Ionospheric refraction depends on the amount of ionization in the F region, which increases near the peak of the sunspot cycle. When the F region is highly ionized, its critical frequency rises and can refract higher HF signals back to Earth. The 10-meter band (around 28 MHz) and the 6-meter band (around 50 MHz) are within the HF/VHF range that can be bent by a strongly ionized F region, so they can support long‑distance (skywave) propagation during solar maximum.

Frequencies in the UHF and microwave ranges — for example 1.25 meters (~222 MHz), 70 centimeters (~430 MHz), and 23 centimeters (~1.2 GHz) — are much higher. Those frequencies are generally too high to be refracted by the ionosphere and instead pass through into space, so they do not rely on F‑region propagation for long‑distance contacts.

Memory aids:

• Higher frequency = less ionospheric bending; lower HF bands (meters) are more likely to be refracted.
• 10 m (~28 MHz) and 6 m (~50 MHz) can be affected by the sunspot cycle; UHF/SHF (70 cm, 1.25 m, 23 cm) usually are not.

Last edited by u98cne2qkykjgcbzrdsqzj2b6qk=. Register to edit

Tags: none

Because the atmosphere refracts radio waves slightly more than it refracts visible light, radio waves bend a bit and follow the Earth's curvature farther before they are blocked. That extra bending makes the radio horizon extend beyond the visual horizon — in effect the Earth "appears" a little less curved to radio waves than to visible light, so VHF and UHF signals can be received at greater distances.

Memory aids:

• Nothing (known) travels faster than light, so a greater radio horizon is not due to higher signal speed.
• Dust can affect radio waves, but that does not produce a consistent increase in horizon distance.

Last edited by kd7bbc. Register to edit

Tags: none