This page is part of the N0NJY General Class self-study course for Technician operators upgrading to General.
Propagation is the soul of HF operating. Every other skill — antenna building, transceiver operation, operating procedures — serves this one purpose: getting your signal from here to there. A skilled HF operator understands propagation well enough to make predictions, investigate failures, and exploit opportunities that a less informed operator will miss entirely.
This module goes beyond ionospheric theory. By the end of it you should be able to sit down at your station, check conditions in two minutes, decide which bands are worth trying and why, recognize the greyline window and use it deliberately, operate NVIS effectively for regional emergency communications, and understand how the solar cycle shapes your operating options across months and years — not just hours.
The single most common propagation mistake new HF operators make is tuning an apparently quiet band, hearing nothing, and concluding the band is dead. Sometimes that conclusion is correct. Often it is not. Before you give up on a band, you need to know why it appears dead, because the cause determines what to do next.
There are four distinct reasons a band can appear empty:
1. The band genuinely is not open for your desired path. This is the real thing — no ionospheric support for the distance and frequency you need. The MUF (maximum usable frequency) for your path is below the band you are trying to use. Solution: move down in frequency to a band that is open, or wait for conditions to change.
2. The D layer is absorbing your signal. On the lower HF bands (160, 80, and 40 meters), the D layer forms during daylight and absorbs signals before they can reach the reflecting F layer. This happens every single day and is completely predictable. A 40-meter DX contact that was easy at 0600 local time becomes impossible by 1000. This is not a band failure — it is the D layer doing exactly what it always does. Solution: either wait for sunset, move to a higher frequency that punches through the D layer absorption, or accept that your contacts on those bands will be regional rather than intercontinental during daylight.
3. A geomagnetic storm is disrupting the ionosphere. When the K-index rises above 4, propagation at mid and high latitudes can degrade severely or disappear entirely on some bands. The higher the latitude of the path, the worse the effect. A K-index of 5 or above during a major storm can produce a nearly complete HF blackout at high latitudes for hours. Solution: check the K-index before concluding the band is useless. If the K-index is elevated, the lower HF bands (40 and 80 meters) often survive a storm better than the higher bands. Equatorial paths are less affected than polar paths.
4. A sudden ionospheric disturbance (SID) from a solar flare has blacked out HF. A major solar flare releases a burst of X-ray radiation that reaches Earth in about eight minutes and causes intense ionization of the D layer on the sunlit side. The result is an HF blackout that can last from minutes to hours. If you were hearing stations clearly and the band went completely silent in seconds, suspect a flare. Check the NOAA space weather page for flare alerts. Solution: wait it out. The blackout is temporary. After a major flare, propagation often improves for a day or two as the ionosphere recovers and solar particles arrive.
You do not need to understand plasma physics to work DX effectively. You do need a working mental model of how the ionosphere behaves so you can predict what it will do under various conditions.
The ionosphere is a region of the upper atmosphere from roughly 60 km to 1,000 km altitude where solar radiation strips electrons from gas molecules, creating a region of free electrons that can refract radio waves. Think of it as a series of imperfect mirrors at different altitudes, each with different properties and different behavior based on time of day, solar activity, and season.
The D layer exists only during daylight. It does not reflect HF signals — it absorbs them, particularly on the lower HF bands. The lower the frequency, the worse the absorption. On 160 meters, D-layer absorption is so severe during the day that the band is essentially useless for anything beyond a few hundred miles. On 40 meters, D-layer absorption eliminates long-distance propagation during the day but allows it at night. On 20 meters, the D layer is much less of a factor because higher frequencies penetrate it more easily.
The D layer disappears within about 30 minutes of sunset and reforms within about 30 minutes of sunrise. This daily cycle is what creates the greyline propagation window described below.
The regular E layer forms during daylight and mostly disappears at night. It can support propagation for distances of roughly 1,000 to 2,000 km on HF. More practically important is Sporadic-E (Es) — irregular, intense patches of ionization that form in the E layer unpredictably. When Sporadic-E occurs, it can support propagation on frequencies well into the VHF range, sometimes to 150 MHz or higher. Es is most common in late May through early July and again in November through December in North America. It is the reason 6-meter operators are always watching their band even when conditions appear dead — an Es opening can materialize in minutes and produce signals from thousands of miles away that sound like they are next door.
The F layer is responsible for essentially all long-distance HF propagation. During daylight it splits into two sub-layers: the lower F1 (around 150–250 km) and the upper F2 (around 250–500 km). At night they merge into a single F layer. The F2 layer is the one that matters most for DX because it is the highest and therefore provides the longest single-hop distances — up to roughly 4,000 km per hop for high elevation angles, more for low angles.
The F layer persists at night, unlike the D and E layers, because the gas at that altitude is thin enough that ions and electrons recombine slowly. This is why long-distance propagation on 40, 80, and 160 meters is possible at night even though the D layer has been absent for hours — the F layer is still there doing its job.
When you transmit at a low angle toward the horizon, your signal travels upward at a shallow angle, hits the ionosphere, and is refracted back to Earth. The point where it returns is called the skip distance. Between your transmitter and that point is the skip zone — an area that receives neither your ground wave nor your sky wave. Stations in the skip zone cannot hear you at all, even if your signal is strong enough to be heard on the other side of the planet.
The skip zone is not fixed. It changes with frequency, time of day, season, and solar activity. As you increase frequency toward the MUF, the skip zone grows longer. As you decrease frequency, it shrinks. Understanding this is why experienced operators move up or down in frequency when they need to reach a specific distance — they are adjusting the skip to put the target location outside the skip zone.
The MUF is the highest frequency that will be refracted back to Earth for a specific path at a specific time. Signals above the MUF pass straight through the ionosphere and are lost to space. The MUF changes constantly with solar activity, time of day, season, and the specific path geometry. On a high-solar-activity day, the MUF for a transatlantic path might be 30 MHz or higher. On a low-activity day at solar minimum, it might be 12 MHz. Operating just below the MUF gives you the strongest signals because the ionospheric refraction is most efficient near that frequency.
The LUF is the lowest frequency usable for a given path before D-layer absorption becomes too severe. If you are below the LUF, the signal cannot survive the round trip through the D layer. Between the LUF and the MUF is the range of frequencies that will actually work for your path. In practice, operating at around 85% of the MUF gives reliable, strong propagation with reasonable margin against sudden changes in conditions.
Most new HF operators understand that solar activity affects propagation. What they often fail to internalize is that this effect operates on a timescale of years. The solar cycle is approximately 11 years long, moving from solar minimum (lowest activity) through solar maximum (peak activity) and back. Where you are in that cycle fundamentally shapes which bands are usable and what is possible on HF.
The SFI is measured daily at 2000 UTC by a radio telescope in Penticton, British Columbia, at a wavelength of 10.7 cm. It is a proxy for the sun's UV radiation output, which is what actually ionizes the ionosphere. The SFI correlates directly with ionospheric ionization: a higher SFI means a more heavily ionized ionosphere, which means higher MUF values and better propagation on the upper HF bands.
| SFI Range | What It Means in Practice | Bands to Focus On |
|---|---|---|
| Below 70 | Near solar minimum. Upper bands largely dead for DX. 10 and 15 meters may be open only briefly or not at all. | 40m, 80m, sometimes 20m |
| 70–100 | Moderate activity. 20 meters is reliable. 15 meters opens during the day. 10 meters unpredictable. | 20m primary, 15m daytime, 40m nights |
| 100–150 | Good conditions. 15 meters open reliably. 10 meters opens regularly. 20 meters excellent for DX. | All bands productive; upper bands rewarding |
| Above 150 | Excellent to outstanding. 10 and 15 meters can be open around the clock. Worldwide contacts on all bands. Rare DX is workable. | All bands; 10m and 15m are priorities |
The K-index is updated every three hours and measures geomagnetic activity on a scale of 0 to 9. It is derived from magnetometer readings at a network of ground stations. A K-index of 0 to 2 represents quiet conditions favorable for propagation. A K-index of 3 is unsettled. At 4, propagation at high latitudes begins to degrade. At 5 and above (a geomagnetic storm), propagation across polar paths can fail completely. At K=7 or above, even mid-latitude HF paths are severely disrupted.
The A-index is a daily average derived from K-index readings. Where the K-index tells you what conditions are right now, the A-index tells you whether the overall day has been disturbed. An A-index below 10 is quiet. Above 50 is severe storm conditions.
Geomagnetic storms arrive in two waves. The first is caused by the X-ray burst from a solar flare, which travels at the speed of light and reaches Earth in 8 minutes, causing a sudden ionospheric disturbance (SID). The second is caused by the coronal mass ejection (CME) — actual solar plasma — which travels much slower and arrives 1 to 4 days after the flare. The CME causes the sustained geomagnetic storm that can last for a day or two. After the storm passes, propagation often improves significantly as the ionosphere is energized by the enhanced solar particle flux.
At solar minimum, experienced HF operators shift their focus to the lower bands: 40, 80, and 160 meters. These bands propagate via lower layers of the ionosphere and are less dependent on high solar flux. At solar minimum, 40 meters at night can provide continent-spanning contacts reliably even when 20 meters is marginal and 10 meters is silent. New operators who get licensed near solar minimum sometimes conclude that HF is disappointing — and then discover a year later, as the solar cycle climbs, that 10 and 15 meters have come alive in a way they had not imagined possible.
At solar maximum, the upper bands demand attention. Ten meters at solar maximum can produce signals from the opposite side of the world that sound clearer than your local VHF repeater. Fifteen meters is open in multiple directions simultaneously. DX that is nearly impossible at solar minimum becomes routine. The key is knowing where you are in the cycle and adjusting your expectations and band choices accordingly.
The greyline is the boundary between the sunlit and dark hemispheres of Earth — the terminator that moves around the globe as the Earth rotates. For approximately 20 to 30 minutes on each side of the greyline passage at any given location, conditions exist that produce some of the best HF propagation of the day. Understanding and exploiting the greyline is one of the skills that most consistently separates experienced HF operators from everyone else.
The greyline works because of the D layer. During the day, the D layer absorbs lower HF signals. At night, it disappears. At sunrise and sunset, it is in transition. For the brief period while the D layer is either dissipating (at your location around sunset) or has not yet formed (around sunrise), the lower HF bands can propagate to distances normally impossible. The F layer is still present and reflecting signals, but the D layer that would normally absorb them is temporarily absent or greatly reduced.
Additionally, a station at your location can simultaneously reach stations that are in darkness (where D-layer absorption is absent) and stations that are in the greyline at the far end of the path. When both ends of a circuit are near the greyline simultaneously, the propagation can be exceptional.
Using the greyline is not complicated, but it requires preparation and punctuality. The window is narrow — often only 15 to 30 minutes at its peak. Here is the practical approach:
Step 1 — Know your greyline times. Your sunrise and sunset times change daily with the seasons. Use a greyline tool (listed in the resources section) to find the current greyline position and your local sunrise/sunset. The greyline map shows which parts of the world are currently in the window.
Step 2 — Know where you want to work. The greyline is most useful for reaching locations that are also near the greyline simultaneously, or that are in darkness while you are in the transition zone. A station in Central Europe and a station on the US East Coast can share a greyline window at certain times of year around their respective sunrises and sunsets.
Step 3 — Be on frequency early. Do not wait until your exact sunrise to start listening. Be on the band 15 minutes before sunrise and stay on for 15 minutes after. The strongest signals often arrive before the precise moment of sunrise as the D layer begins its final dissipation.
Step 4 — Use the right band. The greyline effect is strongest on the lower HF bands where the D layer normally causes the most damage. Forty meters is the primary greyline band for distances of 2,000 to 10,000 miles. Eighty meters can produce spectacular greyline openings to specific parts of the world at certain times of year. On 20 meters, the greyline is less dramatic because the D layer is already less of a factor on that frequency.
Step 5 — Work quickly. The window closes. If you hear a station from an unusual location, respond immediately. Do not spend five minutes adjusting your audio settings. The greyline waits for no one.
NVIS is arguably the most misunderstood propagation mode in amateur radio, and the misunderstanding runs in both directions. Some operators dismiss it as a compromise — a last resort when you cannot get a proper antenna in the air. Others treat it as a niche technique of limited value. Both views are wrong. NVIS is a deliberately engineered propagation mode with specific applications where it dramatically outperforms conventional low-angle HF propagation. Understanding it is especially important if you have any interest in emergency communications.
Normal HF propagation uses low radiation angles — signals launched nearly horizontally toward the horizon, which then skip off the ionosphere to distant points. This works well for distances beyond the skip zone, typically several hundred to many thousands of miles. But it creates a fundamental problem: there is a skip zone immediately around the transmitter where you cannot be heard at all. For a 40-meter station with typical low-angle propagation, the skip zone can be 200 to 500 miles wide. Stations within that radius are effectively unreachable by sky wave.
NVIS solves this by using nearly vertical radiation. The signal goes almost straight up, hits the F layer at a steep angle, and falls almost straight back down. The coverage area is a roughly circular region centered on the transmitter, typically extending from zero to about 300 miles (0 to 500 km) radius. There is virtually no skip zone. Every location within that circle can receive your signal, and you can hear every location within that circle.
For emergency communications after a hurricane, earthquake, or other disaster — exactly the scenarios where repeaters are down, infrastructure is damaged, and you need to communicate reliably across a multi-county or multi-state area — NVIS provides the coverage that no other HF mode and no VHF/UHF system can match without infrastructure.
NVIS works on the lower HF bands, specifically in the range of 2 to 10 MHz. The optimal frequency changes with time of day, season, and solar activity because the F layer height and density change with these factors. Use a frequency that is below the MUF for near-vertical paths — which is lower than the MUF for long-distance paths — but above the LUF where D-layer absorption kills the signal.
| Time of Day | Season | Recommended NVIS Frequency | Notes |
|---|---|---|---|
| Daytime | Summer | 7–10 MHz (40m, upper 40m) | D layer present; need higher frequency to punch through |
| Daytime | Winter | 5–8 MHz (60m channels, 40m) | Lower solar angle reduces D layer intensity |
| Nighttime | Any | 3.5–5 MHz (80m, 60m channels) | D layer absent; lower frequencies work well |
| Transition (dawn/dusk) | Any | 5–7 MHz | D layer forming or dissipating; monitor and adjust |
The 60-meter channels (covered in G1) were specifically chosen by the FCC because they propagate well via NVIS for regional emergency communication. Channel 1 at 5.332 MHz is particularly well suited for daytime emergency NVIS in most of North America. If you are in an ARES or RACES group, know these channels.
Here is the counterintuitive fact that most new operators struggle with: a low antenna is better for NVIS, not worse. The radiation pattern of a horizontal dipole changes dramatically with height above ground. At heights of one-half wavelength or more, most radiation goes toward the horizon — good for long-distance DX, terrible for NVIS. At heights of one-eighth to one-quarter wavelength, most radiation goes nearly straight up — exactly what NVIS requires.
For 40 meters (7 MHz), one-eighth wavelength is approximately 17 feet. One-quarter wavelength is approximately 34 feet. A 40-meter dipole hung at 15 to 25 feet above ground is an excellent NVIS antenna. A 40-meter dipole hung at 60 feet is a poor NVIS antenna but a good DX antenna. The same wire does completely different things depending on how high you put it.
Building the antenna: A 40-meter NVIS dipole is 66 feet of wire (468 / 7.2 MHz ≈ 65 feet, adjust to resonance) split at the center and fed with 50-ohm coax through a 1:1 balun. There is nothing special about it compared to any other dipole — the only special thing is the installation height.
Field deployment options:
Tuning and operation: Cut the dipole for the center of your intended operating range. For 40-meter NVIS, cut for 7.200 MHz (phone) or 7.050 MHz (digital/mixed). If you have an antenna tuner, you have more flexibility. Check SWR before transmitting. With a properly cut dipole at the right height, you should see SWR below 2:1 across most of the 40-meter band without a tuner.
TEP occurs when signals propagate across the geomagnetic equator via a mechanism distinct from normal F-layer skip. It is most common in spring and fall, primarily affects the 6-meter band (50 MHz) and occasionally 2 meters, and can produce contacts of 5,000 to 10,000 miles between stations located symmetrically north and south of the magnetic equator. For a US East Coast station, TEP provides occasional 6-meter contacts into South America that are simply impossible by any other mechanism. TEP is unpredictable but follows seasonal patterns — if you operate 6 meters, watch for TEP openings in April-May and September-October.
Sporadic-E produces some of the most dramatic propagation events on the HF and low-VHF bands. Dense patches of ionization form unpredictably in the E layer and can support single-hop propagation of 500 to 1,400 miles on 10 meters, 6 meters, and occasionally 2 meters. Multi-hop Es can extend this to transatlantic distances on 6 meters. Es is most common in late May through July in North America. When an Es opening occurs on 6 meters, signals often arrive suddenly and are extremely strong — seemingly local — from stations 600 miles away. The opening may last minutes to hours and then disappear as unpredictably as it arrived.
Meteors entering the atmosphere at high speed create brief ionized trails that can reflect radio waves. These trails last from milliseconds to a few seconds. Meteor scatter is primarily useful on VHF (50 MHz and higher) where the trails produce sufficient reflection. During major meteor showers (Perseids in August, Geminids in December), activity increases significantly. Modern weak-signal digital modes like MSK144 (from the WSJT-X software suite) are specifically designed for meteor scatter and can complete contacts using burst durations of fractions of a second.
Tropospheric ducting occurs in the lower atmosphere (troposphere, below about 10 km) when temperature inversions create a refractive layer that traps VHF and UHF signals in a duct, allowing them to travel far beyond line-of-sight distances. Ducting primarily affects 144 MHz, 432 MHz, and higher frequencies. It is particularly common over large bodies of water and in coastal areas, where temperature differentials between the warm ocean surface and cooler air aloft create stable inversion layers. During a duct event, signals on 2 meters can travel 500 to 2,000 miles with no more path loss than a local contact. Ducting can appear and disappear in minutes and is notoriously difficult to predict, though weather pattern analysis and tools like the Hepburn Tropo Forecast provide useful guidance.
During geomagnetic storms, the auroral oval expands toward lower latitudes and signals can reflect off the aurora borealis. Aurora propagation is primarily a VHF phenomenon and produces a distinctive, harsh buzzing or hissing signal quality — SSB audio sounds almost unreadable, CW is raspy and distorted. Despite the signal quality, aurora contacts can be made over distances of 500 to 2,000 miles. Aurora propagation favors paths oriented roughly east-west toward the auroral oval. Operators at latitudes of 40 degrees north and higher are most likely to observe it. When the K-index rises above 5, check 6 meters and 2 meters for aurora propagation while HF is disrupted.
The G3 subelement covers the solar cycle and propagation theory tested in the 2023–2027 FCC General Class question pool. The operator knowledge section above provides the context; this section provides the concise exam answers and all pool questions.
Q1 (G3A01) — What is the effect of a very low sunspot number on radio propagation?
Q2 (G3A02) — What effect does a sudden ionospheric disturbance have on the dayside of Earth?
Q3 (G3A03) — Approximately how long does it take the increased ultraviolet and X-ray radiation from solar flares to affect radio propagation on Earth?
Q4 (G3A04) — Which of the following are good indicators of approaching solar maximum?
Q5 (G3A05) — What is the solar-flux index?
Q6 (G3A06) — What is a geomagnetic storm?
Q7 (G3A07) — At what point in the solar cycle does the 20-meter band usually support worldwide propagation during daylight hours?
Q8 (G3A08) — Which of the following effects can a geomagnetic storm cause?
Q9 (G3A09) — What effect do high geomagnetic activity indexes have on radio communications?
Q10 (G3A10) — What causes HF propagation conditions to vary periodically in approximately 28-day cycles?
Q11 (G3A11) — How long does it take a coronal mass ejection to affect radio propagation conditions on Earth?
Q12 (G3A12) — What does the K-index indicate?
Q13 (G3A13) — What does the A-index indicate?
Q14 (G3A14) — How are radio communications usually affected by the charged particles that reach Earth from solar coronal holes?
Q15 (G3B01) — How might a sky-wave signal sound if it arrives at your receiver by both short-path and long-path propagation?
Q16 (G3B02) — Which of the following is a good indicator of the possibility of sky-wave propagation on the 6-meter band?
Q17 (G3B03) — Why is the F2 region mainly responsible for the longest distance radio wave propagation?
Q18 (G3B04) — What is the maximum distance along the Earth's surface that is normally covered in one hop using the F2 region?
Q19 (G3B05) — What is the maximum distance along the Earth's surface normally covered in one hop using the E region?
Q20 (G3B06) — What is the typical range of MUF values for F-region propagation?
Q21 (G3B07) — What does NVIS stand for, and what is its primary use?
Q22 (G3B08) — What is the reason a 40-meter antenna that is relatively close to the ground provides better NVIS coverage than one mounted at the optimal DX height?
Q23 (G3B09) — What is the approximate maximum distance along the Earth's surface that can be covered in one hop using NVIS propagation?
Q24 (G3B10) — Which HF frequency bands are best suited for NVIS propagation?
Q25 (G3C01) — Which ionospheric layer limits daytime radio communications in the MF range (300 kHz to 3 MHz)?
Q26 (G3C02) — What is meant by the terms "morning radio window" or "greyline" in reference to HF communications?
Q27 (G3C03) — Why is the ionospheric F2 layer the most useful for long-distance communications?
Q28 (G3C04) — What does the term "critical angle" mean, as applied to radio wave propagation?
Q29 (G3C05) — Which of the following propagation modes is most commonly associated with an aurora?
Q30 (G3C06) — Which of the following amateur bands typically allows propagation at distances of several thousand miles via transequatorial propagation?
Q31 (G3C07) — What makes Sporadic-E propagation different from regular E-layer propagation?
Q32 (G3C08) — What does the term "scatter" mean, as used to describe radio propagation?