Radio Propagation Basics: Time of Day, Solar Activity, and Band Conditions

When you first get into ham radio or off-grid communication, it can feel like magic: some nights you work stations a thousand miles away on a few watts, and other days your 100 W HF rig barely makes it across the state.

The secret is radio propagation, how your signal moves through the atmosphere and over the Earth. Propagation changes with time of day, solar activity, and which band you choose. Once you understand those pieces at a high level, you stop guessing and start planning.

In this guide we will keep the science friendly, skip the heavy math, and give you practical rules of thumb you can actually use the next time you sit down at the radio.

What Is Radio Propagation, Really?

Radio propagation is just a fancy way of saying how radio waves travel from one antenna to another. For our purposes, there are three big paths to think about:

Ground wave, skywave, and line of sight

• Ground wave hugs the Earth’s surface. On lower frequencies (like MF and lower HF), it can follow the curvature of the Earth for a few hundred kilometers.
• Skywave is what HF fans love. Your signal goes up, hits charged layers in the ionosphere, bends (refracts), and comes back down far away — sometimes thousands of kilometers from you.
• Line of sight (LOS) is mostly what VHF/UHF and microwave signals do. They travel in straight-ish lines and are blocked by terrain, buildings, and the curvature of the Earth.

Most HF long-distance work is skywave. Most local VHF/UHF work (including repeaters and Meshtastic-style mesh networks) is line of sight. Once you know which path you are using, the rest of the puzzle starts to make sense.

Meet the Ionosphere: Your HF Mirror in the Sky

The ionosphere is a region of the upper atmosphere roughly 60–400 km up that is full of charged particles. Those particles bend HF radio waves back toward Earth and make DX possible.

The ionosphere is not a solid “shell.” It is more like several fuzzy layers that change all the time. For beginners, you mainly care about:

• D layer (~60–90 km) – Shows up in daylight, especially at low frequencies. It absorbs HF energy instead of reflecting it. That is why 80 m and sometimes 40 m can be terrible in the middle of the day.
• E layer (~90–140 km) – Can reflect some HF, especially the lower bands. Sometimes it gets “sporadic E” patches that open up short skip on 10 m, 6 m, and even VHF.
• F layer (~140–400 km) – The main workhorse for HF DX. At night it usually merges into a single F-layer; during the day it can split into F1 and F2.

These layers are powered by the Sun. Sunlight creates them, darkness lets them relax and fade. That is why time of day matters so much for HF propagation.

Time of Day and Your HF Bands

The ionosphere acts differently during the day and at night, which means each HF band has “happy hours” when it usually works best.

The table below is a rough, beginner-friendly guide for common ham bands under “normal” conditions. Real life will be messier, but this gives you a starting point.

Band	Best Time (Typical)	Typical Range	Notes
160 m / 80 m	Night, especially late night	Local to ~600 km; occasional DX	Big D-layer absorption kills daytime; great for nighttime regional nets.
40 m	Late afternoon, evening, night, early morning	Regional to continental	Daytime for closer work, night for longer hops; a true workhorse band.
30 m / 20 m	Daytime, including early evening	Long-haul, intercontinental	Often the best “anytime” DX bands, especially near sunrise and sunset.
17 m / 15 m	Midday when solar activity is decent	Long-haul DX, sometimes quieter	Can be fantastic during good solar conditions; dead during poor ones.
12 m / 10 m	Midday, high solar activity, sporadic E season	Short skip and DX	When open they are magical; when closed, they are really closed.
VHF (6 m, 2 m)	Anytime (local LOS); special openings	Line-of-sight; occasional long-distance	Mostly terrain-limited; 6 m can act like HF during special openings.

Cheat sheet by band

160 m and 80 m:

• Think of these as nighttime regional bands.
• Great for state, region, and emergency nets after dark.
• During the day the D-layer absorbs them and they get noisy and short-ranged.

40 m:

• Often good almost all day, just with different ranges.
• Daytime: good for in-state and nearby states.
• Night: can stretch out to hundreds to thousands of kilometers.

20 m and 30 m:

• The classic DX bands.
• Work well in daylight, at sunrise and sunset, and sometimes all night.
• Great choice when you want to talk across the country or across an ocean.

17 m, 15 m, 12 m, and 10 m:

• These are high HF bands that love an energized ionosphere.
• Best during daylight with decent to strong solar activity.
• 10 m especially can be dead for weeks, then suddenly wide open worldwide.

VHF/UHF (2 m, 70 cm, GMRS, etc.):

• Mostly line of sight; time of day matters less than terrain and antenna height.
• Nighttime can sometimes be a bit quieter noise-wise.
• Special weather patterns can create tropospheric ducting and long paths, but that is a bonus, not something to rely on.

Solar Activity: The Sun Is Your Friend (Mostly)

The Sun controls how “charged up” the ionosphere is. More charged particles usually mean better high-band HF propagation, up to a point.

When hams talk about solar activity and band conditions, they throw around a few common numbers and terms. You do not have to memorize everything, but it helps to know what people are talking about:

• Sunspots / solar cycle – More sunspots generally mean a stronger ionosphere on higher HF bands. The 11-year solar cycle decides whether 10 m is amazing or sad.
• Solar flux index (SFI) – Rough indicator of how ionized the upper atmosphere is. Higher SFI usually helps higher bands like 20–10 m.
• K index / geomagnetic storms – Measures how disturbed Earth’s magnetic field is. High K often means noisy or disrupted bands, especially for polar paths.
• Solar flares and radio blackouts – Big bursts from the Sun can cause sudden HF fadeouts, particularly on the sunlit side of Earth.

For a beginner, the simple version is: good solar activity (without major storms) helps the higher HF bands. Too much disturbance and everything gets messy.

Good solar days vs. bad solar days

On a good day for HF DX, you will typically see:

• Higher bands (15, 12, 10 m) open for several hours in the daytime.
• 20 m strong across long distances nearly all day.
• Reasonable noise levels and stable signals.

On a bad day, you might notice:

• High bands completely closed or only supporting very short-range contacts.
• 20 m weak or noisy, with signals fading in and out.
• Sudden short-term blackouts right after solar flares.

The good news is that even when “conditions” look poor on popular dashboards, you can often still make local and regional contacts on bands like 40 m and 80 m, especially if you optimize your antennas and expectations.

Band Conditions in Plain English

When people say “band conditions are good” or “the band is dead,” they are really talking about a mix of factors:

• Is the band open? — Can the ionosphere support a path at that frequency?
• Noise level — Atmospheric noise, man-made noise, and solar noise.
• Signal strength — How strong your signal is after losses and reflections.
• Fading (QSB) — How much signals rise and fall with time.

Two classic ideas help explain this: MUF and LUF.

• MUF (Maximum Usable Frequency) – The highest frequency that will still refract back to Earth over a given path. Above the MUF, your signal just heads into space.
• LUF (Lowest Usable Frequency) – The lowest frequency that still gets through with acceptable signal strength. Below the LUF, absorption and noise eat your signal.

For a given path and time, you ideally want to operate between the LUF and the MUF. In practice, you just try bands in order and see what works, but this explains why sometimes “more power” helps less than “different band.”

High vs. Low Bands: Pros and Cons

You can think of HF bands as roughly split into lower (160–40 m) and higher (30–10 m). Each group has tradeoffs.

Band Group	Pros	Cons
Low HF (160–40 m)	Great at night for local and regional coverage. Less dependent on high solar activity. Useful for emergency and NVIS-style communication.	More noise (static, power line hash). Daytime D-layer absorption can be brutal on 160/80 m. Physically larger antennas.
High HF (30–10 m)	Smaller antennas, often easier to install. Can support long-distance DX with strong, clear signals. Less atmospheric noise than the low bands.	Heavily dependent on solar activity and time of day. Can be completely “dead” when the ionosphere is weak. Shorter openings, more timing-sensitive.

Before You Key Up: A Simple 7-Step Checklist

When you sit down at the radio and want to know “Can I actually talk to anyone?”, use this quick checklist:

1. Decide your target distance. Local, regional, or long-distance DX?
2. Check the clock. Daytime, night, or near sunrise/sunset?
3. Pick a likely band. Low bands for night/regional, high bands for daytime/DX.
4. Look at current solar/geomagnetic info. Optional, but helpful.
5. Listen first. Tune around and see what you actually hear.
6. Adjust band or mode. If signals are weak, try another band or a more efficient mode.
7. Log what you learn. Make notes so you build your own “feel” for your location and antenna.

After you have done this a few dozen times, you will start to predict what will work just by looking outside, checking the time, and glancing at a band conditions page.

Practical Tips to Make the Most of Any Conditions

Understanding propagation is great, but you still need to get a signal out. Here are some practical ways to tilt the odds in your favor, even when band conditions are not ideal.

• Match band to mission. For emergency or regional coverage, plan on 40 m and 80 m at night. For daytime cross-country, aim for 20 m or higher when solar activity allows.
• Use the best antenna you reasonably can. Height and placement often matter more than raw power. For HF, a simple dipole up as high as you can manage beats a compromised antenna at ankle height.
• Lower your noise floor. Turn off noisy power supplies, chargers, and dimmers. Sometimes just moving your HF antenna away from the house helps a lot.
• Pick efficient modes. When conditions are marginal, CW and digital modes (FT8, JS8Call, etc.) can still get through when SSB struggles.
• Use spotting and propagation tools as hints, not rules. Online maps and dashboards are great, but your own ears and your own antenna are the final word.
• Log your own propagation “diary.” Note which bands work well at which times from your location. After a few weeks you will have your own personal band plan that beats any generic chart.

If you are new to antennas and want to explore more options, have a look at the Radio section and the broader Tech articles on BrokenSignal.tv for build ideas and hardware reviews that complement these propagation basics.

FAQ: Common Beginner Questions About Propagation

Do these propagation rules apply to CB, GMRS, and other non-ham services?

Yes. Propagation is a physics thing, not a ham-radio-only thing. CB sits close to 10 m, so it behaves like a high HF band that can support long-distance skip when the Sun is active or when sporadic E kicks in. GMRS and many handheld services live up in UHF, so they are mostly line-of-sight. Antenna height and terrain matter much more than the Sun, but special conditions can still produce surprise long-distance openings.

How do I know what bands are open right now?

The simplest way is to listen. Spin the dial and see what you can hear at various frequencies and distances. On top of that, you can use online tools like propagation dashboards, MUF maps, and spotting networks that show where other stations are making contacts. Just remember: if a dashboard says “poor,” but you are hearing strong signals and making contacts, then conditions are good for you.

Can I still make contacts when conditions look bad?

Usually yes, especially for local and regional work. You may not be able to work the world on 10 m, but you can often still cover your state or region on 40 m or 80 m. In rough conditions, focus on:

• Using lower-noise modes like CW or digital.
• Improving your antenna height and orientation.
• Trying a different band instead of just cranking up the power.
• Being patient — marginal openings come and go quickly.

Wrapping Up: Turn the “Bands” into a Plan

Radio propagation can seem mysterious at first, but you do not need a PhD in ionospheric physics to use it. If you understand that time of day, solar activity, and band selection all work together, you can choose the right band and the right time for the job.

Start simple: use low bands at night for regional coverage, high bands in daylight for longer paths, and keep an eye on solar conditions for bonus DX. Combine that with a reasonable antenna and a bit of listening, and you will be far ahead of most beginners.

When you are ready to go deeper into specific radios, antennas, and off-grid setups, explore the Radio, Meshtastic, and Tech sections here on BrokenSignal.tv. The more you experiment, the more this high-level propagation theory will turn into instinct — and the more often your calls will get solid answers back.