Section 3.3: How Signals Travel #

When you toss a pebble into a pond, you see ripples spreading out in all directions. Radio waves behave in a similar way, but instead of traveling across water, they propagate through space. Let’s explore the fascinating ways radio signals travel from transmitter to receiver and the factors that can affect their journey.

Ground Wave Propagation #

Imagine you’re dragging a long rope across a field. The rope doesn’t travel in a straight line; it follows the contours of the ground. This is similar to how low-frequency radio waves travel along the Earth’s surface. This method of propagation is called ground wave.

• Most effective for frequencies below 2 MHz (e.g., the 160-meter band)
• Used by AM radio stations and time signal stations
• Can follow the curvature of the Earth for hundreds of miles
• Affected by the conductivity of the ground (travels further over seawater than dry land)

Line-of-Sight (LOS) Propagation #

Picture yourself on a hilltop with a friend. You can communicate easily because you can see each other. This is the essence of line-of-sight propagation.

• Common for VHF and UHF frequencies
• Signals travel in a straight line from transmitter to receiver
• Limited by the curvature of the Earth (typically up to about 100 miles)
• Can be extended by using higher antennas or repeaters on high points

However, even with line-of-sight signals, things aren’t always as simple as they seem. Multipath propagation occurs when signals reflect off various surfaces like buildings or mountains, causing them to take different paths to reach the receiver. When these multiple signals arrive slightly out of sync, they can either cancel out or reinforce each other, leading to signal strength varying greatly, sometimes just by moving your antenna a few feet!

But what if your line of sight to a distant repeater is blocked by buildings or other obstacles? In such cases, you might be able to communicate by finding a path that reflects your signal off a nearby surface, such as a building or a mountain, and redirects it toward the repeater. This technique takes advantage of reflections to bypass obstacles and can be particularly useful when using a directional antenna. By carefully aiming your antenna, you can often find a reflection path that allows your signal to reach the repeater, even when a direct line of sight isn’t possible.

This ability to use reflected signals adds another layer of flexibility to VHF and UHF communications, especially in urban environments where obstacles are common.

Picket fencing is a term used to describe the rapid fluttering or fading effect often heard when a mobile signal is affected by multipath propagation, especially at VHF and UHF frequencies. It sounds like the signal is cutting in and out as you move, similar to driving past a picket fence where the view alternates between clear and obstructed.

Knife-edge diffraction is another phenomenon that can help signals bend around obstacles. Imagine a sharp edge, like the top of a mountain or a building, standing between you and the signal source. The radio waves can “bend” around this edge and continue on to the receiver, making communication possible even when there’s no clear line of sight. This diffraction effect is particularly useful in mountainous or urban areas where obstacles are common. It explains why signals can sometimes be heard beyond an obstruction, a scenario known as Beyond Line of Sight (BLOS) communication.

Sky Wave Propagation #

Imagine bouncing a ball off the ceiling to reach someone across the room. Sky wave propagation works similarly, but instead of a ceiling, radio waves bounce off layers of the ionosphere.

• Most effective in the HF band (3 MHz to 30 MHz), i.e., the 80-meter to 10-meter bands
• Most important for long-distance (DX) communication on HF bands
• Signals can travel thousands of miles with multiple “hops”
• Highly variable depending on time of day, season, and solar activity
• Different layers of the ionosphere reflect different frequencies at different times

Sometimes, signals propagated by the ionosphere can experience irregular fading. This is often due to the random combining of signals arriving via different paths, which can cause them to interfere with each other, leading to unpredictable signal strengths.

For example, during periods of high solar activity, the F region of the ionosphere becomes more ionized, which allows for long-distance communication on bands like 10 meters and 6 meters. The best time for long-distance 10-meter band propagation via the F region is typically from dawn to shortly after sunset during these periods of high sunspot activity. This is because the F region is most effective at reflecting signals during daylight hours, particularly when the Sun is at its peak, enhancing ionization.

Another interesting propagation mode related to the ionosphere is auroral backscatter. When VHF signals encounter the aurora, they can be scattered back to Earth. However, these signals are often distorted and fluctuate in strength, making communication challenging but fascinating for those interested in space weather effects.

Near Vertical Incidence Skywave (NVIS) Propagation #

NVIS propagation is like throwing a ball almost straight up to have it come back down nearby. In radio terms, NVIS allows for short- to medium-range communication over rough terrain or when obstacles block the line of sight. It’s particularly useful in mountainous regions, forests, or for emergency communications.

• Best Bands: NVIS works primarily in the low HF bands such as 80 meters and 40 meters. It is most effective at frequencies between 2 MHz and 10 MHz.
• How It Works: NVIS signals are transmitted almost straight up (at steep angles) toward the ionosphere, which then reflects the signals back down over a radius of up to a few hundred miles. This method helps to cover areas where ground-wave or line-of-sight propagation would be ineffective.
• Ideal Conditions: The D layer of the ionosphere, which typically absorbs lower HF signals during the day, becomes less active at night. Thus, NVIS is often more effective in the early morning, late afternoon, and nighttime. It also tends to work better when solar activity is moderate, as too much ionization can prevent lower frequencies from reflecting back down.

Why Use NVIS? In regions with hills, forests, or urban environments where obstacles block direct signals, NVIS can provide reliable communication where line-of-sight or ground-wave propagation fails. Emergency response teams, military communications, and amateurs in remote areas often rely on NVIS to maintain local and regional communication links.

When setting up for NVIS operation, antennas should be low to the ground—typically 0.1 to 0.25 wavelengths high. For the 80-meter band, that means positioning the antenna only about 10 to 20 feet above the ground. A horizontal dipole or loop antenna works best for NVIS, as these create the high-angle radiation pattern needed to send signals upward.

Sporadic E Propagation #

Sporadic E propagation is another exciting phenomenon that can occur within the ionosphere. This mode is most commonly associated with the 10, 6, and 2-meter bands and can allow for strong signals from beyond the usual radio horizon.

• Sporadic E occurs when patches of intense ionization form in the E layer of the ionosphere, reflecting signals that would normally pass through this layer back to Earth.
• This can result in unexpectedly strong signals over distances of a few hundred to over a thousand miles, often leading to brief but powerful openings on these bands.
• Sporadic E propagation is most common during late spring and summer, with another peak in mid-winter.

This type of propagation can catch operators by surprise, as it allows for long-distance communication well beyond the normal line-of-sight range, making it one of the most intriguing and unpredictable propagation modes in the VHF and lower UHF bands.

Tropospheric Ducting #

Think of the way sound travels further on a foggy day. Tropospheric propagation is somewhat similar, involving the lower atmosphere. Tropospheric ducting is like nature’s own waveguide. When there’s a temperature inversion in the atmosphere, it can create a ‘duct’ that VHF and UHF signals can travel through for hundreds of miles. It’s like your signal hitching a ride on a highway in the sky!

• Affects VHF and UHF signals
• Can extend range beyond line-of-sight
• Enhanced by temperature inversions and weather fronts
• Can create temporary “ducts” that guide signals for hundreds of miles

Scatter Propagation #

Picture throwing a handful of pebbles at a wall. They scatter in various directions. Radio waves can behave similarly when they hit certain obstacles. Meteor scatter is one of the coolest ways to make contacts. When a meteor burns up in the atmosphere, it leaves a trail of ionized particles that can reflect radio waves. It’s like playing ping-pong with a shooting star! These reflections only last for a few seconds, but that’s often enough to make a contact if you’re quick.

• Types include ionospheric scatter, tropospheric scatter, and meteor scatter
• Can provide communication beyond line-of-sight
• Often results in weak but usable signals

The 6-meter band is particularly well-suited for meteor scatter communications. The ionized trails left by meteors provide a temporary but effective means to reflect radio signals, making it possible to communicate over distances well beyond the line of sight.

Factors Affecting Propagation #

Several factors influence how radio waves travel:

1. Frequency: Lower frequencies tend to follow the ground and diffract around obstacles better. Higher frequencies are more line-of-sight but can penetrate the ionosphere.

2. Power: Higher power can overcome path loss and noise, increasing effective range.

3. Antenna height and gain: Higher antennas often have better range. Directional antennas can focus energy in specific directions.

4. Time of day and season: The ionosphere changes dramatically between day and night and across seasons, affecting HF propagation.

5. Solar activity: Sunspots and solar flares can dramatically affect ionospheric propagation.

6. Terrain: Hills, buildings, and other obstacles can block or reflect signals.

7. Weather: Certain weather conditions can enhance or degrade propagation, especially at VHF and above. For example, precipitation, such as rain or snow, can absorb signals at microwave frequencies, reducing their range. However, for lower frequencies like 10 meters and 6 meters, fog and rain have little effect on signal propagation.

8. Vegetation: Trees and other vegetation can absorb UHF and microwave signals, weakening them as they pass through.

9. Atmospheric Refraction: The atmosphere can slightly bend radio waves, allowing VHF and UHF signals to travel slightly beyond the visual horizon. This is why the radio horizon for these signals is typically farther than the visual horizon.

Lastly, multi-path propagation can have significant effects on data transmissions. When multiple paths combine at the receiver, they can cause errors in the data, leading to an increase in error rates. This is particularly important when working with digital modes.

Understanding these propagation methods is crucial for successful amateur radio operation. By choosing the right frequency and mode for the conditions, you can communicate across town or across the globe.

Remember, part of the excitement of amateur radio is the variability of propagation. Conditions that make a contact possible one day might not exist the next. This unpredictability adds an element of challenge and discovery to every QSO (contact).

As you gain experience, you’ll develop a feel for which bands are “open” at different times. You’ll learn to read propagation forecasts, understand the impact of solar activity, and maybe even contribute to our collective understanding of radio wave propagation through your own observations and experiments. That’s the beauty of amateur radio—every operator is potentially a scientist, pushing the boundaries of our knowledge about how signals travel!