Frequency Selection

Part of Radio

Frequency selection determines which part of the radio spectrum to use for a given communication task — a choice that determines range, reliability, and equipment requirements.

Why This Matters

No single radio frequency works best for all purposes. A frequency perfect for local communication may be useless for long-distance work. The frequency that crosses continents at night may fail by day. The band that covers hundreds of kilometers in open country may be blocked by terrain in valleys and mountains. Choosing the right frequency is as important as having working equipment, and using the wrong frequency reliably fails.

The relationship between frequency and propagation is dictated by physics: the atmosphere, ionosphere, and Earth’s surface interact with radio waves in frequency-dependent ways that create dramatically different coverage patterns. Understanding these interactions — ground wave versus sky wave, ionospheric layers, skip zones, critical frequencies — allows you to predict which frequencies will work and which will not for any given path and time of day.

In a post-collapse communication system, frequency selection must be deliberate and systematic. You may have limited equipment, limited spectrum access, and no ability to experiment as freely as a modern amateur radio operator. Getting it right the first time requires understanding the underlying physics rather than guessing.

Propagation Modes

Ground wave propagation: at frequencies below about 2 MHz, radio waves follow the Earth’s curvature. The wave “creeps” along the surface, using the conductive Earth as a waveguide. Range depends on transmitter power and frequency — lower frequencies travel farther. A 1 MHz ground wave can cover 500–1,000 km with a kilowatt transmitter. AM broadcast stations (535–1705 kHz) use ground wave for their primary coverage areas. Ground wave is reliable and predictable but requires large, efficient antennas and significant transmitter power at low frequencies.

Sky wave (ionospheric) propagation: at HF frequencies (3–30 MHz), the ionosphere — charged atmospheric layers at 60–400 km altitude — can refract (bend) radio waves back to Earth. A signal launched upward at an angle may be bent back down to Earth hundreds or thousands of kilometers away. This “skip” or “skip propagation” enables transoceanic communication with modest power. The distance covered by one skip is typically 1,000–3,000 km; signals can skip multiple times for global coverage.

Line-of-sight propagation: at VHF (30–300 MHz) and above, radio waves travel in straight lines and do not follow curvature or bounce off the ionosphere (under normal conditions). Range is limited by the geometric horizon, roughly 4.12 × √(height in meters) kilometers for a given antenna height. A 10-meter antenna height gives about 13 km range; a 50-meter tower gives about 29 km. Useful for local networks, useless for long-distance work.

The Ionosphere and HF Propagation

The ionosphere is formed when solar ultraviolet radiation ionizes atmospheric molecules, creating layers of free electrons that interact with radio waves. The key layers:

D layer (60–90 km): exists only during daylight. Absorbs low HF frequencies (below about 10 MHz) strongly — this is why you cannot hear distant shortwave stations well during the day. Also absorbs MF signals during daylight, restricting AM broadcast stations to ground wave during daytime.

E layer (90–150 km): moderately ionized layer present during daylight. Reflects some HF waves, creating medium-distance skip (a few hundred kilometers). Sporadic-E occurs randomly, reflecting VHF signals to unusual distances.

F layer (150–400 km): the most important layer for long-distance communication. At night it merges into a single F layer; during the day it splits into F1 and F2. The F2 layer persists through the night and is responsible for the longest-distance HF propagation. Its maximum usable frequency (MUF) varies with solar activity, season, and time of day — from 8–10 MHz during solar minimum to 30+ MHz during solar maximum.

The critical frequency (foF2) is the highest frequency that the F2 layer will reflect straight up (vertically). The maximum usable frequency for a given path is approximately 3 times the critical frequency for paths at typical angles. During the day, the MUF is higher; at night, it drops. This means the best frequencies for long-distance daytime communication are in the upper HF range (14–28 MHz), while nighttime communication works better in the lower HF range (3–10 MHz).

Choosing Frequencies for Different Scenarios

Local communication (0–50 km): VHF FM (140–170 MHz) is ideal for portable radios, reliable, and simple equipment. HF ground wave on 3–4 MHz works well but requires larger antennas. For communities without VHF equipment, 7 MHz provides reliable local and regional coverage.

Regional communication (50–500 km): Lower HF (3.5–7 MHz) provides good ground wave coverage and will also work via near-vertical-incidence skywave (NVIS) — described below — to fill in dead zones.

Long-distance (500+ km): HF sky wave, frequency depends on time of day and solar conditions. As a starting point: during daylight hours, use 14–21 MHz; at night, use 3.5–7 MHz. These are reliable starting points that will work for most of the solar cycle.

Near-Vertical Incidence Skywave (NVIS): a technique using frequencies slightly below the MUF to launch the signal nearly straight up. It bounces back from the ionosphere and covers a circular area within about 0–500 km radius. No skip zone — coverage starts at the transmitter and extends outward. Useful for regional communication in mountainous terrain where line-of-sight is impossible. NVIS works at 3–7 MHz during the day, lower at night. The antenna for NVIS is a horizontal dipole at low height (λ/10 to λ/4 above ground) — the low height concentrates radiation straight up rather than toward the horizon.

Practical Frequency Planning

For a post-collapse community radio network, a practical frequency plan might include:

• Primary local: one VHF frequency (around 145 or 155 MHz) for portable/mobile use within the community
• Regional HF day: 7.060 MHz USB (upper sideband voice) or 7.030 MHz CW (Morse) for inter-community daytime communication
• Regional HF night: 3.760 MHz USB or 3.560 MHz CW for nighttime regional communication
• Long-distance HF day: 14.300 MHz USB for day contact with distant stations
• Emergency: 14.300 MHz USB (widely monitored internationally) and 2182 kHz AM (maritime distress)

All community members with radios should know these frequencies by memory and in writing. Post the list at every station. Test the frequencies regularly — propagation changes seasonally and with the solar cycle, and you want to discover problems during drills, not emergencies.

When a planned frequency is not working (no signals heard, high noise), first try frequencies 20–30% higher or lower. If the channel seems dead in both directions, the propagation path is closed — wait for conditions to change or switch to a different mode (e.g., from sky wave to ground wave on a lower frequency). Keep records of what frequencies work at what times of day and season; this log becomes invaluable for planning.