the learning never stops!

High Frequency (HF) Radio




Principles of Radio Communications

Amplitude, frequency, and wave lengths


Figure: Radio Wave Properties, (Eddie's notes)

[Radio Communications, Chapter 1]

  • Radio waves belong to the electromagnetic radiation family, which includes x-ray, ultraviolet, and visible light. Much like the gentle waves that form when a stone is tossed into a still lake, radio signals radiate outward, or propagate, from a transmitting antenna. However, unlike water waves, radio waves propagate at the speed of light. We characterize a radio wave in terms of its amplitude, frequency, and wavelength.
  • Radio wave amplitude, or strength, can be visualized as its height being the distance between its peak and its lowest point. Amplitude, which is measured in volts, is usually expressed in terms of an average value called root-mean-square, or RMS.
  • The frequency of a radio wave is the number of repetitions or cycles it completes in a given period of time. Frequency is measured in Hertz (Hz); one Hertz equals one cycle per second. Thousands of Hertz are expressed as kilohertz (kHz), and millions of Hertz as megahertz (MHz). You would typically see a frequency of 2,345,000 Hertz, for example, written as 2,345 kHz or 2.345 MHz.
  • Radio wavelength is the distance between crests of a wave. The product of wavelength and frequency is a constant that is equal to the speed of propagation. Thus, as the frequency increases, wavelength decreases, and vice versa. Radio waves propagate at the speed of light (300 million meters per second). To determine the wavelength in meters for any frequency, divide 300 by the frequency in megahertz. So, the wavelength of a 10 MHz wave is 30 meters, determined by dividing 300 by 10.

The HF Spectrum


Figure: Radio Frequency Spectrum, (Eddie's notes)

[Radio Communications, Chapter 1]

  • The HF band is defined as the frequency range of 3 to 30 MHz. In practice, most HF radios use the spectrum from 1.6 to 30 MHz. Most long haul communications in this band take place between 4 and 18 MHz. Higher frequencies (18 to 30 MHz) may also be available from time to time, depending on ionospheric conditions and the time of day.



Figure: AM Signal Sidebands, (Eddie's notes)

[Radio Communications, Chapter 1]

  • Today’s common methods for radio communications include amplitude modulation (AM), which varies the strength of the carrier in direct proportion to changes in the intensity of a source such as the human voice. In other words, information is contained in amplitude variations. The AM process creates a carrier and a pair of duplicate sidebands — nearby frequencies above and below the carrier. AM is a relatively inefficient form of modulation, since the carrier must be continually generated. The majority of the power in an AM signal is consumed by the carrier that carries no information, with the rest going to the information-carrying sidebands.
  • You can think of the data as existing in the fat lobes of the signal where as the center has no space for any data at all. Shortwave radio enthusiasts can use either or both sidebands.

  • In a more efficient technique, single sideband (SSB), the carrier and one of the sidebands are suppressed. Only the remaining sideband — upper (USB) or lower (LSB) — is transmitted. An SSB signal needs only half the bandwidth of an AM signal and is produced only when a modulating signal is present. Thus, SSB systems are more efficient both in the use of the spectrum, which must accommodate many users, and of transmitter power. All the transmitted power goes into the information-carrying sideband.
  • In aviation we used to note our frequency as "Upper" or "Lower" to differentiate between the two. You would say, "transmitting 8060 upper," for example. All aircraft HF communication, outside of the military, seems to have gravitated to the upper sideband so we no longer need to say it. (It is assumed.)

Propogation Paths


Figure: Propagation Paths, from Radio Communications, Figure 1-8.

[Radio Communications, Chapter 1]

  • Propagation is defined as how radio signals radiate outward from a transmitting source. Radio waves are often believed to travel in a straight line like a stone tossed into a still lake. The true path radio waves take, however, is often more complex. There are two basic modes of propagation: ground waves and sky waves. As their names imply, ground waves travel along the surface of the earth, while sky waves “bounce” back to earth.
  • Ground waves consist of three components: surface waves, direct waves, and ground-reflected waves. Surface waves travel along the surface of the earth, reaching beyond the horizon. Eventually, surface wave energy is absorbed by the earth. The effective range of surface waves is largely determined by the frequency and conductivity of the surface over which the waves travel. Absorption increases with frequency.
  • Transmitted radio signals, which use a carrier traveling as a surface wave, are dependent on transmitter power, receiver sensitivity, antenna characteristics, and the type of path traveled. For a given complement of equipment, the range may extend from 200 to 300 km over a conductive, all-sea-water path. Over arid, rocky, non-conductive terrain, however, the range may drop to less than 30 km, even with the same equipment.
  • Direct waves travel in a straight line, becoming weaker as distance increases. They may be bent, or refracted, by the atmosphere, which extends their useful range slightly beyond the horizon. Transmitting and receiving antennas must be able to “see” each other for communications to take place, so antenna height is critical in determining range. Because of this, direct waves are sometimes known as line-of sight (LOS) waves.
  • Ground-reflected waves are the portion of the propagated wave that is reflected from the surface of the earth between the transmitter and receiver.
  • Sky waves make beyond line-of-sight (BLOS) communications possible. At certain frequencies, radio waves are refracted (or bent), returning to earth hundreds or thousands of miles away. Depending on frequency, time of day, and atmospheric conditions, a signal can bounce several times before reaching a receiver.

Line of Sight / Minimum Altitude


Figure: Line of sight, (Eddie's notes)

So, at what point does your VHF lose its air-to-ground capability? A little geometry:

( rp + h )2 = rp 2 + dlos 2

Where rp is the radius of the earth (20,025,643 feet at the equator), h is the height of the aircraft, and dlos is the line of sight distance from the aircraft to the horizon. Solving for dlos we get:

dlos = h ( 2rp + h )

Which gives us the line of sight distance available at minium altitudes:

Altitude (feet) Distance (nm)
10,000 104
20,000 147
30,000 180
40,000 208
50,000 233

You can use your VHF or HF for line of sight communications, but to go beyond the horizon, you will need to bounce an HF signal off the ground, water, or the ionosphere.

Sky Wave Propagation


Figure: Sky wave propagation, (Eddie's notes)

[Radio Communications, Chapter 2]

  • The ionosphere is a region of electrically charged particles or gases in the earth’s atmosphere, extending from approximately 50 to 600 km above the earth’s surface. Ionization, the process in which electrons are stripped from atoms and produces electrically charged particles, results from solar radiation. When the ionosphere becomes heavily ionized, the gases may even glow and be visible. This phenomenon is known as Northern and Southern Lights.
  • Why is the ionosphere important in HF radio? Well, this blanket of gases is like nature’s satellite, making HF BLOS radio communications possible. When radio waves strike these ionized layers, depending on frequency, some are completely absorbed, others are refracted so that they return to the earth, and still others pass through the ionosphere into outer space. Absorption tends to be greater at lower frequencies, and increases as the degree of ionization increases.
  • The angle at which sky waves enter the ionosphere is known as the incident angle. This is determined by wavelength and the type of transmitting antenna. Like a billiard ball bouncing off a rail, a radio wave reflects from the ionosphere at the same angle it hits it. Thus, the incident angle is an important factor in determining communications range. If you need to reach a station that is relatively far from you, you would want the incident angle to be relatively large. To communicate with a nearby station, the incident angle should be relatively small.

Ground station HF radio operators worry about the angle of incidence as a way of aiming their signals to a desired distance. Since aircraft are closer to the ionosphere, the achievable angles are far greater as is the achievable distance. The pilot doesn't need to worry about the angle, other than to know how the angle can be affected by atmospheric conditions.

More on that below, under Frequency Selection.

Ionosphere Layers


Figure: Ionosphere Layers, (Eddie's notes)

[Radio Communications, Chapter 2]

  • Within the ionosphere, there are four layers of varying ionization. Since ionization is caused by solar radiation, the higher layers of the ionosphere tend to be more highly ionized, while the lower layers, protected by the outer layers, experience less ionization. Of these layers, the first, discovered in the early 1920s by Appleton, was designated E for electric waves. Later, D and F were discovered and noted by these letters. Additional ionospheric phenomena were discovered through the 1930s and 1940s, such as sporadic E and aurora.
  • In the ionosphere, the D layer is the lowest region affecting HF radio waves. Ionized during the day, the D layer reaches maximum ionization when the sun is at its zenith and dissipates quickly toward sunset.
  • The E layer reaches maximum ionization at noon. It begins dissipating toward sunset and reaches minimum activity at midnight. Irregular cloud-like formations of ionized gases occasionally occur in the E layer. These regions, known as sporadic E, can support propagation of sky waves at the upper end of the HF band and beyond.
  • The most heavily ionized region of the ionosphere, and therefore the most important for long-haul communications, is the F layer. At this altitude, the air is thin enough that the ions and electrons recombine very slowly, so the layer retains its ionized properties even after sunset.
  • In the daytime, the F layer consists of two distinct layers, F1 and F2. The F1 layer, which exists only in the daytime and is negligible in winter, is not important to HF communications. The F2 layer reaches maximum ionization at noon and remains charged at night, gradually decreasing to a minimum just before sunrise.
  • During the day, sky wave reflection from the F2 layer requires wavelengths short enough to penetrate the ionized D and E layers, but not so short as to pass through the F layer. Generally, frequencies from 10 to 20 MHz will accomplish this, but the same frequencies used at night would penetrate the F layer and pass into outer space. The most effective frequencies for long-haul nighttime communications are normally between 3 and 8 MHz.

There are free electrons everywhere and when these electrons attach themselves to molecules in the atmosphere these molecules are said to be ionized. An ionized molecule is good for bouncing radio waves so an ionized layer of atmosphere is good for long distance communications. Where the knowledge shown here comes in handy is when selecting a frequency. During the day the lowest ionosphere is as ionized as it gets and a higher frequency (with the longest wavelength) bounces best. During the night this layer isn't so effective so you want to pass through it. So a lower frequency (with a shorter wavelength) will pass through the D layer on its way to the F layers where a better bounce can be had. That validates the old pilot's rule of thumb: the higher the sun, the higher the frequency.

Atmospheric Ionization Factors

[Radio Communications, Chapter 2]

  • The intensity of solar radiation, and therefore ionization, varies periodically. Hence, we can predict solar radiation intensity based on time of day and the season, and make adjustments in equipment to limit or optimize ionization effects.
  • Ionization is higher during spring and summer because the hours of daylight are longer. Sky waves are absorbed or weakened as they pass through the highly charged D and E layers, reducing, in effect, the communication range of most HF bands.
  • Because there are fewer hours of daylight during autumn and winter, less radiation reaches the D and E layers. Lower frequencies pass easily through these weakly ionized layers. Therefore, signals arriving at the F layer are stronger and are reflected over greater distances.
  • Another longer term periodic variation results from the 11-year sunspot cycle. Sunspots generate bursts of radiation that cause higher levels of ionization. The more sunspots, the greater the ionization.
  • During periods of low sunspot activity, frequencies above 20 MHz tend to be unusable because the E and F layers are too weakly ionized to reflect signals back to earth. At the peak of the sunspot cycle, however, it is not unusual to have worldwide propagation on frequencies above 30 MHz.
  • In addition to these regular variations, there is a class of unpredictable phenomena known as sudden ionospheric disturbances (SID), which can affect HF communications as well. SIDs are random events due to solar flares that can disrupt sky wave communication for hours or days at a time. Solar flares produce intense ionization of the D layer, causing it to absorb most HF signals on the side of the earth facing the sun.
  • Magnetic storms often follow the eruption of solar flares within 20 to 40 hours. Charged particles from the storms have a scattering effect on the F layer, temporarily neutralizing its reflective properties.

Required for Oceanic?

U.S. Requirement

[AC 91-70A, ¶3-3.c.] Notwithstanding the fact that pilots must comply with all regulations applicable to their flight, all aircraft operating over the high seas must equip suitable instruments and navigation equipment appropriate to the route to be flown (§ 91.703, ICAO Annex 2, section 5.1.1, and this chapter). The aircraft must also equip a functioning two-way radio to maintain a continuous listening watch on the appropriate radio frequency and establish two-way radio communications with the appropriate ATC unit (ICAO Annex 2, section It is not acceptable to depend on radio relay operations to satisfy this requirement.

ICAO Requirement

[ICAO Annex 2, §] An aircraft operated as a controlled flight shall maintain continuous air-ground voice communication watch on the appropriate communication channel of, and establish two-way communication as necessary with, the appropriate air traffic control unit, except as may be prescribed by the appropriate ATS authority in respect of aircraft forming part of aerodrome traffic at a controlled aerodrome.

  • Note 1. SELCAL or similar automatic signalling devices satisfy the requirement to maintain an air-ground voice communication watch.
  • Note 2. The requirement for an aircraft to maintain an air-ground voice communication watch remains in effect after CPDLC has been established.

[ICAO Annex 2, §5.1.1] Aircraft shall be equipped with suitable instruments and with navigation equipment appropriate to the route to be flown.

North Atlantic Requirement

[ICAO Doc 7030, §NAT, ¶3.4.1] Within the NAT Region, aircraft equipped for SATCOM voice shall restrict the use of such equipment to emergencies and non-routine situations. An unforeseen inability to communicate by voice radio constitutes a non-routine situation. Since oceanic traffic typically communicates through aeradio facilities, a SATCOM call due to an unforeseen inability to communicate by other means should be made to such a facility rather than the ATC centre unless the urgency of the communication dictates otherwise. Dedicated SATCOM telephone numbers (short codes) for aeradio facilities and air traffic control facilities are published in national AIPs.

Yes, you can use your SATCOM for position reporting if you really needed to, and yes, you do make position reports with CPDLC. But the requirement remains: you need the HF when beyond VHF coverage. You need at least one, you might need two if you don't have CPDLC or a qualified satellite voice system.

More about this: HF Requirement.

Frequency Selection


Figure: Effects of Skywave Paths, from HF Communications Transceiver Pilot's Guide, Figure 1-1.

[HF Communications Transceiver Pilot's Guide, pages 1-3 to 1-5]

  • Because HF radio waves depend upon the ionosphere for reflection, their propagation is affected by changes in the ionosphere. It is changes in the density of the electrically charged particles in the ionosphere which cause propagation to improve or deteriorate. Since the ionosphere is formed primarily by the action of the sun’s ultraviolet radiation, its thickness changes in relation to the amount of sunlight passing through it. Sunlight-induced ionization increases the particle density during the day and the absence of it reduces the particle density at night. At midday, when the sun’s radiation is at its highest, the ionosphere’s thickness may expand into four layers of ionized gas. During the nighttime hours, the ionosphere diminishes, normally merging into just one layer.
  • A good rule of thumb for the time of day is that the higher frequencies are best during daylight (10 to 29.9999 MHz) and lower frequencies work best at night (2 to 10 Mhz).
  • This rule of thumb can be explained by a mirror analogy. It is the electrically charged particles in the ionosphere which reflect or bend radio waves back toward earth like a mirror reflects light. Sunlight induces ionization and increases the density of these particles in the ionosphere during the day. The mirror becomes thicker and it reflects higher frequencies better. When the sun goes down the density of charged particles decreases and the ionosphere becomes a mirror that can only reflect lower frequencies in the HF band.

Higher frequencies are best when the sun is up (above 10 MHz) and lower frequencies work best when the sun is down (below 10 Mhz).


Figure: Maximum usable frequencies, (HF Communications Transceiver Pilot's Guide, Tables 1-1 and 1-2)

  • For any one particular frequency, as the angle at which an HF radio wave hits a layer of the ionosphere is increased, a critical angle will be reached from which the wave will just barely manage to be reflected back to earth. Waves entering at sharper angles than this will pass through this layer of the ionosphere and be lost in space (or may reflect off another layer of the ionosphere). Changing the frequency under the same conditions will change the critical angle at which the HF radio waves will be reflected back to earth. The highest frequency which is reflected back to the earth is called the maximum usable frequency (MUF). The best HF communications are usually obtained using a frequency as close to the MUF as possible since radio waves higher than this frequency are not reflected and radio waves lower than this frequency will be partially absorbed by the ionosphere.

If experiencing HF communications difficulty, select the highest available frequency based off the maximum usable frequency table.

  • You should also be aware of the possibility that you or the ground station you are calling may be in a quiet zone. The linear distance from the point of transmission to the point where the skywave returns to earth is called the skip distance. There may be a quiet zone between the end of the ground wave and the return of the skywave. No communication can take place in this area. At any time, day or night, there is a “window” of usable frequencies created by the reflecting properties of the ionosphere. At night this “window” will normally be in the lower range of HF frequencies, and during the day it will be in the higher range of frequencies.

If HF conditions have been good and a particular frequency appears to be unusable, that frequency's "bounce" may place the receiving station in a quiet zone. Pick a frequency of great enough difference to change the "bounce."

What Can You Do with HF?

Emergency Frequency

[AC 91-70A, ¶3-4.e.] HF 2182 / 4125

Position Reports

Time Hack

Before the days of GPS the frequencies were crawling with time hack stations. Most of those, alas, are gone. The frequencies 5000, 10000, 150000, and 25000 got you something almost everywhere in the world. These days those frequencies are pretty much just in the United States, but you might get lucky.

The National Institute of Standards and Technology (NIST) broadcasts on the following frequencies from the locations noted:

Frequency (MHz) Latitude Longitude
2.5 40°40'52.2" N 105°02'31.3" W
5 40°40'42.1" N 105°02'24.9" W
10 40°40'47.8" N 105°02'25.1" W
15 40°40'45.0" N 105°02'24.5" W
20 40°40'53.1" N 105°02'28.5" W


  • VOLMET Aviation Weather Broadcasts Worldwide
    • AFI Brazzaville h+00, h+30 10057, 13261
    • EUR Shannon h+00, h+30 3413, 5505, 8957, 13264
    • NAT Gander h+20, h+50 3485, 6604, 10051, 13270
    • New York h+00, h+30 3485, 6604, 10051, 13270
    • PAC Anchorage h+25, h+55 2863, 6679, 8828, 13282
    • Hong Kong h+15, h+45 2863, 6679, 8828, 13282
    • Honolulu h+00, h+30 2863, 6679, 8828, 13282
    • Tokyo h+10, h+40 2863, 6679, 8828, 13282
    • Auckland h+20, h+50 2863, 6679, 8828, 13282
    • SAM Buenos Aires h+25, h+55 5601
    • Ezeiza h+15 2881, 5601, 11369
    • SEA Sydney h+00, h+30 6676, 11387
    • Bombay h+25, h+55 2965, 6676, 11387
    • Calcutta h+05, h+35 2965, 6676, 11387
    • Singapore h+20, h+50 6676, 11387
    • Bangkok h+10, h+40 2965, 6676, 11387

Book Notes

Portions of this page can be found in the book International Flight Operations, Part IV, Chapter 2.


Advisory Circular 91-70A, Oceanic and International Operations, 8/12/10, U.S. Department of Transportation

HF Communications Transceiver Pilot's Guide, KHF 950/990, Allied Signal Aerospace, Rev 0, Dec 96

ICAO Annex 2 - Rules of the Air, International Standards, Annex 2 to the Convention on International Civil Aviation, July 2005

ICAO Doc 7030 - Regional Supplementary Procedures, International Civil Aviation Organization, 2 2008

Radio Communications In the Digital Age, Volume 1, HF Technology, Edition 2, Harris AssuredCommunications, October 2005

Revision: 20160816