HF
There is much about HF to be confused about. It is a High Frequency (HF), for example, which turns out to be lower than most of the frequencies we deal with on a daily basis. Younger pilots often hear from the older pilots about just how difficult it was to make a position report "back in the day," but it seems rather easy these days. It is true that technology has made the radios easier to use and the transmitted signals much clearer. But there are still a few things to be gained from learning the technical stuff behind the technology.
Principles of Radio Communications
Amplitude, frequency, and wave lengths
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- 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
- 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.
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.
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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
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- 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.
- 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.
Line of Sight / Minimum Altitude
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)
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
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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.
Ionosphere Layers
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- 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
- 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
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 3.6.5.1). It is not acceptable to depend on radio relay operations to satisfy this requirement.
ICAO Requirement
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.
Aircraft shall be equipped with suitable instruments and with navigation equipment appropriate to the route to be flown.
North Atlantic Requirement
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.
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 3.6.5.1). It is not acceptable to depend on radio relay operations to satisfy this requirement.
ICAO Requirement
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.
Aircraft shall be equipped with suitable instruments and with navigation equipment appropriate to the route to be flown.
North Atlantic Requirement
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.
Frequency Selection
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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).
- 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.
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."
- 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
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:
(MHz) Latitude Longitude
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:
(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
Weather
- 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