High Frequency, HF Radios

If you listen to a group of pilots speaking about International Flight Operations, HF radios become one of the fabled and mysterious features of Oceanic/Remote operations. HF radios provided the first primitive air-to-ground radio contacts for Richard Byrd, Amelia Earhart and many other early aviation pioneers. HF radios allowed World War II bombers and transports to communicate over long distances. Domestic ATC was based on HF voice well into the jet age of the 1960’s. HF remains a viable technology due to its compact size, affordable price and installation simplicity.

How do HF Radios Work?

HF routinely works over thousands of miles for two specific reasons. HF signals are reflected back to earth by ionized layers 60 to 200 miles above the earth modern HF transmitters use an Single-Side Band, SSB process that puts virtually all of the transmitter's power into the audio signal, providing about eight times the "talking" power of a basic am transmitter. HF radio communications can be difficult. HF radio signal propagation is highly dependent on the multi-layered ionosphere for reflection. These layers are formed primarily by the action of the sun's ultraviolet radiation. The ionosphere’s effectiveness as a reflector changes in relation to the amount of sunlight passing though it. The following general rules apply;

            Higher frequencies, those above 10.0 MHz, work better during the day

            Lower frequencies, those below 10.0 MHz, work better during the night

Any frequency may skip over the intended ground station when reflecting off the ionosphere. Solar flares and magnetic storms can severely impact all frequencies especially in polar regions above 60˚N or below 60˚S           

Who Can I call With a HF Radio?

You may have to communicate with the controlling atc facility through a general purpose radio facility. GP stations are staffed radio operators who can relay messages to ATC, your company or both. GP stations can also relay messages from ATC or the company to the pilot. Position reports given to GP radio facilities are automatically relayed to ATC. For example, across the North Atlantic, Gander Radio, New York Radio, Iceland Radio, Shanwick Radio and Santa Maria radio provide HF ATC functions. Communications with these facilities are limited to "flight safety" messages that involve ATC or weather communication. When operating in these areas, you will be assigned both a primary and a secondary frequency to assure radio contact even when one frequency fades or distorts. If your airplane is equipped with SELCAL get a positive check on both frequencies whenever possible, but you must have a check of at least one.

Long distance operational control facilities, LDOC can provide "flight regularity" messages. Operators use a LDOC station to relay pertinent flight information to and from company operations/dispatch centers. Flight-regularity messages must pertain to the operation of the aircraft itself, or to the aircraft load. Personal messages are not acceptable.

Public correspondence stations provide personal or operational telephone service and operates in the HF band. These stations link your airplane with regular landline phone services. AT&T operates three of the USA stations. Two popular stations in Europe are located in Sweden & Switzerland.

A pair of HF transmitters that have been established in the United States to provide a “time hack”.  These two stations are operated by the National Bureau of Standards in Fort Collins, Colorado and Kekaha, Hawaii. These stations broadcast continuously on 2.5, 5.0, 10.0 and 15.0 mhz. All of these frequencies announce every minute by voice and are given as "Coordinated Universal Time, UTC".

A worldwide system of "VOLMET" broadcasts of meteorological information for aircraft in flight. This is like an ATIS broadcast on an HF frequency. These recorded voice broadcasts of current weather observations and forecasts for airports and terminal areas. VOLMET s are broadcast on a fixed schedule. Check your Jeppesen publications and FIR NOTAMSs for the scheduled times. For example, VOLMET broadcasting is the primary means of disseminating SIGMET/AIRMET information to operating aircraft in the NYC oceanic FIR.

2182 khz and 4125khz are designated as international SAR frequencies by ICAO annex 12. In 2013, the u. S. Coast guard terminated its 24/7 voice listening guard of 2182 khz. Maritime information and weather broadcasts transmitted on 2670 khz were terminated concurrently.

How do I Operate a HF Radio?

There are some basic differences to operating an HF vice what pilots are accustomed to with VHF. Because of the signal propagation and the wide rage of frequencies, HF radios require different antenna “tuning” for each frequency.

All HF transceivers include an "antenna tuner/coupler." This tuner will electronically adjust the antenna for each frequency. The operating pilot will know this is happening by a tone in the speaker or headset. The tone is initiated when the transmit button is first pushed. Normal tuning time is around 20 seconds, however some late model HF transmitters will tune in less than one second. If not completed within 30 seconds, suspect a fault mode.  To clear the fault, change the frequency selected to any other and then reset to desired setting. Momentarily key the microphone to attempt another tune cycle. If a system fault is encountered twice while in one range of frequencies, select another usable frequency.

Once the tone stops, the radio is ready to transmit on. Subsequent transmissions on the same frequency will not require additional tuning of the antenna.  The antenna tuner/coupler is housed in a pressurized container filled with nitrogen. This is designed to prevent sparking and prolong the life of the tuner/coupler. Pilots intending to operate HF radios in-flight should preflight this item.

HF transmissions are normally more distorted than VHF. There is often a warbling or static on the frequency. Try retuning the frequency 1 or 2 khz or some small amount off the published frequency. Note the amount required to clear the signal and use this as a calibration factor to apply to other frequencies for a clear signal. If squelch operation is desired, this can result in blocking out weak signals.

Do not tune or try to get an HF check on the ground when near large metal structures. The effect of the structure will cause the coupler to think the antenna has different characteristics than it will when it’s airborne. The tuner/coupler will probably lock out the transceiver when an attempt is made to use it in the air. Caution also during fueling operations or with personnel near the antenna, do not operate the HF transmitter.

What about the Regulations?

Simply stated, an airplane must be equipped with at least one operating hf radio capable of monitoring and communicating with air traffic control any time the airplane is operated beyond the range of ground-based vhf radio communication even if an operator has an operational SATCOMvoice or satellite Datalink system installed.  ICAO Annex 2, Paragraph 3.6.5.1 directs this and also describes HF as a back-up to CPDLC. ICAO Annex 6 states this long-range communication directive again.

14CFR 91.511 provides the foundation of HF radio requirements for USA operators. Reading this regulation is difficult because is mixes navigation and communication elements. Basically stated, 2 VHF’s and 1 HF are required of 91/GA operators. 2 VHF’s and 2 HFs are required of commercial operators in 14CFR 135.165.

Commercial operators may use SATCOMvoice as a back-up to the second HF requirement by the use of MMEL policy letter # 106 dated 6JUN14. Flight plan equipment codes must be updated when implementing this relief.

Commercial operators may also seek to use the OPSPEC/LOA B-045 for operational approval of a single long-range communication system, SLRCS. This approval will only be used in a specific portion of the western Atlantic and Caribbean. B-045 can also approve SLRCS operations into the Gulf-of-Mexico without HF under specific circumstances of a fully functional VHF extended range network of ground stations situated in the GOM.

Bottomline
Yes, you need that old, hard to tune, painful to listen with and dangerous... HF radio