HF Radio Communication: The Basics

HF (High Frequency) radios operate within a frequency range of 1.5Mhz to 30Mhz Single Side Band. An accurate description is "single sideband suppressed carrier amplitude modulated telephony" and is designated "J3E", but is more commonly abbreviated to SSB. All commercial operation in Australia use USB (Upper Side Band).

Radio waves, like light waves and all other forms of electromagnetic radiation, normally travel in straight lines. Obviously this does not happen all the time, because long distance communications depends on radio waves travelling beyond the horizon. How radio waves propagate in other than straight-line paths is a complicated subject, called Propagation, but one that need not be a mystery. For the sake of accuracy the following notes have been taken direct from the ARRL handbook for amateur radio operators. A general knowledge of propogation is a requirement to pass an Amateur Operators Certificate of Proficiency examination. For our purposes, we do not need to pass an examination, just have a knowledge of how it all works.

PROPAGATION

Radio waves travel at 300,000km per second and weaken as they travel weather in the near vacuum of cosmic space or within the Earth's atmosphere. Attenuation grows rapidly with distance because signals weaken with the square of the distance travelled. If the distance between a transmitter and receiver is increased from 1km to 10km, the signal will only be one-hundredth as strong. Radio energy is also lost during refraction, diffraction and reflection, the very phenomena that allows long-distance communication.

Radio waves can be divided into two types, ground wave and sky wave.

A ground wave is the result of a special form of diffraction that primarily affects longer-wavelength vertically polarized radio waves, such as those created when we talk on our radios using a vertical antenna, the most common for mobile operation. It is most apparent at 1.5MHz to 4Mhz, where practical ground-wave distances may extend beyond 200 km. The term ground wave is often mistakenly applied to any short-distance communication, but the actual mechanism is unique to the longer-wave bands.

Radio waves are bent slightly as they pass over a sharp edge, but the effect extends to edges that are considerably rounded. At medium and long wavelengths, the curvature of the Earth looks like a rounded edge. Bending results when the lower part of the wave front loses energy due to currents induced in the ground. This slows down the lower part of the wave, causing the entire wave to tilt forward slightly. This tilting follows the curvature of the Earth, thus allowing low- and medium wave radio signals to propagate over distances well beyond line of sight. Ground wave is most useful during the day at 1.8 and 3.5 MHz, when D-layer absorption makes sky wave propagation more difficult. Vertically polarized antennas with excellent ground systems provide the best results. Ground-wave losses are reduced considerably over saltwater and are worst over dry and rocky land.

A sky wave is then any other type of wave. When the suns rays reach the upper atmosphere they ionize the air particles. This ionization happens in layers, each layer having it's own effect on radio waves.

  • F Layer: at approximately 400km, bends LF, MF & HF waves, VHF and above are not reflected, it is always present and it splits into two layers during daylight hours
  • E Layer: approximately 200km bends LF, MF & HF waves to a lesser degree, it is occasionally very dense and then can bend VHF waves.
  • D Layer: approximately 100km, forms only in daylight, absent at night, absorbs LF & Lower HF waves and more intense during summer

During daylight,

  • The F layer form two layers F1 & F2
  • The chance of ionized E layer is greater
  • The D layer is ionized and absorbs MF waves

During darkness,

  • The D layer disappears and the more distant broadcast stations can be heard.
  • The lower HF frequencies 3-4 MHz, become very useful

The highest frequency that can be used with the ionosphere is called the MUF, Maximum Useable Frequency, and the lowest frequency is called the ALF, Absorption Limiting Frequency. Below this frequency the D layer absorbs radio waves.

Other Phrases

  • MUF, Maximum Useable Frequency is the highest frequency that can be used with the ionosphere
  • ALF, Absorption Limiting Frequency is the lowest frequency below this frequency the D layer absorbs radio waves.
  • The Skip Distance: is the distance from the transmitter to the point where the Sky Wave first returns to earth.
  • The Skip Zone: the area where you are to far from the transmitter to hear it by the ground wave, but still too close to hear it by the Sky Wave.
  • Multi-Hop: The greatest distance a Sky Wave travels by single hop is 4000Km, so distances greater that this are said to have Multi-Hopped. This can extend around the globe, and is the reason Short-wave stations from around the world can be heard.

Now we come to the main reason why HF radio is better than CB. The frequency of the HF radio can be changed to suit the distance and the time of day, for instance if you find yourself in the skip zone and no signals are being received, change to a higher or lower frequency. CB has a set frequency band, 27MHz and this can not be altered, and while some will tell you that they talk to others over any distance, they can not do so reliably, this is the key word, 'reliable'.

It is for these reasons that the RFDS have a number of frequencies that they monitor, some for day time, some for night time, some for short range, and some for long range, a frequency for all times and conditions. As you become more familiar with HF communications you will find that some of these conditions use the same frequency, i.e. short range in daylight and nighttime could use the same frequency. Which leads us to the next point and one which is only half told generally.

Some literature and books on radio expound the theory, 'The higher the sun, the higher the frequency', and in most cases it is quite correct, however, it should also be stated 'The shorter the distance, the lower the frequency'.

So what do we do, who do we believe, how do we know we are right in what we do. Talk to those people in the CMCA that are doing it now, or if you are still unsure enlist the aid of an Amateur Radio Operatoror a radio office in the RFDS.

Making it Work for us.

We now have the basics of propagation, how do we put it into practice. As with most equipment, familiarity by constant usage is very beneficial, when an emergency arises, hopefully not your own, to be able to raise help confidence is a great asset, so use the radio constantly. As a starting point, pull out the manual for the radio, you know, the one you carefully packed away in the plastic cover it came in so it would not get dusty and dog eared. Read it carefully so you understand all the controls, most have a useful purpose and will be used often. Try to understand what the controls achieve, it makes the selection so much easier under stress conditions.

For all contacts with the RFDS you need a Non Assigned Outpost Licence available from the ACA. Your callsign must be given at the start and end of each transmission, or series of transmissions. Only SA (7am -9pm) and NT (7am - 5pm) monitor their frequencies for voice calls, at all other times and states, use the emergency button to raise the operator after selecting your frequency.

Australia is a great country and one of the very few to have a legal HF spectrum allocation for recreational use. To become a recreational user, you must either apply to ACA for your own licence, which can be costly, as a licence is needed for each frequency, or alternatively, join a recreational club such as HF Radio Club, Bush Telegraph or the 4 Wheel Drive Club. For a small fee from $25 pa. You are allocated a callsign under their Land Mobile Licence, and the list frequencies you can use. This ranges from 3MHz to 15MHz and will give you good communication anywhere in Australia most of the time. As with the Outpost Licence Your callsign must be given at the start and end of each transmission, or series of transmissions.

The recreational clubs have a number of strategically located bases, remember, while most offer a degree of assistance for those in trouble, the base stations are monitored by volunteers and may not be available when you need them the most. Please don't bet your life that they can assist. They are very helpful for getting road and weather conditions and for arranging a time to talk with mutual friends, passing on messages from home and sometimes just to say hi and let them know where you are in case somebody is looking for you. For all genuine emergencies please use the RFDS. Selcall is the best way to raise an operator during the day.

Radphone

Now that the RFDS and Telstra have terminated their Radio Telephone services, this feature of communications has been left to the recreational clubs to support. For an extra fee you will be issued with a pin number to have installed in your radio which will then allow direct dialling into the Public Switched Telephone Network (PSTN). In some cases the public can also call your radio from their home, but this is somewhat complicated to explain to a non radio user and it is suggested that they contact one of the bases by telehone and ask for a message to be passed onto you. Most club Radphone services have a time limit of 6 minutes per call, this is to allow others to have reasonable use of the service. If in doubt about any feature call up the base operator and discuss the details with them, most would be pleased to talk with you.

Free to Air

This is a much used statement meaning you can talk on the radio to other club members or the club base stations without any cost, apart from your memberships fees, unlike cell phones or satellite phones there is no time charge. However be aware of other club members and their rights to the frequencies. Keep your overs short and to the point and occasionally ask if anyone else needs the frequency. If you get no response then carry on with your conversation. Please don't just selcall on top of a conversation in progress, it is better to "break in" and ask how long they will be using the frequency. This serves two purposes, first you will find out how long you must wait and second, the other parties will note your wish to use the frequency.

Final Thoughts 

When looking for an HF Radio, study all the details of the types available, ask others that have them installed for advice. Most owners will be pleased to talk radio with you and also demonstrate their unit. Make sure when you finally have a radio installed, that everyone is familiar with it's use and is capable of operating it to get help in an emergency. I once heard the story of a couple that had ventured well off the beaten track, become bogged in mud and the poor fellow had a very bad heart turn and passed out. His wife had never used the new HF radio they had installed, and was unable to make it work. Her husband finally came around after a number of hours and put out an emergency call. They were very fortunate that his heart turn was minor and they lived to tell the tale. Don't put yourself in this picture, use your radio and insist that your partner becomes familiar with it's operation.

If you have any questions you are unable to find answers for, send me an email, I may have an answer for you or be able to point you in the right direction.

Don Crago (S11627)

Festival Wanderers

Email: vk5kdc@internode.on.net

Selcall: 1483

The CMCA would like to acknowledge the ARRL and the use of their Handbook for the text on Propagation.

by Don Crago (S11627)

(Don is a licenced amateur radio operator and has spent a number of years communicating around Australia and overseas via HF radio and satellite.)