Topics:
| 1.1 | Basic theory of short-wave radio |
| 1.1.1 | Frequency and Time |
| 1.1.2 | Distance |
| 1.1.3 | Sunspot Cycle |
| 1.2 | Propagation Predictions |
| 1.2.1 | Computer Prediction |
| 1.2.2 | Calculation by hand |
| 1.2.3 | Experience and Experiment |
| 1.2.4 | Rule of Thumb |
| 1.2.5 | Unpredicted effects |
Short-wave bands are to be found on many broadcast radio receivers. There are various broadcast bands and to tune into them you are given the wavelength,for example 25, 31 or 41 metres. In radio communications the tuning information is given in kHz (kiloHertz) or MHz (MegaHertz), formerly Kc/s or Mc/s. Radio waves travel at the speed of light i.e. 300,000 km per second and because wavelength and frequency are related you can easily translate from wavelength to frequency by the relationship:
| Frequency in MHz | = | 300 __________________ Wavelength in metres. |
For example,a wavelength of 25 metres gives:
| Frequency | = | 300 _____ 25 | = | 12 MHz |
The short-wave band is referred to as the HIGH FREQUENCY (HF) band. The HF band covers frequencies in the range 2 to 30 MHz.To communicate between two different places it is necessary to transmit radio waves to the distant receiver, but radio waves travel in straight lines and the earth is round, so how do we communicate over hundreds and thousands of miles? Such communication is possible because the earth is surrounded by the ionosphere. The ionosphere is made up of several ionised layers which are continually combining and then separating. They occupy a region between 100 and 300 kilometres above the surface of the earth. At certain times the layers will reflect radio waves back to earth, see Wave A in Fig.1. The ability of the ionosphere to reflect radio waves depends upon the electron density of the layer. The higher the electron density the more effective the ionosphere is as a reflector. The sun, shining on the ionosphere, causes the electron density to increase. Hence the density increases from dawn, being a maximum around midday and gradually decreasing towards night. In addition to this daily variation in density, there is also an annual cycle and a 11 year sunspot cycle. It is possible for radio waves to pass straight through the ionosphere without being reflected as shown by Wave B in Fig.1. To be of use for radio communications the radio waves must be reflected back to earth.
Three of the factors which determine whether radio communication is possible are the time, the frequency in use and the distance between the radio stations.
1.1.1. Frequency and Time
The higher the frequency the more likely the wave will pass through the ionosphere and not be reflected back to earth. Therefore the highest frequency that will be reflected and can be used for communication between two places is called the MAXIMUM USABLE FREQUENCY ( MUF ). The MUF varies throughout each 24 hour period and also throughout the year. An example of the daily change for January and July is shown graphically in Fig 1.2. Because Kinshasa and Kisangani are near the Equator the seasonal change in MUF is not very pronounced. The graphs are drawn for a Sunspot Number of 0 which is its lowest value. The values of MUF will also change with sunspot number, in general the higher the sunspot number the higher the MUF. If possible the time around dawn and dusk should be avoided for radio communications because the ionosphere is in a state of change. The ionosphere is reasonably stable during daylight hours.
1.1.2. Distance
The shorter the distance between two radio stations the lower will be the MUF. In Fig 1.3. this can be seen where the distance from Kinshasa to Tondo is 500 kms and the daytime MUF is 8 MHz whereas the distance from Kinshasa to Kisangani is 1,300 kms and the MUF is 12 MHz. With a sunspot number of 10 the MUF for stations up to 100 kms apart would be 7 to 8 Mhz in July in Equatorial Africa. Therefore at higher frequencies no contact is likely. However there will be a ground wave (see Fig 1.1. Wave C) which may extend some 20 kms or more depending upon the surface of the earth. Ranges between 50 to 100 kms can be difficult for HF communications unless a suitable frequency is used and for reliable communication up to a few hundred kms frequencies around 2 MHz may be necessary.
1.1.3. Sunspot Cycle
The electron density in the ionosphere varies according to the 11 year sunspot cycle. The ionosphere is most dense when the sunspot number is high. A sunspot maximum occurred in 1979/80 so the next maximum can be expected in 1990/91 with minimum occurring in 1985 and 1996. An example of the effect of the change of sunspot number was seen in Great Britain. The use of Citizen Band ( CB ) radio for communication became very popular around 1980. The radios operated at 27 Mhz and were illegal at the time. People were able to make CB contacts into Europe and were thrilled with this long distance communication with transmitter power output often of only 4 watts.Gradually over the next few years these contacts became more difficult and few overseas CB stations were heard. The lack of propagation was simply due to the fact that the ionosphere was no longer reflecting the 27 Mhz waves back to earth. This change in propagation would have been expected by anyone aware of the sunspot cycle and its effect on the ionosphere.
One frequently reads that the prediction for HF communication is as much an art as a science. There are nevertheless patterns which can be predicted and are valid guidelines on which to plan the times for your radio contacts. The ionosphere and its effect on radio wave propagation is a vast subject into which research continues and many books are written. A simple treatment of ionospheric propagation can be found in the amateur radio handbooks such as RADIO COMMUNICATION HANDBOOK from RSGB, Potters Bar, EN6 3JW, England. There are various ways to determine at what time the ionosphere will permit communication.
1.2.1. Computer Prediction
Programs which give the MUF between any two geographical locations are available to run on many different computers. To use such programs it is necessary to give the computer the latitude and longitude of each location, the month of the year and the sunspot number. The computer will then calculate the MUF at hourly intervals for a 24 hour period. It should be noted that with some programs the calculated MUF may be up to 2MHz above or below the actual value. Also some programs are not very accurate at ranges below a few hundred miles, though the accuracy increases at longer ranges. The name of one prediction program is MINIMUF. Enquiries about these programs should be made through the supplier of your radio, also the programs are available through amateur radio sources.
1.2.2. Calculation by hand
It is possible to calculate by hand the frequency that can be used for communication between any two stations. This method requires information regarding great circle distances and the zenith angle of the sun amongst other factors. However all the necessary information including step by step instructions for calculation is given in a very useful book PLANNING AND ENGINEERING OF SHORTWAVE LINKS by Gerhard Braun from Heyden and Son, London and Philadelphia.
1.2.3. Experience and Experiment
It is always worth listening to the experiences of other people who claim to have done what you intend to do, but enquire in detail what frequency, time and area they operated their radios in and with what success. Then proceed to verify any information by trial transmissions yourself. If you have no such prior information regarding propagation, then arrange a timetable between your stations to attempt to make contact. The contacts could be every 2 hours from 0600 to 1800 hours, trying each frequency you have for say 5 minutes. After several days you would obtain a general pattern of the best time for contacts. During such experiments if no contact is made your frequency may be too high. Also you may never make contact at frequencies of 11 mhz and above when the sunspot number is low, but when the sunspot number rises towards a maximum such contacts would be possible. It is therefore sensible to obtain some theoretical predictions of MUF before the radios are purchased. Here is an example of lack of information regarding propagation:
A group obtained radios with a frequency around 10 MHz and proceeded to try to use them for contacts at ranges of 70 kms at 1800 hours. When they found it was impossible they assumed that their radios were at fault and wanted them to be repaired. They were advised to try for the same contacts at 1300 hours instead of 1800 hours and were successful.
1.2.4. Rule of Thumb
A "Rule of thumb" method is used in East Africa and is also suitable for Central Africa. This method calculates the best frequency for various radio links operating between 6 am and 6 pm.:
For ranges up to 800 kms use 7MHz. For every 160 kms of range above 800 kms add 1 MHz to the frequency. e.g.
| Kms 800 1120 1600 | Frequency in MHz 7 9 12 |
Note that frequencies below the best frequency may also be usable. For example, 7.5 MHz is used by a group based in Nairobi for links with the rest of Kenya.
1.2.5. Unpredicted Effects
The general pattern of propagation is further varied by disturbances of the ionosphere caused by solar flares. The result can be very poor or no HF radio communication for a few hours or a few days. Expect such events and do not immediately assume your radio is at fault, though it is wise to perform the checks suggested in para. 7.2.3.