or reliable operations and will assist both technical and non-
technical personnel.
Handbook Table of Contents:
Chapters:
(Revised April 1997)
q
1. How Radio Waves Travel
q
2. Aerials

q
3. Transceivers and Power Supplies
q
4. How to Plan your Radio Network
q
5. Costs and Choices
q
6. How to Install a Radio System
q
7. How to Operate a Radio Network
q
8. How to Maintain a Radio Network
q
9. Additional Facilities
q
10. Packet Radio
q
11. Disaster and Relief Communications
Note: The www copy does not include the 34 diagrams. Updated: August 18, 1998
WHERE THERE IS NO TELEPHONE
Chapter 1. HOW RADIO WAVES TRAVEL
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
Return to Table of Contents

1.1 Basic theory of short-wave radio.
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.
1.2. Propagation Prediction
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