Bredhurst Receiving and Transmitting Society

6. Propagation

6a Radio propagation basics

The subject of propagation covers how radio waves travel through the atmosphere. The aim is to transmit and receive a good signal. On HF, it is often desirable to efficiently radiate and receive a strong signal over long distances known by the amateur radio term of as "working DX". International communications are made possible by the radio signal bouncing off of part of the atmosphere called the ionosphere (ref 6.6). The radio wave travels further because it bounces off the ionosphere. Amateur radio stations deliberately operate on HF to use the properties of the ionosphere to achieve long distance communications.

For positioning an antenna, the higher the better - this is why antennas are often on masts. Ideally, the antenna is high enough for the radio wave to clear obstacles e.g. hills and buildings that absorb the radio signal. Greater antenna height is an advantage for both receiving and transmitting. A radio wave is absorbed by obstacles - for example a mobile station will lose radio contact when entering a tunnel.

6a.1 Recall that radio waves travel in straight lines, unless diffracted or reflected.

Diffraction refers to the bending of radio waves in the ionosphere  and if the bending is very great the wave almost get bent right back a bit like reflection.

So Diffraction or bending or  Reflection means that the direction of the radio wave is changed e.g. a HF signal that is moving away from the Earth (towards outer space) is diffracted or reflected back to the Earth by the ionosphere where it can be received by a station far away from the transmitting station.

 Reflection of radio waves by moving objects is used by Radar.

6a.2 Recall that radio waves get weaker as they spread out.

The effect is the same as a torch beam - at a greater distance away, the same light has to cover a greater area so it is less intense. The light from the torch widens and fades away.

6a.3 Recall that at VHF and UHF hills cause "shadows" and that waves get weaker in penetrating buildings but glass windows are more transparent to radio waves.

Radio waves are similar to light. At VHF and UHF, hills cause "shadows". Radio waves get weaker after penetrating buildings, but glass windows are more transparent to radio waves. Solid materials (e.g. walls) absorb and adversely weaken the radio wave.

The range achieved at VHF/UHF is dependent on antenna height, a clear path and transmitter power. Higher antennas are preferable to higher power, because a higher antenna improves both transmit and receive performance. Outdoor antennas perform better than indoor antennas. The surrounding walls reduce the effectiveness of an indoor antenna.

6a.4 Recall that the range achieved at VHF/UHF is dependent on antenna height and a clear path and transmitter power. Understand that higher antennas are preferable to higher power, as they improve both transmit and receive performance. Recall that outdoor antennas will perform better than indoor antennas.

Raising the antenna above obstructions is likely give a clear path for the radio waves to travel. On VHF/UHF, if loss is high, a clear path is much more effective in getting a good signal than a 10 or 100 times increase in transmit power. For example, satellites can be accessed with very low power at a great distance. Typical ranges are a few km with a handheld, and tens of km from a mast-mounted antenna.

Obstructions (e.g. hills and buildings) between two radio stations can block the radio signal, making communication impossible on VHF/UHF. In contrast, where there is a clear path between two radio stations, for example between 2 ships at sea, communication is possible on VHF/UHF.

6a.5 Recall that, at VHF/UHF, range decreases as frequency increases and that in general, VHF/UHF waves have a range not much beyond "line of sight".

At VHF/UHF, range decreases as frequency increases. In general, VHF/UHF waves have a range not much beyond "line of sight". Line of sight is how far you can see. The range of VHF/UHF may be slightly beyond line of sight due to refraction at the horizon. Refraction refers to the bending of the radio wave.

VHF and UHF are normally used for local communications, whereas HF is suitable for long distance communication.

Higher frequencies, that is VHF and UHF, are not reflected by the ionosphere but pass straight through it. VHF and UHF signals go straight through the ionosphere and are lost in space [unless communicating with a satellite or spacecraft]. Therefore, VHF and UHF radio waves are normally used over short distances i.e. line of sight.

6b Ionosphere Basics

6b.1 Recall that the ionosphere comprises layers of 'conductive gases' at heights between 70 and 400kms above the earth.

The ionosphere is a layer of conductive gas at heights between 70 and 400 km. The ionosphere is part of the Earth's atmosphere that reflects HF radio waves back to Earth, giving the advantage of a greater range but it has no real effect on VHF or UHF radio waves that pass through and are thus lost into space.

Make sure you remember that the height of the ionosphere is from 70 to 400km - you may be asked this in the exam.

6b.2 Recall that on HF almost all communication relies on the waves being reflected by the ionosphere.

Recall that HF can provide world-wide propagation depending on how well the ionosphere bends the waves back to the earth.

Recall that this varies with frequency, time of day and season.

Long distance communication is possible on HF when the radio wave bounces off the ionosphere. Worldwide coverage requires several bounces. Worldwide propagation has to depend on how well the ionosphere reflects the waves back to Earth. This varies with frequency, time of day and season.

For worldwide propagation, the radio waves follow the curvature of the Earth because the ionosphere prevents them from leaving the vicinity of the Earth.

When conditions are good for HF radio waves of a particular band (of frequencies) to travel long distance, the band is said to be "open".

 The causes of a band being open are time of day, season, atmospheric conditions, and the number & size of sunspots during the 11 year Sunspot Cycle. It is possible to check whether a band is open by listening to beacons.

EXAM TIP

Make sure you remember that the height of the ionosphere is from 70 to 400km - you may be asked this in the exam.

It is apparent from wrong answers given to questions on this section that students do not understand what are the typical distance ranges that the various frequencies can achieve.

Please note that only HF achieve longest distances

Next is VHF

then UHF.

The syllabus on this page is copyright of Ofcom

Questions

Here are some questions raised by students which do not relate directly to the syllabus but are interesting none the less.

1. If a transmitter were connected to coax and that coax were terminated only in a coaxial plug then nothing would be radiated ??

Yes -- The coax (feeder) is only used to convey the Radio Frequency Current to the Load (antenna). If no load is connected then the current cannot dissipate and will return to the transmitter in the form of a Standing Wave possibly damaging the transmitter.(modern transistorised transmitters have sensors to protect the power amplifiers from excessive reflected components by reducing the forward power)

2. it it the case that when and antenna is connected to the end of a length of coax then it acts as a load into which the RF passes from the Transmitter and causes a current to flow and there by creates an electromagnetic field that radiates.

Correct, connect a load, be it the radiating elements of the antenna or a non-inductive resistor of the correct impedance, which must match the transmitter output impedance and the nominal impedance of the coax (feeder). Then the load will absorb the transmitter output power and there will be no reflected component. (VSWR 1 to 1) a perfect match, and the transmitter will give its maximum output power into the load.

3. Does the AC of the mains radiate into the open space when a load is connected ?

No -- Mains AC frequency 50 Hz will not radiate due to the low frequency, (as we understand radiation) however there will be an electric and magnetic field surrounding the conductors leading from the power station right to the load.

Evidence of these fields can be heard easily. Tune your car radio to a station on medium or long wave and drive under the national grid distribution cables and you will hear a 50 Hz buzz a little way before and a little distance after you pass these cables.

Now imagine how noisy the bands would be if these radiated in the same manner as our radio waves!!!!