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Bredhurst Receiving and Transmitting Society

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6. Propagation

Radio propagation basics

6a.1 Recall the basic structure of the ionosphere: D, E and F layers and their order. Understand that ionisation is caused mainly by ultraviolet rays from the sun.

In the FLC your were introduced to propagation and now we will be taking it several stages further.

You should be aware that HF propagation is predominantly due to ionisation of the ionosphere and that the ionosphere is made up of layers of gases.

We must now consider what these layers of gasses are called. These layers are simply called the D E and F layers there is no A B C layers.

Layer

Approx Height

Diagram

F

400 kms

E

Varies

D

70 kms

Troposphere is below the D Layer

The D layer is nearest to the earth at about 70kms and the top layer, the F layer is at about 400kms. The E layer sits between the D and the F layers.

It is the ionisation of the gases, caused mainly by ultra violet rays from the sun, that makes them partially conductive and which gives the ability to refract (reflect) the HF radio waves back to earth. This "bouncing back and forth can happen several times, for instance to reach Australia with losses in signal strength each time.

6a.2 Recall that the level of ionisation changes with the time of day, the time of the year, and according to the 11-year sunspot cycle. Understand that the sunspot number is an indicator of solar activity and that more sunspots gives better HF propagation as a result of increased ionisation.

The amount of ionisation that occurs is dependent upon :-

  1. the time of day

  2. the time of year (season) and

  3. changes in sun spot activity in its 11 year cycle.

The sun spot activity is an indicator of solar activity. The more sunspots the better the HF propagation as a result of increased ionisation.

Although radio amateurs talk in terms of SUN SPOTS it is the Solar flares that cause an effect on the ionosphere. The effect of x-ray emissions from a flare is evidenced in the ionosphere as a disturbance and greater ionisation.

In November 2003 the greatest ever recorded solar flares took place and caused radio black out (amongst other effects) so too much of the sun's activities can worsen rather than improve communication.

6a.3 Recall that reflection from the F layer is the main mode of HF propagation.

Some students just cannot remember that it is the F layer which is the main mode of HF propagation but may be if you can remember that the word REFLECTS has an "F" in it standing for the F layer. ( In pure terms the use of the word Reflect is not correct but that is the effect of the refraction of the RF in the F layer that makes then appear REFLECTED !!!) BUT for the exam the syllabus is using the word REFLECT !!!

The word propagation means to increase the distance that your radio signal can travel over and above the distance that your radio signal can travel as a Ground Wave.

As you will see from the diagram below that it is the GROUND WAVE are the radio wave hugs the earth but quickly become weaker with a ranges of a few kms at best. Thus at what ever frequency you are operating in the HF bands any distance you achieve is dependent upon bouncing / deflecting off the "F" layer in the ionosphere.

F layer ionisation during daylight enhances propagation

It is the F layer which plays the greatest part in HF propagation. The sun causes the ionisation during periods of daylight with a peak in the early afternoon (local time).

F layer looses much of its ionisation during night particularly at the higher HF frequencies.

Whilst the F layer remains ionised over night it is much weaker than during the day and hence propagation at night dwindles on the higher HF bands.

In Winter F layer remains relatively constant at the daylight levels

During the winter the ionisation tends to remain higher as the WX is colder.

F layer has enhance propagation during sun spot activities

During the 11 year sun spot cycle the more sun spots there are the higher the ionisation which lead to better reflection and the higher the radio frequency that can be reflected.

6a.4 Understand the meaning of ground wave, sky wave, skip distance, and skip zone (dead zone).

There are certain terms associated with propagation that are now introduced :-

GROUND WAVE = the radio waves that hug the earth but quickly become weaker with a ranges of a few kms at best.

SKY WAVE = the radio waves that are reflected back to earth by the ionosphere. Even very low powered HF transmitters are capable of having their signals reflected back from the ionosphere as it is not the power of the transmitter that is needed to make the signal reach the ionosphere, the power is only needed to be sufficient for the signal to be heard by the receiving station. QRP operation on less than 5 watts has reached Australia!

SKIP DISTANCE = the distance from the transmitter to the first point at which returning skywave is received back on earth having been reflected by the ionosphere.

SKIP ZONE = The zone is between the end of Ground Wave coverage and the point at which returning skywave is first received back on earth. Thus the SKIP ZONE (also called the Dead Zone) is signal free as far as communication is concerned as nothing is heard from the transmitting station.

All the distances change as the ionosphere changes so you cannot state the the Skip Zone is such and such a distance and the same goes for the Skip Distance.

Skip distance minus ground wave distance = the skip zone distance

Note:- NO SIGNALS are received in the SKIP ZONE from the transmitting station's antenna location!!!

The Tx indicates where the Transmitting station is located. Stations at A, B, C, and D will hear the signals but for different reasons.

Ground Wave

Stations B and C are near enough to hear the grounds wave signal

Sky wave

Stations A and D are far enough away (outside the Skip Zone or DEAD ZONE) to hear the signals from a bounce off the ionosphere.

Skip Zone also known as DEAD ZONE

The Skip Zone or DEAD ZONE is as it says no signals will be hear from the transmitting station. so anywhere between A and B and C and D will not hear the transmissions.

6a.5 Recall that high atmospheric pressure can cause ducting in the troposphere, which increases the range of VHF and UHF signals.

Recall that the range of VHF signals can occasionally be significantly increased by reflection from highly ionised areas in the E layer (Sporadic E).

What is high atmospheric pressure.

If you look at the weather forecast on the TV you will often see a simplified version of what is called a synoptic chart such as is shown below.

You can see on the chart the letter H and the number 1037 below and L and the number 983 below. The H represents the high pressure area and the L the low pressure area. The numbers relate to what is called a barometric pressure - but you do not need to know any details other than understanding that there is a difference between HIGH and LOW Pressure in the atmosphere.

These HIGH and LOW pressure areas are also called a High pressure system and a Low pressure system. So with that background in mind let's deal with the effect of the High pressure system.

Ducting is the enhancement of VHF and UHF

When there is high atmospheric pressure, as explained above, enhanced VHF and UHF propagation can occur by what is called "Ducting" in the troposphere.

The troposphere is the layers of atmosphere below the ionosphere.

The orange coloured lines represent the VHF and UHF signals which are not reflected / refracted by the F layer but if the signal does get trapped in the troposphere then it is "ducting" that enhances the propagation as if it were flowing through a large pipe.

The basis of ducting is that the VHF and UHF signals get trapped between two layers of the troposphere or between the ground and a layer of troposphere and then are released in a hole in the ducting at a much further distance than is normally expected of VHF and UHF propagation.

From UK ducting across the North Sea water surface may allow microwaves communication with the Netherlands but the path soon disappears at a cliff face even a modest one where the signal is forced up out of the duct.

E LAYER and Sporadic E enhance VHF and UHF

If you have success with DX communication on VHF and UHF then is will be because of the enhanced propagation due to tropospheric ducting.

VHF and UHF can sometimes get reflected like HF when the "E Layer" becomes highly ionised - and the signals are reflected back to earth. Because the effect is not uniform it can be considered as sporadic - hence "Sporadic E" propagation.

6a.6 Recall that VHF and UHF signals normally pass through the ionosphere, and at these frequencies propagation is within the troposphere situated below the ionosphere.

Propagation of the signals at VHF and UHF frequencies are within in the troposphere which is located below the ionosphere.

If no propagation takes place in the troposphere then the VHF and UHF signals are passing into and out of the ionosphere and thereafter are lost into space.

6a.7 Recall that snow, ice and heavy rain can attenuate signals at UHF and above.

it is a fact that you need to learn that Snow, ice and heavy rain can attenuate (reduce the signal strength) of UHF and higher frequency signals.

6a.8 Recall and be able to manipulate the formula v = f x. Be able to calculate frequency or wavelength given the other parameter.

For calculations, the velocity of radio waves will be given.

Some more maths for you to further your understanding of conversion of frequency to wavelength and vice versa.

In the FLC you were able to use a simple table to convert frequency to wavelength and vice versa. That table was a representation of the simple formula which enables you to calculate frequency or wavelength given one or the other.

v = f x

The velocity of light = the frequency x wavelength

Note the uses of a lower case v so that there is no confusion with an upper case V which stand for Voltage.

NOTE: The frequency f is expressed in Hz and the wavelength is in meters.

In the exam you will be given the velocity of light at a figure of 300,000,000 m/s.

For those who find manipulation of formulae difficult another magic triangle has been constructed.

and now for a few examples:-

Example 1

If f = 3MHz what is the wavelength ?

so 3MHz = 3,000,000 Hz

and wavelength = 300,000,000 / 3,000,000 ( answer in m )

so wavelength = 300 / 3 = 100m REMEMBER use Frequency in Hz and wavelength in meters !

Example 2

if wavelength = 6m what is the frequency

so frequency = v / wavelength

so frequency = 300,000,000 / 6 ( answer in Hz )

so frequency = 50,000,000 Hz = 50MHz

Example 3

If the frequency is 1 GHz what is the wavelength ?

wavelength = V / frequency

so wavelength = 300,000,000 / 1,000,000,000 ( answer in m )

so wavelength = 300 / 1000 = 3 / 10 = 0.3m

NOTE:

For the exam we are told that the only units that will be used will be 1,2,3 or 6 eg 1Mhz, 20m, 30Mhz, 6m 1GHz (this means that the leading number will only be 1, 2, 3, or 6)


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