In a region extending from a height of around 50km to over 500km, most of the molecules of the atmosphere are ionised by radiation from the Sun. This region of the atmosphere is referred to as the Ionosphere.
The free electrons in the ionospehere cause HF radio waves to be refracted (bent) and, eventually, relfected back to earth. The greater the density of electrons the higher the frequency that can be reflected back.
During daylight there can be four regions present, these being the D, E, F1 and F2 layers. There (approximate) heights are:
- D Layer: 50 to 90km
- E Layer: 90 to 140km
- F1 Layer: 140 to 210km
- F2 Layer: 210km and above
At some times during the solar cycle the F1 layer may ‘merge’ with the F2 layer to form an “F layer”. At night the D, E and F1 layers become very depleted of free electrons, leaving just the F2 layer for use in communications.
When it comes to the refraction of HF frequencies, only the E, F1 and F2 layers refract HF waves. The D layer however is important as it can absorb or attenuate HF frequencies.
The most important layer for HF communications is the F2 layer. This is because:
- It is present 24 hours a day;
- It allows the longest communication path because of its high altitude;
- It reflects the highest frequencies in the HF range.
The Sun goes through a periodic rise and fall in activity. These cycles affect HF frequency propagation. Solar cycles are from 9 to 14 years in length, and at their minimum only the lower HF frequencies will be reflected by the ionosphere, whilst at solar maximum the higher frequencies will be reflected.
At solar maximum there is also more chance of large solar flares occurring. Solar flares are huge explosions on the Sun that emit radiation that ionises the D layer, which causes increased absorption of HF waves. This is called “short wave fade-out”, and they occur instantaneously affecting lower frequencies the most. Fade-outs can last from 10 minutes to several hours, depending on the severity of the flare.
Frequencies are normally higher during the day and lower at night. After dawn, solar radiation causes electrons to be produced in the ionosphere and frequencies increase rapidly. Maximum occurs around noon. In the afternoon frequencies begin to fall due to electron loss and with darkness D, E and F1 layers disappear. At night communication is by the F2 (or simply F) layer only and attenuation is low. Through the night the maximum frequency gradually decrease, reaching their minimum just prior to dawn.
Sporadic E refers to a largely unpredicable formation of regions of very high electron density in the E layer. Sporadic E can form at any time, day or night, occurring at altitudes of 90 to 140km. It can vary greatly in the area it covers - a few km to hundreds of km - and the time it persists for - minutes to many hours.
Sporadic E can have very similar electron density as the F layer, meaning it can reflect the HF frequencies normally reflected by the F layers. In some cases the sporadic E layer can be transparent allowing the HF signal to pass through to the F layer. However in other cases the sporadic E layer totally obscures the F layer so the HF signal never reaches the F layer at all.
At low to mid altitudes sporadic E occurs mainly during the day and early evening, and is more prevelant during the summer months. At high altitudes sporadic E tends to occur at night.
|Fig.1: Sporadic E -v- F Layer propagation|
Types of HF Propagation
HF frequencies (3 to 30MHz) can propagate to distant stations by a number of paths:
- Ground Wave: travels near the ground for short distances, up to 100km over land or 300km over water.
- Direct or ‘Line-of-sight’: may interact with the earth-reflected wave.
- Sky Wave: reflected by the ionosphere. Can cover great distances.
|Fig.2: Ground Wave, Direct Wave and Sky Wave|
Frequency Limits of Sky Wave
Not all HF frequencies are reflected by the ionosphere. If the frequency is too high it will pass right through the ionosphere and out into space. If it is too low the signal strength will be reduced due to absorption in the D layer. The range of frequencies that are deemed usable depend on a number of factors:
- time of day
- the season
- the solar cycle
- the location
The above factors affect the upper limit, whilst the lower limit is also affected by:
- receiver site noise level
- antenna efficiency
- transmitter RF power
- E layer screening
- ionosphere absorption
The Maximum Usable Frequency (MUF) is the highest frequency usable from a given location at a given time of day in order to communicate long distance. Services such as IPS provide MUF predictions based on several factors.
This is the distance covered by a signal after it has been reflected once back to Earth. The maximum hop length is set by the height of the ionosphere and the curvature of the Earth. For an E and F layer height of 100km and 300km the maximum hop lengths are approximately 1800km and 3200km respectively. Any distance greater than these require multiple hops (e.g. a distance of 6100km will require at least 4 hops by the E layer and 2 hops by the F layer).
The skip zone is the area between where the Ground Wave stops and where the Sky Wave returns to Earth. Skip zones vary in size with time of day, season, and solar activity. Skip zones are generally smaller during the day, at solar maximum and around the equinoxes.
|Fig.3: Skip Zone, Skip (Hop) Distance, Ground Wave and Sky Wave|
Multipath fading occurs when a signal travels from transmitter to receiver via more than one path. This results in multiple received signals each varying in phase and amplitude. Depending on the phase and amplitude of the received signals they may add or subtract from each other, causing the signal to fade in and out.
Travelling Ionospheric Disturbances (TID) may cause a layer to ‘tilt’, causing the signal to become focussed or defocussed. Fading periods of 10 minutes or more can be associated with a TID. A TID travels in a horizontal direction at 5 to 10km/min, affecting higher frequencies first.
Polarisation fading occurs when there are changes to the polarisation of the transmitted signal while travelling along the propagation path. This can result in the receiving antenna being unable to receive parts of the signal. This fading can last for anything from a fraction of a second to a few seconds.
Skip fading can occur around sunrise and sunset if the operating frequency is close to the MUF and the receiving antenna is located close to the skip zone boundary. This is caused by fluctuations in the ionospehere, which is at its most unstable at sunrise and sunset.
Radio noise is caused by both internal and external sources. Internal or “thermal noise” is generated inside the receiver. It is usually negligible in good quality equipment when compared to external noise sources.
External noise can originate from man-made (environmental) sources or from natural (atmospheric and galactic) sources.
Atmospheric noise is most often caused by thunderstorms. Even distant storms can produce significant noise on HF frequencies.
Galactic noise comes from our galaxy. Since only higher frequencies will pass through the ionosphere from above, this usually only affects the higher frequencies.
Man-made noise results from large current and voltage devices (e.g. ignition systems, neon signs, power transmission lines and cables, welders, etc.). Man-made noise is more often vertically polarised, so using a horizontally polarised antenna can help reduce man-made noise considerably.
10 Metre & VHF Propagation
Propagation on 27/28/29MHz and VHF (30 to 300MHz) is primarily line-of-sight. It is for this reason that antenna systems for 27/28/29MHz and VHF need to be mounted as high as possible and clear of obstructions. Antennas should also concentrate radiation at low angles, unless communicating with aircraft, otherwise the signal may pass over the top of the receiving antenna.
27MHz and the lower frequencies of the VHF spectrum can sometimes propagate over long distances. This can occur:
- around solar maximum and during the day when the F layer will often support long range sky wave communications on 27MHz and VHF;
- sporadic E layers can often support 27MHz and lower VHF frequency propagation over around 500 to 1000 nautical miles (1000 to 2000 km). This is more likely to occur at mid-latitudes and during the daytime and summer months;
- 27MHz and VHF can also propagate through ‘ducting’ (temperature inversions) at altitudes of a few kilometres, which gradually bend the signals to follow the curvature of the Earth, possibly travelling up to several hundred nautical miles. UHF signals can also propagate via ducting.