The Ionosphere is a layer in the atmosphere which exists because the very high energy of the sun’s radiation (EUV) ionises molecules in the upper atmosphere and gives them an electric charge. This layer is composed of positively charged ions (hence the name ionosphere) and free electrons. These charged particles act on radio waves in the HF range of frequencies. The Ionosphere is made up of several layers which are labeled as D, E, F1 and F2. During the night most of the layers disappear and the F1 and F2 layers combine.
The Ionosphere is not a stable medium and will vary due to several factors:
- Time of Day (Day/Night)
- Solar Cycle / Solar Activity
- Degree of Latitude
- Sporadic E
The ionosphere is stronger (more effective at attenuating and bending radio waves) during the day. (The sun’s radiation charges the ionosphere). During the night the energy dissipates and the ionosphere becomes weaker and lower. These variations have an effect on the frequency to be used for reliable communication.
Solar Cycle: The sun goes through a periodic rise and fall in solar activity due to Sunspot activity varying in length from 9 to 14 years. This cycle has
a direct effect on the Ionosphere and consequently an effect on the propagation of HF radio waves.
Solar Flares: can cause short and long term disturbances to the density and size of the ionosphere, this can effect HF propagation causing outages:
- 10 mins to 2 hours, Short Wave Fadeout
- 10 mins to several hours, absorption of HF signals at high latitudes
- 1 to 3 days, geomagnetic and ionospheric storms
- Solar flares may even disrupt VHF, UHF and Satellite comms and give airline passengers a high X-ray dose!
Ionospheric Variation Seasonal: The changes in season between summer and winter also have an effect on the Ionosphere. The ionosphere is more intense during summer and therefore useable frequencies are generally higher.
Frequency Variation with Distance: Sky waves can give reliable communications, planning must make sure the radio signal to be used:
- Will bend back at the correct angle
- Will not be absorbed (too low a frequency)
- Will not go through the ionosphere and without returning to the earth (too high a frequency)
Short Range Communication & The Skip Zone: For any frequency there is a Zone where the ground wave has run out of energy and the skywave has not been reflected back to earth. This is called the skip zone and is the major cause of problems with short range (15 – 150KM) HF communication. To solve this problem, 2 things are needed:
- Low frequencies which will not go through the ionosphere so easily at high angles of incidence
- Antennae designed to transmit at thigh angles – called a NVIS antennae. This would produce the kind of reflections from the ionosphere and thus reduce or completely get rid of the skip zone
Limitations of HF: The HF medium when used for communications is particularly susceptible to atmospheric and environmental interference compared to VHF or UHF.
- The ionospheric and it’s constant changes have a major effect on whether or not a signal can be received
- Vehicles and industrial machinery etc produce background or “white noise” which can override a signal being received
- Signals propagate over long distances and hence some users in one place can unintentionally interfere with someone thousands of kilometres away
- Lack of spectrum management
HF Frequency Propagation Summary.
- A Sky Wave reflected by the ionosphere is the most common method of HF communication
- Different HF frequencies are needed to communicate between stations separated by different distances and the wrong choice of frequency can make communication impossible
- Choose your frequency on the basis that “The hotter the time of day and the longer the distance, the higher the frequency
- When planning a network, frequencies need to be chosen taking the solar cycle into account, as well as the distance between the stations and the hours of operation
- Effective short range communication on HF requires a different antenna
Are you interested in Codan HF Training, if your staff need a comprehensive training course then we can help guide you, get in touch to book your team of delegates.