... The Ground Wave
Low frequency communications encompass marine and aeronautical navigation and communications systems, fixed public broadcast system and strategic communications systems. The importance of these services is highest in polar regions where severe ionospheric disturbances often disrupt high frequency communications.
ELF frequencies (3-30 Hz) are mainly used for underwater submarine communications as signals can penetrate up to several hundred metres deep with transmitter powers in the region of 100 MW, but messages are limited to 1-2 characters in length. VLF signals (3-300 kHz) can still penetrate the water with 100-200 kW transmitters but to much lesser depths. Low speed teletypewriters are normally used for navigation and shallow water submarine communications. With suitable transmit powers, ELF and VLF range can be more than 8,000 km (5,000 miles).
At LF frequencies (30-300 kHz) the range using 50-100 kW transmitters varies from 1,600-8,000 km (1-5,000 miles). Here communications with both submarines and surface ships at sea, and also with planes is possible using teletypewriter and Morse Code. There is also an AM broadcast band (144-351 kHz) in Europe and parts of Asia, and the US Air force ground wave emergency network in case of nuclear war operates on 150-175 kHz.
MF frequencies range from 300 kHz to 3 MHz, although the upper part of the band from 1.6 or 2 MHz is often classed with HF. Depending on transmitter power and frequency, ground wave range can range from 160-1600 km (100-1,000 miles). Located within these bands are navigation and other beacon and GMDSS NAVTEX stations. There is also an AM broadcasting band (550-1,600 kHz) which includes low power community radio stations.
Looking at the factors that affect propagation of the ground wave, it becomes easier to predict just how much range is possible in a given situation. This is highly important, especially where, as in the case of community radio stations, transmitter power is restricted.
Communications at low and medium frequencies mostly use the dominant ground wave. These frequencies offer significant propagation stability compared with the HF sky wave and significantly greater range than the VHF/UHF direct wave which is generally limited by line-of-site.
In ground wave communications, range is affected by a number of factors. The greatest determining factor is frequency: the lower the frequency the greater the range. After that, range will also depend on the power of the transmitter, the electrical characteristics (conductivity and dielectric constant) of the terrain over which the signal will travel, antenna efficiency and electrical noise at the receiver site.
Because low frequency wavelengths are very close electrically to the interface between the earth and the air, the ground has a considerable affect on propagation. As the waves travel over the ground, energy is absorbed into the earth. Also the energy travelling closest to the ground sees a higher dielectric constant than that of the air, causing it to travel at a slower rate As a result, the signal is attenuated or loses strength and gradually tilts over in the direction of propagation, causing the electrical field eventually to be shorted out.
Due to the fact that vertically polarised waves suffer less than horizontally polarised ones in this respect, vertical antennas are normally indicated for ground wave communications. Vertical antennas also have the advantage of low take off angles. For example, the attenuation of a vertically polarised ground wave at 2MHz transmitted over medium soil would be around 45dB at 30km whereas a horizontally polarised ground wave would be attenuated by nearly 95dB.
Ground conductivity varies considerably, being influenced by the type of soil, the degrees of moisture and salinity and by the nature of land development. Propagation is much less efficient over the land than over the sea. This is because the rate of attenuation varies considerably over a land path due to frequent changes in ground types while over the sea, there is relatively little variation. Salt water exhibits the highest conductivity, being in the region of 5,000 times more conductive than dry ground. Desert sand, dry soils, urban locations and polar ice are least favourable. If range is to be maximised, therefore, it is essential to have a good earth system.
A: Day time, temperate zone, B: Night time temperatezone,
C: Night time tropical zone
Ambient noise has an even greater impact on the received signal. As the frequency is lowered, noise levels increase, with the result that, received signal to noise ratio becomes an important design factor for any system.
Noise levels are greatest at low equatorial latitudes and lowest in polar auroral zones. Levels vary over a 24 hour period being highest a night. During the daytime the variation is normally in the region of 20 dB but this may increase to 80 or 100 dB when thunderstorms occur in the vicinity of the receiving station. Levels of noise also are subject to seasonal variation, being least in winter and greatest in summer. During tropical monsoons levels can be extreme.Therefore it is during the daytime in winter that greater distances can be achieved.
As a result, advance planning will need to allow for worst noise conditions. Higher power transmitters are normally indicated for tropical regions.
At 2 MHz the surface path normally has a maximum range of about 500km (300 miles). The usual minimum range is about 80km (50 miles). However, under ideal conditions (at midday during winter with low noise conditions) it is possible to communicate on 2 MHz over distances of about 900km (575 miles) by using a 100W transmitter with an efficient antenna/earth system.
Another important factor affecting propagation of the ground wave is that of the sky wave. During the daytime, on the lower frequencies below 3 MHz, sky wave signals are generally absorbed by the D layer of the ionosphere. On dissipation of this layer at sunset, propagation via the F layer can lead to co-channel interference from distant stations, and to selective fading when sky wave and ground wave signals from the same transmitter arrive at the receiver at different times. Fading can also occur on long circuits when sky wave signals take multiple hops to reach the receiver.
Propagation via the sky wave, therefore can cause serious problems at night, yet at the same time be welcomed by radio listeners who, provided there is no co-channel interference, can tune into distant stations.
As mentioned previously, range can vary from 160-1600 km (100-1,000 miles) at MF frequencies but decreases rapidly as the frequency is increased. In order to compensate for high noise levels and attenuation, special attention needs to be given not only to choice of transmitter power but also to translating as much power into radiated signal as possible.
This means that all losses in the antenna/earth system must be kept to an absolute minimum. At these frequencies required antenna lengths are normally impractical to achieve with the result that the antennas are often electrically very short. Therefore efficient antenna design is critical and earth systems need to be custom designed to suit site ground conditions.
Looking ahead, the future for the lower end of the radio spectrum is by no means bleak. Currently, moves are well underway to provide better quality reception for these frequencies by taking advantage of the advances made over the past two decades in digital techniques. The aims of the digital system are to improve the signal to noise ratio and make it independent of the received RF signal, and also to overcome the difficulties caused by the various types of multipath propagation. These techniques also introduce the possibility of offering additional information such as text alongside the audio. Many AM transmitters can be easily modified to carry DRM signals. However, a new type of digital radio receiver is required and before this system can be popularised, costs will need to be such that it is affordable worldwide.
Digital radio based on MPEG4 type audio and speech coding is currently in operation under the name of Digital Radio Mondiale (DRM). It offers near FM quality audio and can be used with data and text displayed on the receiver. Further information including live broadcast schedules is available at http://www.drm.org.