designing for MF and LF communications
When radio began, it started with low frequency communications, which it was thought was the answer to long distance communications. Since then we have seen the development of sky wave communications on HF frequencies, followed by the proliferation of communications system using the VHF and UHF direct wave and microwave. It may be thought by some that low and medium frequency communications are somewhat outmoded, but in reality they are still very much in evidence today…
Communications at low and medium frequencies mostly use the dominant ground wave. These frequencies offer significant propagation stability compared with HF and are relatively immune to jamming. Because attenuation of the ground wave increases with frequency, signal strength is greater and signals can travel much further than at VHF or UHF. The relatively large wavelengths are not as affected by obstructions, such as mountains, trees and buildings, or by weather conditions. However, the wave lengths also mean that data transmissions are very limited and become more so as the frequency decreases.
The MF band (300 kHz to 3 MHz) is used for broadcast communications, international distress frequencies and for search and rescue. Non Directional Beacons (NDBs) at airports and on oil rigs, Navtex coastal stations broadcasting information on weather, navigation warnings and search and rescue alerts, and maritime GMDSS stations retransmitting GPS navigation positioning using differential GPS to obtain greater accuracy all use this band.
The central part of the band has traditionally been the home of AM Radio, and while many commercial broadcast stations have moved up the bands, some have remained and a new generation of low power community radio stations have moved in. The high and low ends of the band are used for military communications.
The LF band (30- 300 kHz), is mainly used for aircraft NDBs, navigation (LORAN) information and weather systems. There are also time synchronisation stations for watches and clocks and some European AM broadcast services located in this band.
Below 30 kHz, signals can penetrate below the water and underground, providing communications for submarines and mines, as well as aircraft and vessel navigation beacons.
Because wavelengths are very close electrically to the interface between the earth and air, range is affected by ground conductivity, which varies considerably, being influenced by type of soil, degrees of moisture and salinity and by the nature of land development. Dry soils and urban locations are less favourable, while salt water exhibits the highest conductivity, being in the region of 5,000 times more conductive than dry ground.
As the waves travel over the ground, energy is absorbed into the earth. 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.
Below 500 kHz atmospheric noise, which increases progressively as the frequency is lowered, determines range rather than wave fading. While wave strength remains fairly static, noise levels during a typical day may cover a range of 20dB. Thunderstorms in the general area of a receiving station may increase levels to 80 or 100 dB on occasion. Levels of noise also are subject to seasonal variation. Typical unattenuated field intensities at 100 nautical miles for 100 watts input are:
| at 300 kHz | 62 uv or 36dB/uv (ref cap) |
| at 400 kHz | 87 uv or 39dB/uv (ref cap) |
| at 500 kHz | 111 uv or 41dB/uv (ref cap) |
| (Reference Moonraker Type 100MF 10 metre vertical) |
While these figures may appear to be satisfactory, in the tropics, which is one of the noisiest areas in the world with respect to radio frequencies, communications difficulties arise. At these latitudes it is not unusual for the ambient or local noise level to reach as high as 36 dB/uv (decibels per microvolt) when you most require the best signal.
This may mean that for 300 kHz the useful range can fall to less than 70 nautical miles and to about 130 or 1150 km (80 or 90 miles) at 500 kHz. This is IF all the other parameters are working perfectly.
Above 500 KHz noise levels decrease progressively and range is determined more by antenna/earth efficiencies and transmitter power. Therefore investment in a good antenna/ground system that maximises radiation efficiency and keeps losses to a minimum is of critical importance. During the daytime, propagated skywave signals are generally absorbed by the D layer of the ionosphere. On dissipation of this layer at sunset, propagation via the F layer can cause ground wave interference on long circuits, resulting in minor signal strength variations.
For ground wave communications, antennas need to have vertical polarisation and are normally quarter wave or below due to the wavelengths. Unless the antenna system is land based and permanent, it is unlikely that it will be practical, or for that matter economical, to have the full quarter wavelength which should be around 250m (820ft) at 300 kHz decreasing to 150m (490ft) at 500 kHz. On oil rigs, for example, frequently the maximum possible antenna length is around 10m (32ft). So, antennas are often electrically short limiting performance and just look like a small capacitance at those frequencies.
As wavelengths within the low frequency band range from 1 to 30 km, this becomes increasing apparent at very low frequencies and is a primary consideration in the design of practical VLF/LF systems. Therefore, for antennas below HF, design objectives are usually to make the antenna appear electrically as long as possible and to provide maximum efficiency.
Greater efficiency can be achieved by using series resonance together with an inductor to neutralise antenna reactance, which is the most practical form of power feed or coupler.
| Therefore the principal tuning component is a high Q inductor. This can be managed by incorporating a top hat of radials at the top of the antenna just above the inductor or by increasing antenna height. The top hat electrically lengthens the antenna, thus increasing the efficiency, and also has the effect of concentrating low angle radiation. In the case of the Moonraker 100MF , the top hat of radials adds approximately 90pf of extra capacitance. To achieve this without the radials you would need to almost double the length of the antenna to around 20 metres (65.6 ft). |
Because low frequency antennas characteristically have low radiation resistance and relatively high capacitive reactance, the most practical form of power feed or coupler employs series resonance in the antenna circuit, using an inductance to neutralise the antenna reactance. The design of this tuning inductance is an important part of the design.
High currents flow through the inductance, so it is also important that resistance or R losses are kept low.
| Keeping radiation system or R losses to a minimum is essential due to the low radiation resistance. Although most losses occur in the ground and tuning inductors, conductors, insulators and structural supports can also contribute to loss. In Moonraker antennas the R losses are kept low by using aluminium as the radiator rather than thin pieces of wire in fibreglass which do not have this advantage. The object is to obtain the highest ratio of radiation resistance to total antenna/ATU circuit resistance. |
Even after taking these and other factors such as low insulation losses into account in the design the radiation efficiencies achieved for antenna and tuning unit are still only around 1.5% at 300 kHz and 4.9% at 500 kHz or 4.9 watts output for 100 watts in! Even these efficiencies are based upon having the antenna in free space with earth losses of 1 ohm and an ATU working Q or 300. It should be possible to obtain 1ohm earth loses on an oil rig installation at sea.
Losses can also occur in antenna conductors, insulators and miscellaneous items of the structural support system, and in some locations antenna icing, snow cover and frozen soil may also contribute. However, the principal loss in most systems occurs in the ground system and in the tuning network inductors.
When an antenna radiates, a circulating current is created with current flowing between the antenna and the earth. Losses occur when the current is absorbed by the earth rather than returned to the antenna. Where antennas are physically short and the percentage of power radiated is therefore low, the quality of the ground system will be a major factor in determining quality and range of communications.
Antennas below a half wave are dependent on a ground system for resonance, and, although a half wave antenna does not need an earth system in order to resonate, it does need one for radiation efficiency. Therefore, when an antenna is less than a wavelength in length, a good ground system is essential to keep the resistance of the earth or earth losses to a minimum.
As not all types of terrain are able to conduct electricity equally, how much current is returned to the radiator depends on the actual degree of conductivity of the ground. The use of a good ground system ensures that these losses are kept to a minimum so that the maximum possible power is available for radiation.
| Type of Terrain | Ground Conductivity |
|---|---|
| Sea Water Pastoral Flat, marshy, densely wooded Rocky Sandy City, industrial | Highest | to | Lowest |
Grounded vertical antennas at sea have the best possible earth system. Wooden and fibreglass vessels need to make a good connection with the water with earth plates located where they will be under water at all times.
For land based communications, sites need to be as flat as possible, especially in the direction of main interest, and soil depth and conductivity should be assessed. Siting a marine base station close to the sea will greatly enhance performance, as will connection with the water table where it is very high.
Ground losses within the site area can be reduced by providing a low reactance low resistance ground system. this may consist of a radial network of wires buried a half metre or so below the surface and extending in all directions to some distance beyond the antenna.
While it is a good idea to have as many radials as possible in a ground system, each installation will have an optimum number directly related to the antenna length, working frequency and type of ground. Above this number, the increase in performance is minimal in relation to the cost of the additional radials. At Moonraker we are always happy to design individual ground systems, especially in the case of MF/LF communications where it is most important.
Naturally there are other aspects that need to be taken into account. For one, the system bandwidth needs to be sufficient to maintain tuning in varying antenna conditions. Wind deflection is an important consideration for installations on oil rigs due to down and side drafts from helicopters, which need to be taken into considerations. For example, the bandwidth of the Moonraker 100MF and tuning unit, which is only a short 10 metres in length, is 1.128 kHz at 300 kHz and almost 2 kHz at 500 kHz at the 3dB down points. This antenna, often used on oil rigs, has a wind deflection of less than 3 metres at the top at 216 km/h. Overall the antenna has been designed to be as compact as possible yet maintain the best efficiency.
Antenna selection depends primarily on frequency, power radiating capability and bandwidth requirements with performance directly related to effective height and volume. With limitations in available site area and budgets, it is important to ensure that the resulting system performance will be sufficient.
Where space permits, T Top and Inverted L system, using multi-wire panels (capacity hats) suspended between two masts, can give good results for low power applications in the upper part of the LF and MF bands. T Top antennas are commonly in use at airports and maritime coastal DGPS stations.
At VLF higher transmitter power and larger top hat panels supported by additional end or side masts and support catenaries are normally required.
and the Trideco, which features one or more rhombic or triangular
shaped multiple wire panels suspended from three or more masts.
The rarely used Valley Span antenna is suspended across land forms, like deep valleys or mountain ridges.
Base insulated towers, freestanding or guyed, are frequently used for low power applications mainly in the MF broadcast band (535-1605 kHz) at heights of 1/6th to 5/8th wavelength. Top loading can be provided by active radials or sections of the upper support guys. This can be more cost effective for low frequencies than the simple vertical radiator or the T or inverted L types.
Top loaded monopoles, like the Moonraker freestanding 10 metre 100MF and guyed 15 metre 150MF are in common use at low frequencies. For the 150MF, loading is provided by additional active radial guys attached to the top of the tower and broken up by insulators at some point down from the top. These top loading radials can also function as support guys. With receiving antennas, performance is generally limited by atmospheric noise and antenna efficiency is not as important. Normally whip type antennas are satisfactory. Other types of antennas, like electrical and magnetic loops are used to enhance reception of the wanted signal and eliminate noise interference.
Article originally available at http://www.moonraker.com.au/techni/shortantennas.htm