...Local Area Communications
- Expanding the Choices
Today the trend in land radio and television communications is often twoards VHF and progressively towards UHF and higher frequencies using a network of repeater stations. The new systems often promise better and better reception qualities but these are frequently not realised when the topography is taken into consideration as these systems rely on line-of-site to deliver their promised advantages - in fact to deliver at all.
Moreover, the cost of installing this networks can be considerable and then there are the charges associated with using them. For many countries and for aid agencies working with developing nations, the High cost means that significant choices have to be made in what can be afforded and when the results are less than satisfactory, the repercussions are great. Those planning the network need to be well up on local ground conditions if repeater locations are to accurately overcome line-of-site obstructions.
At best the VHF direct wave has a line-of-site range of about 80km (50 miles). As the systems use higher frequencies, the problems increase. This is due to the nature of VHF/UHF direct waves which get shorter, travel shorter distances and are less able to compensate for hilly terrain as the frequency increases.
At VHF frequencies the direct wave has the bility to bend and scatter through a process known as diffraction to fill in around corners to some extent. However, as the frequency increases, this ability decreases and the size of the shadow zone where no signals reach reach gets larger.
Obviously these difficulties can be overcome with satellite communications, but operational costs are even more restrictive.
By contrast, HF communications are free-to-air. Normally close-in communications are restricted by a phenomenon known as the skip zone which exists between where the ground wave ends and the first skywave comes in. This is often an area where you are likely to want to communicate within. The extent of the skip zone varies in area depending on frequency, location, season and time of day.
However, it is possible to minimise the skip zone black-out area and communicate within the skip zone by using antennas that exhibit very high angle (near verticle) take off patterns.
This is known as NVIS (Near Vertical Incidence Skywave) Mode.
In this mode the take-off angle is between 80-90( above the horizon, the vertical signal is then reflected down to earth by the ionosphere in an umbrella shape. Communications can work well regardless of terrain making it ideal for mountainous or jungle areas and also for mobile convoys. Going round a corner does not result in lost signals.
With most professional installations, it is desirable to erect an HF 1/2 wave dipole antenna as high as possible above ground to get maximum range. However, this is not suitable if you wish to use NVIS mode as well, because lower angle radiation is favoured at the expense of the higher angles as antenna height above ground is increased.
Using a broadband horizontal dipole erected a minimum of 3m (9.8ft) to a maximum of 5m (16.4ft) above ground it is normally possible to use NVIS mode between 2-16 MHz. With such an antenna consistent results may be achieved in the range 6-12 MHz. A broadband horizontal dipole erected at these heights above ground is fully omnidirectional in NVIS mode operation.
Erecting a broadband horizontal dipole at 10m (32.8ft) above ground will also give NVIS mode but NVIS mode will only be available up to 6 MHz. Erecting the antenna slightly lower than 3m (9.8ft) will make NVIS communications possible on 16-22 MHz but these frequencies are more susceptible to changes in propagation.
Antennas with slanted wires are also very good for both NVIS and medium distance communications. The tilting of the wire gives both high and low angle radiation. These types of antennas typically have the added advantage of being extremely economical as they require less space for erection.
NVIS mode may be used for marine communications as well as land. In marine installations, a vertical whip may be lowered to a horizontal or near horizontal position. The ground may be the sea itself, or, when erected at cabin roof height and lying over a metal deck, the deck itself.
Where it is not desirable for the antenna to be visible whether it be for covert reasons or on sensitive environmental grounds, it is possible to suspend a broadband horizontal dipole in a trench and use NVIS mode. Such an installation may allow you to use NVIS mode across a wider range of frequencies but at reduced efficiencies. It is even possible in extremely arid terrain or on ice or smow, to lay the antenna on the ground.
Projected NVIS radiation patterns can be plotted for individual antenna installations across a range of frequencies.
Suitable antenna types for land NVIS communications are the Moonraker HFB D/S , the HF FD230 and the HF EF230, while at sea the type 23L/D may be used for normal communications in vertical position and NVIS communications in the horizontal laydown position.