GB2057773A - Aerial systems - Google Patents

Abstract

In order to modify or improve the performance of a radio transmitting aerial it is often desirable that the ground around the aerial act as a reflector of the emitted radiation. At present a constant high reflectivity of the ground around an aerial is commonly attained by laying in the ground a grid-like network of wires at a spacing considerably less than the wavelength the aerial is working at, but this is an expensive operation, and a cheap alternative is required. The invention provides a method for improving the reflector effect of the area of ground (bounded by line 12) upon which the aerial (10/11) stands, in which method the ground is permeated with an aqueous, preferably saturated, solution of calcium sulphate (as 13) so as to increase its conductivity and thereby improve its reflectivity. <IMAGE>

Description

SPECIFICATION Aerial systems This invention relates to aerial transmitting systems-that is to say, systems for the transmission of electromagnetic radiation.

In order to modify or improve the performance of a transmitting aerial it is often desirable that the ground around the aerial act as a reflector of the emitted radiation. This it will do provided that the conductivity of the soil is sufficiently high, and thus that its resistivity is sufficiently low (which is in general so if it is less than 10,000 ohm-cm).

The two major factors determining soil conductivity/resistivity are moisture content and the electrolytic materials distributed throughout the volume of soil. The main solid components of soil-various clays and gravels-are very poor conductors of electricity, even when wet (pure water itself is a poor conductor).

However, most soils naturally contain small amounts of minerals, such as calcium and magnesium carbonate, which are electrolytes, and which dissolve in water to form relatively highly conductive solutions. Thus, in general wet soils are reasonably good conductors by virtue of their electrolytic material content.

Unfortunately, the conductivity of many ordinary soils is insufficient for them to constitute a good radiation reflector. This may be because the soil has an inherently low natural electrolytic material content, or because it is too dry, or even because it is permanently too wet (the electrolytic materials having been leached out). Moreover, in some cases the state of the soil may vary between wet and dry, with consequent variations in conductivity, and thus reflectivity, to such an extent that highly undesirable adjustments are constantly being effected to the aerial installation to compensate.

At present a constant high reflectivity of the ground around an aerial is commonly attained by laying in the ground a grid-like network of wires at a spacing considerably less than the wavelength the aerial is working at, but this is an expensive operation, and a cheap alternative is required.

It is known that soil conductivity may be improved by wetting the soil with a highly conductive solution of some suitable electrolytic material such as sodium chloride (common salt), sodium carbonate (washing soda), calcium chloride, copper sulphate, sodium sulphate (Glauber's salt) or magnesium sulphate (Epsom Salts).However, none of these materials provides a satisfactory answer; the high conductivity/low resistivity of aqueous solutions of these materials (for common salt, a typical 20"C resistivity of 250 ohms-cm for a 2.4 gpl solution) tends to go hand in hand with their high solubility (for common salt, about 360 gpl at 0 C), and the latter necessarily means that they are readily leached from the soil so that within a few years- sometimes even within a few months-the highly conductive ground has reverted to its original poorly conductive state, and the treatment then needs to be carried out all over again.

We have now found, rather surprisingly, that calcium sulphate (commonly available in unhydrated form as "anhydrite" and in hydrated form as "gypsum"), a material of low water solubility (about 2 gpl at 0 C), can nevertheless be used to provide a very acceptable soil conductivity, for a saturated aqueous solution has a resistivity as low as 500 ohm-cm at normal soil temperatures (about 10 C).

In one aspect, therefore, the invention provides a method for improving the reflector effect of the ground adjacent an aerial for the transmission of electromagnetic radiation, in which method the ground is permeated with an aqueous solution of calcium sulphate so as to increase its conductivity and thereby improve its reflectivity.

In another aspect, the invention provides an aerial system for the transmission of electromagnetic radiation, which system comprises the aerial itself together with the ground adjacent thereto, this ground being permeated with an aqueous solution of calcium sulphate.

The word "adjacent" used herein (as in "the ground adjacent the aerial") is defined to mean the ground upon which the aerial stands, including the ground extending therefrom away from the aerial up to the limit at which no increase in reflectivity has any significant effect.

The aerial itself may be any of those types used for the transmission of electromagnetic radiation, but the invention is of prime value in connection with the aerials employed for the transmission of short, medium and long wave radiation-that is, for radiation in the 10 to 1000 metre range (a frequency range of 30 MHz to 300 kHz). Aerials for use at these wavelengths/frequencies tend to be flat arrays of dipole or monopole antennae, the arrays extending over an area of ground as large as several hectares.

The ground adjacent the aerial is permeated with the calcium sulphate solution to an extent-in respect of direction, area and depth-sufficient to result in the desired reflectivity. Thus: As regards direction, in many cases the aerial is directional, and the ground may be permeated in whatever direction (from the aerial) is appropriate.

The area of permeation, like the direction, is chosen to ensure that the desired reflectivity is obtained. In general, however, the useful area is bounded by a line a distance from the aerial determined by the effective aerial height (the taller the aerial, the further the line). In most cases this boundary is preferably 2 to 3 times as far from the aerial as the latter is tall-say (for a conventional shortwave dipole array), from 50 to 1000 metres from the boundaries of the array.

The ground is permeated to a depth sufficient to produce the desired reflectivity. The actual depth is determined mainly by the electromagnetic radiation concerned (shortwave radiation is reflected more easily than longer wave radiation), but in general permeation to a depth of about 2 to 3 metres is sufficient.

The soil adjacent the aerial is permeated with the calcium sulphate solution, meaning that the solution is thoroughly diffused throughout the soil volume concerned. Nor- mally the soil is saturated with the solution, so that it holds as much as it is physically capable of holding.

The aqueous solution of calcium sulphate permeating the soil is itself the main conductive portion of the earth reflector. The actual conductivity of the solution per se is not particularly significant (save that it should be high as compared to that of ordinary soil), so that within reason the concentration of the solution is not especially important. Thus, a calcium sulphate concentration of from 1 gpl up to saturation at soil temperature (10'C)- thus, up to about 2.5 gpl-is quite acceptable, having a resistivity of from 1,500 ohmcm down to about 500 ohm-cm, but for sheer ease of preparation it is preferred always to use a saturated solution.

The expression "calcium sulphate" here has its ordinary significance, meaning the chemical itself. In practice, of course, the calcium sulphate employed may take any of the chemical/mineral forms available, and in the majority of cases is available as, and is most conveniently used as, the hydrated material gypsum, in one or other of its naturallyoccurring mineral forms such as selenite (also known as Satin spar).

The calcium sulphate solution may be applied in any convenient way to the ground area to be rendered conductive, but generally it is preferred to top-dress the soil with solid particulate calcium sulphate, to plough this into the top-soil and sub-soil, and then liberally to dowse the whole area with water so as to cause sufficient of the calcium sulphate to dissolve to form a saturated solution which then permeates throughout the desired soil volume.

As will easily be appreciated, it is the advantage of calcium sulphate that it gives good conductivity at low concentrations but is only slightly water-soluble. Thus, despite considerable watering (as in many years of rain or the movement of ground water) the mixed-in solid material will provide a residue of undissolved calcium sulphate to replace that leached out of the soil by that rain or ground water.This retention of a reserve of calcium sulphate can be enhanced by providing the chemical in a suitable range of fairly coarse particle sizes, rather than as a uniform fine powder or large particle size form (very fine particles tend to clog up the soil, impeding moisture flow and - thereby reducing the required effect, while large particles dissolve too slowly); the smaller particles in the range will dissolve faster (to provide the initial concentration), while the larger particles will dissolve more slowly (to provide the reserve). A suitable range of particle sizes is that set by British Standard BS 882 Zone 1 for fine aggregates (this British Standard is reproduced in graphical form in Fig. 1 of the accompanying drawings).

As will be appreciated, transmitting aerials can be located in areas where the ground level is neither flat nor horizontal. In such cases the invention may not be entirely applicable, for in general it is required that the earth reflector portion of an aerial system be flat (i.e., free from bumps or undulations), so as to avoid scattering of the radiated energy, and horizontal, so as to avoid adding an unwanted directional component thereto.

The invention is now described, though by way of illustration only, with reference to the accompanying drawings in which: Figure 1 shows in graphical form the particle sizes recommended by BS 882 Zone 1; and Figure 2 shows a perspective view, in cutaway diagrammatic form, of an aerial system of the invention.

British Standard BS 882 relates to recom mendations for the grading of particle sizes for the aggregate used (mixed with cement powder) in the preparation of concrete. Zone 1 of this standard concerns fine aggregates; the relevance to the present invention lies in that the particle size range of Zone 1 seems eminently suitable for the gypsum particles whose use is preferred in the constructional method of the present invention.

Zone 1 is illustrated graphically in Fig. 1; the graph is a plot of sieve mesh numbers/ sizes against the percentage, by weight, of particles passing therethrough. Upper and lower spectrum lines are shown, and for a sample to meet the standard its spectrum line should fall somewhere between the two.

Fig. 2 (which is not drawn to scale) shows a short wave aerial array consisting of three 50 metre long centre-fad (by means not shown) aerial wires (as 10) arranged side-by-side 10 metres apart and supported by 1 5 metre posts (as 11). The ground under and around the array (bounded by the dotted line 12), extending to 20 metres beyond the array, has been sown with gypsum (graded to BS 882 Zone 1) at a rate of 2 S(g/m2, and this has been ploughed in to the top 20 cms of soil.

The whole area has then been saturated with water, and the formed calcium sulphate solu tion has permeated the treated ground (as 13) to a depth of 2 metres.

Claims (11)

1. A method for improving the reflector effect of the ground adjacent (as hereinbefore defined) an aerial for the transmission of electromagnetic radiation, in which method the ground is permeated with an aqueous solution of calcium sulphate so as to increase its conductivity and thereby improve its reflectivity.

2. A method as claimed in claim 1, in which the aerial is a flat array of dipole or monopole antennae.

3. A method as claimed in either of the preceding claims, in which the aerial is directional, and the ground is permeated in whatever direction (from the aerial) is appropriate.

4. A method as claimed in any of the preceding claims, in which the area of permeation is bounded by a line a distance from the aerial that is 2 to 3 times greater than the aerial is tall.

5. A method as claimed in any of the preceding claims, in which the ground is permeated to a depth of 2 to 3 metres.

6. A method as claimed in any of the preceding claims, in which the soil adjacent the aerial is saturated with the calcium sulphate solution.

7. A method as claimed in any of the preceding claims, in which the aqueous solution of calcium sulphate permeating the soil has a concentration of from 1 gpl up to saturation at soil temperature.

8. A method as claimed in any of the preceding claims, in which the calcium sulphate is used as the hydrated material gypsum.

9. A method as claimed in any of the preceding claims, in which the calcium sulphate solution is applied to the ground area to be rendered conductive by top-dressing the soil with solid particulate calcium sulphate, ploughing this into the top-soil and sub-soil, and then liberally dowsing the whole area with water so as to cause sufficient of the calcium sulphate to dissolve to form a saturated solution which then permeates throughout the desired soil volume.

10. A method as claimed in claim 9, in which the calcium sulphate used has the range of fairly coarse particle sizes defined by British Standard BS 882 Zone 1 for fine aggregates.

11. A method as claimed in any of the preceding claims and substantially as described hereinbefore.

1 2. An aerial system for the transmission of electromagnetic radiation, which system comprises the aerial itself together with the ground adjacent thereto (as hereinbefore defined), this ground being permeated with an aqueous solution of calcium sulphate in accordance with a method as claimed in any of the preceding claims.