GB2150359A - A wide band antenna - Google Patents

Abstract

A wide-band antenna is intended to enable operation in the transmitting or receiving mode between 30 MHz and 1 to 2 MHz. This antenna principally comprises a radiating vertical mast 1 of a height of some ten metres which provides a vertically polarised radiation throughout the frequency band, a conductive spiral 2 secured on to the guy lines 3,4,5,6 securing the mast and a conductor 7. The spiral which is connected in series between the radiating mast and the conductor forms a choke or impedance coil for the high part of the frequency range and an antenna having a circular horizontal polarisation for the low part of the range. The conductor, which has resistor R inserted in it, connects the spiral to the ground, the resistor being intended to absorb the reflections at the low frequencies of the range. The radiating mast may comprise a steel member or a conductive spiral on an insulating member (Fig. 3). <IMAGE>

Description

SPECIFICATION A wide band antenna The present invention relates to a wide-band antenna and in particular to an antenna for telecommunications within the range of decametric waves and within the low range of hectometric waves, up to frequencies of the order of 1 to 2 megahertz.

There exist antennas of the vertical monopole type having bands of utilisation frequencies corresponding to the aforesaid waves. Some of these antennas are utilised for reception and may have two outputs with a switching filter in order to divide the band into two parts for optimisation of each subsidiary band utilised. These antennas having the disadvantage of requiring the installation of two co-axial connecting leads and two receivers.

Antennas of the monopole kind having a height of 10 metres are equally known, which are employed for transmission and have a single input. These require an antenna tuning box with all the restrictions this implies, in order to cover the range from 1.6 to 30 MHz.

More complex antennas are also available, which are utilised for transmission purposes. These comprise a first part formed by monopoles or dipoles lined up according to a logarithmic progression and having two inputs in order to allow utilisation, respectively, of this first part and of a second part which is comparatively similar or wholly dissimilar to the first part, the whole being able to reach a height of 40 metres. If one transmitter only is available, it is necessary to make use of a remotely controlled electromechanical switching system or of a power switching filter.

Biconical antennas also exist, which are formed by linear radiating elements evenly distributed around a vertical mast and tensioned by guy-lines or stays in a manner such as to form the two sides of a triangle of which the third side is formed by the mast. Monoconic antennas are also known, formed by linear radiating elements distributed evenly along the generatrixes of a circular cone having a vertical axis and a downwardly turned apex. These antennas may be utilised for transmission but in view of the range of utilisation frequencies corresponding to the aforesaid waves, their height amounts to around forty metres. These antennas which are high and have a more complex structure than the single monopole, are utilised to secure the best possible efficiency.

The present invention has as its object to avoid or at least to secure a considerable reduction of the aforesaid shortcomings and to discover the best compromise within a height arbitrarily limited to ten metres.

This is accomplished by the combination within one and the same antenna of two kinds of aerials connected in series and of which one is devised more particularly for transmitting the higher frequencies and the other for transmitting the lower frequencies.

According to the invention there is provided a wide-band antenna intended in particular for telecommunications on decametric waves, which comprises a vertical mast comprising a vertical radiating part, an insulating base and means of access to the lower extremity of the radiating part which provide access to the antenna, n electrically insulating guys or stays attached close to the top of the mast (n being equal to at least 3), a conductive spiral having n segments per turn suspended between the guys or stays, this spiral having a first extremity connected to the upper extremity of the radiating part and a second extremity situated above ground, an electrical conductor and a resistor in series with the electrical conductor, said conductor providing a direct connection between the second extremity of the spiral and the ground, and wherein the mean value of the characteristic impedance of the spiral is substantially equal to that of the vertical radiating part and to the value of the resistance at the low frequencies of the utilisation band of the antenna.

A better understanding of the present invention will be gained and other features will become apparent from the following description and the accompanying drawings, in which: Figure 1 illustrates a first embodiment of antenna in accordance with the invention, seen in perspective, Figure 2 illustrates the antenna of Figure 1 as seen from above, Figure 3 illustrates an element of a second form of antenna according to the invention, Figure 4 illustrates a third form of antenna seen in perspective.

Corresponding elements are denoted by the same symbols in these different Figures.

In the following and in the claims, the antennas will most often be described in the transmitting mode, although it is understood that these antennas may just as well be utilised in the receiving mode, and that if mention is made of the radiating part of the mast, it should be understood that for operation in the receiving mode, what is meant is the part of the mast serving the purpose of picking up electromagnetic waves.

Figure 1 shows an antenna comprising a mast 1 held in a vertical position by four guys or stays 3 to 6. Starting from the ground, the mast 1 comprises: an insulating base 11, a vertical conductive cylinder 10 of galvanised steel, having a length of 10 metres and a diameter of 300 mm forming the radiating part of the mast, and an insulating masthead 12 which has secured to it the upper ends of the guys 3 to 6. The access E to the antenna is situated close to the lower extremity of the cylinder 10 forming the radiating part, this access being formed by a terminal integral with the cylinder 10.

The four guys 3 to 6 are metal cables evenly spaced around the mast and having an inclination of approximately 45 degrees, which are not only insulated from the radiating part 10 of the mast by means of the masthead 12 but are also insulated from the ground by means of insulating fasteners A,B,C,D, integral with the guys and denoted diagrammatically by small circles. The comparatively long lower parts of these four guys are interrupted by insulators so that the possible half-wave resonances of the guy line elements lie beyond 30 MHz.

A spiral 2 formed by a stainless steel cable of a diameter of 2 mm runs downwards substantially from the top H of the insulating part to a height of 5 metres on the guy line 3. This spiral is formed by four segments per turn and is wound so as to bear successively on the four guys via insulating connectors such as JI to J5 and J16 integral with the guys. These insulating connectors, like the fasteners A,B,C,D, are denoted diagrammatically by small circles. The spiral 2 is a descending quadrangular spiral formed from 15 segments, the topmost of these segments ending at the connector J16 and being extended by a linkage, still consisting of a cable of a diameter of 2 mm up to close to the top H of the radiating part. This linkage is terminated at its upper extremity by a welded connector, which is not illustrated, which is screwed into the cylinder 10.The mean value of the characteristic impedance of the spiral is selected to be substantially equal to that of the mast.

At its lower extremity K, the spiral 2 is extended by an electrical conductor 7 which is also formed by the same cable as that of the spiral, the conductor 7 connecting the point K to a point on the ground situated close to the foot of the mast 1. A resistor R is inserted into the conductor 7 close to the ground.

Figure 2 is a view from above of the antenna of Figure 1 which clearly shows that the element 2 is a spiral.

The antenna in accordance with Figures 1 and 2 was devised for operation without electromechanical switching of matching elements, in a frequency range extending from 1.6 to 30 MHz. As a matter of fact, when the antenna is utilised for transmission for example, the mast 1 fed via the access E at its base, radiates in vertical polarisation at all frequencies within the range and the spiral 2 acts as a choke or impedance coil at high frequencies.

By contrast, at the medium and low frequencies within the range from 1.6 to 30 MHz, the spiral acts as a transmitting antenna radiating with a circular horizontal polarisation which promotes a zenithal transmi ssion. As for the resistor R, this is a damping resistance of which the value is substantially equal to the mean value of the characteristic impedance of the spiral at the low frequencies of the band, in order to absorb all the reflections possible at these low frequencies. In the example described and where the antenna is intended to operate with a 10 KW transmitter, the resistance R has a resistance of 240 ohms and is able to dissipate 2 KW at 70" Centrigrade.

Measurements have demonstrated that between 30 MHz and approximately 15 MHz, the antenna makes it possible to operate at medium distances (1000 to 1500 km ) thanks to the mast 1, and that at lower frequencies, the antenna enables transmissions over short distances (0 to 300 km ).

Figure 3 is a side view of another embodiment of a mast of an anterna in accordance with the invention, which differs from the antenna according to Figures 1 and 2, only by its mast. This mast 1, having a height of 10 metres, comprises an insulating base 11 integral with an insulating cylindrical tube 10a having a diameter of 300 mm , the tube 10a being topped by an insulating masthead 12 comprising four rings such as 12a for securing the guy lines of the antenna. On the insulating tube acting as a carrier, is helically wound a conductive cable 10b of stainless steel and having a diameter of 2 mm , which forms the radiating part of the mast.

The access E to the antenna is formed by a conductive terminal attached to the insulating tube 10a and whereat terminates the lower extremity of the conductive cable 10b.

Figure 4 is a perspective view of another antenna according to the invention. This antenna 1 comprises a vertical mast 10 of a height of 10 metres, of which the radiating part is a galvanised steel cylinder, this mast being supported by three guys 30,40,50 distributed in vertical planes passing through the mast and set at 120 degrees to each other, the slope of the guys being approximately 45 degrees. A spiral 20 of conductive cable of which one extremity is connected to the mast top runs down to a point K situated at 5 metres from the ground, this spiral comprising three consecutive segments per turn and each segment being supported by its extremities between two guys.

In the embodiment described with reference to Figure 4, the guys utilised are insulating cables so that these guys do not comprise insulator elements either for their fastening to the ground and to the mast top, or for attachment of the spiral. The only insulating member appearing in Figure 4 is the mast base 11 situated just below the access terminal E of the antenna.

As in the embodiment according to Figures 1 and 2, the spiral of the antenna according to Figure 4 is extended by a conductor 7 directed towards the base 12 and which is terminated close to this base by a damping resistor R of which the extremity facing away from the conductor 7 is connected to earth. Here again, the value of R and of the mean characteristic impedances of the spiral 20 and of the mast 10, are substantially equal.

The antenna according to Figure 4 operates like the antenna according to Figures 1 and 2: with the mast providing a vertically polarised radiation at all frequencies of the range from 1.6 to 30 MHz, and the spiral providing radiation with a circular horizontal poiarisation at medium and low frequencies.

The present invention is not restricted to the embodiments described; thus the number of guy lines may exceed four and the spiral may run down to a greater or lesser depth, it being understood that the spiral should not be too close to the ground in view of the risk that its operation may be impaired by what may be described as a short-circuit between the spiral antenna and its image with respect to the ground. Similarly, in the case in which the mast comprises an insulating cylinder whereon is coiled a conductive wire, which forms the vertical radiating part of the antenna (as in Figure 3), the base of the mast may simply be formed by the lower extremity of the insulating cylinder. A pylon formed by angle irons, tubes or bar stock assembled together by bolting or welding may equally be utilised as a mast for the antenna.

Regarding the spiral of the antenna, this need not be formed by a single cable as in Figures 1,2 and 4, but may be formed by several substantially parallel cables joined together at intervals by means of short-circuits, these parallel cables having the action of increasing the apparent dimensions of the conductor of the spiral whereas the short-circuits are intended to prevent resonances occurring within the spiral.

It should be observed that the conductor 7 of Figures 1,2 and 4, which connects the spiral 2 or 20 to the damping resistor R, may instead of being linear be formed by a second spiral wound in the same direction as the spiral 2 or 20 and decreasing down to the damping resistor, in which case, small auxiliary bracing wires springing from the foot of the mast and joined to the other guy lines, support the second spiral. As for the resistor R, this may be situated at another point of the conductor 7, for example close to the point JI of Figures 1 and 2 or to the point K of Figure 4.

Claims (7)

1. A wide-band antenna intended in particular for telecommunications on decametric waves, which comprises a vertical mast comprising a vertical radiating part, an insulating base and means of access to the lower extremity of the radiating part which provide access to the antenna, n electrically insulating guys or stays attached close to the top of the mast (n being equal to at least 3), a conductive spiral having n segments per turn suspended between the guys or stays, this spiral having a first extremity connected to the upper extremity of the radiating part and a second extremity situated above ground, an electrical conductor and a resistor in series with the electrical conductor, said conductor providing a direct connection between the second extremity of the spiral and the ground, and wherein the mean value of the characteristic impedance of the spiral is substantially equal to that of the vertical radiating part and to the value of the resistance at the low frequencies of the utilisation band of the antenna.

2. An antenna as claimed in claim 1, wherein the vertical radiating part comprises a conductive cylinder having a height of some ten metres.

3. An antenna as claimed in claim 1, wherein the mast comprises a vertical insulating support of a height of some ten metres, and a conductive cable coiled on the support which forms the vertical radiating part.

4. An antenna as claimed in claim 1, wherein the guys or stays each comprise a metal cable and insulating fastening means for insulating the metal cable from the ground and from the spiral.

5. An antenna as claimed in claim 1, wherein each of the guys or stays comprises an insulating cable.

6. A wide-band antenna intended in particular for telecommunications on decametric waves, which comprises a vertical mast comprising a vertical receiving part, an insulating base and means of access to the lower extremity of the receiving part which provide access to the antenna, n electri caily insulating guys or stays attached close to the top of the mast (n being equal to at least 3), a conductive spiral having n segments per turn suspended between the guys or stays, this spiral having a first extremity connected to the upper extremity of the receiving part and a second extremity situated above ground, an electrical conductor and a resistor in series with the electrical conductor, said conductor providing a direct connection between the second extremity of the spiral and the ground, and wherein the means value of the characteristic impedance of the spiral is substantially equal to that of the vertical receiving part and to the value of the resistance at the low frequencies of the utilisation band of the antenna.

7. A wide band antenna substantially as hereinbefore described with reference to, and as illustrated in, the accompanying drawings.