GB2141878A - Aerials - Google Patents

Abstract

A radio receiver and/or transmitter contained in a small, eg man-portable, case (2) and having a monopole aerial (12) is connected to the lower end of the aerial by a length of coaxial feeder (13) having its inner conductor connected between the lower end of the aerial and the receiver and/or transmitter, and the lower end of the outer conductor connected to an earth at the case. The feeder is wound as a helical coil colinear with the aerial, and its outer conductor, which carries the aerial counterpoise current, forms a slow-wave structure of such electrical length that the total current distribution over the aerial-plus-feeder has a relatively high value (low impedance) at the lower end of the aerial and a relatively low value at the case, giving good stabiliry and bendwidth, while allowing the lower end of the aerial to be physically close to the case. In one example, using a quarter-wave monopole, the helically-wound feeder has an electrical length of about a quarter wavelength but a physical length about half this and a coil axis length of only about one-fiftieth of a wavelength. A constructional arrangement is disclosed (Fig.5). <IMAGE>

Description

SPECIFICATION Improvements in or relating to aerials This invention relates to aerials, and has particular application to aerials comprising a vertical monopole, ie a vertical radiating element having an electrical length of a quarter-wavelength or less at the operating frequency which, in conjunction with an imaging groundplane, produces a radiation pattern approximating to that of a dipole (viz doughnutshaped in the vertical plane and uniform in the horizontal plane). In such an arrangement a current flows in the imaging plane which balances the current flowing in the monopole. Ideally the imaging plane should be a conducting sheet having a diameter of at least a half-wavelength (X/2). Such a conducting sheet is sometimes termed a "counterpoise" and the balancing current therein the "counterpoise current".

In some practical situations the provision of an ideal counterpoise is not possible. For example in man-portable receiving and/or transmitting equipment using whip monopole aerials, where the counterpoise is effectively provided only by the metallic content of the receiver/transmitter case its electrical size may be too small, at the operating frequency, (eg below about 200 MHz), to provide the aerial with efficient and stable performance characteristics.

One known arrangement for alleviating this smallcounterpoise problem is to mount the whip aerial at the top of a rigid coaxial feeder of similar length so that the feedpoint is effectively halfway up the resulting structure. However, this arrangement has mechanical and electrical disadvantages which will be explained hereinafter. The present invention provides an alternative alleviation of the smallcounterpoise problem which does not have these disadvantages.

According to the present invention, a receiving and/or transmitting equipment comprises a case containing a receiver and/or transmitter, a monopole aerial element, and a connection therebetween; said connection being constituted by a length of coaxial feeder having, at one end, its inner conductor connected to the receiver and/or transmitter and its outer conductor connected to an earthy point at the case and, at the other end, its inner conductor connected to the lower end of the aerial element; said feeder being located in a convoluted configuration between the case and the lower end of the aerial element which enables said lower end to be located close to the case, and having adjacent portions of its outer conductor mutually insulated;; said outer conductor thereby substantially carrying the aerial counterpoise current and being of such related length and configuration that the current distribution over the aerial element plus said outer conductor has a relatively low value at the case and relatively high value at the lower end of the aerial element.

Since the outer conductor of the coaxial feeder thereby carries the main counterpoise current, the size of the counterpoise provided by, for example, the case becomes relatively unimportant.

Preferably, but not essentially, the configuration of the feeder is such that its outer conductor forms a slow-wave structure, thereby enabling the physical length of the feeder to be less than its electrical length, iethan its length in terms of wavelength, and thus enabling location of the lower end of the aerial element closer to the case. Conveniently the configuration is a single-layer helical coil having its axis approximately colinear with the aerial element.

The aerial element, which will usually be of the flexible "whip" type, may have an electrical length of X/4 or less. The uncoiled feeder may have a physical length of less than A/4, eg about A/8 is a typical suitable value for a single-layer helical coil because the group velocity along the thus-coiled feeder tends to be about half that of light so that its electrical length becomes about A/4; when coiled-up, the axial length of such a coil is typically reduced to about A/50. The invention is of principal value atfrequen- cies below about 200 MHz, where X is relatively large (1 sum).

To enable the nature of the present invention to be more readily understood, attention is directed, by way of example, to the accompanying drawings wherein: Figure 1 is a diagram showing the effect of using a monopole 5 with a small counterpoise Figure 2 is a diagram showing the effect of using a monopole with an ideal counterpoise Figure 3 is a diagram showing a known arrangement for alleviating the small-counterpoise problem Figure 4 is a diagram showing an arrangement embodying the present invetion.

Figure 5 is an elevation, in partial section, of a connection device for use in the present invention.

In Figure 1 a flexible X/4 monopole 1 forms a whip aerial extending vertically from a small receiver/ transmitter case 2 and being effectively fed at point 3. The approximately sinusoidal distribution of the current is shown by the interrupted line 4. (Despite the A/4 aerial-length, the current distribution has a half-wave sinusoidal form). Because the current at the feed-point 3 is small and highly dependent on any stray capacitance 5 to ground (directly, or via a human carrier in the case of man-portable equipment), the aerial impedance and electric field are high and variable. Matching problems thereby result and such aerials when thus connected are often inefficient and unstable in performance characteristics.

In Figure 2 an ideally large counterpoise is provided by a metal plate which is at least A/2 in diameter. The balancing current distributions in the monopole 1 and the plate 6 are shown at 7 and 8 respectively (the instantaneous directions of these currents are shown by the small arrows). The current at the feed-point 3 is a maximum at the peak of the sinusoidal distribution, resulting in stable operation, ie the current does not change rapidly with frequency and/or aerial length. This ideal situation is usually unobtainable in practice with man-portable equipment at operating frequencies below about 200 MHz.

Figure 3 shows a known arrangement for overcoming the small-counterpoise problem. Here a flexible aerial element 9 of length approximately X/8 is mounted at the top of a rigid coaxial feeder 10 also of length approximately X/8 so that the feed-point 3' is halfway up the total aerial structure. The counterpoise current flows in the outer conductor of feeder 10, and the resulting overall current distribution is approximately triangular as shown at 11, providing a lower and more stable aerial impedance. All of this current distribution, ie on element 9 and on feeder 10, contributes to the radiation pattern. However, this arrangement has the following disadvantages: (a) Since only element 9 is flexible, the high feed-point 3' can be a mechanical weakness in a whip aerial, especially when the equipment is carried through obstructions of low clearance.

(b) The length of element 9, which determines the tuning characteristic, ie the bandwidth, is only half that of a monopole. This is particularly unfortunate when the total permissible length (element 9 plus feeder 10) is close to A/4 over the operating frequency range (usually quite narrow in such equipments).

Hence this aerial tends to be much more frequencyselective (narrower bandwidth) than a standard X/4 monopole, which is undesirable. (The bandwidth of an aerial depends approximately on the length between maximum and minimum current therein, increasing with this length).

Figure 4 shows an embodiment of the present invention. A flexible monopole 12 is fed buy a helically coiled length of coaxial feeder 13, the lower end of whose outer conductor is electrically bonded to the case 2 and the upper end of whose inner conductor is connected to the monopole 12. The outer conductors of adjacent turns are mutually insulated. The lower end of monopole 12 is rigidly mounted above the case 2 by conventional means not shown. The monopole 12 may have a electrical length of X/4 (as shown) or less.The uncoiled length of feeder 13 may also be A/4 or less; A/8 is a typical suitable value because the group velocity along a helix of relevant dimensions tends to be about half that of light, giving a typical coil axial length of about A/50. (Figure 4 is not to scale, for clarity.) The outer surface of the outer conductor of feeder 13 carries the main counterpoise current, shown as the lower part of the overall current distribution 14, and so provides a low and stable aerial impedance, the feed-point 3" being at peak current as shown, with a current minimum at the case. The size of the counterpoise provided by case 2 is then relatively unimportant. The counterpoise current on the outer conductor of feeder 13 also contributes to the radiation pattern.

Since the coiled-up length of the feeder 13 is short, eg only about X/50, the point of mechanical weakness, iethe rigid mounting at the lower end of the whip monopole 12 (feed-point 3"), is much lower than in the arrangement of Figure 3, indeed not much higher than that for a conventional whip monopole mounted directly on an equipment form- ing the counterpoise as in Figure 1. As in conventional practice, the length of the monopole 12 can be reduced by connecting an inductance at its lower end in series with the inner conductor of feeder 13 (bottom end inductive loading) or a capacitance plate at its upper end (top end capacitative loading).

In one example of the embodiment of Figure 4, for use in the 100 MHzfrequency band, the monopole 12 was 75 cm long (ie AM at mid-band), 4mm in diameter, and made of brass. The feeder 13 was a length of RC 174 PVC-sheathed coaxial cable of characteristic impedance 50 ohms and was 34cm long, ie A/8 when straight but AM as a slow-wave structure when coiled. It was coiled in 5 turns of diameter 2.54 cm with adjacent turns spaced over a coiled-up length of 5cm, the inter-turn spacing (centre-to-centre) being twice the diameter of the coaxial cable.

In the described embodiment the coaxial feeder is wound as a single-layer, constant-diameter, helical coil with its axis colinear with the aerial element, but this is not electrically essential. For example, the coil diameter could taper towards one or both ends or a flat spiral coil could be used; these would also be slow-wave structures. Nor is the coil alignment with respect to the aerial element electrically significant; eg the coil axis could be normal to the aerial element. However, a single-layer helical coil aligned as shown will usually be the most convenient arrangement.

In Figure 4 the outer conductor of the feeder 13 is shown connected to the case 2 itself, here assumed made of metal or otherwise electrically conducting and constituting an earthy point. If an electrically insulating, eg plastics, case is used, the outer conductor can be connected to a suitable earthy point of the circuitry within the case. In either case the connection need not be DC but can be a capacitative one having low impedance in the operating frequency range.

Figure 5 shows the construction of a connection device 24 used in the foregoing example. The coaxial cable 13 is wound in a helical groove in a former 14 of laminated plastics material. A coaxial connector portion 15 is secured to the lower end of the former 14 and engages a matching connector portion on the top of the transmitter/receiver case (not shown). The inner conductor 20 of feeder 13 is connected to the centre pin 16 of connector portion 15 and the outer conductor 18 (the braiding) of feeder 13 is connected to the peripheral sleeve 17 thereof.To the upper end of former 14 is secured a brass insert 21 which is internally threaded to receive the lower end of the whip aerial 12. The upper end of braiding 18 is left open-circuit, while the inner conductor 20 is soldered to the insert 21. The complete assembly is sealed in a PVC heat-shrink sleeve 23. Figure 5 is approximately to scaie, the diameter of former 14 over sleeve 23 being approx 2.5cm and the height (lower end of connector portion 15 to upper surface of inset 21) approx 10 cm.

In the foregoing description the aerial element 12 is a monopole approximately AM in length, but this is not essential. For example the aerial element may be less than AM in length. Thus in another embodiment for use at 100 MHz, the element 12 is approximately A/8 long; in order to tune the aerial and achieve a half-wave sinusoidal current distribution along the aerial element plus coiled-up feeder, the feeder 13 is made 50% longer than in the first-described embodi ment, ie electrically 3A/8 when coiled instead of A/4 (3A3/1 6 when straight), but otherwise similar. In this case the bandwidth is again determined by the total aerial element length and a relatively low feedimpedance is obtained, with a current minimum at the case. However, as the feed-point is not now located at the peak of the sinusoidal current distribution (where the change of aerial current with feed poisition is a minimum), the optimum stabilityimprovement effect obtained with the AM monopole aerial element due to that location is not now present.

Claims (8)

1. A receiving and/or transmitting equipment comprising a case containing a receiver and/or transmitter, a monopole aerial element, and a connection therebetween; said connection being constituted by a length of coaxial feeder having, at one end, its inner conductor connected to the receiver and/or transmitter and its outer conductor connected to an earthy point at the case and, at the other end, its inner conductor connected to the lower end of the aerial element; said feeder being located in a convoluted configuration between the case and the lower end of the aerial element which enables said lower end to be located close to the case, and having adjacent portions of its outer conductor mutually insulated;; said outer conductor thereby substantially carrying the aerial counterpoise current and being of such related length and configuration that the current distribution over the aerial element plus said outer conductor has a relatively low value at the case and a relatively high value at the lower end of the aerial element.

2. An equipment as claimed in claim 1 wherein the configuration of the feeder is such that its outer conductor forms a slow-wave structure, thereby enabling the physical length of the feeder to be less than the electrical length in terms of wavelength.

3. An equipment as claimed in claim 2 wherein said configuration is a single-layer helical coil having its axis approximately colinear with the aerial element.

4. An equipment as claimed in claim 2 or claim 3 wherein the monopole aerial element is approximately a quarter-wavelength long at the operating frequency and the feeder is electrically approximately a quarter-wavelength long.

5. An equipment as claimed in claim 4 wherein the physical length of feeder is approximately oneeighth wavelength long.

6. An equipment as claimed in claim 2 or claim 3 wherein the monopole aerial element is approximately one-eighth wavelength long at the operating frequency and the feeder is electrically approximately three-eighths wavelength long.

7. An equipment as claimed in claim 6 wherein the physical length of the feeder is approximately three-sixteenths wavelength long.

8. A receiving and/ortransmitting equipment substantially as hereinbefore described with refer- ence to Figure 4 or Figure 5 of the accompanying drawings.