455

BEVERAGE ANTENNAS FOR HF RECEPTION

A G P Boswell

GEC-Marconi Research, UK

INTRODUCTION

A long wire antenna suspended horizontally at a small height above ground has been known for a long time to provide directional coverage for reception of long- distance low-frequency groundwave signals. The early examples of this type of antenna were used in transatlantic communications, using typical lengths of lOkm or more in the frequency range between 10 and 1OOkHz. Although the radiation efficiency was low (that is, that as a transmitting antenna the Beverage would fail to radiate most of the incident power), the corresponding losses could be accommodated in a satisfactory receiving system design because of the high level of external noise at these frequencies.

Figure 1 illustrates the general arrangement. The antenna is fed against ground and operates as a travelling-wave antenna with a load resistance at the remote end. The characteristic resistance can be calculated by treating the antenna as a single-wire feeder operated against ground, and this value determines both the feed impedance and the load resistance. The load resistor absorbs some signal power, but without it the antenna would be bidirectional, and hence the load resistor suppresses a lobe which would in any case be in the rearward direction and therefore presumably unwanted. Of course the antenna can be operated in two opposite directions by interchanging the feed-point and load resistance.

Early work on this antenna [l] concentrated on its response to groundwave signals, and the treatment predicted the directivity but not the absolute gain. Long- distance communications then moved to higher frequencies and little interest was shown in the Beverage between 1930 and 1960. Later work by Wait [21 and Knight [3] identified the core problem to be that of predicting the propagation constant of the wave on the wire. A more. complete and recent analysis by King in 1992 [4] allowed the entire radiation pattern to be computed for a larger range of ground conditions than before, but was confined to predicting the directivity of the antenna. The theory treats the antenna as a transmit antenna, but is applicable to the receiving case by reciprocity.

Design of a receiving system at HF requires a knowledge of the absolute gain of the antenna so that the system noise figure (including antenna losses) can be matched to the expected level of external noise. King’s theory can be readily adapted to provide the efficiency by noting that the input power for a given driving current is equal to the square of the current times the real part of the characteristic impedance. Thus the radiated field can be related to the input power to give the efficiency in the usual way.

Unfortunately King’s theory can not be applied practically bccause it assumes a current is introduced on to the wire by an unspecified means. In a practical case the feeding arrangement closely affects the efficiency and therefore cannot be ignored. The work here describes simulations of a practical Beverage over various types of ground.

RESULTS

The theory put forward by King [4], with the extension described above, was recomputed to determine the radiation efficiency of the antenna used in his example, which is wire of length 87.85m. diameter 6mm and height 0.15111, with ground relative permittivity of 8 and conductivity 0.04S/m, operated at 10MHz. A comparison was made between this and a NEC-3 simulation using 59 wire segments and a load resistance of 276 Ohms (calculated from the single-wire transmission-line expression). The radiation pattern shape agreed extremely closely. The maximum gain was predicted to bc -1O.7dBi by NEC-3 and -7.9dBi by the modified King theory. The discrepancy is probably accounted for by the power dissipated in the load resistor in the NEC-3 case.

With the simulation thus verified, a practical case encountered in the field was simulated. This antenna, devised and installed in the past by unknown contractors, uses a wire of length 53m, height 1.22111 and diameter 3”. An additional sloping wire at each end, of length 7m, connects to the feed and load points at a height of 0.16m. The total length is thus 67m. At each end there is no direct carth connection, but a rectangular grid of wires 7.4 by 1.55111 placed under the sloping sections at a height of O.lm, giving close capacitive coupling to the ground plane, an arrangement which appears to be an

Antennas and Propagation, 4-7 April 1995 Conference Publication No. 407,O IEE 1995

456

appropriate form of ground connection for the Beverage. Figure 2 indicates the general arrangement, and the feed and load resistance was taken as 400R. Computed radiation patterns for 8 and 20MHz are given in Figures 3 and 4 as constant-gain contours plotted in the Sanson- FlamsW projection commonly used for HF patterns. After skywave propagation a signal has indeterminate polarisation, and the patterns are given in terms of scalar amplitude regardless of polarisation. The ground constants used here are a permittivity of 15 and a conductivity of O.O03S/m as might be obtained in typical English conditions. The maximum absolute gain levels are -7dBi at 8MHz and OdBi at 20MHz.

Figure 5 shows the elevation angle of maximum directivity and the azimuth and elevation half-power beamdwidths plotted against frequency. The angles vary approximately as the inverse root of frequency, behaviour which indicates that the Beverage acts as an end-fire array of constant length.

The variation in absolute gain over the range 4-28MHz is shown in figure 6. Gain increases with frequency at about 4.7dB/octave. An end-fire array normally exhibits a gain-slope of 3dB/octave. The additional slope can be ascribed to slowly reducing ground losses as frequency increases. This is consistent with the ground conditions chosen, for which the complex dielectric cunsrant has equal real and imaginary parts at 3.6MHz. Thus at higher frequencies the ground is expected to behave mcxe as a dielectric than as a poor conductor.

The match to a 400R load is shown in figure 7 for the range 4-28MHz. plotted on a Smith grid limited to a VSWR of 21. It can be seen that the VSWR of the antenna is less than 2 1 from 4.5MHz upwards, which corresponds to a signal loss of under 0.5dB. The impedance plot is centred on an impedance of approximately (400-j 14O)Q.

As a comparison the simulations were repeated for very dry ground, with a permittivity of 3 and a conductivity of O.OOO1 S/m. The results, shown in figures 8 and 9, show that the pattern shape is changed little by thcsc considerably drier conditions. However thc absolutc gain increased to -2dBi at 8MHz and -3dBi at 20MHz.

The simulations were also carried out for perfect ground, with infinite conductivity assumed. In this case the patterns failed to exhibit any definite beam, and the antenna was deemed to be unworkable. The interesting conclusion is that the Beverage antenna in this application works best with "poor" ground conditions and does not work at all with "ideal" ground conditions (words in quotes indicating the conventional nomenclature).

Further simulations were carried out to show the effect of changing the size of the antenna. With the wire height doubled, the absolute gain increased by 2dB at 8 and 20MH2, the takeoff angle rose by 1-2 degrees, and the azimuth beamwidths at the two frequencies increased by about 15%. The elevation beamwidth was 15% less at 8MHz and unchanged at 20MHz.

With the total antenna length reduced to half its original value, the absolute gain was -12dB at 8MHz and -4.5dB at 20MH2, thus showing a significant reduction The takeoff angles increased by about 50%, the azimuth beamwidths increased to approximately twice the original values, and the elevation beamwidths increased significantly, to 70 degrees at 8MHz and 36 degrees at 20MHz.

CONCLUSIONS

The Bevcrage antenna functions adequately as a directive receiving antenna or array element in the HF range when installcd over lossy ground. A well-formed bcam is produced in the forward direction. The absolute gain is jess than might be deduced from the beamwidths, because of losses introduced by the presence of the ground. Howevcr the antcnna fails completely when installed over a perfectly conducting plane.

The Beverage antenna is adequate for use in a receiving system, in that the losses are small enough in general terms to pcrmit the design of a receiving system that meets the normal requirement of being limited by external rather than intcmal noise. The losses are however sufficiently large to preclude the use of this type of antenna for transmitting in normal conditions. Thus it is suitable only as a receiving antenna, and provides a useful low-cost option in cases where the right amount of flat land is available, especially if the land is dry. It is suitable only for skywave signals as the radiation patterns show poor performance at extremely low elevation angles.

The results show that the beamwidths and take-off angle vary with frcqucncy in a similar manner to an end-fire array. The gain varies rather more quickly, indicating that the ground losses reduce at higher frequencies. The antenna covers a frequency range of 3 or more octaves, and should have a wire length of approximately one wavelength at the lowest frequency of operation. The wire height and diamctcr are not critical.

REFERENCES

1. Beverage, H.H., Rice, C.W., Kellog, E.W., "The Wave Antenna", Trans. AIEE., Feb 1923.

2. Wait, J.R., "Long-wave behaviour of the Beveragc Wave Aerial", Elect. Lett. Vol. 12, pp

457

358-359, 1976. D i r e c t i o n o f f i r e - Knight, P., "Low-frequency behaviour of the Beverage Aerial", Elect. Lett. Vol. 13, No. 1 6th Jan 1977.

King, R.W.P., "Circuit Properties and Complete Fields of Horizontal Wire Antenna", J. Appl. .-, V u , No. 3. 1st Feb 1992.

Load _ _ _ _ _ S o u r c e 3.

7 r / / r r r r / r r r r r / r / r r / i i r i i i i i / r r / r r 4. Figure 1: Beverage Antenna

------a Fiqure 2: Wire-qrid model arrangement

Figure 3: Forward Radiation Pattern

iation Pattern

458

. '--\ .

\ \

'1 \ \, Beanwidth lazimuth) -. . \ 1.

-- \ Beanwidth (e levat ion1 .- -\

\ . '.

1

Takeoff angle

Log frequency

Fiqure 5 : Takeoff anqle and Beamwidths p l o t t e d aqainst loq(freauency)

Fiqure 6: Gain p l o t t e d aqainst loq(freauencv1

Fiqure 7: Match t o a 400 Ohm load

F i a u r e 8: Radiat ion Pattern a t 8MHz w i t h Dry Ground Figure 9: Radiat ion Pattern a t 20MHz w i t h Dry Ground