GB1588872A - Radar radomes - Google Patents

Description

(54) IMPROVEMENTS IN OR RELATING TO RADAR RADOMES (71) I, SECRETARY OF STATE FOR DEFENCE, LONDON do hereby declare the invention, for which I pray that a patent may be granted to me, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to microwave antenna assemblies, and more particularly to microwave antenna and radome assemblies adapted to tolerate undersea environments.

Conventional microwave assemblies for undersea use each normally comprise a microwave antenna with appropriate electronic circuitry, this arrangement being housed within a specially designed radome adapted to exclude water even at appreciably high hydrostatic pressures. The need to exclude water from the antenna environment imposes on the radome construction severe design constraints and high manufacturing standards. In particular, machining tolerances must be high and materials must be of high quality, for the radome must maintain a good liquid seal over a range of external hydrostatic pressures. To meet these criteria, conventional radomes are therefore undesirably bulky, and must be machined to an undesirable degree of accuracy.

It is an object of the present invention to provide a microwave antenna and radome assembly in which the size and machining criteria for the radome can be relaxed whilst maintaining the integrity of the antenna environment against water ingress.

According to the present invention, a microwave antenna and radome assembly comprises a liquid filling the internal volume of the radome not occupied by the antenna, the said liquid having a low dielectric loss tangent at the operating frequency band of the assembly and acting to transfer ambient pressure outside the radome to the radome interior.

At least the first stage of radar signal processing circuitry is preferably also immersed in the said liquid. It has been found, surprisingly, that such immersion produces no significant degradation in the performance of either the radar antenna or the said processing circuitry.

The pressure transfer liquid must necessarily be non-corrosive with respect to the materials with which it is in contact inside the radome, these materials being those of the radome, the antenna and its support arrangement, and those electronic components located within the radome.

The pressure transfer liquid preferably has a dielectric constant of less than 3.0 and a loss tangent value of less than 0.02.

The material of the radome is preferably chosen to have a dielectric constant substantially equal to that of the liquid.

In one embodiment of the invention, the liquid is a mineral transformer oil, and an appropriate radome material is polytetrafluoroethylene (PTFE).

The immersed microwave antenna may be of the static, omnidirectional type, such as for example a bicone antenna.

Alternatively the immersed antenna may consist of a directional, rotatable device normally rotated by a motor drive. In this case, the motor itself may advantageously be liquid-immersed and pressurised at the same hydrostatic pressure as that experienced by the antenna, whereby, in operation, no pressure differential will exist between the motor and the antenna thus avoiding any requirement to pressure-seal the motor drive shaft.

The hydrostatic pressures experienced by the antenna and motor may conveniently be equalised by means of pressure equalisation means, such as for example a flexible diaphragm, located between the vessels containing them. Alternatively, and very conveniently, the motor and antenna may be located in the same body of liquid.

The microwave antenna and radome assembly is preferably adapted to take up liquid volume changes due to temperature and hydrostatic pressure variations. This may be achieved either by incorporating bellows means in the said assembly, or by designing the radome assembly to flex under pressure.

A microwave antenna assembly incorporating a liquid-filled radome of the invention exhibits the major advantage that the problem of sealing the mast of a submarine vessel against water ingress is reduced to mere pressure sealing of electrical leads at the mastlantenna assembly interface. Because of the equalisation of hydrostatic pressure inside and outside the radome, there is no requirement, as in prior art apparatus, for a close-tolerance high pressure seal of the radome internal volume against water ingress.

It has been found that a microwave antenna immersed in a liquid having a relative dielectric constant greater than unity exhibits an operating frequency equal to that of a corresponding but larger antenna surrounded by air. Accordingly, it is a further major advantage of the present invention that antenna size for a given operating frequency is reduced.

In order that the invention may be more fully understood, one embodiment thereof will now be described, with reference to the accompanying Tables and to the drawings accompanying the provisional specification in which: Figure 1 is a sectional view on the line I-I of Figure 2 of a microwave antenna assembly within a radome.

Figure 2 is a base view of the radome and antenna assembly of Figure 1.

Figure 3 is a section on the line III-III of Figure 1, and shows an electronic component assembly suitable for use with the microwave antenna assembly of Figure 1.

Figure 4 is an electronic circuit corresponding to the component assembly of Figure 3.

Figure 5 illustrates an experimental radome test arrangement.

Figure 6 is a comparison graph of field strength against frequency of air-filled and liquid filled radomes.

Figure 7 illustrates the relative size of bicone antenna appropriate to different frequency bands and environments. and Figure 8 illustrates the relative sizes of radomes appropriate for air-filled and liquid-filled operation over the same frequency band.

Figure 1 shows a sectional view of a substantiallv conventional microwave assembly comprising a bicone antenna 1.

The antenna 1 is mounted on a support structure 2 attached to a base unit 3. Figure 2 is a view of the base unit 3 in the direction Z in Figure 1. In Figure 2, the base unit 3 is provided with feed through connectors 4 for DC power supply leads, together with a bellows unit 5, a socket 6, a filler plug 7 and a bleed screw 8. The line I-I in Figure 2 indicates the section from which Figure 1 is derived. Figure 3 is a scrap section along III-III in Figure 1, and illustrates an electronic component assembly 9 to be fitted into a space 10 in the support structure 2. The component assembly 9 consists of the first stage of processing circuitry for radar signals from the antenna 1. The assembly 9 comprises a semi-rigid RF supply lead 11 for connection to the bicone antenna 1, a limiter and detector substrate 12 together with a thick film video amplifier 13. The video output from the amplifier 13 is fed via a coaxial lead 14 to the socket 6 (see also Figure 2). The support structure 2 is fitted with feed-through connectors 16 to permit DC power and bias voltage to be fed to the electronic component assembly 9.

The base unit 3 is provided with an annular flange 17 having an annular recess 18 accommodating an O-ring 19. The bicone antenna 1 and support structure 2 are enclosed within a radome 20 in the shape of a domed cylinder, the dome 21 being hemispherical. The inner peripheral surface of the base region 22 of the radome 20 abuts the O-ring 19 to seal the internal volume 23 of the radome 20; this isolates the bicone antenna from the environment surrounding the radome. A clamp ring 24 is provided to compress the radome 20 against the O-ring 19 in order to achieve a good seal.

After assembly of the arrangement illustrated in Figures 1 to 3, a dielectric liquid (not shown) is admitted through the filler plug 7 to fill the unoccupied internal region 25 of the radome 20. Air is carefully expelled from the radome through the bleed screw 8 until no bubbles remain within the radome 20. Filling the radome 20 with liquid provides internal support against external hydrostatic pressure. The radome thickness required to withstand a given external pressure is accordingly reduced. The thickness of the radome 20 shown in Figure 1 is accordingly appreciably less than that of a corresponding radome designed for air-filled operation.

Figure 4 is an electronic circuit diagram of the component assembly 9 shown in Figure 3, and comprises the first stage of microwave signal processing circuitry. The lead 11 from the antenna (not shown, see Figures 1 and 3) is connected across an earthed limiter diode 30 and thence to a bias coil 31 and detector diode 32. The detector diode 32 is connected to a head amplifier 33 which supplies signal to the video output 34. This is an entirely conventional microwave circuit arrangement and will not be described further.

Having regard to the attenuation of microwave frequency radiation by the majority of solids and liquids, the selection of both a suitable radome material and a suitable liquid to fill the radome requires careful attention.

The environmental, mechanical, chemical and electrical parameters of a suitable pressure transfer liquid are as follows. The liquid is likely to experience hydrostatic pressures in the range 1000 to 1500 lb/in2 and temperatures in the range -400C to 70"C. The density and temperature coefficient of expansion of the liquid must possess values compatible with the engineering design of the radome and antenna assembly, having regard to the temperature variation likely to be experienced. The liquid must be noncorrosive with respect to non-ferrous metals, ceramics and epoxy resins likely to be incorporated in the radome and antenna assembly. The viscosity must be sufficiently low over the temperature range of interest to maintain adequate pressure transfer. The liquid must exhibit as low a dielectric loss tangent as possible in the required microwave operating frequency range, typically 1.0 to 18.0 GHz.

Good electrical insulation properties are also desirable, and the low dielectric loss tangent requirement implies a comparatively low dielectric constant. A dielectric constant in the range 1.5 to 3.0 would be particularly suitable, since few if any liquids have dielectric constants below 1.5, and a value greater than 3.0 would normally imply an unacceptably high loss tangent.

The pressure and temperature conditions correspond to typical undersea and surface environments experienced by apparatus mounted externally of a vessel. There are no critical density requirements since the vast majority of liquids have acceptable densities. The viscosity requirement is essentially that the liquid should exhibit at least some flow properties to achieve adequate pressure transfer at the lowest temperature of interest. around -40"C. The maximum coefficient of expansion which can be tolerated is dependent on the radome assembly and/or bellows design which must accommodate this. The liquid must not corrode either the antenna or any electronic circuitry exposed to it within the radome. The operating frequency range is dictated by the particular application of the receiver, and the loss tangent requirement arises because of the desirability of minimizing attenuation at microwave frequencies; liquids with dielectric constants below l.5 are uncommon, whereas those with dielectric constants above 3.0 typically exhibit unacceptably high values of loss tangent, as is also the case with liquids which are poor insulators.

Table 1 gives the appropriate parameters for seven liquids considered possibly appropriate, at least a priori, for use as a pressure transfer liquid. The liquids are listed for convenience under their manufacturers' names and part numbers. Those liquids produced by BP and Esso Ltd are mineral oils, the ICI and DOW Corning Ltd liquids are silicon oils, and the last three liquids, "Eccoflo 2", carbon tetrachloride and glycol, are common dielectric liquids.

"Univolt" is a Registered Trade Mark.

Of the liquids in Table 1, glycol has an unacceptably high dielectric constant and carbon tetrachloride an unacceptably high freezing point. "Eccflo 2" exhibited no marked advantages, and availability from the US manufacturer was poor. Apparatus filled with silicon oil presents subsequent cleaning problems, although these oils are otherwise suitable. Of the two mineral oils, the BP JS.A transformer oil is preferred since it is soluble in common organic solvents and hence easily cleaned off; it exhibits a low freezing point, and it has a very low loss tangent value. Accordingly, the BP transformer oil was selected as the pressure transfer liquid.

The material chosen for the radome was required to contain the pressure transfer liquid with no appreciable water absorption or leakage at simultaneous high internal and external hydrostatic pressures. Moreover, if a radome were to be envisaged having a thickness comparable to a wavelength, then the radome material would be required to have a dielectric constant as close as possible to that of the pressure transfer liquid, in order to minimize wave reflections. Alternatively, the thickness/wavelength criterion could be avoided if the radome were to be made much thinner than the shortest wavelength of interest. This would require a mechanically strong material.

The requirements to be met by the radome parameters fall naturally into five main areas as follows: (i) Electrical Radome materials with relative dielectric constants in the same range as the pressure transfer liquids are preferred, although a higher relative dielectric constant could be considered if the material is strong, but in both cases a low loss tangent would be required.

(ii) Mechanical Due to the pressure transfer principle the strength of the material is not considered as a first priority as in conventional unsupported (lie. air-filled) radomes.

The material must be able to withstand typical environmental temperature extremes. Its thermal expansion should preferably be in the same range as that of the pressure transfer liquid, although limited variation in volumetric changes could be accommodated.

(iv) Chemical The material must have a low water absorption rate, and be resistant to any fluids that it might come into contact with under operating, cleaning or handling conditions.

(v) Weather It must be suitable for outdoor use and as such be resistant to ultra violet light. A minimum life under these conditions of two years is considered essential. However a life approaching ten years is now thought to be attainable.

Table 2 gives the relevant parameters of a number of plastics materials investigated for use as a radome material. Of these, PTFE was selected on the basis of the compatibility of its dielectric constant with that of the BP JS.A pressure transfer liquid. PTFE also has the advantages of low loss tangent, chemical inertness and low water absorption properties together with good weathering properties. Of the other materials PPO and ionomers were considered unsuitable on the basis of unacceptably high water absorption characteristics, and TPX was not available commercially. If a silicone oil had been selected as the pressure transfer liquid, polyethylene, rigid PVC or ABS could have been used to provide a dielectric constant match. Fibreglass could also have been employed, but in this case the radome thickness would need to have been much less that a wavelength in view of the dielectric constant mismatch with any of the acceptable pressure transfer liquids.

The arrangement of Figures 1 to 4, with a PTFE radome filled with BP JS.A pressure transfer liquid, was tested to determine its efficiency as a microwave detection system.

Figure 5 illustrates the experimental arrangement employed for test purposes to compare efficiencies with and without a dielectric liquid within the radome. In Figure 5. the radome 20 is shown mounted upon a support column 41 of very low dielectric constant. The column 41 and radome 20 are located within an enclosure 42 lined with rf absorbent material 43. RF power is supplied to the enclosure 42 via a transmitting antenna in the from of a waveguide horn 44. A signal generator 45 is connected to supply rf power down a line 46 to a 3db power divider 47 connected both to the horn 44 and to an rf thermistor head 48.

The thermistor head 48 is connected to a power meter 49. The mode of operation of the arrangement of Figure 5 consists of adjusting the rf power level supplied to the enclosure 42 until the antenna assembly indicates a given response level.

Figure 6 displays plots of Field Strength (tangential signal sensitivity) against Signal Frequency in GHz, allowance being made for the effort of the 3db power divider 47.

The field strength units are -dbW/m2, ie the decibel value of that power expressed in W/m2 required at the generator 45 to produce a given detected signal strength at the bicone antenna. Since the bicone antenna and subsidiary circuitry were liquidimmersed, it was impossible to measure to gain of the liquid-filled radome assembly directly, and the foregoing field strength valves provided an alternative and convenient measure of field strength. Accordingly, an increasing negative db figure along the field strength axis represents increasing sensitivity. In Figure 6, curves A and B are field strength plots for liquid-filled and air-filled radome assemblies respectively.

The liquid-filled system exhibits a significant reduction in sensitivity as compared to the air-filled system at frequencies just below 5 GHz. This is a resonance effect, and the resonant or minimum sensitivity region may be shifted to higher frequencies by reducing the radome diameter. In a fully developed system, this minimum would be shifted out of the operating frequency range by the radome size reduction, and the size reduction, and the size reduction would also increase sensitivity by reducing the amount of oil between the radome and the antenna.

Figure 6 also shows operating frequency ranges C and D for the air-filled and liquid-filled systems respectively. The liquid-filled system exhibits an operating frequency range D which is shifted to lower frequencies as compared to the operating range C of the air-filled system. It is a fundamental principle of electromagnetic wave propagation that the linear dimensions of the atenna are directly related to the wavelength to be generated. As a result, microwave antennae for operation in air increase in size as the operating frequency is reduced. In contradistinction to this, Figure 6 shows that one effect of immersing the antenna in a liquid having a relative dielectric constant greater than unity is to increase the apparent size of the antenna by shifting the operating range to lower frequencies.

This effect may alternatively be viewed as achieving a reduction in the antenna size required to receive a particular range of frequencies.

Figure 7 illustrates the reduction in bicone antenna size obtainable for comparative systems by immersion of the bicone in oil having a relative dielectric constant of 2.0.

Side elevation and plan views respectively are displayed for bicone antennae 61 to 64 having relative dimensions appropriate for operation in air or oil envirionments at X-J (international designation I and J) band operation or S-C (international designation E and H) band operation. From Figure 7 it can be seen that the effect of oil immersion on bicone size is to reduce the required linear dimensions by about 50% at X-J band and by about 30% at S-C band. The reduction in the linear dimensions of the antenna is proportional to the square root of the relative dielectric constant of the liquid used.

The radome and antenna assembly described relative to Figures 1 and 2 had an overall base diameter of 120mm. The external dimensions of the radome are suitable for air-filled operation in the frequency range 2.5 to 7.5 GHz, although an air-filled radome would need to be much thicker than that shown in Figure 1. Furthermore, with an oil-filled operating conditions in accordance with the present invention, in addition to its thickness reduction the randome size can be reduced as a consequence of the relaxation in the hydrostatic pressure seal criteria. This radome size reduction is additional to that achieved for the antenna.

Figure 8 illustrates an estimate of a typical size reduction obtainable in the case of I-J band operation. In this drawing a radome 71 suitable for air-filled operation is compared with a second radome 72 appropriate for the corresponding oil-filled case. It should be added that radomes of this type are normally constructed as cylinders with hemispherical domes for reasons of engineering convenience, although strictly a perfectly hemispherical radome would be more desirable for pressurised operation. From Figure 8 it can be seen that the ratio of radome diameters for oil-filled and air-filled use is 0.4, and that for the radome heights is 0.86.

This corresponds to a volume reduction by a factor of 8.7, or alternatively the oil-filled radome has only 11.5% of the volume of the air-filled radome.

It will be appreciated that the volume of the radome assembly is a major design consideration for undersea installation on, for example, a retractable support arrangement provided on a submarine. Any reduction obtainable in the size of the radome greatly reduces the visual and radar target presented when the support arrangement is extended. Moreover. the use of a pressure transfer liquid greatly reduces the complexity of the hydrostatic design criteria affecting the radome construction, since hydrostatic pressures inside and outside the oil-filled radome are equalised and the radome will not deform when submerged. Accordingly, only a mere watertight seal is required, sophisticated pressure seals and close mechanical tolerances in radome dimensions being unnecessary. It will therefore be apparent that the use of a pressure transfer liquid in accordance with the present invention leads to important relaxations in the criteria applicable to radome design, and advantageous reductions in antenna size.

A radome and antenna assembly intended for liquid filling must necessarily be designed to cope with the expansion of the liquid in the operating temperature range, normally -40"C to 700C. With reference to Figure 2, this was achieved in one embodiment of the invention by incorporating a bellows unit 5 in the base 3. The bellows 5 is exposed to the external environment surrounding the radome, ie air or water under pressure, and flexes as the internal hydrostatic pressure varies due to liquid expansion, and thus alters the volume available for occupation by the liquid. The bellows 3 also accommodates the comparatively small liquid volume changes produces by external hydrostatic pressure variations. An alternative to the bellows comprises introducing a flexible diaphragm into a section of the radome exposed to the external environment. With the much smaller antenna and radome (see eg Figure 8) which can be used by virtue of the dielectric effect of the liquid, much less liquid is required than is the case for a liquid-filled radome of conventional external dimensions but less than conventional thickness, as described with reference to Figure 1 and used for experimental purposes. In the case of such a smaller radome, made possible by the present invention, it has been found that flexure of the radome wall is sufficient to cope with liquid expansion, and a bellows arrangement is unnecessary.

Workers in the art of microwave electronics will appreciate that at least the first stages of signal processing electronics (see eg Figure 4) should be located in close proximity to the aerial, in this case the bicone antenna. In Figures 1 and 3, these first stages, comprising component assembly 9, are located within the radome 20.

Accordingly, this proximity requirement entails the microwave circuitry and the video amplifier being immersed directly in the pressure transfer liquid. Prior to testing, it was not known what effect on performance would be produced by immersion of the circuit in a dielectric liquid having a relative dielectric constant greater than unity. The tests (see eg Figure 6) showed surprisingly that no significant deleterious effects on performance were produced by the immersion.

The foregoing description envisaged the use of a bicone antenna, which is an omnidirectional device. It is frequently necessary to employ directional antennae which are steered or rotated for direction finding purposes. In undersea applications, directional antenna steering or rotation necessitates motor drive shafts which pass through the hull of the vessel, since the motor is located within the hull and the antenna outside it. Such an arrangement necessarily involves severe design problems associated with producing a seal around a rotatable shaft, the seal being required to preserve the hull integrity against leaks even at depths of many hundreds of feet. In a further aspect of the present invention, therefore, the motor for a motor-driven undersea radar antenna is located within a pressure transfer liquid maintained at the same pressure which surrounds the antenna.

The motor may be located within the same body of liquid as the antenna, or alternatively the motor and antenna may be located within separate bodies of liquid having pressure equalisation means such as for example a flexible diaphragm, located therebetween.

The present invention is concerned with the use of a pressure transfer liquid in conjunction with a radome and antenna assembly. The use of such a liquid, as has been said, produces advantages of reduced design complexity for undersea applications and further advantages in miniaturisation.

These advantages are by no means restricted to the radome and antenna arrangements hereinbefore described. It is accordingly to be understood that the scope of the present invention is in no way restricted by the foregoing description.

WHAT I CLAIM IS: 1. A microwave antenna and radome assembly comprising a liquid filling the internal volume of the radome not occupied by the microwave attenna, the said liquid having a low dielectric loss tangent at the operating frequency band of the assembly and acting to transfer ambient hydrostatic pressure outside the radome to the radome interior.

2. A microwave antenna and radome assembly according to claim 1 comprising a first stage of signal processing circuitry which is immersed in said liquid.

3. A microwave antenna and radome assembly according to claim 1 or 2 wherein said liquid has a dielectric constant of less than 3.0 and a dielectric loss tangent of less than 0.02.

4. A microwave antenna and radome assembly according to any one of claims 1 to 3 wherein said liquid and the material of the radome are chosen to have substantially the same dielectric constant.

5. A microwave antenna and radome assembly according to any one of claims l to 4 wherein said liquid is a mineral transformer oil.

6. A microwave antenna and radome assembly according to claim 4 or 5 wherein the radome material is PTFE.

7. A microwave antenna and radome assembly according to any one of claims 1 to 6 wherein the microwave antenna is a bicone.

8. A microwave antenna and radome assembly according to any one of claims 1 to 6 wherein the antenna is directional and rotatable by means of a motor.

9. A microwave antenna and radome assembly according to claim 8 wherein the motor is liquid-immersed and is at the same hydrostatic pressure as the antenna.

10. A microwave antenna and radome assembly according to claim 9 wherein a flexible diaphragm is provided, said diaphragm separating the said liquid within which the antenna is immersed from the liquid within which the motor is immersed so as to equalise the hydrostatic pressures of the liquids.

11. A microwave antenna and radome assembly according to claim 9 wherein the antenna and the motor are immersed in the same liquid.

12. A microwave antenna and radome assembly according to any one preceding claim wherein the is provided bellows means to accommodate changes in volume of said liquid due to changes in temperature and hydrostatic pressure.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (12)

**WARNING** start of CLMS field may overlap end of DESC **. finding purposes. In undersea applications, directional antenna steering or rotation necessitates motor drive shafts which pass through the hull of the vessel, since the motor is located within the hull and the antenna outside it. Such an arrangement necessarily involves severe design problems associated with producing a seal around a rotatable shaft, the seal being required to preserve the hull integrity against leaks even at depths of many hundreds of feet. In a further aspect of the present invention, therefore, the motor for a motor-driven undersea radar antenna is located within a pressure transfer liquid maintained at the same pressure which surrounds the antenna. The motor may be located within the same body of liquid as the antenna, or alternatively the motor and antenna may be located within separate bodies of liquid having pressure equalisation means such as for example a flexible diaphragm, located therebetween. The present invention is concerned with the use of a pressure transfer liquid in conjunction with a radome and antenna assembly. The use of such a liquid, as has been said, produces advantages of reduced design complexity for undersea applications and further advantages in miniaturisation. These advantages are by no means restricted to the radome and antenna arrangements hereinbefore described. It is accordingly to be understood that the scope of the present invention is in no way restricted by the foregoing description. WHAT I CLAIM IS:

1. A microwave antenna and radome assembly comprising a liquid filling the internal volume of the radome not occupied by the microwave attenna, the said liquid having a low dielectric loss tangent at the operating frequency band of the assembly and acting to transfer ambient hydrostatic pressure outside the radome to the radome interior.

2. A microwave antenna and radome assembly according to claim 1 comprising a first stage of signal processing circuitry which is immersed in said liquid.

3. A microwave antenna and radome assembly according to claim 1 or 2 wherein said liquid has a dielectric constant of less than 3.0 and a dielectric loss tangent of less than 0.02.

4. A microwave antenna and radome assembly according to any one of claims 1 to 3 wherein said liquid and the material of the radome are chosen to have substantially the same dielectric constant.

5. A microwave antenna and radome assembly according to any one of claims l to 4 wherein said liquid is a mineral transformer oil.

6. A microwave antenna and radome assembly according to claim 4 or 5 wherein the radome material is PTFE.

7. A microwave antenna and radome assembly according to any one of claims 1 to 6 wherein the microwave antenna is a bicone.

8. A microwave antenna and radome assembly according to any one of claims 1 to 6 wherein the antenna is directional and rotatable by means of a motor.

9. A microwave antenna and radome assembly according to claim 8 wherein the motor is liquid-immersed and is at the same hydrostatic pressure as the antenna.

10. A microwave antenna and radome assembly according to claim 9 wherein a flexible diaphragm is provided, said diaphragm separating the said liquid within which the antenna is immersed from the liquid within which the motor is immersed so as to equalise the hydrostatic pressures of the liquids.

11. A microwave antenna and radome assembly according to claim 9 wherein the antenna and the motor are immersed in the same liquid.

12. A microwave antenna and radome assembly according to any one preceding claim wherein the is provided bellows means to accommodate changes in volume of said liquid due to changes in temperature and hydrostatic pressure.