Helical antenna with improved temperature characteristics

Abstract

The operating power range of a helical antenna is increased by the use of a hollow tubular helix. The inner tubular wall supports a capillary transport material saturated with a vaporable fluid and the tubing is partially evacuated before being sealed at both ends. Localized heat energy generated within the helix during antenna operation is continuously distributed to the cooler helix regions by means of the condensation cycle of the fluid. The fluid is returned to the hotter regions by means of capillary transport through the capillary material along the inner tubular walls.

Description

BACKGROUND OF THE INVENTION

High power transmitting antennas in the 2 to 30 mHz range for submarine and shipboard use are primarily of the tunable inductive/capacitive type; that is, an inductive helix in series with and physically arranged end-to-end with a capacitive cylinder.

The inductive element is by design, a metal helix imbedded in an insulating coilform so as to expose the internal surface to the traveling "shorting" mechanism within the helix to effect a continuous tuning characteristic. For practical reasons, the coilforms are usually made of epoxy-bonded or silicone-bonded fiberglass; the choice of the resin system being controlled by the required electrical and thermal properties of the material. In transmitting antennas of lower power rating or of reasonably tall radiators where the potential gradient and flow of current along the helix is mild, the use of epoxy-bonded fiberglass is permitted. In transmitting antennas of higher power rating or of exceptionally short radiators where the potential gradient and flow of current along the helix can be steep and severe, the use of silicone-bonded fiberglass is necessary. The dielectric properties that determine the choice of material, in addition to its dielectric strength, are its dielectric constant and its power factor, the product of which is its loss factor; a factor that measures power loss within the substance when exposed to the effects of an alternating dielectric field. This loss of power, of course, decreases the antenna's efficiency and shows up as heat in the coilform.

By way of example, the dielectric properties at 1 mHz for coilforms of practical materials are:

Dielectric Power Loss Material Constant Factor Factor ______________________________________ Epoxy-bonded Fiberglass 5.0 .020 .100 Silicone-bonded Fiberglass 4.0 .003 .012 ______________________________________

Epoxy-bonded fiberglass will therefore absorb roughly 10 times the electrical energy with a corresponding increase in temperature as will silicone-bonded fiberglass under the same conditions. In high power helical antennas, the use of silicone-bonded fiberglass is necessary. In lower power helical antennas where the flow of current is reduced or in those where best efficiency is not the outstanding requirement, the use of epoxy-bonded fiberglass is permissible.

The principal thermal property that determines the choice of material for the coilform is, of course, its ability to withstand high operating temperatures without detrimental change. Again, fiberglass coilforms have proven to be most practical wherein the chosen resin system determined the permissible upper operating temperature. Some examples of the practical temperature limits are shown in the following table:

Material .degree.F ______________________________________ G-10 Epoxy-bonded Fiberglass 250 - 300 Cycloaliphatic-resin E-b Fiberglass 350 - 425 G-7 Silicone-bonded Fiberglass 450 - 500 ______________________________________

However, in helical antennas, the temperature rise in the coilform is not due only to its dielectric heating. A major contributor is the heat conducted from the imbedded metal helix. This is especially true and significant in regions of the helix where the current is concentrated. Here, the ohmic resistance of the helix material, be it silver or copper, is sufficient to produce local, but very significant I.sup.2 R power or "copper" losses. This locally-absorbed I.sup.2 R heat superimposed upon that of the inherent dielectric heat can cause a destructive temperature rise within the coilform. In ITTDCD type helical antennas, this phenomenon has become the limiting factor in establishing the power-handling capabilities of the antenna.

DESCRIPTION OF THE PRIOR ART

FIG. 1 shows a typical helical antenna for shipboard use. The tuning element 1 is a driven metal tube with one end contacting the helix 2 internally through an assembly of graphite brushes 3, the other end similarly contacting the cylinder 4 internally. This tuning element is motor 5 driven by a spline 7 that not only imparts rotation to the tuning element 1 so that the brushes 3 will slide along the helix 2 turn but also permits an axial movement of the tuning element 1 within the helix so as to permit the brushes 3 to follow the pitch of the helix turn. A series of grooved rollers 6 engaged onto the helix 2 and mounted upon the tuning element 1 imparts the proper axial movement to the tuning element 1. Radio frequency 8 and control cables 9 are shown at the bottom end of the radome 10.

FIG. 2 a illustrates a normal voltage and current distribution along the cylinder 4 and helix 2 of a helical antenna shown schematically for comparison in FIG. 2 b. The voltage curve 11 and the current curve 12 are indicated in relation to the position of the brushes 3. The tuning point is indicated as a line 13 along the abscissa 14 and the point of maximum current is located at 15 and the point of minimum voltage is located at 15 for the antenna where the ordinate 16 indicates increasing current values in arbitrary amounts as indicated by the arrow. Since the current distribution is not uniform along the helix, the greatest ohmic heating and, in turn the maximum temperature rise will occur at the point of maximum current. When the tuning "short" is near the top of the helix, the helix current for the same power input is generally low in any one turn compared to the helix current in the restricted few active turns when the tuning "short" is near the bottom of the helix. Since the helical antenna has been configured to operate in two modes so as to cover the frequency range of 2 to 30 mHz, one-fourth wave mode for 2 to 12 or 15 mHz and three-fourths wave mode for 10 or 12 to 30 mHz, critically high currents will be encountered in two regions over the range. These will, of course, occur in frequencies when the short approaches the bottom of the helix to obtain the higher frequencies in each mode of operation.

FIGS. 3 a thru 3 d schematically illustrate four tuned positions and conditions in a helical antenna; FIG. 3 a and FIG. 3 b show the voltage 11 and current 12 distributions when the "short" is near the top of the helix. FIG. 3 a shows the one-fourth wave mode characteristic; FIG. 3 b shows the three-fourths wave mode. FIG. 3 c and FIG. 3 d shows the voltage and current distribution when the "short" is near the bottom of the helix. FIG. 3 c shows the one-fourth wave mode and FIG. 3 d shows the three-fourths wave mode. Schematic representations of the cylinder 4 and helix 2 along with the location of the brushes 3 are included in order to indicate the relative location of the brushes 3 at points of maximum current 15 and minimum voltage 15. In the lower frequency cases a and b for each mode, the temperature of the helix due to I.sup.2 R losses, in all cases a maximum when the current is a maximum 15, is comparatively low and becomes fairly uniformly distributed over the helix because of metallic conductivity. However, in the latter high frequency cases, FIG. 3 c and FIG. 3 d for each mode, the temperature of the helix due to I.sup.2 R loss is quite concentrated, intense and not sufficiently diminished by metallic conductivity. The heat of this concentrated I.sup.2 R loss flows from the helix wire into the enclosing coilform and supplements its inherent dielectric heating. Since the resulting localized temperature rise in the coilform must be kept within its operating limits, this condition establishes the power-handling capability of the antenna at the particular frequency.

SUMMARY OF THE INVENTION

This invention describes a helical antenna that has heat transfer means incorporated within the antenna helix. The helix is manufactured from a hollow tubing having a layer of capillary transport material on the inner surface wall. The capillary material is saturated with a vaporable fluid and the tubing is partially evacuated and both ends are sealed. The fluid subsequently becomes heated and vaporized to carry the heat from the hotter regions to the cooler condensable regions of the helix. The fluid returns to the hotter zone by means of capillary transport through the capillary material on the inner wall. The continuous vapor transport and fluid return results in a lower uniform temperature distribution throughout the entire helix.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a helical antenna of the type used with the prior art.

FIG. 2 a is a graphical presentation of a one-fourth wave operating mode of the helical antenna of FIG. 1.

FIG. 2 b is a schematic indicating the position of the brushes relative to the cylinder and helix corresponding to the operational mode of FIG. b.

FIG. 3 a is a graphic presentation of the resistive heat load of the antenna of FIG. 1 for a 2mHz one-fourth mode operation.

FIG. 3 b is a graphical presentation of the resistive heat load of the antenna of FIG. 1 for a 12 mHz three-fourths mode operation.

FIG. 3 c is a graphical presentation of the resistive heat load of antenna of FIG. 1 for a 5 mHz one-fourth mode operation.

FIG. 3 d is a graphical presentation of the resistive heat load of the antenna of FIG. 1 for a 20 mHz three-fourths mode operation.

FIG. 4 shows a partial section of the inventive helix tubing illustrating the method of heat transfer.

FIG. 4 a shows an enlarged section of the tubing of FIG. 4 illustrating the structure in detail.

FIG. 4 b shows one embodiment of the helix of the instant invention in a vertically oriented plane.

FIG. 4 c shows an embodiment of the helix of the instant invention including a braided metal wick retaining member.

FIG. 4 d shows an embodiment of the helix of the instant invention having a porous capillary layer integrally formed within the inner surface of the helix tubing.

FIG. 5 shows a graphical temperature distribution profile for both a 20 mHz three-fourths wave mode, and a 5 mgHz V.sub.4 wave mode operation; and for helical antennas of the prior art.

FIG. 6 shows a graphical temperature distribution profile for a 20 mgHz three-fourths mode and a 5 mHz one-fourth wave mode operation for the helical antenna of the instant invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 4 shows a partial section of one heat transfer apparatus according to the invention. The helix tubing consists of a thin wall copper tube 42 coated with a layer of silver 43. This combination of metals provides the good electrical and good thermal properties required for the helix. The wick material 44 consisted in this embodiment of a tightly woven cotton braid. Water is chosen as the fluid transfer material since the input power operating range for the helix resulted in a hot spot temperature in excess of the boiling point of water.

FIG. 4 a is an enlarged representation of the encircled part of FIG. 4 and shows in detail the layer of silver 43, the copper tube 42, the cotton wick 44 and the passage 46 for the water vapor to pass through.

The heat is distributed away from a hot zone 47 within the helix tubing as shown diagramatically in FIG. 4 where the water in vapor form is depicted as small arrows 48 moving away from the hot zone 47 in both directions along the tubing 42. At a point along the tubing sufficiently remote from the hot zone 47, the water vapor 48 reaches a section of the tubing where the water vapor 48 can condense for example at 49 where the heat carried by the water vapor 48 is transmitted to the tubing 42 in the condensation process. The water droplet designated as large arrows 49a wets the wick 44 and commences to move along the wick back to the hot zone 47 by the mechanism of capillary action. Since the helix is exposed on its outer surface to ambient air, the heat which is transported to the tubing in the regions outside the hot zone 47 will be removed from the tubing 42 outer surface by the mechanisms of radiation and convection. This transfer of heat is depicted by the vertical arrows 49b.

Since the tubing 42 is sealed to contain the water vapor 48 and the wick 44 is saturated at a temperature much lower than the boiling point of water, the system must continuously transport heat from the hotter regions of the indicated hot zone 47 to the cooler regions extending throughout the entire tube 42. This condensation cycle therefore continues until the entire heat energy is transported throughout the tubing depending on electrical input power to the helix, the geometry of the tubing and the thermodynamic properties of the fluid.

FIG. 5 shows in both schematic and graphic form the relative temperature distribution for a helix according to the prior art. A partial section of a wire wound helix 52 is depicted indicating one hot zone 57 existing between the two contacts 53. The helix 52 is located relative to the graph in order to make the temperature on the graph correspond to a relative point on the helix 52. The ordinate axis 54 depicts the measured temperature in Fahrenheit degrees at a point along the abscissa 55 depicting a thermal point on the exterior surface of the corresponding helix 52.

The temperature distribution curve for a 5 mHz signal (one-fourth wave mode) 56 is shown for comparison to the temperature distribution curve for a 20 mHz signal (three-fourths wave mode) 57. The maximum temperature limit for the particular helix materials chosen is depicted as a straight line 58 relative to the helix 52. The hot zone 57 for both the 5 mHz 56 and the 20 mHz 57 signals for the helix 52 of the prior art is shown to be excessive since the operating temperatures are higher than the maximum temperature limit 58 for the helix 52.

FIG. 6 shows the temperature distribution curve 63 for the helix of the invention 62 employing the heat transfer properties of the instant invention. The helix 62 depicted along the abscissa 65 of the temperature distribution curve 63 is similar for both the 20 mHz (three-fourths wave mode) and 5 mHz (one-fourth wave mode) signals of FIG. 5 56, 57. The ordinate 64 depicts the direction of increasing temperature. Here the temperature 63 at any point along the surface of the helix 62 is limited to the boiling point of the water which is in the vicinity of 212.degree.F depending upon the internal vapor pressure within the helix tubing. The maximum operating temperature limit for the materials employed is depicted as a straight line at 350.degree.F. 68.

The temperature is more uniformly distributed along the helix 62 surface and is kept to the limit of the boiling point of the liquid used. Note that the operational range of temperature for both frequencies 56, 57 is well below the maximum operating temperature range (350.degree.F) for the helix 62 materials chosen.

The use of a trapped condensable fluid within the helix tubing of a helical antenna has solved a very serious materials and power limitation problem in sea-born antennas. In submarines, for example, when the helical antenna is in its standard or upright position, the capacitive element is above the inductive helix, and the hotter regions of the helix are most usually at the bottom region of the vertical helix winding. This configuration readily lends itself to the consensable fluid thermal transfer distribution of the instant invention. The vertical position of operation permits the return of condensed fluid by means of gravity. When the proper fluid is chosen and the helix tubing is evacuated to create a condition of complete vaporization of the fluid in the region of the hot zone, and a condition of condensation outside the hot zone, then the heat is carried upwards by the vapor where it is deposited at a cooler region. The condensed fluid then drops back into the region of the hot zone under the influence of gravity.

Several other factors may influence the thermal and fluid transport properties of the instant invention as is well known to any thermodynamicist. The pore size of the material chosen for capillary transport, the wetability of the liquid and viscosity determine the ultimate rate of liquid flow.

The position of the helix relative to the horizontal plane determines whether gravity may be an important factor in the fluid transfer rate. FIG. 4 b shows the cross-section of a part of a helix wound from a tubing similar to that described in FIG. 4 a except that the wick 44 is omitted from the tubing 42. The helix is shown relative to the vertical plane 70 and the horizontal plane 71. The tubing 42 is usually wound at a slight pitch represented in FIG. 4 b as a pitch angle 72 existing between the tubing 42 and the horizontal plane 71. This inclination of the tubing provides for the return of the fluid within the tubing to the bottom of the helix after evaporation and condensation has occurred. The operating power requirements of the antenna will, of course, determine the maximum temperature to be distributed throughout the helix. Other thermal considerations include the ambient temperature external to the helix and whether the helix transfers its heat externally by means of normal air flow or whether forced coolant means are to be employed. If the antenna described above for submarine application is to be employed in arctic waters, then the water described within the earlier embodiment would be impractical because of its relatively high freezing point. A number of fluids have been investigated such as ketones (acetone), alcohols (glycerines and glycols) and water solutions thereof, and fluorinated hydrocarbons (freon).

When the method of fluid transport is by means of capillary action, the pore size of the capillary must be optimally determined for the particular fluid used. Some of the capillary materials which have been used for the fluids listed above are cotton, wool, fiberglass and felt, both in woven and in braided form. One of the methods employed for keeping the capillary material against the tubing wall and for insuring that the tubing cross section remains essentially clear of material is to use a concentric annular tubing of braided metal wire. The porosity of the braid allows the liquid and vapor to pass freely through to and from the capillary material captured between the braided metal-wire tubing and the tubing wall. This embodiment is shown in FIG. 4c where along with the silver coating 43 on the cooper tubing copper and the cotton wick 44 there is included a braided tubing 73. The braided copper mesh 73 allows the passage 46 in the tubing to remain clear and holds the wick 44 against the tubing wall.

Other embodiments for the instant invention comprise a combination of the tubing wall and the capillary material where the outer surface of the tubing is impregnable, and the inner surface of the tubing is treated in a manner to create a series of very fine pores for allowing capillary transport along the inner surface within the pores. The method of making the inner tubing wall capillary-active can be by coating a porous metal onto the surface of the tubing or by treating the inner surface of the tubing directly to make it porous. FIG. 4 d shows one embodiment of a copper tubing 42 having a coating of silver 43 on the outer surface, and a series of micro-pores 74 integrally formed into the inner surface of the copper tubing 42. This porous layer extends into the tubing inner surface a distance of a few millimeters and can be etched thereon or the entire tubing 42 could be made of porous copper tubing and the silver 43 coated on the surface to seal the outer pores.

Although the helical antenna having improved operating temperature characteristics has been described for a mobile helical antenna, this is done by way of example only and is, in no way, intended to limit the scope of this invention which may find application wherever an internally cooled helix is required.

Claims

What is claimed is:

1. A helical antenna with improved temperature characteristics comprising:

an antenna housing;

a capacitive cylinder within said housing;

a helix coupled with said cylinder; and

a condensable fluid enclosed within the helix whereby heat energy is distributed uniformly along said helix by the condensation of said fluid.

2. A helical antenna with improved temperature characteristics comprising:

an antenna housing;

a capacitive cylinder within said housing; and

a helix coupled with said cylinder said helix consisting of

a number of turns of a length of hollow metal tubing closed at both ends, a quantity of a condensable fluid within said tubing, and means for transporting said fluid within said tubing

whereby heat energy is distributed by vaporization of said fluid and whereby said fluid when condensed is returned by said fluid transport means.

3. The helical antenna of claim 2 wherein said fluid transport means comprises capillary action.

4. The helical antenna of claim 2 wherein said fluid transport means comprises gravity.

5. A helix having uniform heat distribution properties for use with high power transmitting antennas comprising:

a number of turns of a hollow metal tubing sealed at both ends;

at least one layer of porous capillary material on the inner surface of said tubing; and

a quantity of condensable fluid within said tubing for distributing said heat, whereby said heat is transported within said helix by the condensation cycle of said fluid and said fluid is transported within said helix by capillary transport within said material.

6. The helix of claim 5 wherein said tubing comprises a metal selected from the group consisting of copper, silver, and aluminum.

7. The helix of claim 5 wherein said tubing comprises silver coated copper.

8. The helix of claim 5 wherein said capillary material is selected from the group consisting of cotton, wool, felt and fiberglass.

9. The helix of claim 5 wherein said fluid comprises a liquid selected from the group consisting of water, ketones, and alcohols.

10. The helix of claim 5 wherein said fluid is a fluorinated hydrocarbon.

11. The helix of claim 5 further including a braided metal sleeve within said tubing for supporting said capillary material against the inner walls of said tubing.

12. An improved helical antenna of the type having a capacitive cylinder and a helix coupled within a housing wherein the improvement comprises:

a helix having a number of turns for imparting inductance to said antenna, said helix further comprising a length of silver coated copper tubing partially evacuated and sealed at both ends, a woven cotton wick continuously extending along the inner walls of said tubing, a braided copper mesh extending concentrically within said tubing and supporting said wick against said tubing walls, and a sufficient quantity of water to saturate said wick, wherein heat energy generated by the operation of the antenna is continuously distributed from hotter to cooler regions of the helix by the evaporation and condensation of the water, and whereby the water is returned to the hotter regions of the helix by capillary transport through said wick.