US3587106A

US3587106A - Broad band antennas having spiral windings

Info

Publication number: US3587106A
Application number: US745014A
Authority: US
Inventors: James W Crooks Jr; Ray M Mcintyre Jr
Original assignee: General Dynamics Corp
Current assignee: General Dynamics Corp
Priority date: 1968-07-15
Filing date: 1968-07-15
Publication date: 1971-06-22
Anticipated expiration: 1988-06-22

Abstract

A broad band antenna or feed for a parabolic reflector having a pair of spiral windings skewed with respect to the axis of a conical support surface on which they are wound in alternating relationship. Two such supports are rotatably mounted with their axes offset at an angle from the reflector axis so as to conically scan the target area. The feed achieves maximum illumination of the control portion of the reflector and provides a constant squint angle with respect to the reflector axis over a broad band of frequencies.

Description

United States Patent [72] inventors James W. Crooks, ,Ir.;

Ray M. McIntyre, Jr., both of San Diego, Calif. [21] Appl. No. 745,014 [22] Filed July 15, 1968 [45} Patented June 22, 1971 [73] Assignee General Dynamics Corporation [54] BROAD BAND ANTENNAS HAVING SPIRAL WINDINGS 17 Claims, 7 Drawing Figs.

[52] US. Cl 343/739, 343/761, 343/840, 343/895 [51] Int. Cl l-l0lq H36 [50] Field of Search 343/7925, 840. 895. 761.739

[56] References Cited UNITED STATES PATENTS 2,958,081 10/1960 Dyson 343/895 2,982,964 5/1961 Bresk et a1. 343/895 3,059,234 10/1962 Du Hamel el 211... 343/7925 3,193,831 7/1965 Yang 343/840 3,277,490 9/1966 Williams 343/840 3,454,951 7/1969 Patterson et al. 343/895 2,682,608 6/1954 Johnson 343/895 2,966,679 12/1960 Harris 343/895 Primary Examiner-Eli Leiberman AttorneyM artin Lu Kacher ABSTRACT: A broad band antenna or feed for a parabolic reflector having a pair of spiral windings skewed with respect to the axis of a conical support surface on which they are wound in alternating relationship. Two such supports are rotatably mounted with their axes offset at an angle from the reflector axis so as to conically scan the target area. The feed achieves maximum illumination of the control portion of the reflector and provides a constant squint angle with respect to the reflector axis over a broad band of frequencies.

SUPPORTING CONE OF INSULATING MATERIAL PATENIEII JUII22 IBYI SHEET 1 [IF 5 fPHASE FRONT DISH I 2 TYPICAL E PLANE RADIATION PATTERN 26 OF CONVENTIONAL FEED e EFFECTIVE CENTER OF RADIATION FOR LOW FREQUENCY CENTER OF I2 CONVENTIONAL RADIATION I ,RcP FEED AT SHORT AXIS 0F FEED ROTATION 4 WAVE E AND AXIS OF DISH I DESIRED scum-r ANGLE DESIRED BEAM ANGLE OF MAJOR ILLUMINATION BY I FOCAL D'RECT'ON FEED 95}? l4 FEED DISH PERPENDICULAR TO PORTION OF PHASE FRONT ILLUMINATED I ACTUAL SOUINT ANGLE O ILLUMINATED PORTION OF PHASE FRONT Fig.

INVI'IN'H )R. JAMS w. (woo/(s, JR. RA) M. Mel/VT YRE, JR.

RYm JBQ ATTY PATENTEUJUNZZIB'II 3587.106

SHEET 2 BF 5 DISH L-PHASE FRONT TYPICAL E PLANE RADIATION PATTERN OF FEED WITH SKEWED WINDING EFFECTIVE CENTER OF RADIATION FOR LOW FREQUENCY I RCP FEED WITH SKEWED WINDINGS SQUINT ANGLE ROTATION BEAM DIRECTION LCP FEED WITH SKEWED WINDINGS ANGLE 0F PRINCIPAL ILLUMINATION INVENTORS.

JAMES W CROOKS, JR. RAY M Mc/NTYRE, JR.

BY ATTY.

slsamos PATENTED .JUN22 197i SHEET 3 BF 5 IN VIE N TOR 8 JAMES w. moons, JR. RA r M. Mc/N TYRE, JR

PATENTEUJUN22|9H 3587.106

SHEET 0F 5 SKEW ANGLE INVENTOR. JAMES w. moo/rs, .m. RAY M. m INTYRE, JR.

PATENTED JUN22 l9?! SHEET 5 0F 5 I N VENTOR.

JA MES W. CRO0K$,JR.

RAY M. MCINTYREMR.

BY Qtfwh v ATTY BROAD BAND ANTENNAS HAVING SPIRAL WINDINGS The present invention relates to broad band antennas and particularly to broad band antennas having conical spiral feed windings.

The invention described herein was made in the performance of work under a NASA contract and is subject to the provisions of Section 305 of National Aeronautics and Space Act of 1958, Public Law 85-568 (72 Stat. 435; 42 USC 2457).

The invention is especially suitable for use in a feed for parabolic antennas which are adapted to track space vehicles by responding to signals radiated therefrom. The invention however is also generally useful in broad band antennas, including antennas of a type which do not have a reflector dish as well as in directional antennas where a desired radiation pattern in a direction having a specified angle with respect to the angle at which the antenna is mounted may be desired.

The conventional conical antenna having equiangular spiral windings has a radiation pattern which is directed along the axis of the cone on which the windings are disposed. It has been found that this characteristic is detrimental to a scanning antenna where the axis of the cone is offset from and makes an angle with the axis of the reflector with which the antenna is used as a feed. The term feed comprehends both reception and transmission of signals.

At certain lower frequencies in the band in which the antenna is active although the feed is positioned for proper operation at the highest frequency, instead of scanning conically in synchronismwith the rotation of the feed, the radiation pattern from the reflector remains essentially along the reflector axis. This effect is manifested particularly in the case of the plane of the Electric Vector of linear polarized radiation. Since the antenna does not scan with the requisite pattern on which the servosystem of the tracking system depends, it becomes difficult to lock on to and to track the source of incoming radiation, say a space vehicle or missile. In the event that the ratio of focal length with respect to the diameter of the reflector is small as for example (in deep parabolic dishes) the side lobe energy or side lobe radiation pattern of the feed impinges on the reflector and the feed pattern does not effectively illuminate the whole reflecting surface of the dish thereby compromising antenna performance.

It is therefore an object of the present invention to provide improved scanning antennas.

It is a further object of the present invention to provide improved broadband antennas.

It is a still further object of the present invention to provide an improved conical feed for spiral antenna.

It is a still further object of the present invention to provide a wideband conical spiral type feed which may be used in combination with a dish or by itself as an independent antenna and whereby the radiation pattern of the feed remains at a desired angle with respect to the axis thereof over the entire band of frequencies over which the antenna is active. The angle can be made approximately constant or varied over the frequency range of the antenna by controlling the angle of skew along the winding.

Briefly described, an antenna or feed embodying the invention includes a support structure. A winding is spirally wound around the support structure to define a substantially conical surface or cone. A portion of the windings can be disposed at a predetermined angle with respect to the axis of the cone so that the direction of the radiation pattern remains at a constant angle with respect to the'cone axis. In the event that the aforementioned antenna structure is sued as a feed for an antenna having a reflector, it is desirably supported so that the cone axis is at a predetermined angle to the axis of the reflector. This angle being selected to achieve a desired squint angle. The squint angle of the antenna, when this invention is used as a feed in a reflector, is the direction of the radiation pattern from the reflector with respect to the reflector axis. By virtue of the skewed windings on the feed, the squint angle of the antenna can be made to be approximately proportional to the antenna beam width over the entire wide band of frequencies over which the antenna is active, notwithstanding that the feed may be rotated about the axis of the dish in order to pro vide scanning action.

The invention itself, both as to its organization and method of operation, as well as additional objects and advantages thereof will become more readily apparent from a reading of the following description in connection with the accompanying drawings in which:

FIG. I is a schematic diagram of a conventional antenna of the type having a dish illuminated by a feed on which equiangular conical spiral windings are disposed;

FIG. 2 is a schematic diagram similar to FIG. 1 of an antenna which embodies the invention:

FIG. 3 is a plan view of a conventional equiangular or logarithmic conical feed or antenna;

FIG. 4 is a view similar to FIG. 3 of an antenna or feed which embodies the invention;

FIG. 5 is a sectional view taken along the line 5-5 in FIG. 4;

FIG. 6 is a section view taken along the line 6-6 in FIG. 4; and

FIG. 7 is a plan view of a conical spiral antenna or feed in accordance with another embodiment of the invention.

FIG. 1 illustrates a conventional telemetry antenna which may be used as a receiving antenna for tracking and receiving signals from space vehicles and missiles. The antenna may also be used for transmitting. The antenna includes a dish reflector which is parabolic in form and has a focal point along the axis thereof. The dish is relatively deep, that is to say the ratio of its focal distance (viz. the distance from the focal point of the dish to its surface (F) to the diameter of the dish (d)) is relatively low say of the order of 0.3. The antenna includes a feed 17 which is shown in greater detail in FIG. 3. This feed includes two

conical members

12 and 14, which may be made of insulating material. Each of these members carries a pair of windings 16a and 16b and 18a and 18b. The windings 16a and 16b on the conical member I2 alternate and are wound in the form of equiangular spirals. Windings 18a and 18b on the conical member 14 are similarly wound as equiangular spiral windings but in a sense opposite to the windings 16a and 16b. Thus the windings 16a and 16b will be responsive to one sense of circular polarized radiation whereas the windings 18a and llfib will be responsive to the opposite sense of circular polarized radiation. The conical members are supported on a yoke 20 which may be joumaled for rotation about the axis of the dish.

Transmission lines 19 and 21 are respectively provided in the cylindrical members 12 and I4 and are connected at their front ends to the ends of the windings near the apex of their respective conical members. The other end of the windings I6 and 18 are terminated, say by resistors 22 and 24, respectively, each being connected between the opposite ends of their associated windings. The ends of the transmission lines 19 and 21, which extend from the base of the conical members may be connected by way of cables to a rotating joint for deriving signals form the respective windings I6 and 18. A two wire transmission line may be used for 19 and 21 or alternately a coaxial line with a balun to couple from the coaxial line to the balanced transmission line at the tip of the cone.

Altemately a coaxial transmission line can be used as the spiral winding with a coaxial transmission line bringing the signal in (or out) connected to one of the two spirals in the place of the resistor terminatiomfl" he coaxial transmission line center conductor can be connected from one spiral to the center conductor of the opposite spiral on the same cone at the small end (apex) of the spiral with a small gap between the outer conductor of one winding and the outer conductor of the second winding. The outer conductor of the second spiral winding is shorter to the inner conductor at the end of the gap. This is a conventional method of feeding equal angle spiral antennas from a coaxial transmission line and is sometimes referred to as the use of an infinite balun. The terminating resistor is optional. The antenna, by virtue of the fact that the windings I6 and 18 have smaller diameter portions which spiral outwardly to larger diameter portions is operative over a broad band of frequencies, say from the S band through the UHF band. The effective center of radiation (viz. the point on the feed 17 where the antenna is responsive to different frequencies of radiation) varies with the distance along the axes of the

conical members

12 and 14. The shorter wavelengths will have their centers of radiation near the apex of the cone and for longer wavelengths the center of radiation will move along the axis of the conical members away from the apex thereof.

As shown in FIG. 1, the apex of the conical members and the center of radiation at short wavelengths is close to the focal point of the dish. A typical E-plane radiation pattern at lower frequencies and longer wavelengths is also shown spaced at its center further away from the dish. In order that the antenna be responsive to signals at the longer wavelengths, it is necessary that the antennas scan a relatively wide field of view at lower frequencies. Such a wide scanning field dictates a relatively large squint angle. By a squint angle is also meant the angle between the axis of the dish and the beam direction. The desired squint angle and the desired beam direction is shown in FIG. 1. In order to obtain this larger squint angle at longer wavelengths, it becomes necessary to offset the axis of the conical members from the axis of the dish. The foregoing offset is dictated by the characteristic of equiangular spiral antennas of providing a radiation pattern which is along the axis of the conical members, as shown in FIG. 1. By virtue of this offset the angle of major illumination of the dish at lower wavelengths is not sufficiently broad to evenly illuminate the full dish, especially in the plane of the Electric Vector (E- plane). ln deep parabolas, this results in side lobe energy impinging on the reflector surface; further, compromising antenna performance. ln addition, the effective squint angle may be actually reduced to zero at some wave lengths. This characteristic follows from the fact that the actual direction of the radiation (viz. the direction of the beam) is perpendicular to the phase front from the portion of the dish which is illuminated. The phase front is the surface along which all of the energy is in phase. The phase front of the radiation from the illuminated portion of the dish at any frequency is defined by a surface generated by the continum of points having equal distances along ray paths from the center of radiation of waves at that frequency to the dish and then outwardly from said dish. Two of

such points

26 and 28 at the limits of the angle of major illumination of the dish at a longer wave length in the UHF band are shown in FIG. 1. These points are generated by drawing ray paths from the center of radiation along a straight line to the dish and then from the dish such that these angles of incidence 6, and 6, equal these angles of reflection 6, and 0,, respectively. The distance along the line between the center of radiation to the dish and thence to the point 26 on the phase front is equal to the distance along the lines from the center of radiation to the dish and thence to the point 28. Since the actual direction of the beam or the direction of the radiation pattern of the antenna is perpendicular to the phase front which is illuminated, it may be determined by construction that the actual direction of the beam is approximately parallel to the axis of the dish making the actual squint angle approach zero degrees (i.e. along the dish axis) for radiation of lower frequency and longer wave lengths.

in accordance with the invention this deficiency is overcome by providing a radiation pattern from the feed which is essentially in a direction parallel to the axis of the dish so that the dish is illuminated throughout the frequency range over which the antenna is active. FIG. 2 shows a dish 30 which may be a relatively deep parabolic of the type shown in FIG. 1. The antenna includes a feed 32 including two

conical members

34 and 36. The conical member 34 having windings wound thereon in one sense to provide a left circularly polarized feed element whereas the conical member 36 has windings wound thereon in an opposite sense to provide a right circularly polarized feed. The windings are skewed with respect to the axis of their respective

conical members

34 and 36. The resulting E-plane radiation pattern is effectively parallel to the axis of the dish so that the angle of principal illumination of dish 30 is distributed over approximately the central portion of the dish. The phase front resulting from this illumination may be determined by construction of the same manner as explained in connection with FIG. 1. lt will be observed that two

points

38 and 40 on the phase front along rays at the edges of the angle of principal illumination are approximately at the same distance from the dish axis.

Inasmuch as the actual beam direction is perpendicular to the phase front from the illuminated portion it is apparent that the beam direction will be at the desired squint angle and radiation at longer wavelengths may readily be received by the antenna as it scans a wider area. Accordingly, the invention provides a broad band scanning antenna with adequate squint in that it makes possible adequate scanning at wavelengths at which the squint is inadequate with conventional feed to produce adequate tracking error signals for proper servo performance. As shown in FlGS. 4, 5 and 6 the

conical members

34 and 36 may be hollow cones of fiberglass or other insulating material on which pairs of windings 40 and 42 are wound; the

windings

40a and 40b being wound on the cone 34 while the windings 42a and 42b are wound on the cone 36. The cones are supported on a yoke 44 which may be journaled for rotation about the axis of the dish, as shown in FIG. 2. The windings 40a and 4012 as observed on one side of the conical support alternate with each other and are connected at their ends near the apex of the conical member 34 to a transmission line (not shown) similar to the transmission line shown in FIG. 3. The windings 42a and 42b are wound in a sense opposite to the

windings

40a and 40b and are also alternated with each other. Another transmission line may be connected to the apex ends of the windings 42a and 42b. The other ends of the

windings

40a and 40b near the base of the cone 34 may be terminated by a resistor 46 while the ends of the windings 42a and 42b near the base of the cone 36 may be terminated by a resistor 48. This resistive termination is not essential and may be omitted if desired. A termination is, however, preferred.

In order to enchance the performance of the feed 32, it may be desirable to position a ground plane at the bases of each of the

cones

34 and 36. Alternatively, the ends of the

windings

40a and 40b and 42a and 42b may be terminated by rings 50 (FIG. 7) of conductive material such as wire which is disposed around cylindrical portions 52 extending from the bases of the cones; only one of the cones is illustrated in FIG. 7 to show this construction. A first plurality of resistors 54 is connected between the end of the winding 40a and the ring 50 while a second plurality of resistors 56 is connected between the end of the winding 40b and the ring 50. These resistors are parallel to the axis of the cylindrical portion 52 and are spaced from each other. The spacing between the resistor 54 is desirably logarithmic and increases between resistors which are spaced further away from the end of the winding 40a. A similar space relationship may be provided between the resistors 56 which terminate the end of the winding 40b.

The surface of the

cones

34 and 36 is of course defined by the conductive elements or windings 40 and 42 thereon. As shown in FlG. 5 this surface can be made elliptical in cross section for cross sections taken perpendicular to the axis of the cones and the surface be made conical for cross sections taken perpendicular to the scanning axis (viz. the axis of the dish) as shown in FlG. 6. This is a desirable but not essential feature of the design. The windings 40 and 42 are also skewed with respect to the axis of the respective cones 34 and 36 (viz. the conical surfaces which they defined). As pointed out in connection with FIG. 2, it is desirable that the direction of radiation at the longer wave length be essentially parallel to the axis of the dish. This is accomplished by providing the skew of the windings such that the windings are tilted at a skew angle approximately equal to the angle which the axis of the cones make with the dish axis. The skew angle is identified in FIG. 4. The projection of the windings on a plane perpendicular to the axis of the cones (viz. the conical surfaces defined by the windings) can be beneficially modified to be an elliptical spiral rather than circular spiral as is the case with an equiangular spiral winding, such as is shown in FIG. 3. This angle of skew as shown in H6. 4 may be defined as the angle between (1) line connecting diameterally opposite points on adjacent turns of the windings 42a and Mb, in the plane containing the center lines of the two cones 3d and 36 (2) another line intersecting the axis of the cone 36 in the same place as the first but perpendicular to the axis. in order to obtain an angle of skew such that the desired squint angle is obtained the angle of skew is made approximately equal to the angle which the center line of the conical member 36 makes with the scanning axis, the diametrical line should also be perpendicu lar to the dish axis. llnasmuch as the reduction of the squint angle occurs principally at the longer wavelength it may be desirable only to make those turns of the windings of larger diameter tilt at the requisite angle of skew. The smaller diameter of turns near the apex of the conical members then may be equiangular. [n a particular case wherein the windings Ma and 42b were approximately six turns each only the last three turns of the windings were skewed.

The invention of course may be used in an antenna which does not include a dish. In that event, the apex of the conical members 34% and 36 will point in the direction of radiation (viz. towards the missile or space vehicle providing the source of signals to be tracked rather than towards a reflector and the skewed winding rotated about the center line of the cone to provide scanning. Even if the antenna is not used as a scanning antenna, it maybe advantageously be employed in application where it is desired to have a radiation pattern over a wide band of frequencies directed at a predetermined squint angle. Thus it may be applicable where ease of mounting is desired inasmuch as the axis of the conical member may be referenced in a particular direction with respect to the axis of the nose of an aircraft. The look angle or the direction of the beam from the antenna or the direction of radiation to which the antenna is responsive would then be determined by the squint angle which is defined by the skew of the windings with respect to the axis of the conical member; Instead of rotating the antenna, the vehicle in which the antenna is mounted may be rotated in order to obtain a scanning action.

From the foregoing description, it will be apparent that there has been provided an improved antenna which is particularly adapted for use as a wide band scanning antenna as in telemetry applications and especially where a missile or space vehicle is to be tracked. While illustrative embodiments of the antenna have been described and other applications for the antenna have been suggested, various other applications and modifications in the herein described antenna will undoubtedly suggest themselves to those skilled in the art. Accordingly, the foregoing description should be taken merely as illustrative and not in any limiting sense.

What we claim is:

1. An antenna comprising a. an electrically conductive element spirally wound to define a substantially conical surface having an axis, and

b. at least those turns of said element having larger diameters being skewed with respect to said axis so as to depart from equiangular relationship with respect to the other turns of said element which are not so skewed, adjacent ones of said larger diameter turns which are disposed successively in a direction towards the apex of said conical surface being spaced from each other by progressively smaller distances.

2. The invention as set forth in claim 1 wherein the projection of said turns having larger diameters on a plane perpendicular to said axis is an elliptical spiral.

3. The invention as set forth in claim 1 including a reflector having an axis and a focal point spaced from said reflector along said reflector axis, said surface and said reflector being disposed on opposite sides of said focal point with said reflector axis at a predetermined angle to said surface axis whereby the direction of the radiation pattern of said turns of spirally wound element is substantially parallel to said reflector axis.

4. The invention as set forth in claim 3 including means for supporting said elements on said surface for rotation about said reflector axis.

5. The invention as set forth in claim 3 including a second electrically conductive element spirally wound identically with said first named conductive element except in an opposite sense to define a second substantially conical surface, the axis of said second surface being disposed in the same plane as said first surface axis and said reflector axis, said second surface axis and said first surface axis intersecting said reflector axis at the same point and making the same angle with said reflector axis as said first surface axis.

6. The invention as set forth in claim 5 including means for supporting said elements on said first and second surfaces for rotation about said reflector axis.

7. An antenna comprising first and second electrically conductive elements spirally interwound to define a substantially conical surface having an axis, at least those turns of said elements having larger diameters being skewed with respect to said axis so as to define a skew angle which is different from the skew angle of the remaining turns, the skew angle being defined as the angle formed by a first line connecting diametrically opposite points on turns of first and second elements which are adjacent to each other and a line perpendicular to the axis in the plane containing the axis and said first line.

3. The invention as set forth in claim 7 including a transmission line having two conductors, connected respectively to said first and second elements at the apex of said conical surface,

9. The invention as set forth in claim 7 wherein all of the turns of said first and said second conductive elements have the same skewed relationship.

10. The invention as set forth in claim 7 wherein said first and second elements are so wound that the intersection of a first plane perpendicular to said axis and said surface defines a circle and a second plane intersecting said axis at the same point as said first plane and intersecting diametrically opposite points of adjacent ones of said first and second elements and said surface defines an ellipse.

llll. A unidirectional antenna for providing a radiation pattern in a certain direction comprising a. a pair of windings spirally wound as to define a conical surface having an axis lying at a predetermined angle to said certain direction, and

b. at least a plurality of those turns of said windings having the larger diameters being skewed with respect to said axis so that a plane perpendicular to said certain direction and intersecting diametrically opposite points on adjacent ones of said turns of different ones of said windings defining a squint angle with respect to said axis said squint angle being equal to the difference between and said predetermined angle.

12. The invention as set forth in claim 111 including a transmission line having two conductors connected respectively to different ones of said pair of windings at ends thereof near the apex of said surface, and resistive means connected between the ends of said winding near the base of said surface.

13. The invention as set forth in claim 12 wherein said resistive means includes a circular conductor disposed near the base of said surface, a first plurality of resistive elements each connected between the last turn of one of said pair of windings and said circular conductor at positions spaced from each other around the periphery of said circular conductor, and a second plurality of resistive elements each connected between the last turn of the other of said of windings and said circular conductor at positions spaced from each other around the periphery of said circular conductor.

M. The invention as set forth in claim 13 wherein the spacing of said pluralities of resistive element is logarithmic.

15. The invention as set forth in claim llll including a second pair of windings spirally wound identically with said first pair of windings except in the opposite sense so as to define a second conical surface having a second axis, said second axis with said dish axis bisecting the angle between the axes of said surfaces.

l7 The invention as set forth in claim 16 including means for supporting said surfaces for rotation about the axis of said dish.

Claims

1. An antenna comprising a. an electrically conductive element spirally wound to define a substantially conical surface having an axis, and b. at least those turns of said element having larger diameters being skewed with respect to said axis so as to depart from equiangular relationship with respect to the other turns of said element which are not so skewed, adjacent ones of said larger diameter turns which are disposed successively in a direction towards the apex of said conical surface being spaced from each other by progressively smaller distances.

8. The invention as set forth in claim 7 including a transmission line having two conductors, connected respectively to said first and second elements at the apex of said conical surface.

11. A unidirectional antenna for providing a radiation pattern in a certain direction comprising a. a pair of windings spirally wound as to define a conical surface having an axis lying at a predetermined angle to said certain direction, and b. at least a plurality of those turns of said windings having the larger diameters being skewed with respect to said axis so that a plane perpendicular to said certain direction and intersecting diametrically opposite points on adjacent ones of said turns of different ones of said windings defining a squint angle with respect to said axis said squint angle being equal to the difference between 90* and said predetermined angle.

12. The invention as set forth in claim 11 including a transmission line having two conductors connected respectively to different ones of said pair of windings at ends thereof near the apex of said surface, and resistive means connected between the ends of said winding near the base of said surface.

14. The invention as set forth in claim 13 wherein the spacing of said pluralities of resistive element is logarithmic.

15. The invention as set forth in claim 11 including a second pair of windings spirally wound identically with said first pair of windings except in the opposite sense so as to define a second conical surface having a second axis, said second axis intersecting the axis of said first conical surface and so disposed that the same plane extends diametrically through both of said surfaces.

16. The invention as set forth in claim 15 including a reflector dish having a focal point along the axis of said dish and said surface being disposed on opposite sides of said focal point with said dish axis bisecting the angle between the axes of said surfaces.

17. The invention as set forth in claim 16 including means for supporting said surfaces for rotation about the axis of said dish.