US3727230A

US3727230A - Antenna having a combined dipole and loop portion

Info

Publication number: US3727230A
Application number: US00120397A
Authority: US
Inventors: T Doi
Original assignee: Sony Corp
Current assignee: Sony Corp
Priority date: 1970-11-21
Filing date: 1971-03-03
Publication date: 1973-04-10
Anticipated expiration: 1990-04-10
Also published as: FR2119901A1; JPS5119948B1; BE765339A; ES197885U; NL7107744A; DE2115704A1; ES197885Y; GB1328172A; FR2119901B1

Abstract

An antenna which in one embodiment is formed from a plate and which has openings formed therein wherein the opening comprises one-eighth to seven-eighths of the total area of the plate and which forms a composite antenna having a dipole and loop portion. The antenna may also be formed in printed circuit form as a thin foil upon a supporting member and may be directly mounted on a television set, for example.

Description

United States Patent [1 1 Doi [ 1 Apr. 10, 1973 [54] ANTENNA HAVING A COMBINED DIPOLE AND LOOP PORTION [75] Inventor: Toshitada Doi, Yokaharna, Japan [73] Assignee: Sony Corporation, Tokyo, Japan 22 Filed: Mar. 3, 1971 [21] Appl. No.: 120,397

[30] Foreign Application Priority Data Nov. 21, 1970 Japan ..45/103070 [52] US. Cl. ..343/795, 343/819, 343/822,

8 343/840 [51] Int. Cl. ..H0lq 9/28 [58] Field of Search ..343/794, 795, 803,

[56] References Cited UNITED STATES PATENTS 5/1960 Ghose ..343/795 1/1954 Hickson etal ..343/795 2,580,798 1/1952 Kolster ..343/795 2,841,792 7/1958 Edson eta! ..343/819 3,284,801 1 H1966 Bryant ..343/742 3,020,550 2/1962 2,446,436 8/ 1948 Rouault ..343/818 Primary ExaminerEli Lieberman Attorney-Hill, Sherman, Meroni, Gross & Simpson [5 7 ABSTRACT An antenna which in one embodiment is formed from a plate and which has openings formed therein wherein the opening comprises one-eighth to severieighths of the total area of the plate and which forms a composite antenna having a dipole and loop portion. The antenna may also be formed in printed circuit form as a thin foil upon a supporting member and may be directly mounted on a television set,for example.

9 Claims, 28 Drawing Figures PATENTEB APR 1 0107s sum 01 0F 1 a EQUEWFY FREQUENCY L! k) 2 E INVENTOR. TUSHITAUA DUI PATENTEB APR 1 0 i973 SHEET UEUF 11 coNbucT/imz 3,

CMFUNENT INVENTOR. T05HITADA 1101 PATENTEB 3,727,230

SHEET D30F 11 Ii g- EB a 46 gig v I Q i i 5 I i "cam/c1 a: O

Q I as INVENTOR. TUSHITADA WI 7 c0 PUNENT I PMEMiUAP-M 3,727,230

SHEET ULLUF 11 INVENTOR. T05H I TAUA U01 PATENTEU 0 1m 31727, 230

SHEET [1 5 OF 1 1 INVENTOR.

mun/111A 1101 PATENTED 3,727, 230

SHEET 08 0F 11 ma mu 1 [T RE I NVENTOR.

TOSHITADA D01 PAIENTEU APR 1 01013 sum 07oF 11 FKEQUENL-WMH IWENTOR TOSHITAHA flflI 3% -ELEE 1 PATENTED 1 05975 3,727, 230

SHEET 08 [1F 11 mums) FREQUENCY (MHZ) INVENTOR.

T05HI TADA D01 PATENTED APR 1 01973 3, 727, 230

SHEET 11 HF 11 INVENTOR.

Tmzmm 1101 BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates in general to an antenna as, for example, for television receivers having high gain over a broad band of frequencies.

2. Description of the Prior Art Television antennas must be highly directive to avoid ghosts and to obtain the high gain required to pick up weak signals in fringe areas. Previously, relatively large antennas such as Yagis, comprising a dipole element with reflectors and directors have been used for television, however, such antennas are not suitable for use with television receivers for receiving the UHF signals because the impedance characteristics and the mutual impedance characteristic among the antenna elements, the reflector and the director vary as a function of frequency and therefore the spacing of the electrical links of the antenna elements must be changed in order to receive VHF and UHF signals.

SUMMARY OF THE INVENTION The present invention comprises a very directive high gain antenna suitable for use with television receivers, for example, which is of a small size and inexpensive to manufacture and which can be mounted within a television receiver if desired. The antenna of the present invention is formed from a loop antenna element and a dipole antenna element integrally formed with a loop as by forming an opening in a circular plate wherein the outer edges of the plate form the loop antenna element and pie-segmented portions of the plate which. remain after the opening is formed form the dipole elements. In such antenna, the area of the opening may vary from one-eighth to seven-eighths of the area of the plate to obtain the optimum im pedance for the reception of waves over 'a wide frequency range.

It is an object of the present invention to provide an antenna which has constant impedance over a broad frequency range. Another object of this invention is to provide an antenna with sharp directivity.

Another object of the invention is to provide an antenna which is small in size, inexpensive and suitable for use with television receivers.

Other objects, features and advantages of this invention will be readily apparent from the following description of certain preferred embodiments thereof taken in conjunction with the accompanying drawings, although variations and modifications may be effected without departing from the spirit and scope of the novel concept of the disclosure, and in which:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a plan view of an antenna according to the FIG. 5B is a graph showing the frequency to admittance characteristic of the loop antenna shown in FIG. 5A;

FIG. 6 is a graph showing the frequency to admittance characteristic of the antenna of this invention;

FIGS. 7-10 are plan views of other modified forms of 7 this invention;

FIG. 11 is a perspective view of the antenna of this invention mounted in a television receiver;

FIG. 12 is a plan view of a parabolic antenna element of this invention;

FIG. 13 is a top plan view of the antenna element shown in FIG. 12;

FIG. 14 is a side view of the antenna comprising the antenna element of FIG. 12 and a dipole antenna element mounted forwardly thereof;

FIG. 15 is a graph of the frequency to front-to-back ratio characteristic of the antenna of FIG. 14;

FIG. 16 is a plan view of another modified form of this invention;

FIGS. 17 and 18 are perspective views of other embodiments of this invention;

FIG. 19 is a graph of the frequency to gain characteristic of the antenna devices depicted in FIGS. 17 and 18 and of a Yagi antenna;

FIG. 20 is a graph of a directional pattern of the antenna depicted in FIG. 17;

FIGS. 21A, 21B and 21C respectively illustrate other examples of antennas of this invention;

FIG. 22A is a perspective view of another embodiment of the antenna of this invention; and

FIG. 22B is a detailed perspective view of a component of the antenna depicted in FIG. 22A.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 illustrates an embodiment of the invention comprising a circular conductive plate 1 which has a diameter 1 which is equal to about one-third of the longest wave length of an electromagnetic signal to be received. An opening 2 is formed in the plate 1 in the shape of two sector portions 2a and 2b which are joined at their apexes thus leaving generally pie-shaped portions 3a and 3b of conductive material as shown. The rim portion is formed contiguous to the windows 2a and 2b and has a thickness of t which is small compared to the diameter of the plate 1.

Feed points

4a and 4b are provided at the apex portions of 3a and 3b adjacent the center 0 of the plate 1. In a particular embodiment, the plate had the diameter I of 250 mm and with the angle 6 of the sectors 3a and 3b being 60 and the width t being 10 mm was constructed. In this embodiment the area of the window 2 is five-eighths of the total area of the plate '1. If the total area of the plate before the window is cut is S the area 2 is S.

FIG. 2A is a Smith chart on which a curve a -b -c -d is plotted with the points respectively corresponding to the frequencies of 460 MHz, 500 MHz, 600 MHz and 780 MHz for the antenna of FIG. I. The characteristic impedance of the antenna was 200 ohms (corresponding to a reference point 1.0 on the Smith chart). The

- curve a to d of FIG. 2A shows that the impedance of the antenna of FIG. I remains substantially constant over the frequency from 460 MHz to 780 MHz. A second embodiment of a UHF antenna was constructed in which the diameter 1 was 250 mm, the width t, l()

' characteristic for this embodiment is shown by the curve a -b -c -d FIG. 2B is a Smith chart plotted for a fourth embodiment having the same diameter and width as the other embodiments but with the angle selected as 120. The curve a -b.,-c,-d, illustrates the admittance of the antenna as a function of frequency between the ranges of 460 MHz to 780 MHz. Curve a -d is the characteristic for an antenna having the same diameter and width of the prior embodiments and with the angle 0 being 150. Curve a -d illustrates the characteristic for an antenna having the same dimensions as the other embodiments but with the angle 6 being equal to 180.

The curves of FIGS. 2A and 2B illustrate that the impedance of an antenna may be held substantially constant as a function of frequency over the frequency range from 460 MHz to 780 MHz by selecting theangle 0 of the elements FIG. 3 illustrates the frequency versus admittance characteristics of antennas constructed according to FIG. I in which the diameter 1 was 250 mm, the angle 6 of the sectoral parts 3 was 60", the characteristic impedance was 200 ohms and the width t of the arcuate portions was varied respectively to be equal to 5 mm, mm and 40 mm. Frequency was measured at 460 MHz, 500 MHz, 600 MHz, 800 MHz and 980 MHz, respectively corresponding to the points a, b, c, d and e on the Smith chart. The curve a, e corresponded'to a width of 5 mm of the portion 5, the curve a e, corresponded to the portion 5 having a width of 10 mm, and the curve a e corresponded to the portion 5 having a width of 40 mm. FIG. 3 illustrates that when the width -2 of the arcuate portion 5 is about 5 to mm, that the impedance remains substantially constant over the frequency range from 460 MI-Izto 980 MHz.

I It can be concluded from the above experiments that when the area of the window 2 is from one-eighth to se'ven-eighths of the total area of the conductive plate before the window is removed, that an antenna of excellent impedance characteristics is obtained operable over a wide frequency range.

FIG. 4A illustrates a dipole antenna 6 comprising the segments 3a and 3b of the antenna 1 and FIG. 5A comprises a loop antenna 7 having a diameter 1 and a thickness d. It is to be realized that the antenna of FIG. 1 comprises a combination of the dipole of FIG. 4A and the loop of FIG. 5A formed into an integral antenna. FIG. 4B is the frequency versus admittance characteristic of the dipole of FIG. 4A which resonates at a frequency a little less than a wave length of A/2. The impedance to frequency characteristic of the antenna of FIG. 4A is illustrated in FIG. 4B in which the susceptance component is indicated by full line 66 and is O at the center frequency f and the conductance component is indicated by the broken line 68 and is at a maximum at the center frequency f The loop antenna 7 has a characteristic as illustrated in FIG. SB wherein the curve 76 indicates its susceptance characteristic which passes through 0 at f and curve 73 indicates its conductance characteristic.

It is to be noted that the curves of FIG. 6 showing the characteristic of the antenna of FIG. 1' comprise the combination of the curves of FIG. 4B and FIG. 5B of the antennas of FIGS. 4A and 5A, respectively. For example, solid line curve 8G comprises the susceptance characteristic of the antenna of FIG. 1 and is obtained -by combining the curves 6G and 7G from the antennas 3a and 3b within the range of to V of FIGS. 4A and 5A. The conductance characteristic 88 illustrated in FIG. 6 by broken line is the characteristic for the antenna of FIG. 1 and is obtained by combining the characteristics 68 and 7B of the antennas of FIGS. 4A and 5A, respectively.

It is to be particularly noted that the suscepance characteristic 8G has substantially zero value over a very broad frequency range.

FIGS. 2A, 2B, 3 and 6 illustrate that the antenna of this invention has an impedance which is constant over an extremely wide frequency range which cannot be obtained with conventional antennas. This allows impedance matching of the antenna with the receiver over a wide frequency range thus preventing losses caused by a mismatch between the antenna and the receiver, and, further provides for a broad band antenna. The antenna of this invention may also be made very small as compared to conventional wide band slot or log periodic antennas.

FIGS. 7-10 illustrate other embodiments of the invention. In FIG. 7, for example, a circular plate I is formed with a window 2' which comprises two circular window portions 2a and 2b offset from the centerof the plate and joined by an opening connecting them as shown. The portions 3a and 31) extending inwardly from. the outer portion of the disc toward the opening serve as a dipole and the antenna may be fed at the feed points 4a and 4b as shown.

FIG.8 illustrates a modification wherein the plate 1 is formed with a window 2" comprising two substantially elliptical openings 2a" and 2b" arranged symmetrically with respect to the center and joined by an opening. Feed points 4a" and 4b are provided at the center of the plate and the dipole portions 3a" and 3b" extend into the feed points.

FIG. 9 illustrates a generally square shaped plate 11 which is formed with triangular shaped opening 12 formed in two parts 12a and 12b as shown. The triangular windows 12a and 12b are contiguous at their apexes to form feed points 14a and 14b as shown. The triangular shaped portions 13a and 13b provide a dipole portionof the antenna and the outer rim 15 provides the loop portion of the antenna.

FIG. 10 illustrates an antenna formed from a generally hexagonal shaped plate 21 formed with a generally hour-glass shaped window 22 having an upper portion 22a and a lower portion 22b relative to" FIG. 10. Feed points 24a and 24b are mounted adjacent the center of the plate 21 and the portions 23a and 23b form the dipole portion of the antenna and the rim portion 25 forms the loop portion of the antenna.

It has been discovered that all of the antennas illustrated in FIGS. 7-10 operate with substantially the same characteristics as those of theantenna illustrated in FIG. 1 to obtain the desirable characteristic of the antenna of FIG. 1.

FIG. 11 illustrates the antenna of this invention mounted in a television set. The antenna of FIG. 1 might be formed, for example, by vapor deposition or printed circuit techniques. For example, the antenna 1 may be formed on an insulating plate attached to the wall of the wall of the television cabinet as shown in FIG. 11 and mounted on the inside thereof. A lead supplies the output of the antenna from the feed points 40 and 4b to the RF section of the television set.

FIGS. 12 and 13 illustrate a modification of the invention in which the antenna is parabolic in shape and is used as a reflector. A parabolic shaped plate 100 is formed with

windows

20a and 20b of sector shapes having the apex angles 9 as shown. The

windows

20a and 20b are symmetrically mounted relative to the center 0 of the plate 100. In the embodiment illustrated in FIGS. 12 and 13 the

windows

20a and 20b do not intersect at their apexes so the material of the plate 100 extends between the conductive segments 30a and 30b as shown. FIG. 13 is a top view of the antenna of FIG. 12 and illustrates the depth of the antenna as h. The diameter of the antenna is shown in FIG. 12 as L and t designates the thickness of the rim 50, as shown. The letter I-I designates the center line of, the antenna which bisects each of the sector portions 30a and 30b, respectively.

FIG. 14 illustrates the reflector 100 of FIGS. 12 and 13 mounted relative to a dipole 101 which has

feed points

104a and 104b. The reflector of FIGS. 12 and 13 is designated A and the dipole antenna 101 has a length of l and a diameter w and may be formed of rod material, for example. The dipole l0l and the reflector A are spaced apart by a distance d with the dipole in front of the reflector to intercept radio frequency energy indicated by the R.F.arrow. The center 0 of the reflector A coincides with the center 0 of the dipole element 101, and the dipole 101 is-aligned relative to the reflector as shown.

The antenna device of FIG. 14 has a front-to-back ratio frequency characteristic shown in, FIG. 15 by curve 111. This curve is for an antenna wherein the diameter L of the reflector is 250 mm, the width t of the arcuate portion 50 is 10 mm, the depth h of the reflector is 40 mm, the central angles 0 of the

window portions

20a and 20b are 120, the length l of the dipole 101 is 200 mm, the diameter w of the dipole is 6 mm, and the distance d between the dipole element 101 and the reflector as shown in FIG. 14 is 140 mm. Curve 112 in FIG. 15 is the characteristic for an antenna similar to that shown in FIG. 14 but in which the reflector element contains no windows. It is seen from a comparison of the characteristics 11] and 112 that the elimination of the

windows

20a and 20b causes a substantial decrease in the front-to-back ratio and substantially decreases the directivity of an antenna. The curve 113 in FIG. 15 is a plot of the front-to-back ratio of a loop antenna with a reflector of the same configuration. Thus it is seen that the antenna of this invention illustrated in FIG. 14 provides a substantial improvement over the prior art and is very directive over a wide frequency range.

My experiments with various modifications illustrated has shown that very desirable results are obtained when the windows of the antennas are in the range from one-eighth to seven-eighths of the total area of the conductive plate before the windows are formed. With this range in size of the windows it has been determined that the front-to-back ratio of the antenna does not deteriorate and is good over a wide frequency range.

FIG. 16 illustrates an embodiment of the invention wherein the antenna of FIG. 1 has been modified by the connection of an inductor 14 between the feed points 4a and 4b. Also, the antenna 1 may be formedinto a parabolic shape as illustrated in FIG. 13. By changing the value of the inductive element 14 an apparent change in the shape of the reflector 1 is obtained. Thus the inductor 14 allows the characteristics of the antenna to be varied in a manner analogous to end loading of a dipole, for example. An impedance may also be used with the reflector illustrated in FIG. 14, if desirable.

FIG. 17 illustrates a composite antenna having eight elements with a first parabolic element A such as shown in FIGS. 12 and 13, and a second element B mounted in front of the element A and having a configuration as shown. The element B comprises the radiator and the element A comprises the reflector. Director elements comprise the

dipole antenna elements

15a, 15b, 15c, 15d, 15c and 15f, mounted as shown. A very directive high gain antenna is formed with the structure of FIG. 17.

FIG. 18 has elements A, and dipoles 15a-15f similarly mounted as in FIG. 17, and uses a triangular shaped dipole antenna element 16 as the radiator. This antenna also has high gain and is very directive.

The antennas illustrated in FIGS. 17 and 18 comprise substantial improvements over conventional Yagi antennas. FIG. 19 comprises a plot of frequencies versus gain. The curve 191 illustrates the gain versus frequency response of a conventional Yagi antenna. The curve 181 illustrates the gain versus frequency response of the antenna of FIG. 18, and curve 171 illustrates the gain versus frequency response of the antenna of FIG. 17. It is seen that the antennas of FIGS. 17 and 18 show substantial improvement in gain over' the Yagi antenna as observed from the comparison of the curves 171, 181 with the curve of 191.

FIG. 20 is a plot of the directional pattern of the antenna of FIG. 18 for an incoming signal of 600 MHz. It is to be noted from FIG. 20 that the front-to-back ratio of the multi-element antenna of FIG. 18 is infinite and its directivity is very good.

FIGS. 21 and 22 illustrate additional embodiments of the invention. In these figures modified form of the antenna of FIG. 14 is disclosed in which the dipole and parabolic elements are respectively utilized as a radiator and a reflector and are spaced a distance d apart. The dipole element 101 is connected to impedance matching means 10a which may be constructed of twinlead type solid conductive members. The end of the impedance matching means away from the dipole 101 is connected to a suitable feed means. To obtain good impedance matching the characteristic impedance Z,, of the impedance matching means 10a is selected so as to satisfy the relationship Z V Z, X Z where 2,, and Z are the impedances of the antenna element 101 and the feeder, respectively. The length u of the impedance matching means 10a depends upon the frequency range to be covered.

FIG. 21B illustrates the dipole element of FIG. 21A mounted with a parabolic reflector 100 as shown in FIG. 14. The distance between the dipole 101 and the antenna 100 is designated d and the length I of the dipole 101 is chosen as a function of the working frequency. It is difficult to simultaneously select the optimum length u of the impedance matching means 10a and the distance d between the

antenna elements

100 and 101 and the length I because of the frequency dependency of the self-impedances of the

antenna elements

100 and 101, and the mutual impedance between them. FIG. 21C illustrates the dipole antenna element 101 which is bent back as shown and has an optimum length l. The dipole 101 is coupled to impedance matching means 10a which have the length u which is optimum and is selected based on the impedance 2,, of the dipole 101 at the feed points, the impedance l of the feeder and the operating frequency range. By bending the ends of the dipole 101 back toward the element 100 the effective length between the dipole antenna element 101 and'the antenna element 100 becomes shorter. On the other hand, if the dipole is bent such that its ends extend away from the element 100, the length a of the impedance matching means 10a will become longer. Thus it is possible to adjust the length u of the impedance matching means 100 and the distance d between the dipole element 101 and the antenna element 100 to the optimum value by selecting the angle at which the ends of the dipole 101 are bent relative to the impedance matching means 10a.

FIG. 22 illustrates a practical embodiment of the antenna of FIG. 21. The impedance matching element 10a has its end away from the dipole 101 supported at the center of the antenna element 100 through an insulator 117 as shown in FIG.. 22B. The impedance matching means 10a has its end 10b pivotally connected to the support means 117 to permit vertical movement of the impedance matching means 10a relative to the antenna element 100. A slidable support 17 is mounted on the impedance matching means 10a as shown in FIG. 22B. This arrangement allows the antenna to be folded upwardly to the position shown by dashdot lines in FIG. 22A when the antenna is not in actual use. This is convenient for storing or shipping the antenna when not in use. The antenna element 100 is made of a conductive plate which has a window and which may be supported on a dish-shaped molded resin A backing plate which provides mechanical strength. The

element 100 is supported from a 'rnast 20 mounted on a support base 19 by supporting arm 21.

The antenna illustrated in FIG. 22 has a wide band frequency response and the distance between the dipole antenna element 101 and the antenna element 100 and the length of the impedance matching means 10a can selected at optimum values based on the working frequency range.

The window in the member 100 is formed symmetrically about the center of the conductive plate member although substantially the same results may be obtained with an asymmetrical window. If the window is formed extremely asymmetrically, it may be necessary to include an impedance matching means between the antenna device and the receiver.

I claim as my invention: 1. An antenna comprising:

a conductive plate member; at least a pair of window openings formed in said plate and arranged symmetrically with respect to the center of the conductive plate member so as to form a loop antenna and a dipole antenna in the plate member, the window openings having an area in the range between one-eighth to seveneighths of that enclosed by said conductive plate member such that said conductive plate member functions as a combination loop and dipole antenna; and

a dipole antenna mounted forwardly of said conductive plate member.

2. An antenna as claimed in claim 1 comprising an impedance matching means mechanically connected to said dipole antenna and to said conductive plate member.

3. An antenna as claimed in claim 2 wherein said impedance matching means comprises a twin-lead solid conductive member.

4. An antenna comprising a parabolic conductive plate member, an insulator supporting said plate member, a pair of window openings formed in said plate and arranged symmetrically with respect to the center of said conductive plate member, the window openings having an area in the range betweenone- I eighth to seven-eighths of that enclosed by said plate member, an impedance matching means with one end insulatinglyattached to the center of said conductive plate member and formed of a twin-lead solid feeder, and a dipole antenna element mounted on the second end of said impedance matching means.

5. An antenna as claimed in claim 4 wherein said.

dipole antenna element is connected to the impedance matching means such that said dipole element may be pivoted relative to said conductive plate member.

6. An antenna as claimed in claim 4 wherein the impedance matching means is pivotally attached to said conductive plate member so that it may be folded upwards. I

7. An antenna comprising:

a parabolic conductive plate member;

a pair of window openings formed in said plate and arranged symmetrically with respect to the center of said conductive plate member, the window openings having areas in the range between oneeighth to seven-eighths of that enclosed by said conductive plate member;

a first antenna element mounted forwardly of said conductive plate member and serving as a radiator; and

a second antenna element mounted forwardly of said first antenna element and serving as a director.

8. An antenna as claimed in claim 7 wherein the first antenna element is formed of a second conductive plate member and has formed therein a second pair of window openings having an area in the range between one-eighth to seven-eighths of that enclosed by said second conductive plate member.

9. An antenna as claimed in claim 7 wherein said first antenna element comprises a dipole antenna.

Claims

1. An antenna comprising: a conductive plate member; at least a pair of window openings formed in said plate and arranged symmetrically with respect to the center of the conductive plate member so as to form a loop antenna and a dipole antenna in the plate member, the window openings having an area in the range between one-eighth to seven-eighths of that enclosed by said conductive plate member such that said conductive plate member functions as a combination loop and dipole antenna; and a dipole antenna mounted forwardly of said conductive plate member.

4. An antenna comprising a parabolic conductive plate member, an insulator supporting said plate member, a pair of window openings formed in said plate and arranged symmetrically with respect to the center of said conductive plate member, the window openings having an area in the range between one-eighth to seven-eighths of that enclosed by said plate member, an impedance matching means with one end insulatingly attached to the center of said conductive plate member and formed of a twin-lead solid feeder, and a dipole antenna element mounted on the second end of said impedance matching means.

5. An antenna as claimed in claim 4 wherein said dipole antenna element is connected to the impedance matching means such that said dipole element may be pivoted relative to said conductive plate member.

6. An antenna as claimed in claim 4 wherein the impedance matching means is pivotally attached to said conductive plate member so that it may be folded upwards.

7. An antenna comprising: a parabolic conductive plate member; a pair of window openings formed in said plate and arranged symmetrically with respect to the center of said conductive plate member, the window openings having areas in the range between one-eighth to seven-eighths of that enclosed by said conductive plate member; a first antenna element mounted forwardly of said conductive plate member and serving as a radiator; and a second antenna element mounted forwardly of said first antenna element and serving as a director.