BACK-FIRE LOOP ANTENNA
United States Patent 3833908
A back-fire loop antenna having a loop of one or more turns situated between a pair of reflectors parallel with the plane of the loop, one reflector being larger than the other, there being means for supporting the loop and the reflectors in the required positions.
US Patent References:
SHORT BACKFIRE ANTENNA
Ehrenspeck - April 1969 - 3438043
ENDFIRE ANTENNA ARRAY HAVING LOOP DIRECTORS
Campbell - January 1970 - 3491361
SHORT BACKFIRE ANTENNA
Ehrenspeck - April 1969 - 3438043
ENDFIRE ANTENNA ARRAY HAVING LOOP DIRECTORS
Campbell - January 1970 - 3491361
Application Number:
05/382388
Publication Date:
09/03/1974
Filing Date:
07/25/1973
View Patent Images:
Export Citation:
Primary Class:
Other Classes:
343/837
International Classes:
H01Q19/185; H01Q19/10; H01Q19/10
Field of Search:
343/833,834,837,741
Primary Examiner:
Lieberman, Eli
Attorney, Agent or Firm:
Wenderoth, Lind & Ponack
Parent Case Data:
This application is a continuation-in-part of application Ser. No. 138,700 filed Apr. 29, 1971 and now abandoned.
Claims:
I claim
1. A backfire loop antenna consisting of a driving element in the form of a conductive loop of at least one turn having a circumference approximating one wavelength at the frequency of operation; a first plane conductive reflector parallel to the plane of the loop spaced apart therefrom on the axis thereof; a second plane conductive reflector parallel to the plane of the loop on the axis thereof and on the side thereof remote from the first conductive reflector, the second reflector being smaller than the first reflector; and support means for supporting the loop and the first and second conductive reflectors in their said relative positions.
2. A backfire loop antenna as claimed in claim 1 wherein the support means is a common axial shaft on which the loop and reflectors are mounted and which is of insulating material or from which the loop and reflectors are insulated.
3. A backfire loop antenna as claimed in claim 2 wherein one or each reflector is adjustably mounted on the shaft.
4. A backfire loop antenna as claimed in claim 1 wherein the reflectors are spaced apart a distance not greater than the circumference of the loop.
5. A backfire loop antenna as claimed in claim 1 wherein one or each refelctor is constituted by a conductive mesh.
6. A backfire loop antenna as claimed in claim 1 wherein the first reflector has a minimum linear dimension which is greater than the circumference of the loop.
7. A backfire loop antenna as claimed in claim 1 wherein the second reflector has a minimum linear dimension which is greater than one quarter of the circumference of the loop.
1. A backfire loop antenna consisting of a driving element in the form of a conductive loop of at least one turn having a circumference approximating one wavelength at the frequency of operation; a first plane conductive reflector parallel to the plane of the loop spaced apart therefrom on the axis thereof; a second plane conductive reflector parallel to the plane of the loop on the axis thereof and on the side thereof remote from the first conductive reflector, the second reflector being smaller than the first reflector; and support means for supporting the loop and the first and second conductive reflectors in their said relative positions.
2. A backfire loop antenna as claimed in claim 1 wherein the support means is a common axial shaft on which the loop and reflectors are mounted and which is of insulating material or from which the loop and reflectors are insulated.
3. A backfire loop antenna as claimed in claim 2 wherein one or each reflector is adjustably mounted on the shaft.
4. A backfire loop antenna as claimed in claim 1 wherein the reflectors are spaced apart a distance not greater than the circumference of the loop.
5. A backfire loop antenna as claimed in claim 1 wherein one or each refelctor is constituted by a conductive mesh.
6. A backfire loop antenna as claimed in claim 1 wherein the first reflector has a minimum linear dimension which is greater than the circumference of the loop.
7. A backfire loop antenna as claimed in claim 1 wherein the second reflector has a minimum linear dimension which is greater than one quarter of the circumference of the loop.
Description:
The invention relates to a back-fire antenna which employs a loop.
Back-fire antennas have been proposed by Ehrenspeck in U.S. Pat. Nos. 3,438,043 and 3,508,278. The Ehrenspeck antennas offer greatly increased gain of an end-fire antenna without adding to its length and the multiple reflection of electromagnetic waves between two plane reflectors of different size, with the energy being bound to the longitudinal antenna axis by a slow wave structure. Such antennas have been described as short back-fire antennas and they are directional antennas of a cavity type and a slow-wave end-fire type and are unsuitable for use as indoor antennas in the U.H.F. range.
An object of the present invention is to provide a back-fire antenna operating on a different principle from the short back-fire antenna and suitable for use in the U.H.F. region for television purposes. In particular an object of the invention is to provide an improved antenna suitable for use indoors.
The invention consists in the provision of a back-fire loop antenna. This comprises only: a driving element in the form of a conductive loop of at least one turn; a first plane conductive reflector parallel to the plane of the loop spaced apart therefrom on the axis thereof; a second plane conductive reflector parallel to the plane of the loop on the axis thereof and on the side thereof remote from the first conductive reflector, the second reflector being smaller than the first reflector; and support means for supporting the loop and the first and second conductive reflectors in their said relative positions. The operation of this antenna is described by the theory of multiple images which I have developed and which is described hereinafter.
The loop and each reflector may be mounted on a common axial shaft which is of insulating material or from which they are insulated. Preferably each reflector is adjustably mounted on the shaft so that its position with respect to the loop may be altered. The spacing of the reflectors will depend upon the radiation resistance required for the particular application. However, it should normally be within one wavelength of the radiation to be received or transmitted.
Preferably each reflector is constituted by a conductive mesh.
Preferably the circumference of the loop is approximately equal to one wavelength of the radiation to be received or transmitted. The loop may be of any closed shape but is conveniently circular, square or rectangular.
Each reflector may be of any shape. Preferably the first reflector has a minimum linear dimension which is greater than one wavelength of the radiation to be received or transmitted. Preferably the second reflector has a minimum linear dimension which is greater than one quarter of the wavelength of radiation to be received or transmitted.
The invention will further be described with reference to the accompanying drawings, of which:
FIGS. 1, 2 and 3 show in side elevation three examples of a loop configuration which may be used in an antenna in accordance with the invention;
FIGS. 4 and 5 show respectively in side elevation and end elevation respectively an antenna in accordance with the invention;
FIG. 6 is a diagram of multiple images formed with an antenna according to the invention;
FIGS. 7 and 8 are sketches for the explanation of the theory of multiple images; and
FIGS. 9 and 10 are diagrams of radiation patterns for back-fire loop antennas.
Referring to FIG. 1 the loop 1 comprises a single turn of copper tubing formed into a circle of radius a. The circumference 2π a is approximately equal to λ, the wavelength of the radiation to be received or transmitted.
Referring to FIG. 2 the loop 1 is again of copper tubing formed in a single turn in a square loop. The side of the square is d and 4d is approximately equal to λ.
Referring to FIG. 3 the loop is of copper tubing formed into a rectangle of sides d1 and d2. In this case 2(d1 + d2) is approximately equal to λ.
Referring now to FIGS. 4 and 5 there is shown a loop antenna in accordance with the invention in which a loop 1 as in FIG. 1 is mounted on an insulating shaft together with a first reflector 3. Loop 1 is supported by an insulating strut 1a which is fastened to shaft 2. Reflector 3 is formed of a mesh of copper wire into circular form. The diameter D1 of the circle is arranged to be greater than λ. The reflector 3 is mounted to be adjustable on shaft 2 so that the spacing between loop 1 and reflector 3 can be changed to the optimum value.
Shaft 2 is extended through loop 1 and carries at its end remote from reflector 3 a further, smaller, reflector. Reflector 4 is also constituted by a mesh of copper wire as shown in FIG. 5. The mesh is cut to circular form and has a diameter D2 which is greater than 1/4λ. Reflector 4 is mounted to be adjustable on shaft 2 so that its spacing from loop 1 may be set to the optimum. The optimum spacing for reflectors 3 and 4 will depend upon the particular radiation resistance required for the antenna.
For acceptable performance the dimensions of the antenna have a tolerance of plus or minus 20 percent about the optimum. In use, the antenna is arranged so that the loop 1 is in a vertical plane which is directed towards the transmitter or receiver, as the case may be, from which or to which the radiation is transmitted. Both vertically and horizontally polarised radiation may be received.
The invention is not restricted to the details of the foregoing example described with reference to the accompanying drawings. For example, the loop may be made of more than one turn, it may be of any desired shape and may be constituted by a conductor which is, say, of tubular aluminium or steel or of solid conductive material. The shape of the reflectors need not be circular, but they may be square or rectangular, for example. The reflectors need not be made of mesh and may be solid, perhaps perforated. The material of the reflectors may be any suitable conductive material such as aluminium or steel and need not be copper.
The multiple image theory will now be explained with reference to FIGS. 6 to 10 of the drawings.
As a first approximation, we assume that the two plane reflectors M and R are both infinite in extent in order to formulate the problem in terms of multiple images produced by multiple reflections between the conducting reflectors. The plane reflectors can then be replaced theoretically by two infinite series of images with appropriate direction of current-flow, one to the right of reflector R and one to the left of reflector M. The two series of images A 1 , A 2 A 3 , . . . and A 1 ', A 2 ', A 3 ', . . . are primarily interested in the radiation field on the right-hand side of the large plane reflector, we shall therefore only consider the contributions from the loop antenna itself together with the image to the left of the large reflector. The back-fire antenna is then shown in FIG. 2 for analysis. This, in fact, reduces to an array of identical elements A 0 , A 1 , A 2 , A 3 , . . . spaced alternatively 2D 1 , 2D 2 , . . . .
The electric field in the far field due to the N th radiating element can be written as
E n (θ, φ) = f n (θ, φ) e jkr n/r n (1)
where r n is the distance of the point p in the far field from the N th element.
f n (θ, φ) represents the angular distribution of the radiation intensity of the N th element.
and
k = 2π/λ is the wave number.
Since r n is large, we can use the far field approximation
r n = r - │r p n │ (2)
where
r is the distance from the origin to the far field point.
r is the unit vector in the direction of r.
p n is the position of the N th image with respect to the circular loop antenna situated at the origin.
Combining (1) and (2) gives the far field due to the n th radiating element as
E n (θ, φ) = f n (θ, φ)(e - jkr /r) e jk │r . p n │ (3)
Thus, by the principle of superposition, the far field of the array of (N + 1) elements is simply ##SPC1##
where
ψ n = k (r . p n ) (5)
If the shape of the radiation pattern is identical for each element of the array,
f n (θ, φ) = a n E(θ, φ) (6)
where a n is the complex current in the n th element and is sometimes called the feeding coefficient of the element.
E(θ, φ) is the element pattern.
Putting (6) into (4), the far field of the array becomes ##SPC2##
which can be rewritten without the multiplying factor as
E N = E(θ, φ) f(ψ) (7)
where ##SPC3##
is defined as the array polynomial
Radiation Field of a Circular Loop Backfire Antenna
The electric field intensity of a circular loop at a large distance, assuming a sinusoidal current distribution (I = I o sin pφ') is given by
E θ = j p ΩμI o a(e - ikr /r)(p/2α)J p (α) cosQ cospφ (8) E φ = j p ΩμI.s ub.o a(e - .sup .ikr /4r) . [J p - 1 (α) - J p +1 (α)]sin φ (9)
where
(r, θ, φ) is the position of the far field point p .
a is the radius of the circular loop .
I 1 is the maximum current amplitude .
J p (α) is the Bessel function of order p and α = ka sinθ.
If we choose a co-ordinate system such that the loop lies in the xy plane with the reflectors parallel to it, the position of the n th image from the loop will be given by: ##SPC4##
Substituting into (5) yields ##SPC5##
With the complex current given by the reflection coefficients
a n = α n c jn
the array factor becomes ##SPC6##
Simplication of (13) gives ##SPC7##
The far field pattern of the backfire circular loop antenna is given by ##SPC8##
In order to consider the field due to a series of consecutive images we always have
│n 1 - n 2 │ = 1
Radiation Field of a Short Backfire Loop Antenna
The short backfire loop antenna is essentially a full wavelength circular loop placed midway between two reflectors separated at a distance of λ/2. In this case
D 1 =D 2 =
p = (λ/4)1
ka = 1 (16)
The array factor then becomes ##SPC9##
Hence the far field radiation pattern is ##SPC10##
were calculated using various values of N, and a. A slight departure from D 1 , 2 = λ/4, however, produced a shift of the maximum in the radiation pattern. This is understandable as the reflectors are plane conductors and the image antenna and the object antenna should have a phase difference of . Since a is always less than 1, the effect of varying a amounts to a reduction of the number of images used. Hence a suitable choice of N and a will provide the radiation pattern of any short backfire antenna. The results are illustrated in FIG. 9 and FIG. 10 together with the experiment patterns.
Back-fire antennas have been proposed by Ehrenspeck in U.S. Pat. Nos. 3,438,043 and 3,508,278. The Ehrenspeck antennas offer greatly increased gain of an end-fire antenna without adding to its length and the multiple reflection of electromagnetic waves between two plane reflectors of different size, with the energy being bound to the longitudinal antenna axis by a slow wave structure. Such antennas have been described as short back-fire antennas and they are directional antennas of a cavity type and a slow-wave end-fire type and are unsuitable for use as indoor antennas in the U.H.F. range.
An object of the present invention is to provide a back-fire antenna operating on a different principle from the short back-fire antenna and suitable for use in the U.H.F. region for television purposes. In particular an object of the invention is to provide an improved antenna suitable for use indoors.
The invention consists in the provision of a back-fire loop antenna. This comprises only: a driving element in the form of a conductive loop of at least one turn; a first plane conductive reflector parallel to the plane of the loop spaced apart therefrom on the axis thereof; a second plane conductive reflector parallel to the plane of the loop on the axis thereof and on the side thereof remote from the first conductive reflector, the second reflector being smaller than the first reflector; and support means for supporting the loop and the first and second conductive reflectors in their said relative positions. The operation of this antenna is described by the theory of multiple images which I have developed and which is described hereinafter.
The loop and each reflector may be mounted on a common axial shaft which is of insulating material or from which they are insulated. Preferably each reflector is adjustably mounted on the shaft so that its position with respect to the loop may be altered. The spacing of the reflectors will depend upon the radiation resistance required for the particular application. However, it should normally be within one wavelength of the radiation to be received or transmitted.
Preferably each reflector is constituted by a conductive mesh.
Preferably the circumference of the loop is approximately equal to one wavelength of the radiation to be received or transmitted. The loop may be of any closed shape but is conveniently circular, square or rectangular.
Each reflector may be of any shape. Preferably the first reflector has a minimum linear dimension which is greater than one wavelength of the radiation to be received or transmitted. Preferably the second reflector has a minimum linear dimension which is greater than one quarter of the wavelength of radiation to be received or transmitted.
The invention will further be described with reference to the accompanying drawings, of which:
FIGS. 1, 2 and 3 show in side elevation three examples of a loop configuration which may be used in an antenna in accordance with the invention;
FIGS. 4 and 5 show respectively in side elevation and end elevation respectively an antenna in accordance with the invention;
FIG. 6 is a diagram of multiple images formed with an antenna according to the invention;
FIGS. 7 and 8 are sketches for the explanation of the theory of multiple images; and
FIGS. 9 and 10 are diagrams of radiation patterns for back-fire loop antennas.
Referring to FIG. 1 the loop 1 comprises a single turn of copper tubing formed into a circle of radius a. The circumference 2π a is approximately equal to λ, the wavelength of the radiation to be received or transmitted.
Referring to FIG. 2 the loop 1 is again of copper tubing formed in a single turn in a square loop. The side of the square is d and 4d is approximately equal to λ.
Referring to FIG. 3 the loop is of copper tubing formed into a rectangle of sides d1 and d2. In this case 2(d1 + d2) is approximately equal to λ.
Referring now to FIGS. 4 and 5 there is shown a loop antenna in accordance with the invention in which a loop 1 as in FIG. 1 is mounted on an insulating shaft together with a first reflector 3. Loop 1 is supported by an insulating strut 1a which is fastened to shaft 2. Reflector 3 is formed of a mesh of copper wire into circular form. The diameter D1 of the circle is arranged to be greater than λ. The reflector 3 is mounted to be adjustable on shaft 2 so that the spacing between loop 1 and reflector 3 can be changed to the optimum value.
Shaft 2 is extended through loop 1 and carries at its end remote from reflector 3 a further, smaller, reflector. Reflector 4 is also constituted by a mesh of copper wire as shown in FIG. 5. The mesh is cut to circular form and has a diameter D2 which is greater than 1/4λ. Reflector 4 is mounted to be adjustable on shaft 2 so that its spacing from loop 1 may be set to the optimum. The optimum spacing for reflectors 3 and 4 will depend upon the particular radiation resistance required for the antenna.
For acceptable performance the dimensions of the antenna have a tolerance of plus or minus 20 percent about the optimum. In use, the antenna is arranged so that the loop 1 is in a vertical plane which is directed towards the transmitter or receiver, as the case may be, from which or to which the radiation is transmitted. Both vertically and horizontally polarised radiation may be received.
The invention is not restricted to the details of the foregoing example described with reference to the accompanying drawings. For example, the loop may be made of more than one turn, it may be of any desired shape and may be constituted by a conductor which is, say, of tubular aluminium or steel or of solid conductive material. The shape of the reflectors need not be circular, but they may be square or rectangular, for example. The reflectors need not be made of mesh and may be solid, perhaps perforated. The material of the reflectors may be any suitable conductive material such as aluminium or steel and need not be copper.
The multiple image theory will now be explained with reference to FIGS. 6 to 10 of the drawings.
As a first approximation, we assume that the two plane reflectors M and R are both infinite in extent in order to formulate the problem in terms of multiple images produced by multiple reflections between the conducting reflectors. The plane reflectors can then be replaced theoretically by two infinite series of images with appropriate direction of current-flow, one to the right of reflector R and one to the left of reflector M. The two series of images A 1 , A 2 A 3 , . . . and A 1 ', A 2 ', A 3 ', . . . are primarily interested in the radiation field on the right-hand side of the large plane reflector, we shall therefore only consider the contributions from the loop antenna itself together with the image to the left of the large reflector. The back-fire antenna is then shown in FIG. 2 for analysis. This, in fact, reduces to an array of identical elements A 0 , A 1 , A 2 , A 3 , . . . spaced alternatively 2D 1 , 2D 2 , . . . .
The electric field in the far field due to the N th radiating element can be written as
E n (θ, φ) = f n (θ, φ) e jkr n/r n (1)
where r n is the distance of the point p in the far field from the N th element.
f n (θ, φ) represents the angular distribution of the radiation intensity of the N th element.
and
k = 2π/λ is the wave number.
Since r n is large, we can use the far field approximation
r n = r - │r p n │ (2)
where
r is the distance from the origin to the far field point.
r is the unit vector in the direction of r.
p n is the position of the N th image with respect to the circular loop antenna situated at the origin.
Combining (1) and (2) gives the far field due to the n th radiating element as
E n (θ, φ) = f n (θ, φ)(e - jkr /r) e jk │r . p n │ (3)
Thus, by the principle of superposition, the far field of the array of (N + 1) elements is simply ##SPC1##
where
ψ n = k (r . p n ) (5)
If the shape of the radiation pattern is identical for each element of the array,
f n (θ, φ) = a n E(θ, φ) (6)
where a n is the complex current in the n th element and is sometimes called the feeding coefficient of the element.
E(θ, φ) is the element pattern.
Putting (6) into (4), the far field of the array becomes ##SPC2##
which can be rewritten without the multiplying factor as
E N = E(θ, φ) f(ψ) (7)
where ##SPC3##
is defined as the array polynomial
Radiation Field of a Circular Loop Backfire Antenna
The electric field intensity of a circular loop at a large distance, assuming a sinusoidal current distribution (I = I o sin pφ') is given by
E θ = j p ΩμI o a(e - ikr /r)(p/2α)J p (α) cosQ cospφ (8) E φ = j p ΩμI.s ub.o a(e - .sup .ikr /4r) . [J p - 1 (α) - J p +1 (α)]sin φ (9)
where
(r, θ, φ) is the position of the far field point p .
a is the radius of the circular loop .
I 1 is the maximum current amplitude .
J p (α) is the Bessel function of order p and α = ka sinθ.
If we choose a co-ordinate system such that the loop lies in the xy plane with the reflectors parallel to it, the position of the n th image from the loop will be given by: ##SPC4##
Substituting into (5) yields ##SPC5##
With the complex current given by the reflection coefficients
a n = α n c jn
the array factor becomes ##SPC6##
Simplication of (13) gives ##SPC7##
The far field pattern of the backfire circular loop antenna is given by ##SPC8##
In order to consider the field due to a series of consecutive images we always have
│n 1 - n 2 │ = 1
Radiation Field of a Short Backfire Loop Antenna
The short backfire loop antenna is essentially a full wavelength circular loop placed midway between two reflectors separated at a distance of λ/2. In this case
D 1 =D 2 =
p = (λ/4)1
ka = 1 (16)
The array factor then becomes ##SPC9##
Hence the far field radiation pattern is ##SPC10##
were calculated using various values of N, and a. A slight departure from D 1 , 2 = λ/4, however, produced a shift of the maximum in the radiation pattern. This is understandable as the reflectors are plane conductors and the image antenna and the object antenna should have a phase difference of . Since a is always less than 1, the effect of varying a amounts to a reduction of the number of images used. Hence a suitable choice of N and a will provide the radiation pattern of any short backfire antenna. The results are illustrated in FIG. 9 and FIG. 10 together with the experiment patterns.