Slot antenna

A slot antenna having particular application to the field of UHF broadcasting. The antenna is comprised of a rigid coaxial waveguide which is dimensioned so that electromagnetic energy propagates therethrough in the TE.sub.11 mode. Two longitudinal baffles are provided at diametrically opposing circumferential positions within the waveguide. These baffles stabilize the orientation of the mode (i.e., the mode polarization) within the waveguide so that it is known. Alternate methods of establishing a fixed mode polarization are also disclosed. Slots are provided along the exterior of the waveguide in appropriate circumferential positions relative to the mode polarization. Since the TE.sub.11 mode includes both longitudinal and transverse current components, vertical, horizontal, or circular polarization may be generated through the selection of appropriate positions, shape, and orientations for the radiating slots. In one illustrated embodiment, orthogonal slots are provided at selected locations along the exterior of the cylinder so that circular polarization is transmitted by the antenna. The antenna is preferably center-fed through use of a coaxial feed line which is run along the center of the antenna to a center feed point. Various methods for coupling the electromagnetic energy from the coaxial feed line to the TE.sub. 11 waveguide are disclosed.

BACKGROUND AND FIELD OF THE INVENTION 
The present invention relates to the art of antennas, and more particularly 
to a slot antenna having particular application in the field of UHF 
broadcasting. 
Slotted cylinder antennas have been used effectively in UHF broadcasting 
since the early 1950's. These antennas remain the most popular 
transmitting antennas for these purposes because of the high gain 
associated therewith, as well as the simplicity of the feeding arrangement 
required and their ability to generate desired pattern shapes. 
Prior art slotted cylinder antennas have been arranged and dimensioned so 
that the electromagnetic energy propagates along the interior of the 
radiating cylinder in either the TEM mode or the TM.sub.01 mode. One of 
the reasons that these modes have been selected in the past is that they 
exhibit cylindrical symmetry. The exact position of the slots about the 
circumference of the radiating cylinder is, therefore, not critical. Also, 
although the current lines in the TEM mode are entirely longitudinal, 
horizontal polarization can still be generated with relative simplicity 
(in a vertically oriented slotted cylinder) through the provision of 
longitudinally extending slots, as long as a suitable coupling device 
(such as shown in Bazan U.S. Pat. No. 2,981,947) is provided for each 
slot. This is desirable because horizontal polarization is currently the 
accepted standard for UHF broadcasting. 
There is disclosed herein, however, a slotted cylinder antenna wherein the 
selected mode of propagation within the radiating cylinder is the 
TE.sub.11 coaxial mode. A number of advantages inhere in the use of this 
mode of propagation. The use of this mode allows the construction of a UHF 
antenna having a relatively small diameter, thereby providing a low 
windload of the antenna. Additionally, it is possible to center-feed an 
antenna employing this mode so as to thereby secure the many benefits 
associated with a center fed antenna. Yet another advantage of the use of 
this mode is that the transmission of circularly polarized signals may be 
easily provided for, merely through the provision of pairs of orthogonal 
slots at selected positions along the antenna. This is possible because 
the TE.sub.11 mode includes both longitudinal and transverse current 
components. This is particularly important in view of the growing 
acceptance of circularly polarized antennas as a desirable alternative to 
conventional horizontally polarized antennas. 
A problem associated with the use of the TE.sub.11 mode relates to the 
non-symmetrical nature of the current lines about the antenna axis. 
Because of this lack of cylindrical symmetry, the relative positions of 
the mode and the slots is critical to the attainment of a predictable 
radiation characteristic. The orientation of the mode may wander within a 
coaxial waveguide of conventional construction, however, due to conductor 
imperfections, manufacturing tolerences, or discontinuities within the 
system. Consequently, the desired alignment between the slots and the mode 
is not readily achievable in these waveguides. 
The present invention resolves this problem by structuring the coaxial 
waveguide so that a preferred field orientation exists. When structured 
thusly, the TE.sub.11 mode will be fixed in a known orientation. It is 
therefore possible to position the slots in any desired alignment with 
respect to the mode polarization. 
In accordance with the present invention, a slotted cylinder antenna is 
provided including a radiating structure which is constructed and 
dimensioned so that the electromagnetic energy will propagate therethrough 
in the TE.sub.11 mode, and so that the mode polarization will remain fixed 
in a known orientation. The radiating structure is periodically 
interrupted by radiating slots which are positioned along the radiating 
structure so that the desired transmission polarization and radiation 
pattern are secured. 
In accordance with another feature of the present invention, the radiating 
structure comprises a cylinder having a coaxial feed line disposed 
therein. The coaxial feed line runs along the center of the cylinder to 
the vicinity of the midpoint of the antenna, where a feed point is 
provided for exciting the TE.sub.11 mode of propagation along the 
waveguide defined by the exterior of the coaxial feed line and the 
cylinder. Electrically conductive members are disposed at selected 
circumferential positions within the waveguide so as to establish a 
preferred field orientation for the TE.sub.11 mode. Slots are provided at 
regular positions along the cylinder so as to radiate electromagnetic 
energy therefrom having a desired polarization sense and radiation 
pattern. 
In accordance with yet another aspect of the present invention, orthogonal 
slots are provided along the radiating cylinder so that the polarization 
transmitted by the antenna is substantially circular or elliptical.

DETAILED DESCRIPTION OF THE DRAWINGS 
Reference is now made to the drawings, wherein the showings are provided 
for purposes of illustrating a preferred embodiment of the invention and 
are not intended to limit the breadth of the invention described. Thus, 
although the invention will be described with respect to a top-mount UHF 
broadcasting antenna, it will be appreciated that the invention has 
broader application to RF transmission and reception in general. 
In FIG. 1 there is shown an elevation view of a slotted cylinder antenna 10 
in accordance with the teachings of the present invention. Antenna 10 
includes a cylindrical radiating structure 12 bounded on the top and 
bottom by shorting plates or terminating loads 14 and 16. A warning beacon 
18 may be provided at the upper extremity thereof. At the lower extremity, 
a mounting bracket may be provided for vertically mounting the antenna to 
the top of an antenna tower of conventional design. 
As will be seen more clearly in FIG. 2, antenna 10 comprises three 
coaxially mounted cylinders 12, 22, and 23, together defining an outer 
coaxial waveguide 20 and an inner coaxial waveguide 21. The inner coaxial 
waveguide defined by cylinders 22 and 23 serves as a rigid coaxial feed 
line and runs from the center feed point (FP) of antenna 10 to beyond the 
end of the shorting plate 16. The center conductor 23 may be supported at 
the center of the middle cylinder 22 in any of the manners conventionally 
used in the construction of rigid coaxial transmission lines. The 
protruding bottom end 24 of the rigid coaxial feed line will be connected 
with the rigid coaxial feed line arriving from the transmitter (not 
shown). In this fashion, RF energy is directed from the transmitter to the 
antenna feed point located at the midpoint of antenna 10. 
Although the inner coaxial waveguide is terminated in the vicinity of the 
feed point, the middle cylinder 22 extends the entire length of the 
antenna in order to maintain the coaxial nature of the outer waveguide 20. 
Shorting plates 14 and 16 serve to terminate the outer waveguide 20 at the 
upper and lower extremities by each electrically shorting cylinders 12 and 
22 together. 
In accordance with well-known principles, the outer diameter D.sub.1 of 
middle cylinder 22 and the inner diameter D.sub.2 of outer cylinder 12 are 
selected so that, at the frequencies being broadcast by this antenna, the 
TE.sub.11 mode is propagated through coaxial waveguide 20. This will 
result in an antenna having a relatively small diameter (approximately 
half a wavelength), and a correspondingly low windload. Since the 
TE.sub.11 mode does not have cylindrical symmetry, the radiation 
characteristics of the antenna will depend largely upon the orientation of 
the propagating mode (i.e., the mode polarization) with respect to the 
orientation of the broadcasting slots. To insure that a desired relative 
orientation is achieved, some means must be provided for establishing a 
known and unchanging mode polarization. 
In the illustrated embodiment, a preferred field orientation is established 
through use of two baffles 26 and 28 which are located at diametrically 
opposed circumferential positions within waveguide 20. The orientation of 
these baffles may be seen most clearly in FIG. 2. Baffles 26 and 28 are 
comprised of planer strips of electrically conductive material, and extend 
the length of antenna 10 substantially without interruption. Because these 
baffles electrically connect middle cylinder 22 with outer cylinder 12, 
boundary conditions are established which fix the orientation of the 
propagating mode within the waveguide. With the field orientation thus 
established, the position of the slots may be carefully selected to cut 
across current lines in any desired manner. 
Use of the two baffles effectively creates two half cylindrical waveguides, 
each propagating half of the TE.sub.11 mode. Hence, it is not proper, 
strictly speaking to refer to the mode by the TE.sub.11 notation. However, 
for thin baffles the propagation characteristics are not materially 
effected, and in practice the propagation may be considered to be in the 
TE.sub.11 mode. 
Many other methods of establishing this preferred field orientation could 
alternately be employed. For example, a preferred field orientation could 
be established by the simple expedient of providing the center conductor 
20 with an elliptical cross-section, rather than the circular 
cross-section shown. Alternately, ridges could be placed along the outer 
surface of the middle conductor or along the inner surface of the outside 
cylinder 12 at positions orthogonal to the positions occupied in the 
illustrated embodiment by the baffles 28 and 26. Yet another possible 
method of establishing a preferred field orientation would be to use a 
number of properly spaced pins in place of each of the solid baffles. Each 
of these methods will function to establish a preferred field orientation, 
while also providing suppression of the TEM mode. 
As in rigid coaxial waveguides currently in use, electromagnetic energy 
will propagate through waveguide 21 in the TEM mode. The arrangement used 
to feed this energy to the outer waveguide 20 will in part depend upon the 
method used for fixing the mode polarization. Although all of the 
described methods of establishing the mode polarization will result in 
similar electric and magnetic field patterns within the outer waveguide, 
the use of continuous baffles would serve to completely isolate the two 
sections of the waveguide located on either side of the baffles. 
Consequently, if baffles are used, the two halves of the outer waveguide 
must each be provided with a separate feed arrangement. On the other hand, 
only a single feed arrangement need be provided if any of the other 
described methods are employed, since all of these methods allow coupling 
to occur between the two halves of the waveguide. 
There is illustrated in FIG. 3 a preferred feed arrangement for use when 
baffles are employed for fixing the mode polarization. This feed 
arrangement comprises an annular slot 29 circumferentially located about 
the middle cylinder 20, in the vicinity of the longitudinal midpoint (FP) 
of the antenna. The annular slot will extend entirely about the 
circumference of the middle cylinder, except at the positions occupied by 
the baffles 28 and 26. The inner coaxial waveguide 21 will be terminated 
by a shorting plate 30 at a short distance E beyond the feed slot. This 
additional short section (nominally equal to approximately one-half 
wavelength) functions as a balun. As stated previously, however, the 
middle cylinder 22 extends the entire length of the antenna. 
A second method of feeding the slot antenna when continuous baffles are 
used for establishing the mode polarization is shown in FIGS. 4 and 5. In 
this embodiment, a monopole probe is provided for each of the two isolated 
sections of the TE.sub.11 waveguide 20. The two monopole probes 32 and 33 
extend from the center conductor 23 through respective circular holes 34 
and 35 in middle conductor 22 and are shorted to outer conductor 12. The 
probes will excite either the TE.sub.11 mode or the TEM mode, depending 
upon the configuration of the outer waveguide. Since baffles 26 and 28 are 
included in the outer waveguide, the TEM mode will be suppressed and a 
pure TE.sub.11 mode will propagate. 
FIGS. 6 and 7 illustrate a third feed arrangement. In FIG. 7, the elements 
identified by reference numbers 36 and 38 are intended to represent pins, 
rather then continuous baffles. Since electromagnetic coupling will, 
therefore, occur between the two halves of outer waveguide 20, only a 
single feed arrangement need be provided. This configuration utilizes 
magnetic coupling between the feed line and the TE.sub.11 waveguide 20. A 
loop 40 extends from the center conductor 23 through a circular hole in 
middle conductor 22 and attaches to the periphery of middle conductor 22. 
This loop couples to the magnetic fields of the TE.sub.11 mode. 
If continuous baffles were employed in place of pins 36 and 38, dual loops 
could be used to provide coupling to each of the isolated sections of 
outer waveguide 20, as in the first two feed arrangements. Similarly, if 
either of the first two feed arrangements were used with an antenna where 
coupling occurred between the two halves of the TE.sub.11 waveguide, only 
one-half of the described feed arrangements would be required. 
The antenna thus constructed will, of course, be provided with slots cut in 
the outer cylinder 12 so as to provide the antenna with preselected 
radiation characteristics. The number, spacing, shape, and dimensions of 
these slots will vary in accordance with the specific radiation 
requirements of each antenna. 
It is a property of the TE.sub.11 mode that both longitudinal and 
transverse current components are present. Assuming the axis of the 
antenna is vertically oriented, a slot provided at a position which 
intercepts longitudinal current lines (i.e., a transverse slot), will 
radiate vertically polarized electromagnetic signals. Similarly, a slot 
oriented in such a manner as to intercept the transverse current lines 
will radiate horizontally polarized signals. If a slot is provided in each 
of these orientations, then both vertically and horizontally polarized 
electromagnetic signals will be radiated by the antenna. Since it is also 
a property of the TE.sub.11 mode that the longitudinal and transverse 
currents are always in phase quadrature, the vertically and horizontally 
polarized signals radiated by the respective slots will also be in phase 
quadrature. Consequently, an elliptically polarized electromagnetic signal 
will result. 
At any given instant in time, the phase and amplitude of both current 
components (longitudinal and transverse) will be found to vary as a 
function of both longitudinal and circumferential position along the 
antenna. In the longitudinal direction, the phase and amplitude of the two 
current components will vary sinusoidally and in phase quadrature with 
axial position and with a periodicity of one guide wavelength. The 
orientation of amplitude and phase variations in the circumferential 
direction will be fixed by the presence of the baffles. Thus, a reference 
plane RP (FIG. 2) may be defined which is orthogonal to the plane of the 
baffles. The transverse current component will vary in phase and amplitude 
as a function of the sine of the circumferential angle .phi. from the 
reference plane RP, while the longitudinal current component will vary in 
phase and amplitude as a function of the cosine of the circumferential 
angle .phi. from the reference plane RP. The two components will thus 
always be in phase quadrature, as stated previously. 
The embodiment illustrated in FIGS. 1 and 2 includes an array of 
longitudinally extending slots so that horizontally polarized signals are 
radiated therefrom. In order for the transmitted beam to have a wavefront 
which extends broadside to the antenna, the slots must be positioned so 
that the signals radiated from all of the slots will be in-phase. This may 
be accomplished by providing slots 50 equally spaced by approximately one 
guide wavelength along the length of the antenna, where each of the slots 
is located at the same circumferential angle +.phi., with respect to the 
reference plane RP. 
In order to prevent the grating lobes which would otherwise exist due to 
the high slot spacing of slots 50, additional, intermediately spaced slots 
52 may be provided. Slots 52 are axially spaced apart from slots 50 by 
approximately one-half of a guide wavelength. If slots 52 were provided at 
the same angular positions +.phi., as slots 50, the signals radiated 
thereby would be in phase opposition with the signals radiated by slots 
50. To prevent this, slots 52 are instead located at an angular position 
-.phi., with respect to the reference plane RP. Since, it will be 
recalled, the transverse current component varies in phase and amplitude 
with respect to the reference plane as a sine function, the signal 
radiated by a slot at -.phi., will have the same amplitude but opposite 
phase of a signal radiated by the same slot positioned at +.phi.. The 
total phase displacement between slots 50 and 52 will thus be 360.degree. 
so that the signals radiated thereby will be in phase, as required. 
Since the signals propagating along the upper and lower sections of antenna 
10 have even symmetry with respect to feed point FP, proper phasing of the 
upper and lower portions of antenna 10 may be insured by providing slots 
50 and 52 at (even) symmetrical positions on either side of feed point FP. 
The first slots on either side of feed point FP will be displaced 
therefrom by a distance C. This distance, which will generally be less 
than one-half wavelength, will be selected to be as close as possible to 
feed point FP without coupling to the undesirable modes which exist in the 
immediate vicinity of the feed point. 
The antennas horizontal pattern may be controlled by adjusting the 
circumferential positions .phi. of the slots. For an omnidirectional 
horizontal pattern, slots will be cut into the TE.sub.11 waveguide at four 
equally spaced circumferential positions about the antenna. To insure that 
proper phasing is maintained between slots, those slots located at 
+.phi..sub.1 and (180.degree. -.phi..sub.1) will be provided at common 
longitudinal positions, while those slots located at -.phi..sub.1 and 
(180.degree.+.phi..sub.1) will be located at other common longitudinal 
positions which are spaced from the first longitudinal positions by a 
distance B of approximately one-half of a wavelength. 
As is well-known, the conductance of the slots will vary with the position 
of the slots .phi. with respect to the center of the waveguide (in this 
case, the reference plane RP). As is also well-known, phase shift of the 
wave propagating within the TE.sub.11 waveguide will vary with the 
conductance of the slots. Since this, in turn, affects the slot phasing, 
it may be necessary to adjust the slot spacing B to compensate for phase 
shift introduced by the actual angular slot position .phi. employed. 
Furthermore, the longitudinal slot spacing can be adjusted so as to 
deliberately alter the slot phasing from the described resonant condition 
to account for beam tilt and for null fill. The manner in which these 
factors are taken into account in calculating the slot positioning is 
well-known and will not be dealt with herein. 
There is illustrated in FIG. 8 an embodiment 60 of the invention which 
employs crossed (orthogonal) slots 62 for broadcasting elliptically 
polarized signals. As state earlier, this is possible because the 
TE.sub.11 mode includes both longitudinal and transverse current 
components. Furthermore, since these current components are always in 
phase quadrature, it is possible to provide the necessary quadrature 
phasing between the vertical and horizontal components of the elliptically 
polarized signal simply by cutting the crossed slots at the same 
longitudinal and transverse location along the antenna. This secures the 
further desirable feature that the horizontal and vertical components 
radiated by the two slots thus have a common phase center. 
For crossed slots radiating a substantially circularly polarized wave, the 
energy radiated by a slot will be proportional to the length of the slot 
S, for any given angular waveguide position. The slot lengths along the 
array may thus be adjusted to provide any desired amplitude distribution. 
The ellipticity of the radiated signal will depend, of course, upon the 
relative magnitudes of the horizontal and vertical components thereof. 
This ratio, commonly referred to as the axial ratio, may be adjusted by 
careful selection of the circumferential position .phi. at which the slots 
are located, it being noted that the relative magnitudes of the 
longitudinal and transverse current components will vary with 
circumferential position .phi.. 
As previously stated, slot locations along the antenna will be separated by 
approximately one guide waveguide. Unlike the previous embodiment, 
however, it is not possible to include intermediate slots at positions 
-.phi..sub.1 on the other side of the reference plane. This is because, 
although the phase of the transverse current component shifts by 
180.degree. on either side of the reference plane, this is not the case 
with the longitudinal current component. The longitudinal current 
component varies with the cosine of the circumferential angle .phi., and 
thus has both the same magnitude and the same phase at corresponding 
angles on either side of the reference plane RP. Because of this although 
circular polarizaton will result from crossed slots on either side of the 
reference plane, the sense of the circular polarization (left-hand or 
right-hand circular polarization) will differ on the two sides of the 
reference plane. 
It is therefore possible to radiate elliptical or circular polarization of 
either sense merely through a selection of the side of the reference plane 
upon which the slots are cut; slots may, however, only be placed on one 
side of the reference plane. Furthermore, if the baffles and the reference 
plane be considered as separating the antenna cross-section into four 
quadrants, the same sense of elliptical or circular polarization will 
result from crossed slots cut into diametrically opposing quadrants 
although the signals radiated therefrom would be in phase opposition if 
the crossed slots were provided at common longitudinal positions. To 
insure proper phasing, crossed slots provided in diametrically opposing 
quadrants must thus be spaced longitudinally one from the other by a 
distance of approximately one-half of a guide wavelength. 
A remaining factor which must be considered is that the quadrants 
corresponding to each circular polarization sense will change at the feed 
point due to the even symmetry of the current components with respect to 
the feed point. For the radiation of a single circular polarization sense, 
then, the crossed slots must be cut into different quadrants on different 
sides of the feed point, as illustrated. 
Depending on the polarization of the antenna, the total length of the 
antenna will be selected to be an integral number of guide wavelengths of 
half guide wavelengths. The number of slot positions provided (i.e., the 
number of "layers") will be selected in accordance with the desired gain 
of the antenna. These factors are well-known and are not believed to 
require elaboration. 
Having thus described two possible embodiments of the present invention, no 
further effort will be made herein to catalog the various alternative slot 
positions and orientations which may be employed. The vast range of 
alternatives will be immediately apparent to those skilled in the art so 
that no additional showing will be necessary. It will, for example, be 
noted that the crossed slots shown in FIG. 8, although illustrated as 
oriented along longitudinal and transverse directions, need not 
necessarily be so oriented. As long as the slots are orthogonal, no 
specific disposition of the component slots is required. Furthermore, 
slots oriented at 45.degree. with respect to the antenna axis will 
intercept both longitudinal and transverse current components, and, 
depending on their circumferential location, will radiate elliptically or 
circularly polarized waves. 
In view of this it will be appreciated that although the present invention 
has been described with reference to preferred embodiments, any number of 
alterations therein may be made without departing from the spirit and 
scope of the invention, as defined in the appended claims.