Wideband slot antenna with low VSWR

A slot antenna is described. The slot antenna includes waveguide having a first side, a second side, a third side, and a fourth side, a closed end and an open end. The second side extends substantially perpendicularly from a first end of the first side. The third side extends substantially perpendicularly from a second end of the first side. The fourth side extends between the second side and the third side with the fourth side substantially parallel to the first side. The sides and the closed end form a cavity. A feeding point is located substantially midway between the first end of the first side and the second end of the first side. A T-Bar is located inside the cavity, the T-bar having a center member extending from the feeding point into the cavity and a cross member having a length extending across the cavity between the second side and the third side. The cross member is shaped as a stepped cylinder having a first diameter in a central portion along the length of the cross member and having a second diameter in an outer portion along the length of the cross member.

FIELD OF THE INVENTION 
The present invention relates generally to antennas. More specifically, a 
wideband slot antenna with a low VSWR is disclosed. 
BACKGROUND OF THE INVENTION 
With the advent of digital television, a need has arisen for broadband 
multi-channel antennas suitable for radiating signals at UHF wavelengths. 
In some jurisdictions, various regulations require stations to begin 
broadcasting digital signals and also to continue broadcasting analog 
signals. Stations may be assigned frequency bands that are close together 
or far apart for such multiple broadcasts. As a result, it would be 
extremely useful if a broadband antenna could be developed that would 
radiate efficiently from 470 MHz to 860 MHz, the UHF television band, and 
would also have a low VSWR at its input terminal in that band. Typically, 
a VSWR of about 1.15 or less is required in a television transmission 
system. 
Dipole arrays may be designed that are capable of broadband operation 
across the UHF band and that meet the VSWR requirements for television 
transmission systems. However, there arc disadvantages to using dipole 
arrays. A dipole array assembled in a large panel on a tower has a large 
wind load and tends to be less robust mechanically than other antenna 
designs. Dipole arrays also tend to be mounted on large structures to 
accommodate the mechanical loads of the panels. This makes it hard to 
achieve desirable radiation patterns for the antenna system. 
Slot antennas have been used in physically demanding environments such as 
on airframes for narrow bandwidth applications with success. Slot 
antennas, however, have not been developed that can operate across the UHF 
band with the low input VSWR required for a television transmission 
system. A waveguide slot antenna consists of a length of waveguide short 
circuited at one end and open circuited at the other. The open end is 
usually terminated in some type of ground screen, and the antenna is 
excited by a coaxial to waveguide transition. 
FIG. 1 is a diagram illustrating a slot antenna with a crossbar transition 
between the coaxial cable to the waveguide. A slot antenna 100 is shown. A 
waveguide 102 is closed on one end and open at open end 103. Slot antenna 
100 is fed by a 50-ohm coaxial cable line 104 that is connected to the top 
of waveguide 102. The outside of the coaxial line is electrically 
connected to the waveguide. The center conductor of the line is 
electrically connected a T-shaped bar 106 that extends downward into the 
waveguide cavity. T-shaped bar 106 includes a cross member 108 that 
extends the length of the cavity and is terminated at the sides 110 of the 
cavity. Such an antenna is generally referred to as a T-bar fed slot 
antenna. 
Such antennas have been the object of considerable study in the prior art. 
A cavity backed rectangular slot antenna is described in Reference 1, 
"Antenna Engineering Handbook", Henry Jasik, First Edition, McGraw-Hill, 
1961, which is herein incorporated by reference for all purposes. The 
original design work was published in Very High Frequency Techniques, 
compiled by the Radio Research Laboratory and published by McGraw-Hill in 
1947. The VSWR of two T-bar fed slot antennas investigated is shown in 
FIG. 2A. The VSWR results for the earlier investigation of such T-bar slot 
antennas is shown in FIG. 2B. 
Another investigation of the design of T-bar fed slot antennas is provided 
in Reference 2, "Some Important Parameters in the Design of T-Bar Fed Slot 
Antennas, by E. H. Newman and Garry A. Thiele, IEEE Transactions on 
Antennas and Propagation, January 1975, pages 97-100, which is herein 
incorporated by reference for all purposes. The VSWR of a tuned T-bar fed 
slot antenna is shown in FIG. 2C. Newman, et al., further describes how to 
design T-bar fed slot antennas for various bandwidths. 
Although the performance of such antennas described in the prior art is 
good, the VSWR across a broad bandwidth of 1.8.lambda. corresponding to 
the UHF frequency spectrum is too large. The VSWR is greater than 1.5 at 
several points and in particular, tends to increase substantially at high 
frequencies within its operating band. The VSWR of the antennas described 
in FIGS. 2A and 2B increases to above 2 at the high frequency end and even 
the tuned antenna shown in FIG. 2C has a peak VSWR of greater than 2 at 
the high frequency end of its operating band. 
In spite of the considerable work that has been done on T-bar fed slot 
antennas, the performance of such antennas has not been improved to the 
point where such antennas may be used with success across the entire UHF 
frequency spectrum for television transmission. Other techniques are 
needed to enable the use of slot antennas for such applications. It would 
be useful if a slot antenna could be developed that could meet the design 
requirements for television transmission across the UHF band. 
SUMMARY OF THE INVENTION 
A broadband low VSWR slot antenna is disclosed that can be used for the 
transmission of television signals in the UHF band. The slot antenna is 
fed using a modified T-bar that has a smaller diameter at the ends of the 
T-bar which are attached at the ends of the slot or cavity than at the 
center of the T-bar. The opening of the cavity is attached to a ground 
plane and one or more probes is added to the ground plane at a location 
opposite the feeding point where the cavity is fed. The slot antenna may 
be further modified by extending the sides of the cavity where the ends of 
the T-bar reach the sides of the cavity. In a addition, a ridge may be 
added to the cavity. A dielectric radome may be included in front of the 
cavity and a pair of covers may be added to the sides of the cavity to 
compensate for the radome. 
An array of slot antennas is also disclosed. The array includes slot 
antennas stacked on top of each other and spaced circumferentially in a 
cylindrical array with their T-bars vertically oriented. 
It should be appreciated that the present invention can be implemented in 
numerous ways, including as a process, an apparatus, a system, a device, a 
method, or a computer readable medium such as a computer readable storage 
medium or a computer network wherein program instructions are sent over 
optical or electronic communication lines. Several inventive embodiments 
of the present invention are described below. 
In one embodiment, a slot antenna is disclosed. The slot antenna includes a 
waveguide having a first side, a second side, a third side, and a fourth 
side, a closed end and an open end. The second side extends substantially 
perpendicularly from a first end of the first side. The third side extends 
substantially perpendicularly from a second end of the first side. The 
fourth side extends between the second side and the third side with the 
fourth side substantially parallel to the first side. The sides and the 
closed end form a cavity. A feeding point is located substantially midway 
between the first end of the first side and the second end of the first 
side. A T-Bar is located inside the cavity, the T-bar having a center 
member extending from the feeding point into the cavity and a cross member 
having a length extending across the cavity between the second side and 
the third side. The cross member is shaped as a stepped cylinder having a 
first diameter in a central portion along the length of the cross member 
and having a second diameter in an outer portion along the length of the 
cross member. 
In another embodiment a slot antenna is disclosed. The slot antenna 
includes a waveguide having a first side, a second side, a third, side, 
and a fourth side, a closed end and an open end. The second side extends 
substantially perpendicularly from a first end of the first side. The 
third side extends substantially perpendicularly from a second end of the 
first side and the fourth side extends between the second side and the 
third side. The fourth side is substantially parallel to the first side 
with the sides and the closed end forming a cavity. A feeding point is 
located substantially midway between the first end of the first side and 
the second end of the first side. A T-Bar is located inside the cavity. 
The T-bar has a center member extending from the feeding point into the 
cavity and a cross member having a length extending across the cavity 
between the second side and the third side. A ground screen is attached to 
the open end of the waveguide and extending substantially perpendicularly 
away from the four sides of the waveguide. A conductive probe extends 
substantially perpendicularly from the ground screen in a direction away 
from the cavity. 
In another embodiment, a slot antenna array is disclosed. The slot antenna 
array includes a plurality of slot antennas, Each slot antenna includes a 
waveguide having a first side, a second side, a third, side, and a fourth 
side, a closed end and an open end. The second side extends substantially 
perpendicularly from a first end of the first side. The third side extends 
substantially perpendicularly from a second end of the first side and the 
fourth side extends between the second side and the third side. The fourth 
side is substantially parallel to the first side, the sides and the closed 
end forming a cavity. A feeding point is located substantially midway 
between the first end of the first side and the second end of the first 
side. A T-Bar is located inside the cavity, the T-bar having a center 
member extending from the feeding point into the cavity and a cross member 
having a length extending across the cavity between the second side and 
the third side. The cross member is shaped as a stepped cylinder having a 
first diameter in a central portion along the length of the cross member 
and having a second diameter in an outer portion along the length of the 
cross member. The plurality of slot antennas are arrayed about the 
circumference of a circle in a circular array with the open ends of the 
slot antennas facing radially outward. 
These and other features and advantages of the present invention will be 
presented in more detail in the flowing detailed description and the 
accompanying figures which illustrate by way of example the principles of 
the invention.

DETAILED DESCRIPTION 
A detailed description of a preferred embodiment of the invention is 
provided below. While the invention is described in conjunction with that 
preferred embodiment, it should be understood that the invention is not 
limited to any one embodiment. On the contrary, the scope of the invention 
is limited only by the appended claims and the invention encompasses 
numerous alternatives, modifications and equivalents. For the purpose of 
example, numerous specific details are set forth in the following 
description in order to provide a thorough understanding of the present 
invention. The present invention may be practiced according to the claims 
without some or all of these specific details. For the purpose of clarity, 
details relating to technical material that is known in the technical 
fields related to the invention has not been described in detail in order 
not to unnecessarily obscure the present invention in such detail. 
A modified T-bar fed slot antenna is used to meet the VSWR and bandwidth 
requirements for television transmission across the UHF band. As is 
described below, a number of modifications are used to achieve the desired 
performance. Such modifications include stepping the diameter of the cross 
member of the T-Bar, adding one or more probes to a ground screen 
connected to the open end of the slot antenna, adding a ridge inside the 
cavity of the slot antenna near the feed point of the antenna, and 
covering the ends of the slot when a dielectric radome is used. In 
addition, a plurality of slot antennas may be arrayed in a circular or a 
cylindrical array. Cross coupling between the arrayed antennas tends to 
further lower the VSWR. In a cylindrical array, antennas stacked 
vertically above each other are fed by transmission signals that are in 
phase. 
It should be noted that in the following discussion, the design of antenna 
for the UHF band between 470 MHz and 860 MHz is described. As is described 
in Jasik and Neumen et. al., which were previously incorporated by 
reference, the size of a slot antenna may be scaled to work at different 
bandwidths. Therefore, the modifications disclosed herein to UHF slot 
antenna may be applied to other slot antennas if scaled appropriately. 
Where appropriate, dimensions or sizes will be described in this 
specification both in terms of a size used for the UHF antenna of the 
preferred embodiment as well as in terms of the a minimum wavelength, 
center wavelength, or a maximum wavelength, referring to the band in which 
the antenna is intended to operate. In addition, it should also be noted 
that each of the techniques disclosed herein may be used separately or 
together with certain of the other techniques in different embodiments. 
The T-bar slot feed system excites two modes in the waveguide cavity. One 
of the modes is the principal radiating mode. It produces an electric 
field perpendicular to the direction of radiation away from the cavity. 
The principal mode is referred to as the TE.sub.0,1 mode. The other mode, 
the TM.sub.1,1 mode, operates at a frequency below waveguide cut-off and 
does not radiate a significant amount of energy. The stored energy in the 
second mode causes the slot to have a higher-than desirable VSWR. 
Additionally, the T-bar slot feed can be shown to have a non-radiating 
transmission line mode that contributes significantly to the reactance at 
the input of the slot. By suitable modification of the T-Bar structure and 
how it is terminated at the cavity walls, the reactance at the input of 
the slot can be significantly reduced, particularly at the low frequency 
end of the band of interest. 
The T-bar can be viewed as a section of transmission line in a trough 
cavity which has a well-known characteristic impedance. Using a line which 
has a uniform impedance of about 80 Ohms results in a high reactance at 
the input to the slot at the low frequencies, where the line is about 112 
electrical degrees long. At the higher frequency end of the band, the 
T-bar is approximately 180 electrical degrees long and always presents a 
low reactance at the input to the slot, regardless of its characteristic 
impedance. By varying the characteristic impedance of the T-bar along it's 
length with a higher impedance near the short-circuit end, the line 
presents a lower reactance at the low frequency end of the band. This 
enables an optimal input reactance at both ends of the frequency band of 
interest. 
In one embodiment, the characteristics impedance of the T-bar is varied by 
changing the diameter of the T-bar along its length. FIG. 3A is a diagram 
illustrating a T-bar fed slot antenna 300 fed by a T-bar having a stepped 
diameter. The slot antenna is fed at a feeding point 302. The T-bar 
includes a center member 304 extending downward from feeding point 302. 
The cross member of the T-bar includes a center portion 306 having a large 
diameter and two outer portions 308A and 308B having a smaller diameter. 
In addition to stepping the impedance of the T-bar, the termination of the 
T-bar at the ends where it meets the cavity walls can be modified to 
compensate the input impedance further. Use of an open-circuit, 
quarter-wave (at the median frequency) transmission line produces 
compensating reactances, which further improve the input reactance of the 
slot at the band edges. In one embodiment, the transmission line is 
provided by a cavity extension where the ends of the T-bar meet the cavity 
walls. The length and impedance of the cavity extension may be adjusted in 
different embodiments to provide a desired reactance. 
FIG. 3B is a diagram illustrating a T-bar fed slot antenna with cavity 
extensions for terminating the ends of the T-bar. A T-bar 311 has outer 
ends which extend beyond the cavity into a cavity extension 312 and a 
cavity extension 314. The characteristic impedance of the transmission 
line extending beyond the cavity and the length of the transmission line 
is controlled to lower the VSWR of the slot antenna. 
The T-bar characteristic impedance and termination partially compensate the 
overall slot impedance such that the VSWR can be lowered to about 1.3:1 
across the entire UHF band. The stepped-impedance and open-circuit 
termination techniques thus improve the performance of the slot antenna, 
but not to within the requirements set forth above for television 
transmission. 
In order to further reduce the VSWR to the very low levels required for 
television transmission, one or more probes is added to a ground screen 
attached to the front of the open end of the slot antenna near the slot 
aperture on the side of the slot that is opposite the side on which the 
slot is fed. The non-radiating TM.sub.1,1 mode can significantly effect 
the impedance of the slot since the band of operation is still close to 
the cut-off frequency of that mode. The use of probes near the slot 
aperture lowers the variation of the slot impedance within the band of use 
and to produce an overall VSWR of 1.15:1 or less across the whole UHF 
band. The radiation from the probes is very small and produces a signal of 
orthogonal polarization to that of the principle radiation, thus the 
effect of these probes is to reduce the overall VSWR of the slot without 
significantly effecting the efficiency of the slot in it's principle 
radiating mode. 
FIG. 3C is a diagram illustrating a T-bar fed slot antenna with a ground 
screen extending perpendicularly from the sides of the cavity. The cavity 
is bounded by four sides, 321a, 321b, 321c and 321d. One end of the cavity 
is closed, and the other end is open. The open end is attached to a ground 
screen 322 which extends from each of the four sides in a substantially 
perpendicular direction. A T-bar 324 is inside the cavity. A probe 326 
extends perpendicularly from ground screen 322 on the side of the ground 
screen that is opposite a feeding point 328 where a coaxial cable may be 
attached. Probe 326 further reduces the VSWR of the slot antenna. 
FIG. 3D is a diagram illustrating a slot antenna having two probes 
extending from a ground screen. The slot antenna is fed by a T-bar 330 
that extends inside the slot cavity from a feeding point 331. A ground 
screen 332 extends outward from the sides of the cavity at the open end of 
the cavity. A pair of probes, 334 and 336 extend perpendicularly from the 
ground screen in the direction away from the cavity. Probes 334 and 336 
are symmetrically spaced about the center of the cavity on the ground 
screen on the side opposite feeding point 331. 
FIG. 3E is a three dimensional diagram illustrating a T-bar fed slot 
antenna 340 fed by a T-bar having a stepped diameter. The slot antenna is 
fed at a feeding point 342. The T-bar includes a center member 344 
extending downward from feeding point 342. The cross member of the T-bar 
includes a center portion 346 having a large diameter and two outer 
portions 348a and 348b having a smaller diameter. The antenna 340 
comprises a wave guide 350 having a right side 352, a top side 354, a left 
side 356, a bottom side 358, a closed back end 360, and an open front end 
362. 
FIG. 4 is a diagram illustrating a preferred T-bar fed slot antenna 400 
designed using the above described techniques for the UHF band. A slot 402 
is 0.62 .lambda. long where .lambda. is the wavelength at the lowest 
operating frequency (470 MHz), 0.2 .lambda. wide and 0.21 .lambda. deep. A 
center member 404 of the T-bar extends into the cavity at a feeding point 
406 and the cross member of the T-bar includes a center portion 408 made 
of a larger 0.065 .lambda. (1.625 inch) diameter tube of characteristic 
impedance 80 Ohms. Outer portions 410a and 410b of the center member of 
the T-bar are two 0.01 .lambda. (0.25 inch) lengths of transmission line 
of 200 Ohm impedance that are connected at the two ends of the slot. The 
termination of the T-bar lines utilizes an open-circuit 0.25 .lambda. 
section of coaxial line of 66 Ohm characteristic impedance. 
Since they are non-radiating terminations, the terminating lines are 
extended beyond the wall of the cavity in cavity extensions 412a and 412b. 
In some embodiments, the terminating lines may be omitted and the T-bar 
connected directly to the sides of the slot if the slot is used in an 
array where it is not feasible to include the cavity extensions because of 
space constraints. In such cases, the stepped diameters of the T-bar may 
be adjusted so that the performance may be nearly as good as with the 
extensions. A ground plane 420 is included around the open end of the 
slot. On the surface of the ground-plane, two short probes 422a and 422b 
that are about 0.125 .lambda. high are placed symmetrically about the 
centerline of the slot on the side of the ground plane opposite the 
feeding point, 0.155 .lambda. on either side. 
A ridge 430 is included inside the cavity at the edge of the open end on 
the side where the feeding point is located. The ridge is 0.5 inch high by 
0.5 inch wide by 7.8 inch long. If a dielectric radome (not shown) is used 
to cover the opening, then two conductive covers 440a and 440b are used 
over the ends of the slot as shown in FIG. 4. In one embodiment, a 
polyethylene cover is placed about one third of an inch over the opening 
and two conductive covers extend 0.5 inch over the slot on each end of the 
slot. 
It should be noted that the preferred T-bar fed slot antenna described in 
FIG. 4 combines a variety of the techniques taught herein and that such 
techniques may be used individually as well in accordance with the spirit 
and scope of this invention. 
FIG. 5 is a graph illustrating the VSWR of the slot antenna shown in FIG. 
4. The VSWR is below 1.10 over most of the UHF frequency band and extends 
above 1.10 but below 1.15 only at the very low end of the band. Thus, the 
techniques described above function to significantly lower the VSWR of the 
slot antenna, enabling the slot antenna to be used for broadband 
television transmission in the UHF band. 
So far, a single cavity slot antenna with a T-bar feed has been described. 
Using the described techniques, the VSWR of such an antenna has been 
lowered to less than 1.15 across the entire UHF spectrum. Arrays of such 
antenna are generally used to achieve suitable radiation patterns and 
coverage areas. Typical systems use four or eight elements arrayed around 
a circle and stacked four, eight, sixteen or thirty-two elements high to 
create high gain arrays. The impedance of a T-bar slot antenna in an array 
is substantially the same as the impedance of such an antenna by itself. 
The VSWR of slot antennas in an array is slightly improved by mutual 
coupling between elements, particularly at the lower end of the band of 
interest. Referring back to FIG. 5, it is only at the lower end of the 
spectrum that the VSWR is above 1.10 and almost reaches 1.15. In an array, 
the VSWR is less than 1.10 across the entire band. Thus, using the 
techniques described herein, it is possible to provide an array of slot 
antennas that operates across the entire UHF band with a very low VSWR 
that meets the requirements for television transmission. This achieves a 
very low VSWR for the array over the whole television transmission band. 
FIG. 6A is a diagram illustrating a cylindrical array 600 of slot antennas. 
The array includes stacks of slot antennas such as the stack of slot 
antennas comprised of a slot antenna 602A, 602B and 602C. The antennas are 
stacked vertically and arranged in a circular array so that the ground 
plane extending from each of the openings of the slot antenna forms a 
cylinder with the cavities of the slot antenna forming holes in the 
cylinder. 
FIG. 6B is a block diagram illustrating a feeding arrangement used to feed 
the antennas in one of the stacks included in array 600. A transmitter 610 
sends a signal to a splitter 612. Splitter 612 splits the signal into 
three signals. The three signals are fed to slot antennas 614A, 614B, and 
614C from splitter 612 using three lines, 616A, 616B and 616C which are 
all of the same length. Thus, the slot antennas in the stack are all fed 
in phase. It has been found that feeding the slot antennas in phase 
improves the VSWR of the array and also improves the signal quality. Prior 
art circuits for feeding arrays stacks of slot antennas have used a single 
line with multiple taps. Feeding the slot antennas in the stack using 
equal length transmission lines increases the cost of feeding the slot 
antennas but it has been found that this arrangement enables the stack of 
slot antennas to be used across a large bandwidth as is desired. 
FIG. 7A is an antenna pattern illustrating a signal transmitted by an array 
of slot antennas that includes two stacks separated by 45 degrees. The 
pattern is directional away from the slots and substantially uniform 
within a 50 degree path. FIG. 7B is an antenna pattern illustrating a 
signal from eight slot antennas evenly spaced circumferentially around a 
circle. The pattern is substantially omniazimuthal. 
Although the foregoing invention has been described in some detail for 
purposes of clarity of understanding, it will be apparent that certain 
changes and modifications may be practiced within the scope of the 
appended claims. It should be noted that there are many alternative ways 
of implementing both the process and apparatus of the present invention. 
Accordingly, the present embodiments are to be considered as illustrative 
and not restrictive, and the invention is not to be limited to the details 
given herein, but may be modified within the scope and equivalents of the 
appended claims.