Slot antenna

A stripline radiating element for use in a flat plate antenna array. The radiating element is comprised of a stripline sandwich including first and second stripline boards. A U-shaped slot is etched in the ground plane of the first stripline board and an open circuit transmission line is disposed between the two sandwiched boards for coupling energy to the slot. The inner dimensions of the slot form a strip transmission line with one end thereof, which is opposite the slot portion, being terminated in a short circuit which is formed by plated through holes between both ground planes of each individual stripline board. The length of the open circuited strip transmission line is adjusted to resonate with the slot susceptance and the reactance of the short circuited transmission line.

BACKGROUND OF THE INVENTION 
This invention relates to antennas and more particularly to a stripline 
slot antenna element suitable to be used in flat plate antenna arrays. 
Stripline slot antennas are well known in the art. These antennas are 
generally formed by etching a radiating aperture (slot) on one ground 
plane of a stripline sandwich circuit. The stripline sandwich comprises a 
conducting strip, and a transmission line insulatively disposed between 
two ground planes. Energy is coupled to the slot over the transmission 
line with the electric fields propagated thereon confined within the 
dielectric boundaries between the ground planes. To maintain mode purity, 
to prevent moding problems, prior art stripline antennas have required the 
use of cavities formed opposite of the radiating aperture. These cavities 
are usually formed by either placing plated through holes at predetermined 
distances about the radiating aperture, or by using rivets between the 
ground planes. Another method is to form a physical cavity on the ground 
plane opposite the radiating slot. 
The use of cavities has limited the bandwidth performance of these prior 
art antennas. Typically, the bandwidth of such stripline antennas are 3% 
to 5%. Hence, flat plate antenna arrays comprised of such antenna elements 
are typically limited to bandwidths of 2% to 3% and an efficiency factor 
of no greater than 35%. 
Because the slot is itself a relatively broadband radiator, if the cavity 
could be eliminated, the bandwidth performance of a slot antenna element 
could be improved. Such an improvement would give rise to an associated 
increase in an array efficiency factor. 
Thus, a need exists for eliminating a requirement for cavity backed slots 
in order to provide stripline slot antennas having improved bandwidth 
performances. 
Accordingly, it is an object of the present invention to provide an 
improved slot antenna element. 
It is another object of the present invention to provide a stripline slot 
antenna which requires no resonant cavity. 
It is a further object of the invention to provide a stripline slot antenna 
of a particular configuration requiring no cavity and which is suitable to 
be utilized in flat plate antenna arrays. 
SUMMARY OF THE INVENTION 
The foregoing and other objects are met in accordance with the present 
invention by providing a stripline slot antenna element suitable to be 
used in flat plate antenna arrays. 
According to one feature of the invention, the stripline antenna element is 
formed in a stripline sandwich circuit including first and second 
dielectric boards having parallel opposed ground planes of copper clad 
material. The radiating element of the antenna is formed by etching a 
rectangular slot in the ground plane of the first board. A feed network 
comprising a strip transmission line and microstrip line is disposed 
between the ground planes. The stripline portion is asymmetrically 
disposed between the two ground planes to facilitate stripline to 
microstrip transition without generating undesirous TM modes and to 
optimize the bandwidth of the slot element. A U-shaped radiating slot is 
thus formed between the ground plane of the first board and the input end 
of the microstrip matching line. The opposite end of the microstrip line 
is shorted to both ground planes with the length thereof being chosen to 
cancel the positive susceptance of the slot admittance. 
In accordance to another feature of the invention, a microstrip line is 
formed on one ground plane surface which has one end thereof terminated in 
a short circuit to both ground planes of the stripline sandwich circuit. A 
U-shaped slot is formed between the edge of the microstrip line and the 
upper ground plane. An open circuited conduction strip is disposed between 
the two boards in spatial relation to the microstrip line. Input energy is 
propagated in a TEM mode along the strip line feed network and is radiated 
from the U-shaped slot. The length of the open-circuited strip line feed 
network is adjusted to resonate with the slot susceptance and the short 
circuited microstrip reactance. 
The matching of the slot impedance provides a strip line antenna element 
exhibiting a bandwidth on the order of 10% to 15% for ground plane to 
wavelength spacing ratios of 0.07 .lambda..sub..epsilon.r.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIGS. 1 and 2, there is illustrated strip-line slot antenna 
element 10 of one embodiment of the present invention. It is to be 
understood that the slot antenna elements hereinafter disclosed may be one 
constituent radiating element of a multielement flat plate antenna array. 
Slot antenna 10 is shown as comprising two copper-clad dielectric boards 12 
and 14 which may be bonded together to form a stripline sandwich circuit, 
as is known in the art. A flat conducting strip 16 is disposed between 
upper ground plane 18 and lower ground plane 20. A radiating aperture 22 
is formed in upper ground plane 18 of rectangular shape. Aperture 22 may 
be formed by etching using known techniques. Conducting strip 16 includes 
stripline 24 and microstrip line 26 which form a matching network. As is 
observed, a U-shaped radiating slot 28 is formed between ground plane 18 
and microstrip transmission line 26. The end of microstrip line 26, 
opposite the input feed, is short circuited to both ground planes 18 and 
20 by, for example, plated through holes which are shown typically by 
reference numeral 30. Similarly, mode suppression is provided by plated 
through holes 32. It is to be understood that plated through holes 30 and 
32 may be provided by rivets, screws and other means, the choice of which 
depends on the designer. 
In operation, the length, l, of microstrip line 26 is chosen to produce a 
negative susceptance which cancels the positive susceptance of the slot 
admittance. This establishes a real conductance input value at the 
microstrip line input. The conductance input value can be readily matched 
using a well known quarter wave length transformer section, which may be a 
portion of strip line 24 (not shown). Input energy which is applied to 
stripline 24 is conducted in essentially a TEM mode and radiated from slot 
28. Energy is applied to stripline 24 either by end-launching or by the 
use of right angle connections as is understood. 
It has been shown by R. F. Harrington in an article entitled, 
"Time-Harmonic Magnetic Fields", McGraw-Hill, 1961, pages 182-183, that 
the aperture admittance of a capacitive slot radiator for small values of 
ka; i.e., a/.lambda. &lt; 0.1: 
##EQU1## 
where: W = slot length 
.eta. = 377.OMEGA. 
a = slot thickness 
Moreover, it is known that to a first approximation, the admittance of a 
shortcircuited microstrip line is equal to: 
EQU -j/z tan .theta. (3) 
where: Z = microstrip line impedance 
EQU .theta. = 2.pi.l/.lambda..epsilon.r 
.lambda..epsilon.r = wave length in dielectric 
Hence, the length, l, of microstrip line 26 is determined by setting 
equation 3 equal to equation 2 such that: 
##EQU2## 
Thus, by adjusting the quantity, 1, a real conductance value, G.sub.A for 
the antenna element is derived which is equal to the value as shown by 
equation 1. 
Turning now to the remaining Figures, there is illustrated stripline slot 
antenna 40 of another embodiment of the invention. Antenna 40 is 
fabricated in the same manner as antenna 10 and comprises copper-clad 
dielectric boards 42 and 44 bonded together, for instance. Disposed 
between upper and lower ground planes 46 and 48, respectively, is 
open-circuited stripline 50 adapted to receive and couple energy to 
U-shaped slot 52. The slot is formed between the edge of microstrip line 
54, which is short circuited by plated through holes 56, and upper ground 
plane 46. Plated through holes 58 are supplied for mode suppression as 
before. U-shaped slot 52 is formed by etching the copper-clad material 
from ground plane 46. 
In a similar manner as previously discussed, the length, L, of microstrip 
line 54 is chose such that the transformed slot susceptance is cancelled 
by the negative short circuit susceptance. The length of open-circuited 
strip transmission line 50 is then adjusted to resonant with the slot 
susceptance and short circuited microstrip reactance of microstrip line 54 
to match the input of antenna element 40 to approximately 50 ohms. 
Several slot antenna elements have been fabricated using the concepts as 
described above. For a maximum voltage standing wave ratio (VSWR) of 2:1 
and a ground plane spacing ratio S/.lambda. .perspectiveto. 0.07, 
bandwidths from 6% to 16% were exhibited as the slot dimension, W, was 
varied from 0.44.lambda. to 0.5.lambda.. 
Thus, what has been described is a unique stripline slot antenna element 
having minimum slot dimensions and increased bandwidth. The antenna is in 
the form of a U-shaped radiating aperture. The impedance of the aperture 
is matched by microstrip matching lines. The reduced slot size and 
increased bandwidth characteristics allow for the construction of flat 
plate antenna arrays having higher efficiency characteristics.