Broadband antenna element

A broadband antenna element capable of operating over greater than an octave band of frequencies is disclosed. The element comprises an open-ended rectangular waveguide section having a loop radiator, formed by shorting an insulated probe to one of the broad walls of the waveguide section, disposed therein. Notches are provided in the narrow walls and flanges are provided on the broad walls of the waveguide section for matching purposes. The insulated probe extends through a hole formed in the rear wall of the waveguide section to connect with a stripline feed network.

BACKGROUND OF THE INVENTION 
This invention relates generally to antenna elements and in particular to a 
broadband waveguide antenna element suitable for use in antiradiation 
missile seeker applications. 
Manned aircraft have been, and will continue to be, one of the principal 
means of weapons delivery in modern warfare. Manned aircraft combine a 
capability for accurate delivery of projectiles with the capability of 
reconnaissance and surveillance, utilizing personnel within the aircraft 
for location and identification of ground targets. Improved radar 
processing techniques, such as synthetic aperture mapping, then may be 
used to supplement the senses of the personnel to provide capability of 
attacking ground targets under adverse weather conditions and at night. 
Therefore, if allowed to roam freely in the airspace over the battlefield, 
manned aircraft can be the decisive factor in any ground engagement. 
To counter the threat posed by manned aircraft, highly effective 
ground-based anti-aircraft defense systems are being developed and 
deployed to reduce the probability of penetration of battle areas by 
manned aircraft to make the sustained use of such aircraft impractical. 
The common feature of such systems is the use of some form of radiation, 
such as radar, for the functions of search, acquisition, tracking or fire 
control of airborne vehicles, including manned aircraft. One most 
effective way to counteract the systems being discussed is, of course, to 
provide guided missiles which, when launched from an aircraft, sense the 
radiation and home in on the source of such radiation to deliver 
appropriate ordnance to such source. The radiation from the aforementioned 
defense systems may lie at any frequency within a wide frequency band and 
because such radiation may have one of several polarization senses, a 
missile seeker designed to home in on the source of such radiation 
(sometimes hereinafter referred to as an antiradiation missile (ARM) 
seeker) must be capable of operating over a similarly wide band of 
frequencies and must be responsive to any one of several polarization 
senses. 
An antenna including a matrix of stripline tapered notch elements, as 
described in an article entitled, "A Broadband Stripline Array Element," 
by L. R. Lewis, M. Fassett and J. Hunt, IEEE Antenna Propagation Society 
Symposium at Atlanta, Ga., June 1974, has been developed for ARM seeker 
applications. In such elements, a notch is etched away on both ground 
planes of a stripline and the stripline center conductor is arranged to 
excite a voltage across the notch. The matrix of elements is mounted 
orthogonally with respect to a feed network, requiring a right angle 
transition to be made between each element and the feed network. Such 
right angle transitions are extremely difficult to match over a wide 
frequency band with the result that impedance mismatches between the 
elements and the feed network may seriously degrade the seeker antenna 
performance. In addition, the physical size of such elements and the 
manner in which the matrix of such elements is mounted orthogonally to the 
feed network makes vibrational damage in missile seeker applications quite 
likely. 
The gain of the stripline tapered notch element being discussed is a 
function of the length of the element. Thus, an element having a length of 
approximately one-half wavelength at the highest operating frequency has a 
gain of from 1 to 3 db over an octave band, while an element of the same 
width but having a longer notch has greater than 4 db of gain over the 
same frequency band. In instances where space available for an antenna 
does not allow long elements, as in missile seeker applications, the gain 
of such elements is limited. 
Thus, there exists a need for a broadband antenna element, suitable for 
missile seeker applications, which may be integrated to a stripline feed 
network without causing severe mismatch problems and which is not 
susceptible to vibrational damage. 
SUMMARY OF THE INVENTION 
With this background of the invention in mind, it is an object of this 
invention to provide an antenna element having greater than an octave 
bandwidth. 
It is another object of this invention to provide an antenna element which 
may be integrated with a stripline feed network without mismatch problems. 
It is a further object of this invention to provide an antenna element 
which exhibits greater than 6 db gain over greater than an octave band of 
frequencies. 
These and other objects of the invention are attained generally by 
providing a hybrid radiating element comprising a loop radiator mounted 
within a rectangular waveguide terminated by a short circuit. A first end 
of the loop radiator is connected to the E-plane wall of the waveguide 
section. A second end of the loop radiator is connected to a probe 
extending through an iris in the shorted end of the rectangular waveguide. 
The loop radiator both excites the predominant TE.sub.10 mode within the 
rectangular waveguide which radiates into space and itself radiates 
directly into space so that the operating frequency of the element is 
extended beyond the cutoff frequency of the rectangular waveguide.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to FIG. 1, a broadband hybrid antenna element 10 (hereinafter 
sometimes referred to simply as antenna element 10) is shown to include a 
waveguide section 12 having a pair of broad walls (as that marked 14), a 
pair of narrow walls (as that marked 16), and a short circuited end 18. A 
probe 20, covered by a teflon dielectric sleeve 22, extends into waveguide 
section 12 from an iris (not shown) in the short circuited end 18. The 
length of probe 20 is approximately one-quarter wavelength at the center 
band frequency. The diameters of the probe 20 and the teflon dielectric 
sleeve 22 are dimensioned to provide a 50 ohm structure through the iris 
in short circuited end 18. Diametrically opposed flats (not numbered) are 
provided on teflon dielectric sleeve 22 to permit the insertion of the 
sleeve within the waveguide section 12 and to prevent rotation of the 
sleeve. A strip of metal tape 24, a first end of which is soldered to 
probe 20 and a second end of which is soldered to a broad wall 14, as 
shown, connects the probe 20 to the waveguide section 12. The strip of 
metal tape 24, the probe 20, and the broad wall 14 form a loop to allow 
the TE.sub.10 mode to be excited within the waveguide section 12. The 
width of the strip of metal tape 24 is controlled for the purpose of 
matching antenna element 10 over the 8 to 18 GHz band. The optimum width 
for the strip of metal tape was determined to be 0.100.+-.0.002 inches. 
Metal tabs 26 are included on the broad walls 14 for matching purposes. 
Notches (not numbered) are provided in the narrow walls 16, also for 
matching purposes. 
In operation, at frequencies above the cutoff frequency of waveguide 
section 12, the antenna element 10 operates as a TE.sub.10 mode waveguide 
radiator. At frequencies below the cutoff frequency for the TE.sub.10 
mode, the radiation from antenna element 10 is primarily from the loop 
formed by the probe 20, the metal tape 24, and the broad wall 14. 
Referring now to FIG. 2, the interconnection between antenna element 10 and 
a stripline 30 is illustrated. Probe 20 and teflon dielectric sleeve 22 
are shown to extend through an iris (not numbered) in the short circuited 
end 18 of the waveguide section 12 (FIG. 1). The teflon dielectric sleeve 
22 is terminated flush with the outer surface of the short circuited end 
18. A horseshoe-shaped ring 32, having a plurality of tapped holes (not 
numbered) formed therein, is soldered as shown to the outer surface of the 
short circuited end 18. The upper dielectric board 34 and the upper 
groundplane 36 of the stripline 30 have cutouts (not numbered) provided to 
accommodate the horseshoe-shaped ring 32. A second horseshoe-shaped ring 
38 having a plurality of clearance holes (not numbered) contained therein 
is soldered to a tinned cutout (not numbered) provided in the lower 
dielectric board 42 and the lower groundplane 43 of stripline 30. A pin 44 
is attached to the center conductor circuitry on the upper surface of 
dielectric board 42. Pin 44 is pre-tinned and is designed to be soldered 
within a cylindrical cavity (not numbered) provided in probe 20. Once pin 
44 is soldered to probe 20, horseshoe-shaped rings 32 and 38 are joined 
together by means of screws 46 which pass through the clearance holes 
provided in ring 38 and engage the tapped holes provided in ring 32. 
For ARM seeker applications, an elliptically or circularly polarized seeker 
antenna is desired so that either a linearly or circularly polarized 
source may be attacked. In order to obtain essentially circular 
polarization, adjacent ones of the antenna elements would be orthogonally 
disposed with respect to each other and each orthogonal pair of antenna 
elements would be fed in phase quadrature. Methods for determining 
element-to-element spacing and for obtaining quadrature feed signals are 
matters involving ordinary skill in the art and will therefore not be 
recounted. 
The antenna element 10 has been built and found effective to provide a 
minimum of 6.0 db of gain over the 8 to 18 GHz frequency band. 
The dimensions of the element just mentioned were: 
Dimension a=0.150 inches 
Dimension b=0.620 inches 
Dimension c=0.100 inches 
Dimension d=0.235 inches 
Dimension e=0.250 inches 
Dimension f=0.350 inches 
Diameter of coaxial probe 20=0.050 inches 
Diameter of teflon sleeve 22=0.162 inches 
Having described a preferred embodiment of this invention, it is now 
evident that other embodiments incorporating its concepts may be used. For 
example, the antenna element could be fed by a coaxial cable instead of 
the stripline network shown. Also, if it were desired to reduce the weight 
of the element, the waveguide section could be formed from a plated foam 
material. 
It is felt, therefore, that this invention should not be restricted to its 
disclosed embodiment but rather should be limited only by the spirit and 
the scope of the appended claims.