Blockage-free space fed antenna

A space fed antenna with a folded path to reduce the distance between a phased array and the focal point of such array is shown to be made up of: (a) a first plurality of vertically polarized antenna elements incorporated in the phased array; (b) a second plurality of cross-polarized antenna elements disposed to form a polarization-twisting screen at a distance from the first plurality of vertically polarized antenna elements equal to one-third the focal length of such elements; and (c) a third plurality of latching circulators connected between selected pairs of cross-polarized antenna elements in the polarization-twisting screen so that such pairs also may function as a feed array for the space fed antenna.

BACKGROUND OF THE INVENTION 
This invention pertains generally to optically fed antenna systems and in 
particular to an antenna of such type having a folded feed wherein 
aperture blockage is eliminated. 
Optical or space feeds for lens array antenna systems are preferred in many 
applications because such feeds suffer significantly less loss than 
equivalent corporate feed networks and are less costly to build. Further, 
the relatively low weight of optically fed antenna systems makes such 
systems attractive for use in aircraft. However, in some applications 
(especially in aircraft installations) sufficient space is not always 
available to allow placement of the feed at the focal point of the array 
as is necessary for satisfactory operation. Consequently, only optically 
fed arrays with small focal length to diameter (F/D) ratios have been 
utilized in many applications. Unfortunately, with very small F/D ratio, 
the size of the nominal focal spot becomes very small. When the focal spot 
is small, good array illumination control is difficult to attain without 
intentionally moving the feed off the focal plane. An off focal plane feed 
may be just as effective as a focal plane feed in a monopulse antenna, 
provided both sum and difference main lobes are subtended over the 
frequency range of interest for the maximum scan condition of the main 
lens assembly. Unfortunately, however, an off focal plane feed greatly 
increases the complexity of the antenna system installation, especially in 
aircraft applications. 
A known alternative to an off focal plane feed to mitigate the problems 
inherent with a small F/D ratio is a folded feed technique. The most 
effective feed using such a technique has "polarization-twist" elements to 
reduce the depth of the feed to one-third focal length. In such a folded 
feed, a horizontally-polarized multi-mode horn (or similar feed) is 
positioned in the center of a polarization-twisting surface. Vertically 
polarized energy incident on the outer face of vertically polarized 
antenna elements is passed to be incident on the polarization-twisting 
surface for conversion to horizontally polarized energy directed back 
toward such antenna elements for a final reflection back to the 
horizontally polarized multi-mode feed horn. While such an arrangement is 
effective in folding the optical path between antenna elements and feed, 
it suffers aperture blockage because the vertically polarized energy 
originally passed through the antenna elements and directly incident on 
the multi-mode feed horn is not polarization-twisted as required. 
SUMMARY OF THE INVENTION 
With the foregoing background of the invention in mind, it is, therefore, a 
primary object of this invention to provide an improved folded feed for an 
optically fed antenna, such feed introducing no aperture blockage. 
The foregoing and other objects of this invention are generally attained by 
providing, within a folded optically fed antenna, a polarization twisting 
arry and an antenna feed array made up of similar arrays of so-called 
flared notch radiating elements, such elements being orthogonally disposed 
with respect to one another to provide either vertical or horizontal 
polarization. The elements in the polarization-twisting array are joined 
together to produce a reciprocal polarization twisting operation, while 
the elements in the antenna feed array are connected through latching 
circulators to combine the polarization twisting function as well as 
permit transmission and reception. Additionally, compensation is provided 
in each of the flared notch radiating elements in the 
polarization-twisting array to eliminate any differential time delay 
between such elements and the flared notch elements in the antenna feed 
array.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to FIG. 1, an optically fed antenna 10 employing a folded 
feed according to this invention is shown to include a lens 11 having 
elements 13 (FIG. 2), a polarization-twisting array 15 and an antenna feed 
17 disposed at the center of the polarization-twisting array 15. An 
anechoic chamber 19 is formed, as shown, by enclosing the space between 
the rear face (not numbered) of the lens 11 and the polarization-twisting 
array 15 in any conventional way to reduce sidelobes resulting from 
unwanted reflections. The focal length of the optically fed antenna 10 
here is one-third the focal length of a conventional lens antenna system 
having a lens 11 of the same diameter because vertically polarized energy 
incident on the face of the lens 11 will be reflected two times in the 
anechoic chamber 19 before being passed through the antenna feed 17 to a 
radar receiver (not shown). That is to say, vertically polarized energy 
(such as that indicated by ray 21) that is incident on the face of the 
lens 11 will first be passed through the elements 13 to the 
polarization-twisting surface 15 for conversion to horizontally polarized 
energy and redirection back to the rear face of the lens 11 for reflection 
finally back to the antenna feed 17. 
Referring now to FIG. 1A, it may be seen that the elements making up the 
polarization surface 15 and the antenna feed 17 of FIG. 1 are disposed to 
form a planar array in which the elements making up the antenna feed 17 
are located centrally of the elements making up the polarization twisting 
surface 15. 
Referring now to FIG. 2, an element 13 of the lens 11 is shown to include a 
rear radiator 23 (which is vertically polarized), a reciprocal phase 
shifter 25, a circulator 27, a field effect transistor (FET) power 
amplifier 29, a combined transmit/receive (T/R) and polarization switch 31 
and a front radiator 33 (dual polarized). In the receive mode, the T/R 
polarization switch 31 is effective to direct energy received by the dual 
polarized front radiator 33 (via a low noise FET amplifier 35, the 
circulator 27 and the reciprocal phase shifter 25) to the rear radiator 
23. In the transmit mode, energy incident on the rear radiator 23 is 
passed (via the reciprocal phase shifter 25, the circulator 27, the FET 
amplifier 29 and the T/R polarization switch 31) to the front radiator 33. 
It should be noted here in passing that the impedance match of the rear 
radiator 23 to the anechoic chamber 19 (FIG. 1) becomes a consideration at 
the F/D ratio here achievable because any substantial mismatch may 
represent a substantial loss of power. Fortunately, because the array 
element 13 is an active device, loss of power is not a severe limitation. 
Further, the anechoic chamber 19 (FIG. 1) between the rear face of the 
lens 11 (FIG. 1) and the polarization-twisting array 15 (FIG. 1) assures 
that stray energy reflected from the rear face of the lens 11 (FIG. 1) 
will be well terminated, thereby avoiding the creation of regions of high 
sidelobes due to multiple reflections between the polarization-twisting 
array 15 (FIG. 1) and the rear face of the lens 11 (FIG. 1). 
Referring now to FIG. 3A, each element of the polarization-twisting array 
15 is shown to be made up of a horizontally polarized stripline flared 
notch radiator 41H and an orthogonally disposed, vertically polarized 
stripline flared notch radiator 41V. Such radiators, as well as the 
construction of an "egg-crate" array of such radiators, are described in 
detail in U.S. Pat. No. 3,836,976 issued Sept. 17, 1974, inventors George 
J. Monser et al, and assigned to the same assignee as the present 
invention. The feeds (not shown) for contiguous pairs of horizontally and 
vertically polarized radiators 41V, 41H are connected together to provide 
the polarization-twisting function. Thus, in the receive mode, vertically 
polarized energy received by the radiator 41V is passed directly to 
radiator 41H from which it is radiated as horizontally polarized energy. 
In the transmit mode, the converse holds. That is to say, horizontally 
polarized energy transmitted by the antenna feed 17 (FIG. 1) is reflected 
by the rear face of the lens 11 back to the horizontally polarized 
elements 41H. The energy received by such elements is retransmitted as 
vertically polarized energy from radiators 41V. Such vertically polarized 
energy traverses the vertically polarized rear radiator 23 (FIG. 2) of the 
array element 13 (FIG. 2) to be phase shifted, amplified and 
retransmitted. It should be noted here in passing that if time delay 
focusing of the energy from the polarization-twisting array 15 were 
desired, that function could be provided by controlling the electrical 
length of the interconnection between the feeds of the contiguous 
horizontally and vertically polarized elements 41H, 41V. 
Referring now to FIGS. 3B and 4, an exemplary one of the antenna feed array 
elements is shown to be comprised of a pair of orthogonally disposed 
flared notch radiators 43H, 43V connected together through a latching 
circulator 47. The latter is provided to interface the feed element with 
either a transmitter or receiver (neither of which is shown), while still 
providing the polarization-twisting function. That is to say, with the 
sense of the latching circulator 47 as shown, vertically polarized energy 
incident on radiator 43V will be passed to radiator 43H. Horizontally 
polarized energy from radiator 43H will then be reflected from the rear 
face of the lens 11 (FIG. 1) back to radiator 43H and then passed via the 
latching circulator 47 and a T/R switch 49 to a radar receiver (also not 
shown). In the transmit mode, the sense of the latching circulator 47 must 
be reversed so that the energy radiated from the feed element will be 
initially horizontally polarized as required for proper operation of the 
folded feed system. The sense of the latching circulator 47 may be 
reversed by simply reversing the applied magnetic field. As mentioned 
briefly hereinabove, the provision of the latching circulator in the feed 
elements allows the latter to retain their polarization-twisting 
capability, thereby eliminating the aperture blockage problems in prior 
art folded feeds employing polarization-twisting techniques. 
It should now be appreciated that compensation for the differential time 
delay between the feed elements and the elements (not numbered) of the 
polarization-twisting surface 15 (FIG. 2) resulting from the inclusion of 
the latching circulator 47 in the former must be provided. Such 
compensation may simply be in the form of an additional length of line 
between the orthogonally disposed contiguous horizontally and vertically 
polarized radiators 41H, 41V forming each of the passive radiating 
elements. 
Referring back now for a moment to FIG. 1, it is noted that in an X-band 
application a 30 inch diameter lens 11 was comprised of 1,605 elements 13 
and the antenna feed 17 was comprised of 31 feed elements (FIG. 4). 
Having described a preferred embodiment of the invention, it will now be 
apparent to one of skill in the art that many changes may be made without 
departing from the inventive concepts disclosed. Thus, for example, the 
latching circulator 47 (FIG. 4) provided between the horizontally and 
vertically polarized radiators 43H, 43V, respectively of the feed element 
may be replaced with a conventional circulator and a transfer switch to 
reverse the sense between transmit and receive modes. It is felt, 
therefore, that this invention should not be restricted to its disclosed 
embodiment, but rather should be limited only by the scope of the appended 
claims.