Multipurpose Submarine Antenna

This invention relates to antennas and more particularly to a multipurpose submarine antenna.

Multipurpose submarine antennas are constructed to handle several types of radiations simultaneously. As a result, these structures have a relatively high packing density and space within the structure is at a premium. In particular, one prior art submarine antenna contains as one of its radiators a short, linearly polarized "door knob" fat monopole to handle IFF (identification friend or foe) transmissions. There is no more room within the structure to fit an antenna to receive GPS (global positioning satellite) signals and to compound the problem, the GPS signals are circularly polarized. In addition, GPS transmissions require antenna coverage for all angles in the upper hemisphere extending from +20.degree. to overhead. This means that even if one were willing to take the polarization loss of the present IFF antenna, the pattern loss would be prohibitive at high angles.

An object of the present invention is to provide a multipurpose antenna to provide simultaneous IFF and GPS antenna radiation to circumvent the problems mentioned hereinabove.

Another object of the present invention is to provide a multipurpose submarine antenna providing IFF and GPS antenna radiation without sacrificing gain due to pattern and/or polarization loss and to perform this in the same space now occupied by the prior art IFF antenna alone.

A feature of the present invention is the provision of a multipurpose submarine antenna comprising: an IFF antenna disposed symmetrically about a vertical axis; first means coupled to the IFF antenna to symmetrically feed the IFF antenna; a GPS antenna disposed coaxially of the vertical axis and above the IFF antenna; a gap having a predetermined width disposed between the top of the IFF antenna and the bottom of the GPS antenna; second means disposed coaxially of the vertical axis and within the IFF antenna to feed the GPS antenna; and a conductive shield disposed coaxially of the vertical axis surrounding the second means and interconnecting the IFF antenna and the GPS antenna for isolation therebetween.

Above-mentioned and other features and objects of this invention will become more apparent by reference to the following description taken in conjunction with the accompanying drawing, in which:

FIG. 1 is a cross-sectional view of a multipurpose submarine antenna in accordance with the principles of the present invention; and

FIG. 2 is a cross-sectional view taken along line 2--2 of FIG. 1.

The linearly polarized IFF antenna 1 and the circularly polarized GPS antenna 2 are disposed symmetrically and coaxially about a vertical axis and arranged in a common structure. GPS antenna 2 is disposed above IFF antenna 1 and includes a flat ungrounded spiral 3 located in a cavity 4/ground plate 5 assembly to produce and receive circularly polarized radiation. Spiral 3 is centrally fed at connection 3a by a three wire balun 6 which incorporates a shield 7 (FIGS. 1 and 2) that connects ground plate 5 to the conducting plate 8 of IFF antenna 1. Shield 7 spaced from balun 6 forms part of the antenna isolation mechanism between the GPS antenna 2 and the IFF antenna 1.

A non-conducting standoff 9 is employed immediately below ground plate 5 of the GPS antenna 2 to prevent shorting the tapered split cylinder 10 (FIGS. 1 and 2) of IFF antenna 1 to ground plate 5. A gap 11 is provided between the GPS antenna 2 and the IFF antenna 1 to enable implementing a glassing process discussed hereinbelow and to aid in the IFF antenna matching. Once the width of the gap 11 has been predetermined, this width is fixed.

IFF antenna 1 includes a split tapered cylinder 10 (FIGS. 1 and 2) over a conducting plate 8 to produce and receive linearly polarized radiation. The split 12 of cylinder 10 is illustrated in FIG. 2. The reason for providing this configuration is because two in-phase feed points are required to symmetrically drive IFF antenna 1 in order to allow for three wire balun 6 to be located in its illustrated central position. The two symmetrical feed points 21 and 22 are driven in a balanced manner by the IFF strip line power divider 13 through transmission lines 19 and 20. The strip line construction of power divider 13 will allow the GPS transmission line 14 to pass through the center of power divider 13 if desired. Transmission line 14 is supported in plate 8 by a dielectric sleeve 14a. The split tapered cylinder 10 and conducting plate 8 are electrically isolated from each other by standoff 15. A non-conducting sleeve 16 (FIGS. 1 and 2) is provided between shield 7 and split tapered cylinder 10 to provide electrical isolation therebetween and, hence, electrical isolation between antennas 1 and 2.

In order to enable the multipurpose antenna structure of the present invention to operate in a free flooding environment, such as when the submarine dives, the combined structure is filled with a dielectric material 17 and 18, such as glass, and sealed to the respective metallic members of the multipurpose antenna structure by a process similar to that used to make glass to metal seals. The purpose of dielectric material 17 and 18 is to prevent the sea pressure during a submarine dive from forcing sea water into the antenna and transmission lines associated therewith. This is the glassing process mentioned above.

The driving point impedance for each arm of the power divider cannot be determined by any other means but actual measurement. A major reason for this is the fact that the base diameter of split cylinder 10 adjacent plate 8 is not negligible compared to the major diameter adjacent GPS antenna 2 and so there will be appreciable base capacitance. This is the reason for gap 11 previously mentioned. Since IFF antenna 1 is made approximately 1/4 wavelength long, the gap capacitance can be adjusted (much as in transmission line matching) to negate the base capacitance since the capacitors are interconnected by balun shield 7.

While I have described above the principles of my invention in connection with specific apparatus it is to be clearly understood that this description is made only by way of example and not as a limitation to the scope of my invention as set forth in the objects thereof and in the accompanying claims.