Communications: directive radio wave systems and devices (e.g. – Radar transponder system – Radar transponder only
Reexamination Certificate
2001-12-27
2003-12-02
Sotomayor, John B. (Department: 3662)
Communications: directive radio wave systems and devices (e.g.,
Radar transponder system
Radar transponder only
C342S175000
Reexamination Certificate
active
06657580
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to transponders and in particular to transponders using antenna-based retro-reflectors, and to positioning/tracking systems utilising such transponders.
A retro-reflector is a device which reflects incident radiation falling upon it back upon its incident path. It is known to use a retro-reflector as the basis for an active or passive transponder; the rationale behind this is the establishment of a communication link by reflecting an interrogating signal back towards its source, thus eliminating the need for a transmitter and its associated equipment at the target.
In the application of radar to terrestrial traffic control there is an increasing need to detect, track and identify craft within the surveillance area. This is of particular relevance to harbour and airport traffic management, particularly considering small vehicles with inherently low radar cross sections (RCS). At present, simple retro-reflectors comprising corner structures are sometimes employed on small boats to increase their RCS. Clearly, a requirement of future transponders employed in these fields are simplicity, light weight, low cost and low power consumption.
Long range identification is traditionally achieved by actively transmitting a response to an interrogating radar. Such a method is employed by the military “Interrogate Friend or Foe” (IFF) system. However, a disadvantage of this approach is the requirement for a transmitter and signal processing equipment on board the target. Also, the response signal is transmitted omnidirectionally, which requires a transmitted power which is excessively large to establish the communication link and also pollutes the local radio environment.
It is known to use a retro-reflective antenna array in a transponder. Such arrays are based on the Van Atta array first described in U.S. Pat. No. 2,908,002. The Van Atta array comprises a single or multi-dimensional array of antennas in which antenna pairs are joined by transmission lines of equal electrical length.
FIG. 1
of the accompanying drawings shows a single-dimensional Van Atta array comprising six dipole antennas
1
to
6
. The dipoles are linked in pairs by means of respective transmission lines as follows:
Dipoles
1
and
6
linked by transmission line
7
;
Dipoles
2
and
5
linked by transmission line
8
; and
Dipoles
3
and
4
linked by transmission line
9
.
The transmission lines
7
,
8
and
9
are all of equal electrical length, and it can be shown that the result of such linkage is that an electromagnetic wave front A to F incident on the array will be reflected away from the array back along the direction of incidence.
Consider a plane wave incident at an angle &thgr; to the x-axis. If we take points A to F in the incident wave front, energy from point A is received by antenna
1
and re-radiated by antenna
6
, energy from point F is absorbed by antenna
6
and re-radiated by antenna
1
, and so on for the other antenna pairs. The path lengths AF, BE and CD are all equal and the radiated power from each antenna therefore adds constructively in the direction &thgr; i.e. the reflected wave is in the same direction as the incident wave.
A problem arises in practical use of the Van Atta array because of the requirement that all of the transmission times have to be of the same electrical length. Fabricating a two-dimensional planar array, for example, is next to impossible if the extra complication and expense of crossovers is to be avoided.
BRIEF SUMMARY OF THE INVENTION
In a first aspect of the present invention, this problem is avoided by providing that the length of the transmission lines interconnecting the individual antennas in the array are of a length l given by:
l=A±n
&lgr;
where:
A is an arbitrary length;
n is zero, or an integer;
&lgr; is the wavelength of the electromagnetic wave to be retro-reflected.
If n=0 for all transmission lines then this, in effect, defines a Van Atta array since the transmission lines in such an array are all of an equal arbitrary length. Therefore, it is further provided in the present invention that at least some of the transmission lines have a non-zero value of n. It is therefore inherent in the present invention that not all the transmission lines have the same length.
The elongation of any one of the transmission lines
7
to
9
by an amount equal to a multiple of the wavelength of the incoming signal means that the outgoing signal, when it reaches the plane A-F, will have the same phase as it would have had if the corresponding transmission line had not been lengthened. Therefore the power from each antenna still adds constructively in the direction &thgr;, and the array continues to operate in a retro-reflective manner.
However, the arrangement is now frequency sensitive because the outgoing signal will only arrive in phase at plane A-F if the extra length of the interconnecting transmission line is a multiple of the incoming wavelength. This might be perceived as a disadvantage but in fact is often actually an advantage because, particularly in security or military applications, it provides an extra, albeit small, degree of security. In practice, it is not a difficulty that a single interrogating frequency has to be used, particularly when it is borne in mind that the antennas themselves are already frequency selective. Furthermore, the device can act as a filter returning to an interrogating transceiver only a single frequency or, in practice, a passband of frequencies.
The use of unequal length transmission lines, as described above, enables two-dimensional retro-reflective antenna arrays to be fabricated onto any of the known planar media, such as microstrip, stripline, or dielectric or optical waveguides but would also enable arrays to be built in conventional cavity waveguides. Even if the array is non-planar, the ability to make some of the transmission lines of different lengths to others considerably eases the physical design problem of interconnecting pairs of antennas in a two dimensional array. In fact, the invention gives almost complete freedom as to the length of the transmission lines which greatly assists in the design process. Clearly, however, there is an upper limit on length above which transmission line losses will become unacceptable. Also large length differences will give rise to differentials in the loss characteristics as between different transmission lines which may degrade operation if carried to excess.
Given the above constraints, the length A can literally be chosen arbitrarily and can even be zero, but a zero value would impose a pointless limitation on the transmission line lengths, namely that they would all have to be equal to a multiple of the wavelength. More sensibly, the length A may be chosen to be either the smallest or the largest convenient physical length, and the remaining lengths will either be the same as the arbitrary length or, where this is not possible due to physical or other constraints, will be greater or smaller than the arbitrary length by an amount equal to an integer multiple of the wavelength. Thus it is possible for any one array to have just one or two transmission lines which are different in length from the rest or, at the other extreme, every single transmission line may have a different length all subject, of course, to the length formula quoted above.
In an embodiment of the invention, means are provided for modulating the retro-reflected signal so that the target may identify itself to the interrogator. Conveniently, said modulation means comprises a modulation device connected in at least some, and preferably all, of the transmission lines in order to modulate the signal passing from one antenna of the array to another via the transmission line. Each modulation device may comprise active or passive circuitry.
The modulation means may be realised by any device which is operable to modulate the signal so that modulation products are produced in the frequency domain. Thus the modulation may comprise switching in t
Edwards John David
Thornton John
ISIS Innovation Limited
Sotomayor John B.
Volpe and Koenig, P.C
LandOfFree
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