High frequency radio frequency (RF) communication and radar systems typically use a phased antenna array to control the direction of the electromagnetic transmission. Phased array antennas are inherently narrow band antennas in which the scan angle varies as a function of the true time delay or phase delay between the microwave radiation from each adjacent antenna element.
In order to control the beam direction of the transmission, the previously known scanning antennas have utilized feed networks that vary either the phase or time delay between the feed point for the antenna and the individual antenna array elements. A broadside or undeflected beam occurs when the input signal reaches the individual antenna array elements at the same time and phase. In practice, the beam direction can be varied ±θ degrees off center from the broadside direction by varying the phase or time delay of the signal to the individual antenna elements.
In order to control the direction of the beam transmission from the antenna, many of the previously known antenna arrays have utilized variable phase networks wherein one network is connected between the signal input to the antenna array and each antenna element. These previously known antennas, however, have not proven wholly satisfactory in operation.
One disadvantage of utilizing variable phase networks to control the beam direction for the phased antenna array is that the variable phase networks are expensive and this expense increases dramatically as the number of antenna elements increases.
A still further disadvantage of these previously known variable phase networks is that the previously known systems have utilized switches to selectively connect transmission line segments between the signal input to the antenna and the various antenna elements. Since each transmission line section introduces a preset time delay or phase shift to its associated antenna element, the deflection of the beam from the broadside beam direction is limited to a number of discrete angles relative to the broadside beam direction. Furthermore, signal losses associated with these switches are unacceptable for many high frequency applications, i.e. applications where the wavelength is in the millimeter range, such as 35 gigahertz.
A still further disadvantage of these previously known variable phase networks is that the circuitry necessary to effect the variable phase, particularly when a high number of antenna elements is involved, is necessarily bulky in construction. In many applications, for example when the antenna is used in an aircraft, the space requirements for these previously known systems exceed the available space limitations of the aircraft. This, in turn, necessitates undesirable compromises in the utilization of the available aircraft space.