Dynamic Polarization Control

Dynamic polarization control is a method where the radiating structure can adjust its polarization dynami- cally, and entirely electronically to match the polarization of any receive antenna regardless of its polarization or orientation, as shown in Figure 4.3.

DPC Radiator

Horizontal Linear

Polarization _{Vertical Linear} Polarization Right-handed Circular Polarization Left-handed Circular Polarization Horizontal Linearly Polarized Receiver Vertical Linearly Polarized Receiver Left-handed Circularly Polarized Receiver Right-handed Circularly Polarized Receiver

Figure 4.3: Dynamic Polarization Control (DPC) radiator can switch between various electromagnetic polar- izations to match different receivers regardless of their polarization or spacial orientation.

4.2.1

Dynamic Polarization Control with Multiple Antennas

DPC using multiple antennas uses the transmission of two or more antennas with independent polarizations to create a third polarization that can change simply due to the input drives of the two transmitting antennas. One way to achieve DPC is to produce a linear polarization that can be dynamically controlled through the drives of two circularly polarized antennas, one RHCP and one LHCP. If they are equal in amplitude, then the resulting field is going to be linearly polarized, with a polarization angle related to half of the difference in phase between the two circularly polarized fields as shown below.

tudeE1andE2be defined as

E1=A[sin(ωt+φ1)uˆx+ sin(ωt+φ1+π/2)uˆy], (4.3) and

E2=A[sin(ωt+φ2)uˆx+ sin(ωt+φ2−π/2)uˆy]. (4.4) These fields have a similar amplitude (A), but have independent phases (φ1andφ2), and are both circularly

polarized in opposite directions. The combination in the far field is the sum of the two electric fields:

Etotal=E1+E2=A[sin(ωt+φ1)ˆux+sin(ωt+φ1+π/2)ˆuy]+A[sin(ωt+φ2)ˆux+sin(ωt+φ2−π/2)uˆy]. (4.5) This equation can be simplified by letting∆φ=φ1−φ2andΣφ=φ1+φ2, resulting in

Etotal= 2A[cos(−∆φ/2) sin(ωt+ Σφ/2)uˆx+ sin(−∆φ/2) sin(ωt+ Σφ/2)ˆuy] (4.6)

or

Etotal= 2Asin(ωt+ Σφ/2)[cos(−∆φ/2)ˆux+ sin(−∆φ/2)uˆy]. (4.7)

Thus,Etotalin theXandY dimensions are always in phase regardless ofφ1orφ2, and linear polarization

is always created. The magnitude of the field,

mag(Etotal) = 2A, (4.8)

which is also independent ofφ1andφ2. The angle in space of the field,

angle(Etotal) = ∆φ/2−π/2, (4.9)

which shows that the angle of the polarization is dependent only on the difference between the two phases of the original fields. Finally, the phase of the field,

phase(Etotal) = Σφ/2, (4.10)

means that the phase of the electric field can be controlled independently from the polarization angle or the amplitude of the field.

These results suggest that as long as the amplitudes of the RHCP and LHCP fields are equal, the resulting field will always be linearly polarized, as can be seen in Figure 4.4. The common mode of the phases of the fields will determine the phase of the output field. The differential mode of the fields will determine the

polarization angle. The common mode of the amplitudes will determine the amplitude of the output field, and if the amplitudes of the input fields are different from each other, the differential mode of the amplitudes will determine the axial ratio of the polarization.

X

Y

X

Y

X

Y

Figure 4.4: The superposition of two circularly polarized fields in the far field with opposite handedness (LHCP and RHCP) results in a linearly polarized field, whose polarization angle is determined by the phase difference of the original two circularly polarized fields.

In this way, if the desired output polarization is linear, but the polarization needs to be changed dynami- cally, as in the case of mobile antennas, full non-constant modulating signals can be sent through the common modes of the amplitude and phase, and polarization matching can be maintained by adjusting the differential mode of the phases.

Another way to accomplish this is to use linearly polarized transmit antennas (Figure 4.5), but in order to achieve a full 180◦range of polarization angles, the amplitudes of each antenna must be adjusted even when the desired amplitude of the output field remains constant. Using two circularly polarized fields reduces the requirements for the drivers of the antennas by keeping the phase and amplitude of the output field tied to the common mode of the input phase and amplitude and only requiring a phase shift between the antennas to produce the change in polarization angle.

X

Y

X

Y

X

Y

X Linear Polarization Y Linear Polarization _{Linear Polarization}

Polarization Angle

Figure 4.5: The superposition of two linearly polarized fields that are in phase will also result in a linearly polarized field, where the polarization angle is determined by the relative amplitudes of the original two linearly polarized fields.

4.2.2

Dynamic Polarization Control Through a Single Multi-Port Driven Radiator

A single port antenna in a linear medium has a fixed polarization at a given frequency as long as it is not rotated or otherwise manipulated mechanically. With only a single port at a given frequency, amplitude and phase of the incoming signal are the only controls available. The amplitude cannot change the polarization of an antenna in a linear material or it would violate superposition. A passive antenna in a linear medium is also time invariant, which means that phase changes also do not affect the operation or polarization of a single port antenna.

This fundamental limitation is lifted once more than one port is introduced. In the previous section, the second port was on a second antenna, but by using an MPD antenna, introduced in Chapter 3, additional ports can be utilized within the same antenna. The multiple ports enable the designer to set relative phases and amplitudes at different points on the antenna, which can lead to different current distributions, and thus can affect the radiated electromagnetic field and its polarization. The analysis of the MPD from Chapter 3 shows that a single spoke driven differentially produces a linear polarization aligned along the spoke. By combining these polarizations at different amplitudes and phases, the polarization of the resulting electromagnetic field can be controlled, as will be discussed in the following section.

4.3

Proof of Concept of a 2x1 Multi-Port Driven Radiator Array with