As featured in Everything RF

 

Communication links from antenna often use dual polarization as it allows a doubling of the information capacity of a wireless channel. Circularly polarized waves can offer better propagation with reduced fading and multipath losses because it is possible to receive all orientations of the signal. Signals propagating in rectangular waveguide have linear E-field as the energy is in the TE₁₀ mode, so to transmit and receive circular polarization, via a conical or lens horn antenna for example, requires a polarizer. Ideal characteristics for such an instrument would be a good axial ratio (low cross-polarization components) over as wide a band as possible, with low VSWR and low residual insertion loss.

There are many ways to convert between linear and circular electromagnetic waves. Flann Microwave, for example, currently offers three different types of polarizer – namely the dielectric and compared below.

Dielectric Vane Polarizer

In the vane polarizer, a rectangular-to-circular waveguide transition includes, in the circular waveguide part, a dielectric vane of a length which give a 90° phase shift (λ/4 delay). This vane is called a quarter-wave plate. With the angle between the E-field of a linearly polarized wave and the vane set to 45°, the component parallel to the vane will be delayed by 90° relative to the perpendicular component, which suffers no delay. The result is equal orthogonal components 90° apart which is a mathematical description of circular polarization. Whether the vane is at +45° or-45° relative to the E-field arriving at it determines whether the circular polarization is left-hand or right-hand.

 

Figure 1 – Dielectric vane polarizer

 

In truth, the magnitude of the two orthogonal components will not be equal across the band and will only be so at either end of the range (see figure 2). The phase shift may also vary with frequency, typically up to 1 or 2 degrees. The combined effect of these variations can be seen in the axial ratio for the instrument (Figure 3).

 

Figure 2 – Through coupling (S₁₂ in dB) to the two orthogonal modes (magnitude only) for a WR-28 (WG22) dielectric vane polarizer (Flann model 22648)

 

Figure 3 – Simulated axial ratio for a WR-28 dielectric vane polarizer

 

Perfect circular polarization means an axial ratio of 0 dB, which is not achievable in practice and there is always some degree of ellipticity. The simulated axial ratio shown includes the limitations of the basic design, bit it does not include the effect of manufacturing tolerance which may introduce additional asymmetry. Examples of this asymmetry include non-perfect roundness of the circular waveguide section and misalignment of the flange.

The vane works in reverse, and circularly polarized waves arriving at the vane will have one component attenuated, resulting in a linearly polarized wave whose orientation will depend on the rotation of the circular wave (left or right hand).

Dielectric vane polarizers can be full waveguide band devices. In the example above full-band is 26.5 to 40 GHz, and the axial ratio is better than 0.9 dB (26 dB cross-polarization) over the whole range. The mode-suppressing vane (shown in Figure 1) has a conductive layer parallel to the E-field of undesired (cross-polar) modes to prevent their reflection back to the rectangular port. Use of the dielectric vane, which does have some loss, limits the power which can be handled by this type of polarizer. The vane is necessarily thin, and its power handling depends on how it is mounted.

Septum Polarizer

The Septum Polarizer uses a stepped septum to convert between linear and circular polarization. The instrument has two rectangular ports which combine at a stepped septum into a square cross-section waveguide (Figure 4). The action of the septum converts approximately half the energy to the opposite polarity with a phase shift of around 90°, so there are both TE₁₀ and TE₀₁ modes, the component parts of a circularly polarized wave. The step lengths and heights are optimised to give the best axial ratio over the target frequency band. A linear sloping taper would work, but steps offer better performance (more equal orthogonal E-field components and correct phase shift). Driving one rectangular port produces right-hand polarization and driving the other produces left-hand polarization, at the common port, which can be either square or circular.

The square size is set by the desired frequency range and therefore the rectangular ports are custom sizes too. The feed to the septum polarizer uses tapered or stepped bends from a standard rectangular waveguide size.

For circular to linear conversion (de-polarization), the component perpendicular to the septum splits either side of the septum, while the phase-shifted parallel component effectively enters single-ridged waveguide and propagates at a lower phase velocity. The resulting fields cancel at one rectangular port and add in the other depending on the sense of the incoming circular polarization.

The figures below show a WR-12 (WG26) septum polarizer designed for E-band operation between 71 and 86 GHz, with the simulated coupling to the orthogonal modes and the theoretical axial ratio in dB. Isolation between rectangular ports is better than 20 dB.

 

Figure 4 – Septum polarizer in WR-12 (WG26), with and without the E-field (HFSS simulation)

 

Figure 5 – Coupling (S₁₂ in dB) to the two orthogonal modes (magnitude only) for Flann model 26782 septum polarizer

 

Figure 6 – Simulated axial ratio, better than 0.4 dB over more than half the waveguide band

 

Corrugated or Iris Polarizer

Corrugated polarizers are able to produce right-hand or left-hand circular polarization from a linearly polarized input and can operate at relatively high power.

Each polarizer is optimised for the frequency band of interest and designs are generally suitable for use over about 40% of a waveguide band.

The guide cross-section is square with ridges (corrugations) on two opposite sides (the other sides are flat). This is shown in Figure 7. The ridges are the same in the two faces. A circular waveguide interface is provided at each port with a rectangular to circular transition at one end, forming the full corrugated polarizer assembly. The linearly polarized incident wave is launched into the square guide at 45° to the ridges, which results in it being split into TE₁₀ (parallel) and TE₀₁ (perpendicular) components. The ridges (which can also be described as irises, and the instrument as an iris polarizer) appear inductive to the TE₁₀ mode and capacitive to the TE₀₁ mode, meaning phase advancement and delay respectively. When phase offset between components equals 90°, there is perfect circular polarization at the output port.

The guide is somewhat overmoded and relies on mode-matching to achieve the best axial ratio over a wide band. Rigorous mathematical analysis of the corrugations is possible but complex. A suitable corrugation design can be achieved by optimising towards specific performance goals in a 3D-simulation tool such as Ansys HFSS. The aim is to keep the number of corrugations and instrument length to no more than is needed, be careful use of higher-order modes.

The figures below show a WR-90 (WG16) corrugated polarizer designed for wideband operation, with the simulated coupling to the orthogonal mode, the phase offset between them, and the axial ratio in dB. Although this instrument can operate over as much as half the waveguide band, depending on the degree of ellipticity considered acceptable, the instantaneous bandwidth of the signal to be polarized may be limited to a few hundred MHz due to the higher modes excited.

 

Figure 7a – Corrugated polarizer in WR-90 (WG16), showing the input polarization at 45°

 

Figure 7b – Corrugated polarizer with E-field (HFSS simulation)

 

In the example shown above, flanges can be arranged to allow the corrugated polarizer to be manually positioned for either right-hand or left-hand circular polarization (i.e., by turning through 90°)

 

Figure 8 – Coupling (S₁₂ in dB) to the two orthogonal modes (magnitude only) for Flann model 16651 corrugated polarizer

 

Figure 9 – Variation of phase offset between the two orthogonal modes

 

Figure 10 – Simulated axial ratio, better than 0.4 dB over 50% of the waveguide band, better than 0.2dB over 45% of the band

 

Improved axial ratio and bandwidth may be obtaining using a quad-ridged structure, but this makes both the design and manufacture more difficult. Other possibilities exist for introducing the inductive and capacitive discontinuities needed to make this type of polarizer work, and have been described in the literature, for example central ridges along the length of the waveguide or the use of dielectric posts.

Comparison of Instruments

Of the three waveguide polarizer types described, the dielectric vane polarizer is usually considered to be a full-band instrument which can offer reasonable axial ratio, while the other two types are usable over 40-50% of a waveguide band with superior axial ratio and higher average power rating.

Both septum and corrugated polarizers would typically be machined split-block devices, to allow for accurate manufacture of the internal features. There are possibilities for putting a radius on the sharp edges of the septum or corrugations to reduce the risk of field breakdown when there is a high peak field requirement.

The vaned polarizer can be electroformed as the internal features are the vanes, which can be put in place after making the body of the instrument. Manufacturing costs may determine the most suitable solution for a given application. As wavelengths go down into sub-millimeter territory, vanes become more difficult to make and mount, and the machined features of the other polarizers require expensive and very precise CNC machines. New ways in which polarizers can be made, such as using metamaterials, have been the subject of research in recent years.

 

Written by Peter Young