Page 362 - Maxwell House

342 Chapter 6

modulator for polarization measurements of the cosmic microwave background. We refer the
reader for details to the original publication [12].
6.8.3 Faraday Rotation Isolator

24
The WR isolator based on Faraday rotation schematically shown in Figure 6.8.2 and consists

Bias Magnet Ferrite Rod
Forward
Wave

Reflected by
Load Wave
Polarization
Absorbing Films
(R-cards)

Figure 6.8.2 Schematic of Faraday rotation isolator
of several parts: input and output WRs, the section of WC with centered ferrite rod, permanent
magnet magnetizing ferrite rod longitudinally, transitions converting TE10-mode in WR into
dominant TE11-mode of WC and back, and two very thin absorbing films called R(resistive)-
cards. The latter is a dielectric film like Kapton or PET (polyethylene terephthalate) with
inhomogeneously deposited the resistive layer of conductive absorber like graphite. Since the
card thickness ≪ , the vertically polarized E-field passes the card practically without loss
due to the induced in it voltage = ⋅ → 0. Meanwhile, the horizontally polarized E-field

induces in the same R-card much higher voltage = ⋅ due to w >> t and rapidly decays

propagating over the card. We take a few steps in our explanation:
1. The forward wave incoming from a generator or any other RF source is polarized vertically
(red vector in the left WR).
2. This wave continues its movement through the WR→WC transition keeping its vertical
polarization (longer red solid vector in WC). Therefore, the mutual orientation of E-field
and R-card guarantees low loss propagation of this wave.
3. The linear polarized TE11-mode in WC reaches the ferrite rod and splits into two CP
waves () with different propagation coefficients . The bias (typically bellow the
±
±
resonance to reduce external magnet weight) and the rod length are selected such way that
the wave E-field vector turns 45° counterclockwise as Figure 6.8.2 demonstrates for the
forward wave.
4. Therefore, the forward wave passes the second R-card practically with no loss and
transforms itself into TE10-mode in WR running to the load. Such transformation is smooth
due to the output WR is turned 45° too.
5. The return wave reflected from the not perfectly matched load is polarized vertically
relative to the output card (large blue dot vector). If so, it reaches the ferrite rod without
any difficulties and then splits into two CP waves () with different propagation
±
coefficients the same way as the forward wave.
±

24 Public Domain Image, source: http://slideplayer.com/slide/5695445/. Picture was updated by the
authors.