I refer to Riihimaa 1967:
and post a copy of Equation 1.
The number of Faraday rotations is inversely proportional to the square of the frequency.
Therefore, the number of rotations at a frequency f1 is:
Number of Faraday rotations = f1^2/(f2^2-f1^2)
where f2 is the frequency of an adjacent band , f2>f1
If the Faraday modulation bands are centered at 20 and 20.3 MHz, the number of rotations is 33.1
If the angle between the magnetic field direction and the line of sight is close to 90 degrees, the number of rotations will be close to 0 and the Faraday modulations observed with a linearly polarized antenna may disappear.