SM 5 BSZ - Polarisation Control for the Receiver with Diode Mixers
(April 19 1997)
 The H and the L signals are the amplified signals from a pair of orthogonal linearly polarised antennas. The antennas may be cross yagis in X or + configuration. With the phase difference between H and L adjusted to zero once and for all, rotation of the polarisation plane means adding H*cos(u) and L*sin(u) where u is the angle between the desired polarisation plane and the horizontal plane. (L is also denoted V on this page.) It is easy to produce DC voltages corresponding to sin(w) and cos(w), where w is the turning angle of a potentiometer. Approximate sine and cosine function generator for use in a polarisation control system The H and L signals can be multiplied by the DC voltages representing sine and cosine for the potentiometer shaft angle with any electronically controlled attenuator, and the Schottky diode ring mixer is a suitable one since it automatically provides a sign reversal when the control signal reverses sign. I am using SBL1 from Mini-Circuits. A schottky diode ring mixer works as a four quadrant multiplier. The IF port works down to DC, the 2.2k resistor gives a suitable current at 5V for the mixer to still be a linear multiplier. The IF port is loaded by 50 ohms for the overtones of H or L. Combining the sine / cosine generator with two mixers gives a device that produces a signal A, corresponding to physically turning the polarisation plane by w: A=H*sin(w) + L*cos(w) The approximate sine / cosine generator goes from -90 to 90 degrees so it allows turning the polarisation plane through 180 degrees covering all linear polarisations provided the phase angle between the two input signals is very close to 0 (or 180) degrees. By the addition of two more mixers another signal B is obtained, and it corresponds to a polarisation plane perpendicular to w: B=H*cos(w) - L*sin(w) This way of rotating polarisation is currently in use in my 144MHz station - and it works fine. I do not often use the B signal, because parallel with this system I use a computerised adaptive polarisation system, and that provides the polarisation angle even from very weak signals on a LED display. Electronic transformation of the orthogonal pair H and V to a new orthogonal pair A and B. This transformation produces an electronic rotation of the polarisation planes while keeping orthogonality. The resistor and transformer is a hybrid arrangement producing the sum of two signals while isolating them from each other. The hybrids at the outputs can be omitted - they are for improved dynamic range, and this early in the receiver chain it may be unnecessary. To SM 5 BSZ Main Page