Separate systems is a good ideaWith two antenna relays and two preamplifiers mounted near the feed points of a pair of orthogonal antennas, the linear combining of the signals in order to achieve polarisation switching can be separately optimised for transmit and receive. On the transmit side a simple select one or the other, horizontal or vertical, will be satisfactory for stations running full EME power. With full power, a 3 dB loss now and then in transmit is not a serious limitation, and when you call CQ, marginal stations will call at times when polarisation is aligned - usually you would not have known what polarisation to select anyway. For the transmit side power handling capability is an important aspect, if two preamplifiers are used, two cables may be brought down for the transmitter so the polarisation switching for the transmitter can be placed at some convenient serviceable place without introduction of feeder losses on the receive side.
For the transmit side you may select one of these solutions Simple Polarisation Switching or if you want a really sophisticated system you may find some hints here: Using The Switchless Combiner for Polarisation Control
Once you have two cables with amplified receive signals from your cross yagis coming into your schack, you have the possibility to start simple and gradually refine your equipment without worrying about tower climbing, rain, ice and snow....
On the receive side, signal levels are low and some attenuation is allowed as long as amplification of earlier stages is high enough. It is easy to design very flexible systems using cheap "junk box" components and if you have more than one receiver, they may have independent polarisation adjustments.
Simple relay configuration
The figure shows how to use a non shorting relay. If you have a shorting one, the two cables have to be shortened to 0.25 and 0.75 wavelengths to make the unused one not load the used one. This circuit adds the two signals V and H in phase or in antiphase depending on the positioning of relay 1. By switching off the supply voltage to one of the preamps, the pure H or V signal will reach the receiver at a 3dB lower signal level. This is a very easy way of arranging linear polarisation switching in 45 degree steps. I have been using it several years with good results. The phase difference between the H and V signal has to be exactly 0 or 180 degrees. This will not happen automatically and some adjustment is needed. One way is to insert a bandpass filter in each cable V and H, possibly combined with an amplifier each. The phase is adjusted by slightly detuning the filters. Cutting one or the other cable will give large phase shifts in case the filters need too much detuning. For some practical hints look here: How to calibrate an adjustable polarisation antenna
This simple 180 degree phase shifter is the same idea as Two relays for the "x" configuration By adding a second relay, circular polarisations can be obtained. Such a second relay has to be shorting on one side and open on the other. The same idea is applied in: Three relays for the "x" configuration
For switching between different cable lengths in the receive system, PIN diodes are conveniently used. Also JFET switches, schottky diode and ordinary diode switches work fine. The requirements on attenuation and isolation are low for a rx polarisation switching system.
Continous polarisation rotation for the receiverWith 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 B*sin(u) where u is the angle between the desired polarisation plane and the horizontal plane. This can be done with any kind of electronically controlled attenuators, and one way is to use schottky diode mixers: Polarisation Control for the Receiver With Diode Mixers Of course two high frequency potentiometers and a phase inverter may be used for continuos polarisation adjustment, but it is less straight forward to get an angle readout and it is not easy to produce a simultaneous rotation of two perpendicular polarisations. If you want to have access to all possible polarisations, linear, elliptic and circular, a 0 to 90 degree phase shifter has to be added in H or L. I am using a dual phase shifter that shifts the phase difference between H and L by +/- 45 degrees. For details, look at: An electronic Phase Shifter for the Receiver
A switchless combiner may be used for continuos rotation of two orthogonal polarisation planes.
A switchless combiner may be designed from two hybrids and a dual phase shifter. In this way the design becomes symmetric, the phase shift of the two channels of the phase shifter is changed in opposite directions. The phase difference between the two channels of the phase shifter has to be adjusted from -180 to +180 degrees to allow rotation through 180 degrees for the polarisation plane. Electronically adjustable phase shifters may be designed as varicap controlled filters. Here is the one I currently use to vary the phase difference +/- 45 degrees for the H and V signals in order to go from linear to circular polarisation. An electronic Phase Shifter for the Receiver
The stereo receiverOnce you have amplified signals from two orthogonal antennas available in the shack, a two channel receiver such as FT1000 to bring the two independent signals all the way down to audio will allow a truly unpolarised receiver by the use of "the ultimate signal processor" which you have between your ears. The transverter needed to go from 144 MHz to 28 MHz has to have two mixers with a common oscillator. Adding a mixer and associated amplifiers to a conventional transverter is a simple task.
Listening in stereo will produce a loss of about 1dB compared to an antenna with perfectly matching polarisation. The advantage is that all signals are heard simultaneously regardless of polarisation - also Oscar class stations with circular polarisation come in at -1dB as compared to the correct circular polarisation. The advantage is in speed, which is a good thing particularly in EME contests. If you use a stereo system and hear a really weak signal, it is very easy to add or subtract the audio signals to produce a single signal corresponding to the matched polarisation. Then using the same signal for both ears will recover the lost 1 dB. It is also easy to combine the audio signals simultaneously in the orthogonal way to produce a signal containing noise only. Feeding the orthogonal signal to a computer running a FFT spectrum like FFTDSP by AF9Y will allow a precise control of polarisation - just by turning potentiometers for minimum on the screen. Even if the signal is weak - and turning for maximum is difficult due to the QSB, this method gives the optimum receive signal easily and fast.
Linear combination of audio signals
Two ganged potentiometers, both linear with R=1k ohm can be used to produce multiplication of an audio signal with an approximate sine and cosine for the turning angle of the potentiometer.
The figure shows an approximate way to translate the shaft angle of a pot to approximate sine and cosine coefficients which are multiplied by the incoming voltage. If the transformer has a step up of 10 in voltage, the attenuation of the resistor network is compensated. This circuit gives a straight line approximation, and for use in a polarisation control system this is insignificant, it causes a small amplitude error and a slightly non-linear degree scale on the pot. Of course an op amp can replace the transformer. What is necessary is a phase inversion so one pot can be fed with a "push pull" signal.
By use of two units with U=H and U=V respectively, it is easy to produce:
A=H*sin(u) + V*cos(u)
This result is obtained just by summing the output voltages, just connect the H*sin(u) and the V*cos(u) signals through one resistor each (at least 2.5k) to an audio amplifier. The negative sign needed for the B signal is obtained by moving the 2.5k resistor to the other side of the transformer for the V*sin(u) signal.
If you do not like the idea of a 4 ganged pot, just use two separate pots. Then there are two possibilities:
A=H*sin(u) + V*cos(u)
This is two independent pots, turning the polarisation independently for audio channels. The difference between the A and B is a 90 degree rotation, when u and w are separate pots it is better to make both have the same zero point by connecting both of them like the A pot. Of course you may use any number of audio channels if you are a multi operator station with several operators in a contest.
The other arrangement is the following:
A=H*sin(u) + V*cos(w)
Here the A and B signals are always orthogonal. If the pots are not turned equally, the audio level will change. This is the configuration to use together with a FFT spectrum analyser. Normal use is with H and V to each ear and the B signal to the FFT. If a weak signal needs improvement, turn the pots for the B signal to disappear on the FFT screen, and switch for the A signal to both ears.
In this section it has been assumed that the H and V signals have conserved their phase relation through the receiver, all the way to audio frequencies. This will not happen automatically, and a separate phase adjustment is the best solution. One convenient place for a phase shifter is in the local oscillator signal(s) feeding the first mixers. One possibility is this one: An electronic Phase Shifter for the Receiver If the phase shifter can vary the phase difference between the signals by 90 degrees, all polarisations can be obtained, from circular to linear.