As many operators of satellites will know, it’s common for a satellite to fade out during a pass, even when the satellite is well above the horizon. There are a few factors behind this, of which I’ll not go into here, but the reason behind the fading is the polarization of the satellites downlink is changing as it moves across the sky with respect to your antenna, think cross polarization. The same effect also impacts the uplink to the satellite, but the uplink and downlink are not linked, i.e. they are independent of each other.
Most satellite stations on VHF (2m) and UHF (70cm) comprise of a crossed-yagi type antennas which have elements in the Horizontal and Vertical plane, usually each of the driven elements, one in the H-plane and one in the V-plane, are fed with some amount of phase offset in order to make the antenna polarization RHCP (Right Hand Circular Polarization) or LHCF (Left Hand Circular Polarization). However as mentioned before, the satellite will appear to change its polarization with respect to your antenna as it moves across the sky, only spending some of its time in the fixed polarization of RHCP or LHCP.
Changing the phase at which the RF is delivered to each of the antennas driven elements can change the polarization of the antenna, so the simple method to counteract the fading seen as a satellite passes overhead, is to keep change the phase delivered to the driven elements. This can easily be done remotely using relays.
I must state at this point, much of my work on this project was taken from the information published by Howard G6LVB here; http://www.g6lvb.com/remotepolarization.htm
Above is the diagram which shows how changing the phase to each of the driven elements in a crossed-yagi can result in a different polarization. Also note that it only requires 3 wires to do this, Ground, Relay 1 control (RL1) and Relay 2 control (RL2). Below shows the coax needed to make a phase switch box for 2m and 70cm.
To make the job of calculating the coax lengths needed easier, I created a Microsoft Excel sheet (.xlsx). This calculates the coax lengths in mm based on the frequency of operation and velocity factor of coax entered. It can be downloaded here: http://www.geekshed.co.uk/files/G6LVB_Phasing_Length_Calculator_v1.0.xlsx
The reasons “L” is 2cm is that is the space between the 2x MD-951 relays I used (left image). However I think I maybe should perhaps have used 5 cm, as there is 3cm of 50-ohm path inside the relay (right image) which should be taken into account – Although VF is shown as 0.66 for this section, I’m not exactly sure what the velocity factor of this actually is, but it can be calculated, which is something I’ll do if I build another system.
To make the phase switch box you can use any form of relay, but try and use a type known to be good at the frequency you plan to use it on. I’ve made 2 of these phase switch boxes now, the first used Schrack RTE24012 relays and worked fine for 2m, a little down on 70cm but done the trick and met the required target losses for the build.
The 2nd unit used MD-951 relays and was slightly better at 2m and 70cm.
The coax between the relays was coiled up and it all fitted into an outdoor masthead box, they type you normally find preamps in.
The control box is made up of 2×4 position rotary switches, one for each band. The 4 positions provide Vertical, Horizontal, RHCP and LHCP choices. As the ground is shared between each band, only 5 wires are actually needed. Ground, Relay 1 VHF control, Relay 2 VHF control, Relay 1 UHF control and Relay 2 UHF control.
A bit of added weight! A fishing lead weight.