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Satellite TV: Multi-Satellite

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Disq and Motorized Dishes

A conventional motorised system uses one Dish with one LNB. The dish is rotated on a polar mount by a motor. The Analog or Digital satellite receiver has the motor control (on newer systems using 22KHz tone bursts called Disq) built in. Each channel has a menu option to select which "satellite". In addition a setup menu stores the dish position for each "satellite".

A Sky Digibox has no motor control or menu software.

Even with a multiple output LNB and multiple receivers you can only tune into one satellite at a time.

Fixed dish and multiple LNBs

If you have a Digibox and an 80cm or 90cm dish (perhaps off an older analog system you can mount multiple LNBs on it to either feed several receivers at the same time or one receiver via a change over switch arrangement.

This works because as you move the LNB east the reflection source is further west, so if the main LNB on the arm is aligned for 19E then to the right (looking from LNB to dish) or East gives the further west of 13deg.

Due to the width of the LNB and the focal length of typical 80 or 90cm dishes, 13 and 19 degrees are side by side (approximately). A 120cm dish will let you "see" satellites only 3 degrees apart. The closest spacing at 90cm is about 5 degrees.

Because of the distance north we are at from the equatorial plane the satellites orbit on, the LNB must be lowered as you move it East and raised as you move it toward zero degrees (assuming the dish is lined up on 19E).

As you move away from the true focus to "see" a satellite further east or west of the aligned position the signal is reduced. However it is not much reduced for 6degrees. An 80cm 19E dish with an offset LNB for 28.2 Degrees East (mount an LNB further to Left or West), has quite a reduced signal compared with the main LNB, but because the dish is so much bigger than the Sky Mini dish, the Sky Digital signal is improved.

View of three LNBs on one dish

28.2 E ---------19.2E -- 13E

Sky Digi---SkyAna--HotBird

Conclusion

If you have an 80 or 90 cm dish and more than one satellite receiver then the multi-LNB approach is good. If you want to watch 5 or more satellites, or satellites at very different angles, then you may need a specialist receiver with Disq motor control and a professional motorised polar mount. For many satellites you may need a 120cm dish rather than the smaller sizes.

DIY changeover for LNBs

A manual changeover switch is cheap but needs two coax feeds to the living room. A specialist remote switch at the dish is usually Disq operated, which the Sky Digibox won't do. My solution is a change over relay switch at or near the dish with a control wire to the Digibox Scart or other control switch in the living room. See Making an LNB changeover relay switch.

Interfacing the SCART

The Digibox Scart output on/off is 0V for off and 5V or 12V for on, depending on the conditions. It can't directly drive the relay. Your need a 9V or 12V power supply (a small plug top adaptor will do), two resistors and one transistor. Total cost less than £5. Detials soon

DIY motor drive and Polar Mounts

If you want to, you can make a motorised polar mount using an old £10 car wiper motor to drive it. An old redundant 286 or better PC with a joystick port (or sound card with joystick port) can control it. One axis of the "joystick" is an optically isolated sensor mounted on the dish to measure the position. The other axis is a knob marked in degrees in a little box beside your viewing chair. The PC drives the motor left, right or off from relays connected to the parallel port. The program operates the motor to match the two joystick signals. The motor supply needs to have two limit switches with diodes across them to stop the motor but allow it to reverse in case the software has a problem!

I'll publish detailed plans later.

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