Sunday, 21 October 2012

Goggle Box (3)

We have looked at the terrestrial television provision on a boat. The next one is broadcast television through the use of satellite technology.

Satellite technology has moved on from my first experience of using "OSCAR" satellites (Orbital Satellite Carrying Amateur Radio) which were in (LEO) Low Earth Orbit. I would transmit a signal on UHF to a satellite passing overhead. The satellite would retransmit the signal on VHF back down to earth. I could communicate with fellow radio amateurs in other countries. The first amateur satellite, simply named OSCAR 1, was launched on December 12, 1961, barely four years after the launch of world's very first satellite, Sputnik II used an array of antennas that could track the satellite as it passed overhead. The commonly accepted definition for LEO is between 160 kilometres (100 miles) and 2,000 kilometres (1,200 miles) above the Earth's surface.

Today, broadcast television is transmitted from the ground up to powerful satellites that are in GEO (GEostationary or GEosyncronous Orbit) The satellites then retransmit the television signals back down to earth. geostationary orbit is a circular orbit 35,786 kilometres (22,236 miles) above the Earth's equator and following the direction of the Earth's rotation. An object in such an orbit has an orbital period equal to the Earth's rotational period and thus appears motionless, at a fixed position in the sky to ground observers. The GEO orbit also overcomes one problem with satellites in LEO. There is no need for an antenna system to track the satellite.

There is another change, which is away from using Yagi-Uda type antennas to one using a satellite dish. The change is because the television signals are no longer in the UHF television band. But are now broadcast on one of the microwave bands. The most common application of the "parabolic reflector" is in satellite dishes. A satellite dish is designed to receive the microwaves from satellites, which transmit broadcast quality images, such as television. Satellites are not limited to TV but are used for other kinds of data, such as the Internet, telephone calls and weather images.

The early transmissions from satellites required quite large antennas to be able to collect enough of the very weak transmission signals. Modern satellites are now use high power transmitters which means that satellite TV to the home now requires much smaller dishes. Times have changed and there are automated satellite locating systems available (at a not insignificant price) that can be installed on your boat. Which will automatically sense the satellite location and point the dish. However, I prefer to use the "Armstrong Method" and do it by hand, ear and eye.

Because the geosynchronous satellites are hovering in orbit over the equator, the satellites are not directly overhead for us. They appear to be much further down towards the horizon.  So one possible problem with satellite television on your boat is to ensure that trees or other objects are not blocking your view of the satellite. So you will need to establish the satellite position to ensure that you are pointing your dish in the right direction. For this you will need a boy scout with the orienteering badge.

There are one or two bits of kit that you can obtain to replace the boy scout to help you point the dish. The first is a basic marching compass. So that you can find which direction is South. The direction that we are most interested in lies between South East through South to South West. 

Now comes a critical bit that confuses a lot of people. The geostationary satellites which have to be parked over the Earths equator can only move either to the east or to the west if they are to remain stationary in our sky. From the due south position we could expect to receive signals up to about 40 degrees either side of due south. When you look up the data for a satellite it will be described as being at a specific location either East or West of south measured in degrees. Say 1 degree West or 13 degrees East. satellite due south of the UK would be at an angle of about 25 degrees above the horizon. The further east or west the satellite is, the closer to the horizon it will be. So you can see why we need to have a clear aspect towards the south.

There is little room for error when setting up a satellite dish. The beamwidth of a dish is very narrow. Mounting a dish on a boat can bring additional problems. The slightest movement of the boat can be enough to move the dish off the satellite.

But all is not lost. There is a simple solution to the problem. Mount the dish on a short pole with a weight on the bottom and place the antenna on the towpath. The boat may move about a little but the dish will stay in the same place. Unlike a terrestrial antenna a dish does not have to be high off the ground. Just a view of the satellite unobstructed by trees and buildings.

You can also purchase a small folding tripod which will do the job equally as well. If its a large dish, then some additional stability is gained from using tent pegs to secure the feet. One trick I have seen used is to use the base and bottom section of a picnic table parasol. The plastic container bottom being filled with sand.

I use a small electronic device (SLX Amazon £10) that lets me tune the dish to the correct satellite. As a signal is detected a tone is given out. The tone goes higher in frequency as you turn the dish onto the satellite. I just tune for a constant, loud, high pitched squeal.

Note: Television transmissions from the Astra satellites can be received on an 50cm dish. However, if you are wanting to include other satellite broadcast you may need a bigger dish.

Here is the Astra 2A, 2B, 1N footprint. Most of the UK falls within the 50cm dish range. The outer rings would require a 90 cm dish or larger.

I have created a poll that will run between now and the 1st of January 2013. It's not very scientific. Its a simple question "Will CART Succeed". You have three choices Yes, Unsure and No. Now you can vote for one of the three choices. If you change your mind before the poll closing date. You can come back and change your vote. (I wish we could do that for politicians and the Independent IWA members on Council) You will find the poll option on the right hand side just below the members pictures.


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