Sat antenna

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Quick and dirty, just to have something here.

Different antennas

Not all satellite antennas are using a parabolic dish; for example, satellite phones. For TV though, almost without exception, one uses a parabolic dish.

The parabolic dish is shaped to reflect and focus the signal from the satellite. It can be used to focus any signal, like sound, light, FM radio signal, and so forth. All telescopes, as well as most sound focusing equipment (like they often use from the sideline on football matches) uses the same principle. The free space loss of signal from the satellite is over 300 dB, a factor of 1×1030. Thus, an effective antenna is a necessity.

The formula to make a parabolic: y = x2

There are two major types of parabolic dishes:

  • The classic antenna which focuses the signals to the center of the antenna. This antenna uses the center of a parabolic arch for designing the dish. The signals are focused at the center of the antenna, and one usually has three rods extending out to the point where the signals are focused. This is where one puts the LNB. This is used for most professional installations, and virtually every antenna with diameter above 4 meters is of this type.
  • The offset antenna. The offset is a part of a parabolic arch as all parabolic dishes are, but by using the side of the arch rather than the center of the arch, one get an antenna that is easier to assemble, and the antenna has a lower elevation, which gives less problems with dirt, snow and other stuff ending up in the antenna. One also avoids having supports for the LNB obstructing the signal path.

Low noise block converter (LNB/LNC)

An LNB/LNC is the unit you put where the signals from a parabolic dish is focused. It receives the signals from the satellite, and converts them from a jaw dropping >10 GHz (Ku band) or >3 GHz (C band) to L band ~950 MHz to ~2,100 MHz, which is more compatible with coaxial cable. Some say that L band is less than 1,550 MHz, and some say 2,500 MHz. Wikipedia uses the word roughly.

The reason it is called a block converter, is that is takes a frequency block, and converts it down to a different frequency band. The mixing frequency in a LNB can be anything, but the usual frequencies are 9,750 MHz and 10,700 MHz (Universal LNB). The frequencies one can receive will then be 9,750 + 950 MHz to 9,750 + 2,100 MHz, and the same for 10,700 MHz mixing frequency. For other mixing frequencies, just change the 9,750 MHz number.

The LNB uses a very easy system to choose which mixing frequency to use. Since satellite operators want to utilize the frequency bands as efficiently as possible, you can also choose Vertical or Horizontal alignment (for C band it is circular polarisation, and you have right-hand circular polarization, RHCP, and left-hand, LHCP). This means that you have high and low frequency band (for dual band LNBs. They also come in single band), and you have vertical and horizontal, giving a total of four different blocks. low v., low h., high v., high h. For larger installations, the LNB will have one output for each of them, and you have a DisEqC switch that allows you to connect a virtually limitless number of tuners. The satellite tuner supplies the LNB with power, so that it can run the mixing frequencies circuits. The voltage that the tuner supplies chooses the polarization. 13 V selects horizontal, 18 V selects vertical. The tuner also supplies a 22 kHz signal to choose what mixing frequency to use. One can also control a DisEqC switch with this signal, so that a tuner can choose between multiple LNBs.

Finding the satellite

There are numerous ways of doing this. First though, you want to make sure that there are no buildings or trees between you and the satellite. Se below to help you find the direction.

The easiest way is using an instrument for this. It costs a bit, so you could try to do it without first. For digital receiver you want an free to air channel, cause the decryption of a channel makes it appear on the screen slower. Find one using lyngsat. You also want a compass. Using the Satellite Calculations page by Jens Sætre, you can find the direction the antenna is supposed to point azimuth (left/right) and the elevation (up/down). Please also make sure that your LNB is in the correct position. Having this wrong will make the adjustments much harder, maybe even impossible. Using the compass and the calculations, as well as the elevation indicator on the antenna (if you cannot find one, look at other installations nearby for a hint) you should move it about SLOWLY until you get a picture.

Pointing the antenna

This is the part that most people, also the professional installer, fails. Pointing of the antenna is important for signal level and quality. The bigger the antenna, the more important it is to point it correctly.

Here are the problems:

  • The antenna has a roll off. This means that if it is not spot on, but almost, there should be no problems, until you add the other problem.
  • The satellite is not stationary. It is situated in geosynchronous orbit, around 35,786 km above ground level, above the equator, and is affected by the sun's gravity, the moon's gravity, solar storms and so on.

These things causes the satellite to move about quite a bit. Seen from down here, it is not , but given a series of unfortunate events, it could be crucial for your satellite signal. Seen from earth, the satellite moves in the sky in figure of eight. It goes round this path once a day, in sync. with its orbit. What you need to do is to point the antenna while the satellite is in the centre of the figure of eight. If you point the antenna at one of the extremes, and at the same time miss it a bit, you will will end up with much weaker signal at the opposite time of day. Several dB is easy. I have seen more than 10 dB fluctuation on 1.2 m antenna.

The satellite's orbit is corrected every second week or so to compensate for all these forces moving it about. This means that there is not a specific time of day when the satellite is aligned correctly. You will have to calculate this. The reward though, is more stable TV signal, all day through.

So, how to do it?

You need ephemeris data (Intelsat ephemeris data), also called 11 parameters data. Intelsat is kind enough to supply centre of box data too, but not all satellite operators do this. The Satellite Calculations web page by Jens Sætre is great for this. Unfortunately you will have to input the 11 parameters yourself unless you use Internet Explorer, where you can upload the parameters as a text file. So, get ephemeris data from operator (if more than one satellite on that position, get the center most of them), input the data on the satellite calculations page, and calculate the centre of box times. Point your antenna at that time. For offset antennas, you can gain most signal level adjusting the azimuth (left/right). For traditional antennas, it does not matter.

  1. First, adjust azimuth. Using the signal-meter, move one way. When it starts dropping, stop, mark the spot, and go the opposite way until you find the same signal value. mark the spot. The best signal level is in the middle between those spots.
  2. Do the same with elevation (up/down).
  3. Then again, do azimuth, just to be sure.

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