AGC signal strength
AZ Azimuth | REL: Relative Azimuth | EL: Elevation | POL: Polarization | Linear Polarization | Circular Polarization | Crosspol or Copol | Parabolic dish | AGC | BUC | LNB Low Noise Block DownConverter | VSAT - Very Small Aperture Terminal |
Obviously, the azimuth of the antenna must match the azimuth of the satellite, or it will not acquire the satellite. This satellite azimuth depends only on the satellite longitude and the position of vessel.
The most common reasons for an antenna not finding the satellite is that it is pointing to the wrong place in the sky. This could be due to compass error, antenna alignment errors, the wrong ship position from the GPS, or the wrong satellite longitude selected.
The Azimuth is counted clockwise from North through 360 degrees. The Azimuth does not change unless the ship moves to a different location. The azimuth stays the same as the ship turns.
The Relative is counted clockwise from the bow through 360 degrees. The Relative changes with heading, as the ship turns.
(See limited azimuth TV antenna Relative description)
Zero degrees is looking at the horizon and 90 degrees is looking at the zenith (or directly overhead).
Polarization is often abreviated to "Pol" ( spelled without the E but pronounced like pole).
In order to re-use the same frequency for more than one signal, the satellites employ different polarization techniques.
Two types of polarization are used on the satellite; Linear (Vertical or Horizontal) and Circular (RHCP and LHCP).
With linear polarization, the satellite can transmit one signal at a frequency of say 11592 MHz, in the horizontal plane and then transmit another signal using the same frequency in the vertical plane and this doubles the capacity of the satellite. The receiving antenna at the other end can discriminate beween the two pol signals and receive just the required signal on the pol of choice, or receive both signals on two separate, vertical and horizontal LNBs.
The same applies to circular polarization where one signal may be transmitted in a spiral pattern in a clockwise or right hand (RHCP) direction and a second signal in a spiral pattern in a counter-clockwise or left hand (LHCP) direction. The receiving equipment at the other end can discriminate between the two pols and use either one, or both of the signals.
There is typically more than 30 dBs of isolation between a horizontal and vertical signal, or the LHCP and RHCP signals, so the two will not interfere with each other when they arrive at the receiving antenna.
Linear polarization transmits the signal in two different planes, either horizontal or vertical.
With linear polarization it is important that the receiving antenna is closely aligned with the transmitting antenna by rotating the feed, in order to maximize the signal on the required pol and eliminate the signal on the opposite pol.
Correct polarization adjustment is extremely critical on transmitting VSAT terminals to avoid transmitting and causing interference on the opposite pol. When comissioning a VSAT terminal a crosspol isolation lineup is conducted with the teleport to ensure that there is at least 30 dB of isolation on the opposite pol.
Most Ku-Band VSAT systems use linear polarization and most transmit on one pol and receive on the opposite pol referred to as crosspol. Some satellites in the Asian regions transmit and receive on the same pol or Copol.
Most Ku-Band TV satellites around the world use linear polarization. In the USA TV networks like DIRECTV or DISH use circular polarization which eliminates the need to align the polarization skew with the satellite.
C-Band satellites use either linear or circular feeds. Most Ku-Band TV antennas use a universal linear LNB. The TV satellites in USA for DIRECTV and DISH Network use circular polarization. This eliminates the need for the antenna pol skew to be aligned with the satellite.
With any reflective surface, the angle of incidence is equal to the angle of reflection. Whatever angle the signal hits the dish it will bounce back off at the same angle.
The shape of the parabola is such that the signals coming in from the satellite are reflected off the surface toward a common focal point.
In the same way, the transmit signal from the feed toward the dish is reflected back in a parallel beam toward the satellite.
The term originates from a reference voltage in radio receivers that would automatically turn down the gain of the receiver as the signal increased. Today in satellite terms it refers to the strength of the signal.
On a SeaTel antenna, for example, you may have an AGC of 1400 when you are off the satellite, and 1800 when you are on the satellite. The importance is the difference between these two levels and not necessarly the actual level.
It contains an upconverter that changes the frequency from the IF frequency (usually L-Band) up to the RF frequency (C-Band, Ku-Band or Ka-Band). It also has the final power amplify that boosts the signal to the transmit power level. It is called a BLOCK up converter as it converts the entire block of signals from the IF to RF frequencies. (A regular up converter would only convert a narrow band of the required signal to RF frequencies).
It is most often called a BUC (pronounced like buck) but some perfectionists refer to it as a B.U.C (like bee you see)
Super high radio frequencies, like Ku-band and C-band, are extremely sensitive to losses and noise, thus requiring sophisticated and expensive equipment and cables to process and transport the signals.
The LNB, converts the higher frequency radio signals, recieved by the dish, to lower, intermediate frequencies, that can be handled and transported using lower cost equipment and cables, down to the satellite receiver, or modem.
LNB stands for Low Noise Block converter, because it converts the entire block or range of Ku-Band or C-Band signals received from the satellite, down to a range of IF frequencies, usually L-band(1.5Ghz) in the case of Ku-band systems, and 70Mhz in the case of C-Band systems.