Satellite Loop Primer
Kindly written for UHF-Satcom.com by Mr. N. Syder from the USA.
A satellite loop is a test used by a technician to test out a satellites transmit and receive path all at once. This test is normally performed on Geo synchronous satellites but also on some polar orbiters. More than once a technician has received a complaint that the satellite circuit is down and that they need to figure out what the problem is. When you have the dish, an outage is always your fault until you prove otherwise. This test is used by commercial and military stations.
A little background is needed here; Most satellites have transponders configured to downlink from the same area where the uplink signal was received. This could be a spot beam, a global, or hemispheric coverage transponder. The transponder needs to be configured this way or you will not get back the signal you send up. They also need to be configured so that if you go up C band it comes back down C band, or Ku up / Ku down. There are some satellites that are operating in a cross strap mode. For example, here in the States, you go up C band, but in Europe, it comes down Ku band.
Something that a VSAT operator needs to know to perform a satellite loop is if their terminal is a single or dual oscillator unit. A single oscillator unit means that 1 oscillator is used for the transmit and receive halves of the unit. This means that the transmit and receive frequencies need to be in the same 40 MHz slice of RF spectrum. A dual oscillator unit means you can set the transmit and receive frequencies independent of each other.
Most satellite providers provide a transponder layout for users to consult. Lets walk through one to see how it is done:
Click on the above link, the click on Networks, then under networks click on satellites. Next, click on coverage maps. You can wait for the opening to play out or click on skip. Now find IA-5 at 97 degrees west. Click on it. Now look in the lower right corner where it says “Space craft performance / Transponder frequency”. Click on it, a PDF file will open up. This file will tell you a whole lot about the satellite and its transponder layout. You will also notice that they were kind enough to furnish info on all of their satellites in one file. You might want to copy it to disk or print it out. Here are some more satellite owner/operators where you can get a lot of the same info:
That is the short list; there are others to be found using your favourite search engine.
Now that you know the transponder layout, you need to know what the transponder translation frequency is. What’s that? It is the frequency that is mixed with the uplink frequency to change it into a downlink frequency. Satellites are nothing more than a mixers/repeaters in the sky. Most C band satellites use 2225 MHz, for Ku it is normally 2300 MHz. Those are the standard, but you will run into exceptions to the rule. Remember that info you saved? Check it for non-standard translations.
Lets figure out a up and downlink using a known translation. Say you put up a signal at 6125 MHz, where do you look for it at?
6125 MHz = (C band TX freq)
- 2225 MHz = (C band Receive translation) 3900 MHz = (C band RX freq)
So you would look at 3900 MHz for your downlink. If you are given the receive frequency, add 2225 and it will give you the uplink frequency.
Before a tech transmits to a satellite, he will check to make sure his dish pointing is correct and polarity are correct. Next he will choose the correct IF. (Intermediate Frequency) and RF (Radio Frequency) frequencies to transmit and receive his own signal. A VSAT terminal needs to be looked at and determine what kind of IF it uses. Does it use 70 or 140 MHz, L band, or RF. from an up converter? Most modern VSAT terminals use L band for transmit and receive IF., The next most common is the 70 or 140 MHz style. It is rare nowadays to run into a VSAT package that uses RF in and out.
Here's how 70 or 140 MHz IF input works: if you are dealing with 70 MHz, you have a 40 MHz wide swath of RF, from 50 – 90 MHz, that he VSAT package can use. Normal C band transponders are 40 MHz wide, so the up converter RF frequency that is in a VSAT terminal is set for the centre frequency of the transponder in use. So if your VSAT TX RF frequency is the centre of a transponder, then 70 MHZ equates to the centre frequency of the transponder after conversion.
For example, you are using 70.00 MHz as an input, the RF centre frequency in the VSAT TX is 6125 MHz, your RF output is 6125 MHz. If you input is 65 MHz, your RF output would be 6120 MHz.
140 MHZ input works the same, except you are now working with an 80 MHz wide IF bandwidth, from 100 MHz to 180 MHz.
If the terminal uses the 70 or 140 MHz IF. in, you can set it the centre RF freq (TX and RX) on the VSAT terminal and use 70 (or 140, depending on the model) MHz to feed it with, and it will be right on frequency.
If it is L band input, the L band input will determine the RF output. Using L band as an IF. input gives you access to the whole 500 MHz of 1 side of the satellite. To convert C band TX to a L band IF. input take the C band frequency and subtract 2225 MHz to get your C band receive frequency, then subtract 5150 MHz, which is the oscillator frequency that mixes with the C band frequency to give you L band.
6125 MHz (C band TX freq) -2225 MHz (C band Receive translation) 3900 MHz (C band RX freq)
3900 MHZ (C band RX freq) -5150 MHZ (Oscillator Frequency) -1250 MHz (disregard the – sign)
So the L band frequency is 1250 MHz. You would use this for both the TX and RX frequencies.
With C band to L band conversion, you will notice that there is a inversion - this is due to the use of a high side oscillator running at 5150MHz.
Ku band is a little different. You need to know in what portion of the Ku receive band the satellite downlinks in. Does it downlink in the 11.7 to 12.2 GHz band, or the 10.95 to 11.7 GHz band? If it uses the 11.7 to 12.2, the oscillator frequency is 10750 MHz. If it uses the 10.95 to 11.7 GHz, use 10000 MHz
A note on C to L and Ku to L band conversions. C band to L band conversion cause an inversion, I.E. signals on the high end of C band turn into a low end frequency on L band. There is no inversion with Ku to L band conversion.
You are now at the point that you can transmit. Usually a carrier is used to first access the satellite so the satellite access centre can look at it. This carrier normally uses about 5 or so watts. This carrier is used to determine if you are on the correct polarity and to look at cross pole. Cross pole is how much of your signal is on the wrong polarity. Narrow bandwidth carriers use less power than large carriers. A narrow voice signal wouldn’t need much space at all. A 4 Megabit wide data carrier would take 3 times as much. You can use almost any signal to perform this test. Common test signals are analogue/digital video using colour test bars, or even a narrow voice signal. Decide what kind of signal you are going to use by knowing how much space you have available to you for your test.
If you are performing a satellite loop test and you aren’t in contact with the satellite access centre, the normal way to set your power on the satellite is to use a spectrum analyzer on the downlink side of your antenna. You start out with a low power carrier and slowly walk it up in power until your carrier is no taller in amplitude than any other carrier on the transponder you are on. If you run the carrier up in power so that it is larger in amplitude than all of the rest of the carriers, you could start robbing RF power from other carriers, or even in rare cases, cause a transponder to reset. This will definitely draw attention to you when complaints start rolling into the satellite access centre and they start looking for the culprit.
So now you can test out your TX and RX now by putting up a known signal and looking for it on your downlink, be it a voice signal, digital carrier, or analogue/digital video. If you receive back what you are putting up and it looks good, you know your setup is working.
Disclaimer As always, the information contained herein is for educational purposes only. Anything stupid that you may do with this information is your problem.