Computing for the maximum transmit cable length of a VSAT Internet system?

Note the BUC gain and power supply requirements. Note your minimum and maximum modem output levels.

Using the BUC gain work out the appropriate input level for saturation.

Calculate the modem levels for various BUC output levels and try to design a system that allows the BUC to saturate with the modem perhaps still 5 dB below its maximum output. Operating any equipment at its extreme limits is not recommended. That gives you the dB loss for the transmit IFL cable, review the cable spec at L band to find the length. If you will be operating a very small carrier, with the BUC at very low power, e.g. 0.1W then take this into account, or use a lower power BUC. You dont want to get into the situation where you can't reduce the power sufficiently, even with the modem ats it minimum output. A longer cable may provide useful L band attenuation while allowing the DC and 10 MHz reference through.

Using the power requirements of the BUC, e.g. 55W at 15 to 24V.What power can your modem put out without overheating etc. If marginal then use an external power supply with cooling fan etc.

For higher powers, like 55W, it helps to use the maximum voltage at the modem end, in this case 24V. This minimises the current (amps) and thus the voltage and DC power loss in the cable. Solid copper centre wire is mandatory. 50ohm cable may be helpful as it has a thicker centre wire, if the modem and BUC can match 50ohms. Copper coated steel centre wire must be avoided!. Be conservative. Ask if the DC current taken by the BUC is smooth. It may comprise spikes at the switch mode power supply frequency and may spike also when the modem transmits a burst. Work out the cable ohms and compare with the cable specification versus length. DC power is very demanding on the coax centre pins, centre socket springs and the sheath wire connection at the plugs. A smear of electronic grade silicone contact grease can help exclude oxygen and moisture from the metallic points of contact.

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