SatSig topic:

Plinear transmit power : VSAT carrier line up

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Nov 16^th, 2016 at 1:23am

Hi,

Could you please suggest the following to understand what power output should we put in to the modem and understand the margin which could remain from the BUC?

Required power to transmit: 8 watts
BUC size: 25 watts
BUC gain: 60dB
Cable loss: 10dB
Splitter loss: 3dB

What is also should be taken in scope as a loss in WGL? And how much Tx power output should be put in the modem in order we transmit at 8 watts?

Ps. We do not have a power meter in order to get the exact transmit power, and our spectrum works on L band.

Thanks for your help.

Nov 16^th, 2016 at 1:59pm

First make sure your antenna is pointed accurately at the satellite.

Contact the satellite operations centre and ask for help.

They will ask you to transmit a low level carrier and they will measure it. For this test they may request that you transmit CW or unmodulated carrier as this is easier to measure accurately. Check you know how to control the modulation on/off in your transmit modem.

They will ask you to increase the level gradually till your uplink power (EIRP) is correct. Your EIRP is the simple addition of your antenna transmit gain and your transmit power into the antenna feed, e.g. 50 dBi antenna transmit gain + 9 dBW transmit power = 59 dBW EIRP. I'm assuming an 8W BUC connected directly to the feed. 10log(8W) = +9 dBW. At this time make a record locally for yourself of your L band measured levels and the transmit level setting on your modem output. Write this all down with details of the test equipment and cables used at the time.

Having done this, you are now transmitting at the correct level and are good to operate, but we still have little idea about how many watts you are actually transmitting. I would guess somewhere between 2 and 16 watts!.

The 8W was presumably a pre-calculated link budget figure based on your nominal earth station transmit gain and the satellite sensitivity. The actual transmit power now may be significantly different from 8W.

The correct way of measuring your transmit power is to insert a high loss calibrated cross-guide coupler with a power meter head and power meter. Examples: Meters: HP/Agilent 432A 436A. Sensor: HP8684A Search eBay. The meters are cheap, under 100. The sensors generally much more, so if you get one take care and don't burn it out. Search eBay "cross waveguide coupler" under 100.

Check the documentation for your BUC. It may have telemetry with alarms, voltage and local oscillator phase lock status and an in-built power meter.

If you are transmitting more than one carrier simultaneously then you must have some spectrum monitoring on your output to assure that you are not transmitting intermodulation interference to other uses of the satellite. A high loss calibrated cross-guide coupler provides the point where an SHF spectrum analyser may be connected.

Nov 16^th, 2016 at 3:05pm

Thanks for the information, but I need to know what Tx power output has to be set on the modem from 0 to -40 assuming WGL and Plin 44dBm to get 8 watts on the transmit power out of 44dBm?

What is the calculation for this?

Nov 16^th, 2016 at 4:01pm

8 watts is 10log(8) dBW = +9 dBW
Gain of BUC = 60 dB (spec value, typically several dB +/-)
So input power to the BUC + 60 dB = +9 dBW
So input power to the BUC = -51 dBW
Convert to dBm by adding 30
Input power to the BUC = -21 dBm
Cable loss from modem to BUC = -13 dB
Output power from modem needs to be = -21 + 13 = -8 dBm

WARNING: DO NOT USE THESE FIGURES FOR YOUR INITIAL TRANSMISSION.

Reasons:
The maths regarding the levels and the maths regarding frequency may be wrong.
You may damage your BUC.
You may be on the wrong satellite and cause severe interference to someone.
You may be on the wrong polarisation and cause severe interference to someone.
You may be on the wrong frequency and cause severe interference to someone.
The original suggestion that 8W would be appropriate may be wrong. The actual requirement may differ by several dB.

Transmit only when told on the phone by the NOC.

Start with the modem in CW mode and minimum output level, e.g. -40 dBm.
Only when the NOC confirm you are on correct satellite, polarisation and frequency, increase the power in small steps with them monitoring all the time.
If you lose the phone contact switch off immediately. If the NOC can't see your signal switch off as even the most weak CW signal is detectable at the NOC.