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VSAT technology and installation >> iDirect Forum: hubs and terminals >> Power calculation for iDirect HUB https://www.satsig.net/cgibin/yabb/YaBB.pl?num=1307951551 Message started by adnabil11 on Jun 13^{th}, 2011 at 8:52am 
Title: Power calculation for iDirect HUB Post by adnabil11 on Jun 13^{th}, 2011 at 8:52am
Hi all;
I would like to ask you about my power consumed in Watt as I have 2 carriers: 1 1st carrier with Tx power= 9.5dbm 2 2nd carrier with Tx power= 12dbm I use 30W BUC, and I need to know the exact power (in Watt) consumed from this BUC. Regards; 
Title: Re: Power calculation for Idirect HUB Post by Eric Johnston on Jun 13^{th}, 2011 at 10:19am
To measure BUC power output:
You make the measurement using a high loss crossed waveguide coupler and a power meter. This shows my portable test set up. You connect the Ku band BUC output to the waveguide end on the left. The power goes mainly straight through the guide and is absorbed in the finned dummy load on the right. You connect a power meter head to the N type connector on the first grey coupler. The calibration is 40.0 dB at 14 GHz and 39.9 dB at 14.5 GHz. Other couplers have other nominal loss, like 50.0 dB. The second, black, coupler does a similar job and has an SMA output connector for a spectrum analyser. This shows an HP/Agilent power meter head and meter. Read the calibration factor off the table printed on the head, typically values for about 10 spot frequencies between 10 MHz to 20 GHz, including 50 MHz. Set the calibration factor using the table that comes printed on the sensor head for 50 MHz. Calibrate using the 1 mW 50 MHz reference before use. Connect the power meter sensor head to the N type socket on the waveguide. Set the calibration factor using the table that is printed on the sensor head for the actual freq, say 6.2 GHz. The meter may be set to display W, mW, uW or nW or dBm or dB(rel). Set dBm Example: The power meter reading is  13 dBm. So BUC Power = 13 + 50 = +37 dBm = 7dBW = 5 watts. Note that the power reading is the total RMS power, so if you are transmitting multiple carriers it is the addition of all of them. All the above gives highly accurate results. Warning 1: Power meters have expensive and sensitive sensor heads. Be careful not to burn them out. Make sure you know the maximum input rating and have a suitable high loss coupler or attenuator in front. Warning 2: Always terminate the BUC output waveguide, either into the antenna feed system or an appropriately sized dummy load. Microwave power heats , and may burn, the human body. Never look down a waveguide as the power, and even quite low power, will cook your eye lens, which has no cooling blood supply. Conversion: Watts to dBW: 10 log (watts) = dBW Watts to dBm: 10 log (watts) + 30 dBm dBW to dBm: dBW + 30 dBm dBm to dBW: dBm  30 dBW dBW to watts: divide by 10, =, 10^x watts dBm to milliwatts: divide by 10, =, 10^x milliwatts Try to learn some or all of these: +20 dBW = 100 watts +10 dBW = 10 watts +6 dBW = 4 watts +3 dBW = 2 watts 0 dBW = 1 watt 3 dBW = 0.5 watt 10 dBW = 0.1 watt 0 dBW = +30 dBm Best regards, Eric. 
Title: Re: Power calculation for Idirect HUB Post by Eric Johnston on Jun 13^{th}, 2011 at 10:32am
The only information you give is:
1 1st carrier with Tx power= 9.5dBm 2 2nd carrier with Tx power= 12dBm The first thing I need to say is that you really should try to avoid transmitting multiple carriers. Are you sure you can't arrange things so that you transmit just one carrier. You are making big problem and inefficiency for yourself in terms of output back off and intermodulation interference. If you add the two carriers together the total power is 7.6 dBm. To do this calculation go to Accessories, Calculator, View Scientific mode. Take each number in turn and enter is as divided by 10, then do +/ to make it negative e.g. 9.5 dBm enter 0.95 change sign +/ now reads 0.95 do 10^x now reads 0.112 Write this down thus 0.112 12 dBm enter 1.2 change sign +/ now reads 1.2 do 10^x now reads 0.063 Write this down below the result above thus 0.112 0.063 Repeat if more than 2 carriers add up the column of figures thus 0.112 0.063  0.175 do log result is 0.757 In your head multiply by 10 and the answer is 7.57 dBm. Best regards, Eric. 
Title: Re: Power calculation for Idirect HUB Post by Eric Johnston on Jun 13^{th}, 2011 at 10:56am
If you know the power into the cable towards the BUC and the combiner / splitter / cable losses you can work out the level at the input to the BUC.
e.g. TX power at modems = 7.6 dBm Combiner loss (assume 4 inputs) = 7 dB Monitor point (10 dB) splitter loss = 1 dB Cable loss 10 dB TX powrer at input to BUC = 25.6 dBm Assume that BUC gives full 30W (14.8 dBW) rated power output with 20 dBm input. Power out = 14.8  5.6 = 9.2 dBW = 8.3 W. Note that the gain of the BUC is not known until accurately measured. Typical specifications allow for a wide range iof variations between different BUCs, different frequencies and temperatures. Measure yours. Some BUCs, particularly the high power types, have in built power measurement. Record the DC current taken and plot this against the measure power output. If you really need to transmit multiple carriers per BUC you must have a monitor point at the BUC output and a suitable spectrum analyser to plot your output spectrum. Measure the power spectral density of the intermods. Consider using an extra BUC on the opposite polarisation or an extra antenna with new BUC rather than having two or more carriers per BUC. Best regards, Eric. 
Title: Re: Power calculation for Idirect HUB Post by adnabil11 on Jun 13^{th}, 2011 at 1:41pm
Thank you very much for your clarification

Title: Re: Power calculation for Idirect HUB Post by DGN1 on Nov 16^{th}, 2016 at 4:03pm
Hello Eric,
1) How to assume that BUC gives full 30W (14.8 dBW) rated power output with 20 dBm input? How did you get the 20dBm input and leave 5.6dBm and minus from 14dBW? 2) Did you apply the gain of the BUC as well which is usually 60dB? Can you provide an accurate calculation please? Thanks. 
Title: Re: Power calculation for Idirect HUB Post by Admin1 on Nov 16^{th}, 2016 at 5:21pm
I think you are referring to this:
TX power at input to BUC = 25.6 dBm Assume that BUC gives full 30W (14.8 dBW) rated power output with 20 dBm input. Power out = 14.8  5.6 = 9.2 dBW = 8.3 W. The gain of a BUC is the difference between input and output. Input = 20 dBm. Output = +14.8 dBW = +44.8 dBm. Gain = 20+44.8 = 64.8 dB. Assuming you are in the linear region, i.e. at or below the 1dB gain compression point. If you reduce the input level by 3 dB, e.g.from 20 dBm to 23 dBm then the output power will reduce from +14.8 to +11.8 dBW = 15.13 watts. 15.13 watts = 10 ^ (11.8/10) = 10 ^ 1.18 
Title: Re: Power calculation for Idirect HUB Post by DGN1 on Nov 17^{th}, 2016 at 1:10am
Hi,
I am getting a wrong outcome, please see below: The total input to the BUC is 29.5 dBm (16.5dBm (Tx Power Output on the modem)  13dB (WGL)) using Comtech modem. So according to the provided calculation: Gain 60dB  Output 44dBm BUC = 16dB Input Gain? Eric inserted: BUC input level = 29.5 dBm BUC gain = 60 dB BUC output level = 29.5 + 60 = 30.5 dBm or +0.5 dBW Next is to find out what is the transmit power level from the BUC: Power out = 14 dBW  13.5dBm = 0.5dBW = 1.12 W. ? This is very unrealistic that the 25w BUC might have been transmitting at 1.12W and another 23.88W is a margin, maybe the calculation what I have made is wrong? Please correct me. Thank you. 
Title: Re: Power calculation for Idirect HUB Post by Admin1 on Nov 17^{th}, 2016 at 10:13am
I suspect that the actual gain of your BUC is more than 60 dB, in which case you are probably transmitting something between 2 and 6 watts.
If the NOC is happy with your uplink EIRP then all is well. It is not unusual for a BUC to be operated in clear sky well below its rated power. You have margin in hand for a 6 dB uplink power control range or for an increase in the transmit bit rate at some future time. I don't understand "WGL". I think you mean 10 dB of cable loss plus 3 dB splitter loss. If it means waveguide loss then this alters things significantly. It would be most unusual to have 13 dB of waveguide loss. You should put the BUC much closer to the antenna feed. 
Title: Re: Power calculation for Idirect HUB Post by DGN1 on Nov 18^{th}, 2016 at 12:00am
Hello,
There is still a doubt, if the calculation is correct, then we could have not transmitted 10Mbps with only 1.12W BUC using the provided calculation and additionally have a margin of 23W. There should be at least 12w according to the LBA as it was approved by the NOC. Kindly advice if we have been missing something? The gain is about 60~65dB in case of variation but not more. Thank you. 
Title: Re: Power calculation for Idirect HUB Post by DGN1 on Nov 18^{th}, 2016 at 12:03am
Also, did you take in scope that Comtech Tx Power Output settings start from "40" to "0" and our settings set on "16dBm"
Hope this would help to make an accurate calculation. 
Title: Re: Power calculation for iDirect HUB Post by Admin1 on Nov 18^{th}, 2016 at 1:11pm
You say the Comtech modem output may be adjusted from 40 dBm to 0 dBm. Your setting of 16.5 dB for operation is just fine. I would not advise a design configuration that involves operation close to 40 dBm or close to 0 dBm as this leaves little scope for later inservice adjustments.
If your BUC gain is 65 dB then the power out is 1.12W x 10^(5/10) = 1.12 x 3.16 = 3.54 W The link budget analysis will show a required uplink EIRP capability based on satellite assumptions, such as edge of uplink beam, full transponder operation, full nominal interference levels, transponder at low gain setting, end of life minimum satellite output etc. A satellite's actual parameters such as uplink G/T and downlink EIRP may be significantly better than specified, particularly in some locations within the coverage beams and during the early years of operation. If things deteriorate you may be asked to adjust your power in future as the satellite conditions change. Assuming your BUC is connected directly to the antenna feed, your uplink EIRP is the addition of the BUC output power and your antenna transmit gain. Basically you don't have a power meter and want to know how much power your BUC is putting out. Do you have any telemetry from the BUC that shows what is its power out ? There may be some way you could plot a curve of BUC output power versus output from the modem. With permission from the NOC you could turn your modulation off and reduce the modem output power from 16.5 dBm to say 36.5 dBm in 2 dB steps while measuring your own carrier on the downlink using your spectrum analyser. Hopefully the graph of carrier power (BUC output) (y axis) and modem (x axis) would be a straight line. Increasing the level above 16.5 dBm needs NOC permission but I would not rule this out. Ask. If you are not allowed to go up, the reason will be that your higher power would overload (i.e saturate) the transponder and cause all the other users of the transponder to lose service. You may also cause unacceptable interference (with your CW carrier) to users on the opposite polarisation on the same satellite and to users on nearby satellites around the orbit. If you are allowed to go up, do it cautiously in 1 dB steps and plot the graph as you do it, extending the straight line upwards. When the line curves over and the vertical gap to the extended straight line is 1 dB, stop. You have reached the 1dB gain compression point. If you go further you risk damage to the BUC but it will eventually reach its saturated output power, which may be 25 W or more. If you can measure the DC input volts and current to the BUC while doing all this, then a graph of that may help also. This transfer characteristic curve for a nominal 2W BUC shows the idea. I do recommend getting a calibrated high loss crosswaveguide coupler and power meter. Look on eBay for HP/Agilent power sensor head and meter. The old analogue ones work perfectly well. 
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