Power calculation for iDirect HUB

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.

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.

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.

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?



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.

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.

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 in-service 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 cross-waveguide coupler and power meter.  Look on eBay for HP/Agilent  power sensor head and meter. The old analogue ones work perfectly well.