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Transmit Cable Maximum Length

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Sep 3rd, 2012 at 2:42am  
Guys,

How can we compute maximum transmit cable length on our VSAT Internet system? For example we are using Belden 1694A and Belkom 8 W Buc.

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Reply #1 - Sep 3rd, 2012 at 1:24pm  
Cable spec sheets include loss figures, but that alone won't give the answer you seek. Modem type, cable length, transmitter type/power source, and ancillary hardware are among other variables that have to be taken into consideration. In most cases, that's a question that must be answered by the provider.

Belden is one of the manufacturers that DOES provide spec sheets for specific cable types. So you'll be able to come up with that number yourself. If you use the ancillary hardware recommended for that specific cable, it shouldn't affect the total loss significantly. That brings us to the transmitter. If it's got a dedicated external power source, that too should not affect total loss. But if the transmitter derives power from the modem via cable conductor, it goes back to a provider question.

//greg//
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Eric Johnston
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Reply #2 - Sep 3rd, 2012 at 6:33pm  
Find the spcification of the BUC, e.g. http://www.belcommicrowaves.com/Media/Doc/SPARK_Sm.pdf

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.

Best regards, Eric
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Reply #3 - Sep 4th, 2012 at 2:26am  
Thank you for your reply Greg and Eric,

Here's the spec of the modem, cable and buc that I am going to use.

Modem:
Output Signal Level = -45 to -2.5dBm
DC Power = 24V 1A

8W BUC:
Output power (at -1dB Gain compression)...........+39.0dBm (min.)
Gain(Nominal)................................................58±3 dB
Gain FlatnessOver any 36MHzband.............±0.75 dB max
Over frequency range..........................±2.0 dB max
Gain Stability (over temp.for constant freq.) ±2.0 dB
Ref. signal...External 10MHz(std.) 0dBm ±5dB

1694A Cable
Nom. Attenuation:
Freq. (MHz) = 1500.000    
Attenuation (dB/100m) = 26.215 dB
 
What is the formula I can use to compute the maximum transmit cable?

Thanks

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« Last Edit: Oct 3rd, 2012 at 11:14am by Admin1 »  
 
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Reply #4 - Sep 4th, 2012 at 4:39am  
Although it sounds like a simple question, I dont think there is a clear answer what kind of solution you can implement on the field.

The way I see it, there are three main issues you will need to consider:

1. The complete link budget, which the modem-cable-BUC is only part of it; but other parameters should be considered such as the data rate and modulation, bursting or not, receiving dish size etc. Also if you will be using C or Ku and what margin do you need (though I believe you are using 8W C band).

2. DC handling of cable. As Eric mentioned, it is not recommeneded to set equipment to their maximum power. Also, I've also noticed small number of of BUC's that got faulty after relative short time in service (one year or less), when receiving too low voltage (cable attenuation and/or bad connectors). I dont know if it was just a coincidence and I dont have any explanation why it happened.

3. Signal attenuation. Again - it is not recomended to transmit on maximum power, it is defintly not healthy for the BUC, and I believe it is not healthy for the modem either. If really required, maybe you can either use stronger BUC or add an active line amplifier along the line.
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Reply #5 - Sep 4th, 2012 at 4:32pm  
Quote:
What is the formula I can use to compute the maximum transmit cable?
Well, I suppose an algorithm could be constructed, but it's not worth the time. We need to know some modem specs, only you have that. I can provide the cable specs: http://belden.com/techdatas/english/1694A.pdf . So given the info accumulated here to date:

A. Transmitter is powered by the modem, operating voltage is delivered by cable center conductor. Cable center conductor resistance (see cable sheet above) is 6.4 ohms per 100 feet. Transmitter works optimally with 24vdc input power, but can work down to 15vdc. However as stated, the lower the input voltage - the harder the transmitter has to work. So what you need to find out is what the DC output of your modem is, then - using the 6.4 ohms/1000ft value - work out how far the modem can push DC operating voltage to the transmitter before dropping below the optimum 24 volts.

B. To be on the safe side, you should do the same type calculation for the signal level. Determine the (a) modem output level in dBm, and  (b) modem output frequency in MHz. Refer to cable spec chart for the "attenuation per 100 feet" value at that frequency. Unfortunately the transmitter data doesn't include an input signal spec. I don't know if it would pay to use the external -15 to +7 numbers or not. Eric will have to address that. Anyway, when you know the modem output in dBm and the transmit input requirement in dBm, refer to the answer in feet that you got from paragraph A above. Then use the cable attenuation chart to determine signal loss in dB at the end of that length of 1694A.

Ideally, you want both A and B to be within input tolerances at the transmitter. If not, you have to adjust to length of the cable accordingly. But in reality, I'll go back to my original answer; it's information that the provider should be able to give you.

//greg//
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« Last Edit: Oct 3rd, 2012 at 11:11am by Admin1 »  
 
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Eric Johnston
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Reply #6 - Sep 4th, 2012 at 11:01pm  
If your modem has a DC Power capability of 24V 1A that means it can only output 24W.  You can only use the modem to supply a BUC that consumes less the 24W.  Preferably 20W or less.
See http://www.belcommicrowaves.com/Media/Doc/SPARK_Sm.pdf
A 1W BUC or the new higher efficiency 2W version (mBLX-2-LP)  both need 16W of DC power and would be suitable.

Clarify the DC power requirement for your exact model of Belcom 8W BUC.

Do your really need an 8W BUC which needs a 55W DC power supply ?  Consider instead, using a 2W BUC and a dish twice the diameter (4 times the area and 4 times the gain, +6 dB ). Clarify with your service provider what is your maximum uplink EIRP (BUC power plus antenna transmit gain) under rain fade and UPPC set to max.  Your location in the uplink beam contour pattern matters, also the bit rate of your uplink carrier. Halving your uplink bit rate halves your required BUC power.

If you really must use an 8W BUC then investigate the modem.  Does if have the possibility of an external power supply for the BUC ?
If the modem has no such DC power injection facility, investigate the BUC.  Is a special configuration of the BUC available with a separate DC power input socket ?
I would suggest an 80 W external DC power supply. You may need your own "bias T" DC power injector. In this case check it will DC block the modem power but permit the 10 MHz reference and L band signal to pass OK.

As mentioned above it is important to provide the BUC with its rated DC power. If you operate with an inadequate power supply your transmit signal will be distorted and you will cause interference to other adjacent services.
The BUC has its own internal switch mode power supply and should not be operated with abnormally low input voltage or intermittent voltages.  ALWAYS POWER OFF at the AC mains wall switch before working on the BUC TX cable.

Regarding signal levels:
BUC Output power (at 1 dB Gain compression.)...........+39.0dBm (min.)  = 9 dBW = 7.94W
Gain(Nominal)................................................58±3 dB

Input power for +39 dBm out = +39 - 58 = -19 dBm
If the cable loss is 10 dB the modem output power = -9 dBm.
The modem can put out -2.5 dBm if necessary so will be happy at say -5 dBm if the gain of the BUC is 4 dB below nominal (with extreme frequency and temperature tolerance).
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« Last Edit: Sep 5th, 2012 at 1:24pm by Admin1 »  
 
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