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    Switches Need Power to Operate
    #1
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    Most fta users probably give little if any thought as to how much power is needed to operate the switches that are installed in most fta systems. I think almost everyone knows that a ku satellite dish motor, and the lnb need power to operate, but the switch or switches may need the same amount of power that the lnb or motor does.

    Earlier this year, I put together a digital meter that shows the voltage and milliamps that were being used by the antenna system or part of the antenna system. Use of Ohm's Law (Volts X Amps = Power in Watts) allows for the calculation of power being used by the antenna system or a particular part of the antenna system. This simple meter can be used for many different things in a fta system, but I will focus mainly on switches and the power they need to operate for this thread.

    Here is a picture of my digital meter, shown connected to the back of an Octagon SX88 fta receiver .
    [Only registered and activated users can see links. ]

    The parts needed to construct this digital meter are a length of coax with two connectors installed, and the meter, which can be found here:
    [Only registered and activated users can see links. ]

    Most fta receivers have around 450 milliamps maximum current rating. This is the amount of power the receiver can produce for long periods of time. It is very easy for switches to use or consume about 25 percent of the receiver power output. Here are some examples:

    1. Ecoda 22 KHz switch, 14 milliamps.
    [Only registered and activated users can see links. ]

    2. 3X4 Multi-Switch, 67.7 milliamps
    [Only registered and activated users can see links. ]

    3. 4 Position Diseqc switch, 26.2 milliamps
    [Only registered and activated users can see links. ]


    3X4 Multi-Switch, Ecoda 22KHz switch, and 4 Position diseqc switch, 108 milliamps.
    [Only registered and activated users can see links. ]


    Adding a lot of switches may take away or rob the lnb or ku motor of the power these parts need to operate properly! A typical consumer fta lnb may need 60-200 milliamps of power to operate correctly. A ku motor may use up to 350 milliamps of power at peak or at start-up, and then may need 150-200 milliamps to move the dish. Add all of this up and you will see that it is easy to exceed 450 milliamps.

    Most of the fta receivers that I have tested will produce more than 450 milliamps for a short period of time.

    A receiver tends to run hotter when it is required to produce more power for the lnb port.

    A 22 KHz switch will provide power to both sides of the switch at all times!
    So if you have two lnbs that draw 200 milliamps per lnb, the receiver will need to produce at least 400 milliamps when these two lnbs are connected to a fta receiver using a 22KHz switch!

    It is a good idea to understand how much power your antenna system needs to operate, and how much power your receiver is capable of producing. I think the amount of power that switches need to operate is mostly overlooked, or is not completely understood. EB
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  3. Collapse Details
    #2
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    May 2016
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    Nice setup, it can be useful to diagnose a funky switch, or measure the current on a non active LNB, (not pointing to a satellite and tuned to a transponder) but it can't measure the DC current on an active setup if that amp meter is in-line between the LNB and the receiver, it will cause a total loss of signal to the receiver.

    You need a very special circuit between the LNB and the power supply to do that without significant signal loss.
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    #3
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    I use a milliamp meter inserted in the coax behind the receiver so I can see the ku motor move. Installing this meter into a running or working fta system does not have any effect on the signal that I can see. Weak signals are still weak and strong signals are still strong.
    Of course there are losses anytime something is inserted between the receiver and lnb, but most lnbs have around 50 db of line amplification which more than covers the loss of inserting this meter into a working fta system.
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    #4
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    Well the problem is that your changing the 75 ohm line inductance by inserting a device never meant to be directly in-line with an RF circuit, this could affect the readings as it's changing things.

    We used a specialty modified receiver that had ports designed for DC/AC voltage and DC/AC current measurement on an active RF circuit, these tap points were directly in line with the receivers internal power supply to the LNB.

    The RF signals on the coax can affect the readings on some (but not all ) current meters and volt meters.

    And the 50 dB of line amplification is not on the output, but on the incoming signal from the satellite to the first mixer stage, there is some amplification on the IF output but that is at a fixed level, the receivers tuner will try and compensate for the additional loss due to the impedance mismatch from the milliamp meter.

    They make in-line inductive DC current taps for 50 ohm systems, but I have not seen a cheap one for a 75 ohm system, these type of taps do not directly connect to the RF signal path.
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    #5
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    Apparently, the impedance of inserting this meter into the line does not change much because I do not see any noticeable loss in received signal when this meter is inserted.
    This is a simple test meter that can be built for less than twenty dollars, and I find that it works exceptionally well. I do not see the rf having any real negative effect on this meter either. Granted, it is not a 10,000 dollar piece of test equipment, but the milliamp values this meter shows closely agree with the readings of other amp meters. The test meter may also be easily removed if any problems arise.

    The line between the receiver and lnb is also highly amplified because there is no other real choice besdies line amplification except to use a very large feed line. Each switch that is inserted between a receiver and lnb has a multitude of electronic parts in it, and the switch also hasa few db of insertion loss. Next, add the loss of 10-20 db in the 100-200 feet of low grade coax cable, and add the loss of the connectors. So the line amplification between the receiver and lnb has no choice but to be very large in db.
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