Things to ponder for those thinking about running multiple LNB’s on their system.

First off most FTA receivers where designed to run using a single LNB, looking at a single satellite, a single LNB will run just fine out to 200 feet from the receiver if you use good coax. (RG-6 quad with a pure copper center or RG-11)

The more LNB’s you add to the system the more the current drain is on the receivers power supply, and if you add an un-powered switch to the system you will add even more current drain to the receivers power supply.

So it is best to get the DC current ratings for all powered equipment attached to the system, you also need to know the overall DC resistance of the run of coax, (expressed as ohms per foot) other stuff like ground blocks and any un-powered switches can add to the current loss.

90% of the FTA receivers out there can only supply 500 milli Amps of DC current (1/2 an Amp) at 14 or 18 volts DC, when you add cheap coax to the system the DC resistance (not the 75 ohm impedance of the coax, that is something different) goes up and your over all system voltage will drop, if its too high the DC voltage will be too low to properly run the LNB(s) and you may get channel or transponder loss or the LNB may not even turn on.

If you plan on running more than 2 LNB’s it is best to use an externally powered switch, this is true for the triple LNB’s from Dish as they need at least 650 mA to run correctly, this is over the recommended DC current output of most FTA receivers.

Now comes the big problem, and that’s using the FTA receiver with a power injector like the DPP power inserter for the DPP44 switch, this type of power inserter uses the coax to supply the switch with 20 to 22 volts of DC voltage at around 1 amp of current, this will run the switch (or DPP+ LNB) just fine, however it will not let the DC control voltage from the FTA receiver to pass through as the power inserter has a DC block on the receiver input side, this will cause all sorts of problems for the FTA receiver, it may even damage the receivers power supply.

So plan your system carefully.




Here are some current ratings to work with:

LNB’s

Single LNB current rating, 150 to 200 mA DC.

Single DPP LNB current rating, 185 mA to 200 mA DC.

Dual DP LNB, 350 to 400 mA DC.

DP twin/quad LNB, 375 to 425 mA DC.

DPP twin LNB, 600 to 650 mA DC.

(anyone with another type/brand of LNB's current spec's please PM the info to me and I will add it to this list, the same for the switches)

Switches.

22 KHz switch, 20 to 25 mA DC.

Standard 4 in 1 out DiSEqC switch, 25 to 35 mA DC.

5 x 4 multi-switch, 240 to 260 mA DC.

5 x 8 multi-switch, 250 to 270 mA DC.

DP 34 switch, 180 to 600 mA DC, this depends on how many LNB’s are attached.
(This switch is powered by the receiver attached)

DPP44 switch, this switch is externally powered and can supply up to 1500 mA of DC current but only works well with DP LNB’s.


There are other switches out there and some have there own built in power supplies so look at their specifications before you buy them as all this can vary by 10 to 30%.


Coax DC loss per foot (DC resistance)

RG-6 with copper plated steel center conductor, .028 ohms per foot, .28 ohms per 10 foot, 2.8 ohms per 100 foot.
DC resistance of the shield, .009 ohms per foot, .09 ohms per 10 foot, .9 ohms per 100 foot.

RG-6 with copper center conductor, .0064 ohms per foot, .064 per 10 foot, .64 ohms per 100 foot.
DC resistance of the shield, .009 ohms per foot, .09 ohms per 10 foot, .9 ohms per 100 foot.

.
RG-6 quad shield with copper center conductor, .0064 ohms per foot, .064 per 10 foot, .64 ohms per 100 foot.
DC resistance of the shield, .0046 ohms per foot, .046 ohms per 10 foot, .46 ohms per 100 foot.

Why is this important?
Because the DC resistance of the coax will cause a DC voltage drop in the overall system, it may look small but it can cause a big problem at the LNB(s) with long runs of coax and several switches inline.

Other things can affect the current, things like ground blocks, lightning arrestors and couplers add to the DC resistance of the system.