LONG Coax Run?

Discussion in 'FTA / Non Small Dish Satellite Area' started by Michael Karnes, Jan 24, 2020.

  1. Michael Karnes

    Michael Karnes New Member

    1
    0
    Jan 24, 2020
    I have a "one off" temp install....I am modulating four Direct TV chanels over coax....I need to make a 1200-1400 ft run.....sure, I know....LONG!

    Can I use RG-11, a couple of 35Db amps in line and end up with a useable signal on the four channels on the end of the run? End will get split over 4 displays.

    I have read you can "conceivably" run RG-11 up to 1000 meters....That seems far fetched.

    HELP!!!
     
  2. P Smith

    P Smith Mr. FixAnything

    26,146
    533
    Jul 25, 2002
    W.Mdtrn Sea
    I would test it - take a reel of RG-11 2000 ft and assemble all boxes in one garage/room; if signal will drop below threshold, then I'll try RF amplifier with adj gain
     
  3. Brad_73

    Brad_73 New Member

    16
    16
    Jul 16, 2019
    Southeast, GA
    Yes you can, it is easy. But there is a LOT of information missing. But first, let's start with this:
    A couple of things, you do not want to use "a couple of 35dB amps in line". I'm also assuming you mean those crappy cheap 35dB amps for less than $50. You need to use just 1 good launch amp for your needs. Cascading multiple amps into one another, is going to raise the noise floor a lot. Unless you are using expensive push/pull amps like the cable company has out on the poles. But depending on your setup, you might not need an amp at all. Let's get into the design below.

    What modulators are you using? THIS is your starting signal level and without knowing this, it is impossible to calculate out the rest.
    Making some assumptions: Most good modulators start out with a least 35dBm signal. Next, you need to choose the modulation channels. If the 4 channels you are creating, are going to be the only signals on the cable, then choose the lowest channels in frequency you can. Some modulators allow VHF modulation but the most common start in the UHF range. You need to have at least 1 channel space between each modulated channel (broadcast quality modulators can put channels side by side - they cost $$$$) - (cheap crappy modulators need 2-3 channel spacing, to keep from interfering with each other.). Again assuming good modulators, lets choose channels 14, 16, 18, and 20. You will be using frequencies between about 470-512MHz. With RG-11, that is about 42dB of loss over 1,200 ft.
    Next, what amp to choose. You have about 7dB loss at the 4-way combiner for the modulators, leaving you with a 28dB signal to go down the coax. That is a HOT signal and a cheap amp will 'clip', you will get sever ringing and over-modulation in the signal sent down the coax. A commercial grade launch amp will do the trick. I looked it up and Solid Signal has a Pico Digital CA-30/550 for only $99. That has a 30dB gain and a max output of 43dBm (at 80 channel load). That last part is important, subtract the gain from the max output will tell you the max input the RF amp can take before clipping (13dB in this case) but with only 4 channels it can output a higher signal. You would need an RF meter (for exact measuring) but my experience would guess at least 50dB output, which means you want to turn the gain down slightly (set about 2/3) to boost that 28dB input to 50dB of output (20-24dB of gain on the dial - full being 30dB).
    With about 50dB of signal going in, you will get about +8dB signal coming out the end of that 1,200ft run. Thru another 4way splitter and you have +1dB headed to your TVs. Even with more cable loss, you should be well above -5dB; which is a good signal to each TV. You would start running into problems, until you drop below about -15dB (depending on the tuner).

    Hope that helps you get started. Again, more information about all the equipment involved would help. Some modulators have over 45dBm of output and you might get by with no amp at all.

    [edit]: Wanted to add this. That 42dB of loss on the RG-11 @ 1,200ft. is a generic calculation. Not all coax cable is the same and your actual loss is greatly going to depend on the quality of that RG-11 coax. Use some cheap copper clad steel (CCS) center with 40%braid coax and expect to get higher losses. Use some great solid copper, quad shield, swept test coax and you could end up with less loss than that.
    Generally a good coax manufacture will have a spec sheet, that will show you the loss at each frequency at what distance. And that is usually a worst case scenario, so better performance can be expected with careful installation (not over pulling and deforming the cable or bending it sharply).
     
    Last edited: Jan 24, 2020
    P Smith likes this.

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