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What about feedline loss?

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  • What about feedline loss?

    With all this talk about which is the right antenna nobody has mentioned the RG-174 feedline they all seem to ship with. At 2.4ghz this tiny stuff will lose at least 2.5db/meter! The connectors introduce loss too.

    I ordered one of those magmount 5.5dbi deals for $15 and tried it with no mods. It was better than no external antenna, but I knew it could be better. Then this past weekend I cut off all but 5" of feedline and soldered it right to the circuit board of my Dell 1180 usb nic. I put the nic down inside the brakelight assembly near the roof of my truck and had just enough feedline to run out to the roof. Then I ran the USB cable back to the computer. It's MUCH better. My favorite guest AP signal/noise ratio went from ~22 to ~45.

    Remember that the i in dbi is "isotropic" indicating that it's being compared to an imaginary antenna radiating in free space. In reality that thing probably gets less than 5.5db gain. With nearly 5' of feedline and a connector you might be down to 1.5db or less by the time your signal passes from antenna to nic.

    Has anybody else done anything like this with their antennas?

    (I've gained a LOT from this site by lurking! This is the first time I felt like I actually had something to offer - thus my first post.


    -- divideoverflow

  • #2
    Maybe a shielded wire could cut some of that loss. I know mine's supposed to be coming with about 5'. Are there other options for the wire itself? Most of the time my antenna will sit in the back window. but every now and then It'll go on the roof; which means I may need that other 3 feet or so.

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    • #3
      RE: Feedline loss...

      Originally posted by welthqa
      Maybe a shielded wire could cut some of that loss. I know mine's supposed to be coming with about 5'. Are there other options for the wire itself? Most of the time my antenna will sit in the back window. but every now and then It'll go on the roof; which means I may need that other 3 feet or so.

      Unfortunately it's already shielded. Coaxial cable such as rg174 has an inner conductor surrounded by a braided shield. All RF cables have characteristic loss. The higher the frequency, though, the more energy will leak out. 2.4 Ghz is really microwave (ovens are at 2.5!) and at these frequencies there really are no good cables. CBs operate at 27mhz and can get away with thin cable, but 2400mhz is just too much.

      Choices are really thick cable or mounting the antenna+wireless nic together and feeding it with usb cable instead of RF cable.

      Keep in mind that I'm just pointing out that this would be _better_ than a long cable. It doesn't mean 5' of cable won't work. You'll just not really get any gain out of the antenna (a 5dbi) after all that cable so it'll be about the same as the normal internal antenna if it wasn't inside the car. It's much better to have the antenna outside the car than inside, imho.

      -- divideoverflow

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      • #4
        No, you need something with a larger center cunductor such as LMR400 or better. I have a lot of experience with microwave equipment, and that is the only way.


        Toups

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        • #5
          I will be running an automatic gain amplifier that will ensure that I get close to the full permitted 36dB EIRP at all times. The plan is a 100mW PCI card to the amp that is set for about 32dBm, about half a meter down RG174 to a 5dbi mag mount.

          While this setup should provide very good range, the big downside is that the amplifiers like this cost a lot, and so they're probably not a very good option for most installs. I'd say unless you can get one for free or very cheap it would not be worth it...

          I will post some details, graphs, and comparisions to a no-amp setup when I get the wireless system fully operational. Right now, I just have the little 5dbi antenna screwed into the PCI card inside the trunk. It works better than I expected that it would, but it is still pretty bad.
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