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Audio QotD: Antenna Preamplifiers

Discussion in 'Question of the Day' started by mbenonis, Oct 27, 2008.

  1. mbenonis

    mbenonis Wireless Guy Administrator Premium Member

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    Chicago, IL
    Here's a relatively simple audio QotD. If you have an antenna that requires an RF preamplifier, do you put the preamp at the antenna, or at the mic rack. Why?

    BONUS: How do you power the amplifier?
  2. jowens

    jowens Member

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    Centerport, New York, United States
    RF Pre-amp at the antenna... that way you're boosting the cleanest part of the signal, not after signal loss/contamination down the long cable run... assuming long cable run, b/c short cable runs rarely require an RF preamp.
    Should be powered via the antenna splitter sending the power down the RG-6 cable.

  3. waynehoskins

    waynehoskins Active Member

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    Arlington, TX
    As to location, it goes in the same place the big-shot microwave guys put their transverters: right at the feedpoint (or as close as you can get), so that the signal that's applied to the preamp has as high a signal-to-noise ratio as possible and is as high above the noise floor as possible.

    As to power, either DC supplied at the antenna site, or more commonly applied down the coax. I imagine the DC would be directly applied to the coax (or even better, low-passed onto it for good measure), and the RF would be capacitively-coupled onto the feedline, the end result being RF that's cleanly biased up 5 or 12 or whatever volts. For this to work, since it's biased onto an unbalanced line, the power supply should be really clean, though because it's an FM signal we're talking about, it's not as critical because there is no amplitude-modulation component to the signal.
  4. Chris15

    Chris15 CBMod CB Mods Premium Member

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    Sydney, Australia
    Noe I know we've been down the impedance mismatch line before, but I would contend of you have an RF situation that needs a preamp, you should use 50 ohm coax. RG6 is 75 ohm...

    The Shure stuff runs on 12V DC I think it actually starts life as 12.7v, but by the end of x metres of cable...

    Location... hanging off the back of the helical seems to work well much of the time :twisted:. Or if you are using the Shure batwings, use the version with attached preamp. Otherwise, as noted, as close as is mechanically sound to the output of the antenna...

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