Resistor noise at HF frequencies?

Discussion in 'Wireless & RF Design' started by MikeML, Mar 1, 2015.

  1. MikeML

    Thread Starter AAC Fanatic!

    Oct 2, 2009
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    I am building a remote coax switch containing a relay to switch between HF dipoles. Power level is <1kW at 3 to 30MHz.

    I want to put a resistor or RF choke from coax-center-conductor to shield (RG-8X) to drain any static charge while the inactive dipole is open-circuited. The active antenna is DC grounded through the relay by a balun between the antenna switch and the transceiver.

    What resistor should I use? Carbon or Metal Film? Resistance? Wattage? Does the resistor raise the noise level the receiver sees? What value of resistance is high enough not to heat during transmit while still bleeding off static?

    Should I not use a resistor at all; rather use an RFChoke with a low DC resistance?
     
  2. alfacliff

    Well-Known Member

    Dec 13, 2013
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    non inductive, resistance quite a bit higher than the line impedance.. like a 100k across 50 ohm. should bleed off static. even a little leakage stops that nasty static buildup.
     
  3. MikeML

    Thread Starter AAC Fanatic!

    Oct 2, 2009
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    Using LTSpice as a calculator, 100K resistor dissipates 0.54W at 1kWcw in a 50Ω system. Can a 2W carbon resistor stand 330Vpp?

    Bleed.gif
     
  4. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    Normaly we take that the resistors can handle about 200 Volts.
    Put 2 resistors of 47 K or 3 resistors of 33 K in series.

    Bertus
     
  5. Lestraveled

    Well-Known Member

    May 19, 2014
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    I had a 340 foot horizontal loop antenna when I lived in Las Vegas. I used a Johnson Matchbox with it and fed it with 450 ohm latter line. It would develop enough static charge that it would arc inside of the matchbox. I used a grounding clip on one of the antenna lugs when not in use.
    For a resistive static bleed off resistor I would use large carbon composite resistors like the RCR 1 watt (vintage) resistor.

    73 KG7VH
     
  6. #12

    Expert

    Nov 30, 2010
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    The manufacturers tell that a 1/2 watt resistor can sustain 350 volts and a 1 watt resistor can sustain 500 volts. Ref: The Mouser Catalog.
     
    Last edited: Mar 10, 2015
  7. MikeML

    Thread Starter AAC Fanatic!

    Oct 2, 2009
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    A generous forum member gave me (and even paid for mailing) a dozen 3 megΩ, 1Watt, AllenBradley, RCR resistors. Thanks M...
     
  8. alfacliff

    Well-Known Member

    Dec 13, 2013
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    why not use a folded dipole? there would be no static across the feedline. folded dipoles are used by comercial and military users.
     
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