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Capacitors for RF ground

Discussion in 'Boat Electronics' started by Michael Porter, Feb 8, 2009.

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  1. Can anyone (Larry?, Bruce?) help with a spec for capacitors for RF
    ground for a HF radio? I want to keep the DC away from the (aluminum)
    Michael Porter
    Michael Porter Marine Design
    mporter at mp-marine dot com
  2. Bruce

    Bruce Guest

    I did look into capacitor grounding some time ago and whoever I talked
    to said shove a bunch of capacitors in parallel into the system. This
    was based on having enough parallel capacitors to carry the signal
    amperage as well as voltage. I asked whether impedance was important
    and was told "probably not as long as you have enough capacitance."

    Larry will probably have more to say on the subject....

  3. You need to define the Frequency Range of the Radio/Antenna System, and
    Power Output, that your trying to DC Isolate/AC Couple. If your dealing
    with just HF Frequencies (3-30 Mhz) and less than 150 Watts PEP, it is
    very easy to get a .5uf non-Polarized 600 Volt Cap, to do the job. As
    you Lower the Frequency, and or Raise the Power Level, you will need
    larger Capacitance and Higher Voltage Ratings. The whole idea here is to
    have the Capacitor be as low of impedance, as possible, at the Lowest
    Frequency of the System, and the highest Voltage Rating available which
    will allow the Highest Power Level.
  4. Larry

    Larry Guest

    Hmm....Let's assume you have a vertical antenna like a Shakespeare or
    insulated backstay or just a longwire strung between some masts.

    When the antenna is near 1/4 wave resonance, its feedpoint impedance
    gets very low, somewhere around 15-20 ohms. Using simple math:

    P = I squared R so I, the current in the antenna and ground system,
    equals the square root of P, 150 watt marine radio divided by R, the 15
    ohms or even lower with your nice metal grounded hull.

    This capacitor is in series with this current from the ground on the
    tuner to the nicely grounded hull. Doing the math you get a hair over 3
    AMPS of RF current going through that cap at 150 watts. This CURRENT is
    the problem. Simple cheap capacitors, with their little wire leads just
    touching the plates in a cheap cap, simply cannot handle this kind of RF
    current flowing in them. The caps get hot from the resistance of the RF
    current flowing through their tin foil plates and will eventually fail,
    probably shorting out as they melt the dielectric so you won't even
    notice it...shorting ground to the hull isn't certain death, won't blow

    So, we SHOULD, to be fair, use an RF capacitor that can handle at LEAST
    4-5 Amps of RF current....a transmitting capacitor.....Strike ONE....

    Now, this series capacitance in series with such a low impedance at or
    near resonance (worst case scenario for the series cap) requires us to
    have a very LOW capacitive reactance so the cap acts like a virtual
    short to the RF, while acting like an open to the DC you're rightly
    worried over...galvanics. So, capacitive reactance is equal to 1/2piFC
    the bigger the C, the lower the capacitive reactance, the better it will
    work. We don't want this capacitor to be part of the tuning solution,
    we want it to act like an RF short. Your lowest frequency, the F in the
    equation is around 2 Mhz, if you use the old marine band (again worst
    case scenario). So the Xc (capacitive reactance) should be much less
    than (I like less than 10% of) that 15 ohms or 1.5 ohms at 2 Mhz.
    Filling in the blanks and solving we get about .05 microfarads. There
    won't be any appreciable voltage across this capacitor so almost any
    voltage rating will do as they are normally quite high voltage
    transmitting caps. .05uF or bigger capacitance and 5 Amps are what
    we're looking for....round numbers.

    About the cheapest place I know of to get real transmitting caps is
    Surplus Sales of Nebraska, who specialized for years in the military's
    mistakes and pork barrel purchasing but has become more than just a ham
    junk store more recently. They do have lots of great RF stuff real ham
    operators love. The caps in question are here:

    The most bang for the buck for what we're looking for is the 2nd cap up
    from the bottom of this webpage on the right hand side.
    ..22 uF at 2500 VDC, easily big enough in capacitance to provide a really
    low capacitive reactance.
    16 AMPS rated...much more than we really need in this application for
    safe operation at 150 watts.
    But, alas, at $49, if you'll compare to any other caps you'll find the
    most bang for the buck....easily fitting our needs for CURRENT and

    The $18 one just to the left of it would work at .001uF if we never used
    a frequency below 8 or 10 Mhz....but that's not realistic. More of them
    in parallel would work fine, but the costs are higher than the .22 at

    The .22 looks like it would be very easy to drill a hole into a
    convenient frame member and bolt the cap solid to the metal frame, then
    use two big flatwashers to the bolt on the other end to hook the tuner
    ground strap to for best low resistance connection. Do coat both ends
    in heavy grease before you're done to protect these irreplaceable bolts
    from the bilge water corrosion in the swamp. Grease 'em good and
    they'll be fine for many years.

    Oh, and please find a place where you can bolt the cap so that the
    mounting bolt that's holding it to the hull is on the BOTTOM, or TOP,
    preferably the bottom so the cap is sitting upright on top of its
    mounting...NOT hanging out sideways with the weight of the cap always
    torquing on the bolt trying to pull it out of the cap....not good....

    Biggest bang for the buck.

    Now, we'll hear a huge blast of noise from all those using 49 cent
    ceramic orange drop ceramics with the tiny wire leads. My reasoning and
    logic comes from being around broadcast AM all these years....where some
    really HUGE EXPENSIVE capacitors are just waiting for the 50,000 watt
    blowtorch transmitter to make them into a BOMB!.....always great fun!

    .....60 amps or so into a 15 ohm vertical tower 1/4 wave high.
    Those capacitors are simply HUGE!
  5. Larry

    Larry Guest

  6. Not the same animal..... RF Caps like Larry is talking about are
    NON-Electrolytic NON-Polarized......
  7. Larry

    Larry Guest

    (Richard Casady) wrote in
    Most computerized music instruments use a 1Farad 5V tiny little button of
    an electrolytic capacitor in them, instead of a big button battery that
    leaks, any more. It can't provide current, but will stay charged when the
    unit is unplugged for months, supporting the static memory the
    registrations are stored in so you don't need to reload them.

    But, you're talking about DC power supplies, not RF coupling. RF current
    through a capacitor is a whole nother ball game!
  8. Larry

    Larry Guest

    .....with very low series inductance...virtually none.
  9. Larry

    Larry Guest

    (Richard Casady) wrote in
    I had a massive, high-current electrolytic with battery-sized ears sticking
    out the top of it to connect to bus bars....a real brute. It was something
    like 1.2 Farads at 250 volts, but I can't remember exactly. It's very
    unusual for that voltage to be so much capacitance.

    Charging it up to 200 volts took many minutes as the power supplies
    available had limited current capabilities. Once charge, this beast was
    straight from hell. Discharging it across a steel bar welded it solid to
    the bar with a bang that could empty the electronics shop...(c;]
  10. Larry

    Larry Guest

    (Richard Casady) wrote in

    Wait until you see the massive ultracapacitor EEStor is making for the Zenn
    Canadian electric cars. Given enough power, it will recharge in seconds,
    not hours, as no chemical battery will be involved. The target is 125
    miles at 80 mph all electric. Amazing technology coming....
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