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Attempting to build a surge-arrestor for my HF receiver

Discussion in 'Electronic Design' started by Dave, Mar 17, 2007.

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  1. Dave

    Dave Guest

    I am trying to protect my HF RF amplifier from static coming down the feed
    line, and would like to bounce some ideas off a few actual EE's. I
    currently have a neon bulb going from the center conductor of my coax to
    ground, but this is far from satisfactory. It does nothing for charges
    under the rated 125V. One person suggested putting a couple of back-to-back
    diodes in parrallel with the neon bulb, and I had the idea of putting a 100K
    multi-watt resistor in series with these diodes.

    This would (I believe) protect the input to the amplifier by taking anything
    over .6V to ground, and slowing the dissipation of a large pulse to protect
    the diodes somewhat and possibly even cause the neon bulb to light up (an
    indication of a less-than-ideal environment in which to operate a radio.)

    Does this sound like an effective means of protecting said equipment from
    incidental static charges following the coax in through my window? I have
    already had a .22uF 50V polyethylene capacitor linking the coax to my tuning
    capacitor blown by static, and replaced it with a .1uF 630V capacitor. This
    "fuse" has so far remained intact, but I would like to augment it some and
    further protect the project I spent so much time putting together and
    troubleshooting. Anyone with any ideas is welcome to comment, cuss or
    discuss. I am not an EE, and am open to any and all feedback.


  2. Phil Allison

    Phil Allison Guest

    "Dave" <

    ** Err - why not just ask for advice instead ?

    We are not brick walls, no need to bounce anything off us.

    Your antenna just needs a resistive path to ground included in the feeder.

    The best answer is an RF choke.

    100uH is probably enough.

    ........ Phil
  3. Dave

    Dave Guest

    Hello Phil,

    Well, asking for advice is what I am trying to do. Sorry if the tone was a
    little too stiff. Just trying not to be presumptuous.

    Only problem with the resistive path to ground built into the feeder is that
    I am trying to build the static protection into the device I am trying to
    protect. It may not always be hooked up to this feedline, and I would like
    it to stay protected.

    Thanks for the feedback. I will probably add such a feature to the feedline
    in addition to whatever I do with the RF amplifier.

  4. Phil Allison

    Phil Allison Guest

    "Phil Allison"

    ** You failed.

    ** Is this dude for real ????

    ** Poor diddums - what a conundrum.

    ........ Phil
  5. Tom Bruhns

    Tom Bruhns Guest

    There are quite a bunch of protection devices available. There are
    small gas discharge gaps (much like your neon lamp, but specifically
    rated for very low inductance and relatively low firing voltage and
    fast firing). There are some low capacitance ESD protection chips in
    0603 SMT packages that seem to do a good job. And I can recommend
    also using RF PIN diodes back-biased to a level a bit above the peak
    RF voltage you expect from your antenna under normal operating
    conditions. This last solution can be implemented with one PIN diode,
    anode to the antenna terminal and cathode to a low-inductance
    capacitor to ground, with the capacitor biased through a resistance to
    a zener diode. You might consider a couple 0.1uF 50V (or better,
    200V) ceramics in 0805 SMT packages in parallel for the capacitance,
    and about 100 ohms off to +10V. Duplicate that with reverse
    polarities for a -10V clamp. Use robust PIN diodes; for HF at 50 ohms
    nominal, PINs with reverse-biased capacitance under 1pF should be
    fine. This sort of clamp should not degrade the receiver's IIP3
    materially; there are a couple significant advantages to using PINs
    instead of fast switching diodes in this application.

    It can also be a help to put a very simple bandpass filter between the
    antenna input terminal and the clamp (and the receiver input):
    assuming a 50 ohm input resistance for the receiver, just a capacitor
    sized to give a 3dB corner with 50 ohms at the lowest frequency of
    interest, in series with an inductor sized to give a 3dB corner
    somewhat above the highest frequency of interest.

  6. Dave

    Dave Guest

    Hello Tom,

    Wow. Thank you for your post, and the ideas/suggestions you offer. I have
    to ask however, what is a PIN diode? And what is the IIP3 of a receiver? I
    am sorry I have to ask, but I have been out of the loop for over 20 years,
    and am just now trying to get back into electronics. And I know next to
    nothing about RF. I cannot tell you how much I appreciate your post
    however, and hope I don't frustrate you with my questions.

    Best regards,

  7. ehsjr

    ehsjr Guest
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