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Attempting to build a surge-arrestor for my HF receiver

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.

Thanks,

Dave

Reply to
Dave
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"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

Reply to
Phil Allison

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.

Cheers, Tom

Reply to
Tom Bruhns

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