I want/need to put up an outside antenna, ideally without inviting lightning into the house. One idea that occurred to me was to use inductive coupling to pass signal through the glass of a window, with low-pass filtered constant ground on the antenna so a lightning pulse will go to ground instead of shattering the glass to get in.

Are there good reasons not to do this ("it won't work because X" or "you can't filter the lightning" or similar)? If it'll work, what's the power limit likely to be (would it stand up to 100 W HF as well as 50 W VHF)?


1 Answer 1


That is possible with this constraint: the coupling factor between the indoor and outdoor inductor needs to be high. Otherwise the losses will be high. For limited frequency range there is an option to use resonant circuits on both sides. At the cost of bandwidth reduction there is an almost lossfree solution possible.

This is without the effect of losses in the glass; probably there will be some metal in the glass. When losses are converted into heat you will feel that heath or see the glass splinter or hear it break. Start with low power.enter image description here

  • $\begingroup$ I didn't think that glass window panes here in North America contained metal. Maybe tinted glass? Another good way to test its loss is to get a small piece and put it in a microwave oven. It shouldn't get hot unless it's lossy. Nice answer, though. :-) $\endgroup$ Commented Jul 31, 2021 at 11:33
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    $\begingroup$ Don't try this on the tinted glass that looks like a mirror -- that has metal in it, and it will totally block this. Also, lightning acts like a wide band spike with a bandwith from 0hz - 1GHz, so this circuit will still conduct some lightning, within the frequency range of the circuit and until the inductors saturate. Hopefully that won't be much. It might turn a milisecond spike into a smaller nanosecond spike or something like that. $\endgroup$
    – user10489
    Commented Jul 31, 2021 at 12:22
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    $\begingroup$ @MikeWaters Actually, the float glass usually used for windows contains traces of iron -- that's why it's green if seen edge on. Iron-free glass (Schott sells this in panes, or any borosilicate is iron-free) won't have that green tint. Not sure if there's enough conductivity to cause eddy current losses, though... Also, for your microwave test, glass sex toys are a thing because they can be warmed in the microwave, despite being made of borosilicate (though they are colored, perhaps that contributes). $\endgroup$
    – Zeiss Ikon
    Commented Aug 1, 2021 at 18:54
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    $\begingroup$ @MikeWaters Float glass hasn't ever used mercury. Molten tin is used; mercury boils well below the working temperature for floating glass. AFAIK, the only use of mercury by glaziers was in early methods of silvering glass second surface mirrors. $\endgroup$
    – Zeiss Ikon
    Commented Aug 2, 2021 at 11:00
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    $\begingroup$ @MikeWaters Due to too much stuff and too little space, the window that's accessible from where my shack will be isn't really accessible, nor is there a practical way to replace the glass vs. replacing the entire sliding pane assembly (early 2000s vintage mobile home). $\endgroup$
    – Zeiss Ikon
    Commented Aug 2, 2021 at 11:03

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