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In my house, I’ve got a mag mount antenna mounted on a piece of sheet metal on top of rubber and the tip of the antenna goes into my ceiling and possibly contacts or is near to home insulation. I was just wondering if this was a hazard for fire in any way or if I’m OK. The antenna's max is 75 W, but my radio generally never puts that out.

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The peak voltage at the tip of the antenna is a function of the input power and the field impedance at that tip. The impedance varies based on the geometry of the antenna, but for a typical mag-mount whip it's probably not more than 5000Ω. Voltage $E$, power $P$, and resistance $R$ are related by

$$ P = {E^2 \over R} $$

$$ E = \sqrt{PR} $$

With the worst-case estimate of 5000Ω and 75W, that makes the RMS voltage at the tip of the antenna:

$$ \sqrt{75\:\mathrm W \cdot 5000\:\Omega} = 612\:\mathrm{V_{rms}}$$

If you are concerned about arcing, the peak voltage is probably more relevant:

$$ \sqrt 2 \cdot 612 \:\mathrm{V_{rms}} = 866 \:\mathrm{V_{peak}}$$

So the question is then if 866 V is a hazard. That's high enough that I personally wouldn't take any chances putting it in direct contact with anything.

Even if you never start a fire, the antenna is really designed to have air around it for proper operation, not a ceiling. I'd suggest a different antenna setup.

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I think you will be safe as long as there is no metal behind the ceiling. Ideally I would try to have a gap.

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  • $\begingroup$ You can try some experiments (outside, and safely) with a piece of paper or something to simulate your situation. At a power levels under 75W into 50 ohms (below microwave frequencies), I'd be shocked (no pun intended) if you could even get a tingle in your fingers much less enough heat/arc to ignite even the driest tinder. $\endgroup$
    – Duston
    Aug 4, 2017 at 21:17
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    $\begingroup$ @duston 5 W you can just feel is warm, 50 W can cause deep burns if you touch the antenna. Source : myself and several friends at various stages. $\endgroup$
    – tomnexus
    Aug 4, 2017 at 22:10
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    $\begingroup$ I can verify the 50W comment, but an insulated ceiling shouldn't try to conduct any power away like a 70% saltwater human body does. Unless there is something metal right above it. $\endgroup$
    – SDsolar
    Aug 5, 2017 at 4:04
  • $\begingroup$ Is the ceiling dry? Is there foil paper on the insulation? Are the structural members wood? Wood conducts a little. Or maybe they are steel? Even worse. Is there any electrical wiring in the ceiling? Are the answers to these questions going to be true forever? Even if the installation isn't hazardous today, it might be difficult to verify that, and it might not always be true. $\endgroup$
    – Phil Frost - W8II
    Aug 5, 2017 at 16:15
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If your antenna is a 1/4 wave vertical, the peak voltage on it will be at the very end of the antenna. So, please don't touch it while you are transmitting.

An antenna that is well-matched has an input impedance of 50 ohms (for most radios) and an output impedance of 377 ohms (this is the impedance of free space, and all an antenna really is, is a matching device from your radio's transmitter to free space).

If you are transmitting 50 watts into a perfectly efficient, perfectly matched antenna, the voltage at the tip is the sqrt(50*377) = 137 volts. You might arc this to a piece of metal if it were within a few thousandths of an inch, and it might conduct a tiny bit of current into high impedance materials.

One concern is the effect on the impedance of the antenna having dielectric materials in the near field. Having things like drywall and fiberglass right around part of the antenna might change its impedance enough to harm the radiation pattern and efficiency.

I would try to remove any materials that are near the antenna.

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    $\begingroup$ Why is the field impedance at the tip of the antenna equal to the impedance of free space? $\endgroup$
    – Phil Frost - W8II
    Aug 5, 2017 at 14:25
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    $\begingroup$ @user2104506 I've never seen a formula in this application that included the Z of free space. Do you have a reference? A related post is here. $\endgroup$
    – Mike Waters
    Aug 6, 2017 at 22:10
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    $\begingroup$ It appears that this idea of matching to 377 ohms (discussion here) came from the book The Electromagnetic Field by Albert Shadowitz. $\endgroup$
    – Mike Waters
    Aug 6, 2017 at 22:29
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    $\begingroup$ Sure, but the tip of the antenna isn't free space... $\endgroup$
    – Phil Frost - W8II
    Aug 6, 2017 at 23:40
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    $\begingroup$ Assuming a PEC and using boundary conditions, please describe the voltage at various points along the length of the described antenna. $\endgroup$
    – Glenn W9IQ
    Aug 8, 2017 at 23:39

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