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Author Topic: Fire Hazards with Attic Antennas  (Read 28894 times)
W6RMK
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Posts: 651




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« Reply #15 on: November 09, 2013, 07:17:36 AM »

So, we have a bunch of anecdotes "It didn't start a fire in my attic", but that doesn't really address the original question of safety.  This is in the same category of "I put up 80 feet of unguyed Rohn 25 and nobody was killed when it came down".

I'll provide a counter example:  I have started a tree on fire with a 40m dipole.  Yes, it was a lame installation (lightweight ropes through trees dragging the wires from my apartment balcony).

I and others have also started inadvertently fires with tesla coils, which is, granted, a lot higher E field strength than most ham antennas.

So here are the issues with attic antennas:
1) Attics are dry and full of stuff that burns
2) A small smoldering fire is going to be hard to detect, because most people don't have smoke detectors in their attics
3) There's often lots of other wires and conductors in attics, and since attics tend to be crowded, your antenna is likely to be close to those wires, so the chance of inducing a current into them is fairly high.
4) The attic is close to human occupation, so RF safety is going to be tricky.  You've got lots of "other stuff" in the near field, so using the "dipole above the ground" safe-harbor limits probably isn't appropriate.

That said, I think you could safely put up an antenna in the attic:
1) appropriate insulators and clearances
2) check if you're coupling to other stuff.  Does the VSWR or feedpoint Z change with power?  That's a pretty good sign that something "bad" is going on.
3) be conservative on the RF exposure.

I'm not sure that a fan dipole or fat elements would reduce the voltage at the end of the elements, it would depend on the construction details.  The other thing is that what you're really worried about is the E-field (whether it will spark) more than the absolute voltage.  A physically large element would have a lower field, but not all "cage" type constructions have lower E-field

The RF exposure issue is a big deal: if you're running a kilowatt, I'd say you'd have a hard time meeting the limits with an attic antenna.  You're hardly in the "far field" so trying to use a point source approximation isn't valid (a lot of the online calculators do this..because it's in OET Bulletin 65).  You can work out all your averaging intervals and duty cycles: if you do RTTY contests you're probably in a stickier situation than if you are running SSB and mostly listening.

FCC and EPA tests a few years back measured fields from a 20m attic dipole as ranging from 7-100 V/m http://www.arrl.org/table-9-2.   Bear in mind that the exposure limit is in the 20-60 V/m range, depending on frequency and whether you consider it occupational exposure or general public.
Run up to a kilowatt, and the fields go up by a factor of about 3 (square root of power).

BTW, the easy to use rule of thumb from RSGB is that if you're running 400W, the minimum safe distance from a dipole is the frequency in MHz: 14 feet on 20 meters.
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KK4OBI
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Posts: 5




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« Reply #16 on: November 19, 2013, 02:15:04 PM »

Ah, memories. It was 1945. WWII was just over. The ban on amateur radio was lifted. Everyone was jumping back to hamming however they could.  Dad (W0KCL, 1933) rigged a dipole in the attic with drooped ends and fired up his rig and 1000w final. After a while we smelled smoke.  There was a steel glider-type porch sofa near the end of one of the dipole ends. RF arced over and set the cushions on fire.

After the firemen left and dad resumed hamming, I noticed that the kitchen lights would not turn off when dad was transmitting. I noticed also the neighbor also having this problem. RF was everywhere. We even could hear his voice transmissions outside where a bad solder joint in the gutter was acting like a diode and demodulating his RF. Ah, memories. This was back when you tuned for minimum standing waves by running a fluorescent tube along the feed line or touching it with your tongue for the least acid taste.
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WB6BYU
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Posts: 13237




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« Reply #17 on: November 19, 2013, 03:37:38 PM »

Several factors affect the voltage at the end of an antenna even when the
power level is held constant:

1) the wire diameter:  a fat wire (like a cage) will have lower voltage

2) the radiation resistance:  a shortened antenna (coil loaded, folded, or
matched with an antenna tuner) will have higher voltage at the ends than
a full-sized one.

3) The capacitance at the end of the antenna.  Adding a capacity hat or
something like the old copper toilet ball will lower the voltage.


So a 20m dipole fed with ladder line will have higher end voltages when
operated on 40m than on 20m.  Since many attic antennas are folded or
otherwise shortened, voltages may be higher than expected.

How much voltage are we talking about?  If we assume a standard dipole
has a 50 ohm radiation resistance and the wire represents a 600 ohm
transmission line, then we'd expect an impedance around 7200 ohms at
the ends.  Practical experiments with end-fed antennas suggest that
something in the 2000 to 5000 ohm range may be more typical.  (The
higher the impedance, the higher the voltage.)  Let's assume 5000 ohms
for a start.  Running 100W the voltage across 5000 ohms would be
707V RMS or 1000V peak.  At 1.5kW it would be almost 4000V peak.

Basically similar to what we used to encounter in the plate circuit of
tube transmitters at similar power levels.

1000V isn't too big of a deal - the insulation on standard house wiring
is usually rated at 600V.  An inch of dry air is more than necessary -
we often use variable capacitors with far less spacing than that in
antenna tuners, where they may be subject to high voltages as well.
But providing a few inches of space around the end of the wire is
probably sufficient in most cases.

A foot of clearance is probably enough at high power, though you
will need to give better consideration to the types of insulators you
use, especially with shortened antennas:  ceramic would be better
than some scrap plastic that you don't know the properties of. 
Designing your clearances for 10kV should give enough of a safety
factor to handle most types of antennas, though I'd still be wary
of any sort of grossly shortened antenna.

(To be honest, I'd be wary about running a kW to an attic antenna
anyway for other reasons.)
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W6RMK
Member

Posts: 651




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« Reply #18 on: November 19, 2013, 10:12:24 PM »

Several factors affect the voltage at the end of an antenna even when the
power level is held constant:
absolute voltage actually isn't the important thing here.. it's the field. A 1 foot diameter sphere at 10kV isn't a problem. A needle point at 10kV is. 
Quote
1) the wire diameter:  a fat wire (like a cage) will have lower voltage
Not all cages have lower field. A fat *solid* conductor with a rounded end will have a lower field.  A solid metal tube that just "ends" will not. Some cages will have a lower field, some won't: a lot depends on the details of the construction. Are the ends of the wires in the cage connected with a ring of some sort? what's the radius of curvature, etc.

A fat dipole (or bicone, or fan, etc.) has a wider SWR bandwidth, but that doesn't change the voltage as much.
Quote
2) the radiation resistance:  a shortened antenna (coil loaded, folded, or
matched with an antenna tuner) will have higher voltage at the ends than
a full-sized one.
I'm not sure about that one. A physically small antenna often has a large reactive component to the feedpoint Z, and a lot of stored energy in any case. Stored energy makes for high voltage in the C and high current in the L.   But it's not because of the low radiation resistance.  I would say that small antennas have low radiation resistance AND high stored energy: more correlation than causation.
Quote
3) The capacitance at the end of the antenna.  Adding a capacity hat or
something like the old copper toilet ball will lower the voltage.
It's not the capacitance that is changing the voltage: The voltage is the same (for the most part).. what you're really doing is increasing the radius of curvature, so for the same voltage, the field is smaller (volts/meter), and volts/meter is what causes sparks.  Actually, having a capacity hat or ball or toroid will *increase* the peak voltage. A sharp point will have corona and arc, which limits the voltage, while the gently curved surface of the ball will support a much higher voltage before breakdown.  Van deGraaff generators and Tesla Coils are fine examples of this.
Quote

So a 20m dipole fed with ladder line will have higher end voltages when
operated on 40m than on 20m.  Since many attic antennas are folded or
otherwise shortened, voltages may be higher than expected.

How much voltage are we talking about?  If we assume a standard dipole
has a 50 ohm radiation resistance and the wire represents a 600 ohm
transmission line, then we'd expect an impedance around 7200 ohms at
the ends. 
I'm not sure where that 600 ohms comes from.  And how you get from 50 to 7200.  It's actually pretty hard to calculate what the voltage at the end of an antenna is, especially with a simple equation. Trying to back into it by looking at the feedpoint Z of a doublet running at twice the resonant frequency isn't a particularly good way.

The only real way to do it is with a finite element model, and integrate along the conductor. NEC can do this, but it won't give you total voltage: you'd have to add up the voltages along each segment.  Nor does NEC give you the E-field at the surface of the wire (because NEC is a method of moments code, and it is worried more about accurately solving for the *current* in each segment).  One of the other 3-D FEM codes would probably give you an accurate E-field, but most hams aren't running HFSS or similar. And even then, for resonant structures, it's hard to predict breakdown.

Quote
Practical experiments with end-fed antennas suggest that
something in the 2000 to 5000 ohm range may be more typical.  (The
higher the impedance, the higher the voltage.)  Let's assume 5000 ohms
for a start.  Running 100W the voltage across 5000 ohms would be
707V RMS or 1000V peak.  At 1.5kW it would be almost 4000V peak.

Basically similar to what we used to encounter in the plate circuit of
tube transmitters at similar power levels.

1000V isn't too big of a deal - the insulation on standard house wiring
is usually rated at 600V.  An inch of dry air is more than necessary -
we often use variable capacitors with far less spacing than that in
antenna tuners, where they may be subject to high voltages as well.
But providing a few inches of space around the end of the wire is
probably sufficient in most cases.
This is where the problem is.  The deal isn't so much the absolute voltage, it's all about the field strength in V/m. If the field is bigger than 3kV/mm (70kV/inch), air breaks down.  What you need to do is know the radius of the conductor.  A bare 1mm radius conductor (AWG 18) will break down at 3kV.  Or, more important, a 10mm radius conductor ( a bit less than 1/2 inch), but with a blob of solder that has a sharp point that's less than a mm across will break down.

Or some semiconductive dust with sharp corners.

If the voltage is less than about 350 V, it can't break down (called the "minimum sparking voltage") regardless of how close the gap or small the radius.
Quote

A foot of clearance is probably enough at high power, though you
will need to give better consideration to the types of insulators you
use, especially with shortened antennas:  ceramic would be better
than some scrap plastic that you don't know the properties of. 
Designing your clearances for 10kV should give enough of a safety
factor to handle most types of antennas, though I'd still be wary
of any sort of grossly shortened antenna.

The rule of thumb is 1" clearance per 10kV (factor of 7 safety), and 1/4" radius of curvature per 10kV (factor of about 2).  That's surprisingly fat: 1/2" diameter for 10kV.

For breakdown along an insulator surface, the distance needs to be at least 3 times the free air breakdown (this is why insulators have grooves or fins). So, for 10kV, free air breakdown is 1/7th inch, so you'd want the surface distance on a *clean* insulator to be about 1/2".

A real issue you're neglecting here, though, is that an attic antenna is next to other conductors. And while your antenna may be nice and smooth, and suitably curved, it could be inducing a high voltage somewhere near by.  Like that roofing nail sticking through. Or that sheet metal screw holding the AC duct, etc.   There are lots of well documented stories of people having problems with this running Tesla Coils in their garage, and wondering why the ceiling caught on fire or smoldered, when none of the sparks went anywhere near there.

If you go see the HV display at the Boston Museum of Science, you can see this effect demonstrated really, really well, as the big Van deGraaff charges, and you see the corona on all the other stuff around it.  You see it with their Tesla Coils too, but it's harder to spot.
Quote
(To be honest, I'd be wary about running a kW to an attic antenna
anyway for other reasons.)

yeah, the RF exposure issue is a dicey one, for sure.

You also might be cooking the electronics in things like your solid state fluorescent ballasts (been there, done that) or smoke alarms (not a good thing) or garage door opener (killed one of those)
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N4GKS
Member

Posts: 84




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« Reply #19 on: November 20, 2013, 06:21:24 AM »

The risk is enough that I wouldn't do it for the simple fact that your homeowners insurance would have a cow if they found out. Also, if it did cause a fire, would insurance cover it? It's a risk no matter how minuscule. I'm sure some of you have seen this video:

http://www.youtube.com/watch?v=_2-4MLonbkk
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W6RMK
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Posts: 651




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« Reply #20 on: November 20, 2013, 06:35:44 AM »

Excellent video demonstration of the problem.
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AA4PB
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Posts: 12832




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« Reply #21 on: November 20, 2013, 06:43:32 AM »

That video is designed for worst case - PVC pipe for an insulator and the side right up against the tree. That's kind of like saying you can get electrocuted if you stick your finger into an outlet therefore we shouldn't put electrical outlets in our houses.

If you use proper installation techniques you won't have any arcing problems with 100W to an attic antenna. If you do it incorrectly, as shown in the video, you can burn down your whole neighborhood.

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N4GKS
Member

Posts: 84




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« Reply #22 on: November 20, 2013, 11:40:33 AM »

That video is designed for worst case - PVC pipe for an insulator and the side right up against the tree. That's kind of like saying you can get electrocuted if you stick your finger into an outlet therefore we shouldn't put electrical outlets in our houses.

If you use proper installation techniques you won't have any arcing problems with 100W to an attic antenna. If you do it incorrectly, as shown in the video, you can burn down your whole neighborhood.



I've seen a number of ham shacks that had fire hazards galore. Ladder line run down walls full of insulation, no grounds, 10 plugs plugged into one outlet with screw in fuses. I'm just saying running an antenna in your attic with dry wood and plenty of air rushing through the vents fanning an RF fire is a chance I wouldn't take for an S9 signal to anywhere. With the modern rigs, you can buy a cheap computer and remote from a friends house who has room for a wire in his yard and would not mind. Or, put up a pole you can raise at night and lower before daylight if your neighbors are uptight @$$holes.
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W6RMK
Member

Posts: 651




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« Reply #23 on: November 21, 2013, 07:17:53 AM »

That video is designed for worst case - PVC pipe for an insulator and the side right up against the tree. That's kind of like saying you can get electrocuted if you stick your finger into an outlet therefore we shouldn't put electrical outlets in our houses.

If you use proper installation techniques you won't have any arcing problems with 100W to an attic antenna. If you do it incorrectly, as shown in the video, you can burn down your whole neighborhood.


Yes. I agree that video is a worst case.  I started a fire in a tree with less than 50W.

But the challenge, even at 100W, is "what is proper installation" in an attic?

It's not like it's a well controlled environment.  Even at 100W it's easy to get high voltages that can start a fire, and those voltages might be on something that is NOT the antenna.  It's easy to make sure the antenna is sufficiently well insulated, has appropriate radius of curvature, and far away from things.  Not so easy to know if something else happens to be resonant, and sharp edged.  It's not like you need to see 5 foot tesla coil style sparks for there to be a problem. A steady corona discharge as you work PSK31 might do the trick to start the smolder.

If I had to do an attic installation, and I were concerned, here's what I would do:
hook up a good measurement system that can see short duration changes in reflected power from the load. (a peak reading meter of some sort)

Borrow an amp, so you can run 2-3x the maximum power you expect to run.

Fire it up and sweep the frequency at low power and look for anomalies in reflected power (narrow band resonances).  Record the reflected power at each frequency.  Ramp up the power, and do the sweeps again, look for a power dependent change in reflected power.  If you see that, you have a good clue that something is breaking down with the change in voltage.

If you can run all frequencies, at powers that are well over what you ever expect to run, then you're probably in pretty good shape from a fire hazard standpoint.  Then, you just have to worry about RF exposure.  (and there, since there are "conductive things" in most people's attics, the peak exposure may be nowhere near the antenna)



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WA9UAA
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Posts: 314




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« Reply #24 on: January 14, 2014, 09:43:41 AM »

Does a full wave loop antenna have any thing near the high voltage points that exist on the ends of a dipole?
73,
Rob
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KF5KWO
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Posts: 52




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« Reply #25 on: January 27, 2014, 10:50:39 AM »

I'm considering putting up a dipole or loop in my attic. Would putting up a smoke detector or two (long attic) be advised, or overkill?  I'm thinking it would be a good idea, especially to give peace of mind to the family.

73 de Jeff, KF5KWO
Helotes, TX
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AA4PB
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Posts: 12832




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« Reply #26 on: January 27, 2014, 11:33:45 AM »

It'll probably get set off by the RF from the antenna.
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KF5KWO
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Posts: 52




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« Reply #27 on: January 27, 2014, 01:53:06 PM »

It'll probably get set off by the RF from the antenna.


Ah, thanks.  I now recall that my wire loop around the eaves of the house was setting off my alarm a few years back when I would use it on 20. 80/40 were fine, just not 20.

73 de Jeff, KF5KWO
Helotes, TX
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