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Author Topic: Attic Dipoles a Fire Hazard?  (Read 713 times)
W0FM
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« on: August 14, 2001, 12:01:15 PM »

I am (unfortunately) a 40 year veteran in the use of attic antennas.  Currently, I use several multi-band dipoles in the attic of my two story home with surprisingly acceptable results. Until now I have never run more than 100 Watts to these attic dipoles. Now, I am adding an amp to the station. In reading every post I can find on attic antennas I recall seeing a few references to fire hazard. I cannot remember ever reading anything about potential fires caused by attic antennas in well known ham publications, but, then again, I can't say I was ever looking for the subject. All of the elements of my attic dipoles are made with insulated wire and the ends are tied off with insulators and nylon cord. At the nearest point, an insulated wire element comes within maybe an inch or two of the wooden attic truss work. There is no metal nearby to cause possible arcing.  While I am aware of the RF energy exposure issues to occupants of the house (I am usually alone in the basement of the two story house when I have time to operate), should I also have an additional concern about a fire hazard if I increase my power to these antennas? Are there documented cases of fires started by attic dipoles and, if so, what were the potential circumstances? Thanks. Terry, WØFM  
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W0JOG
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« Reply #1 on: August 14, 2001, 12:37:03 PM »

I wouldn't worry about it, Terry.
What you describe is sound construction practices and I've never seen that much heat generated in a resonant antenna. The coax will likely melt down first. c(;<)

73 de Vern, W0JOG
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NB6Z
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« Reply #2 on: August 14, 2001, 12:51:09 PM »

I think you would have more risk of fire in the shack than in the roof. Keep proper fuses on the amps supply...
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G4AON
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« Reply #3 on: August 14, 2001, 02:31:00 PM »

Besides the obvious problem of being near high levels of RF, there is a possible arcing hazard due to high voltages being induced in nearby wiring, and other metal work, both in the loft and upper floors of the house.

Running QRP to a loft antenna is OK, running 100 Watts is getting marginal and more than 100 Watts doesn't sound safe at all!

To check out your compliance with FCC regulations see the following site which will automatically calculate the field strength for you. The figures given assume an outdoor antenna, an indoor antenna could give much higher field strengths due to re-radiation from house wiring, plumbing, etc.

http://n5xu.ae.utexas.edu/rfsafety/

Also see the ARRL page on RF safety at:

http://www.arrl.org/news/rfsafety/

Dave

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KE3YO
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« Reply #4 on: August 15, 2001, 08:28:02 AM »

I'd just like to know how you run 100 watts through an attic antenna and avoid making every telephone in the house unusable, much less 1000 watts.  I've tried that before and had nothing but RFI problems.  
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W0FM
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« Reply #5 on: August 15, 2001, 10:22:20 AM »

For Mark, KE3YO:  Good question, Mark.  I should have provided more info on my particular situation, because it is quite unique.  You see, after having to live with attic antennas for years, I had a little bulb go off when I finally was able to design and build my own home from the ground up.  I saw this as a great opportunity for a ham who already knew he would face antenna restrictions from both the neighbors and the XYL!

The house is two story with 9 foot ceilings.  That makes about 10 FT per floor.  From the floor in the attic to the peak of the roof is a little over 17 feet.  So, my attic dipoles are around 37 feet above ground.  When I designed the house, I specified to the sub contractors that their installations should have little or no wiring in the attic.  I walked through with each of them prior to them doing their installs (the builder still hates me!).  With a few exceptions, all telephone, A/C, thermostat, stereo and alarm wiring stops at the outlets on the second floor or below.  Shielded cable was used on everything except the 120VAC.  The HVAC system was designed so there is no metal ductwork or pipes in the attic.  The whole works was designed very RF friendly.  Obviously, I viewed this as a "once-in-a-lifetime" opportunity.  I also had the architech design a clear space in the attic, free of studs and trusses, that was "supposed" to be roughly 20x20 feet and 8 feet tall (like a small room without drywall), so that I could mount and rotate a small tribander up there.  The attic is floored off with plywood and there are plywood "catwalks" built in two directions over the bare rafters so tweaking antennas would be easier and I would not unexpectedly drop in on someone on the floor below!  All that added only $887.00 to the total cost of the home.  
I do experience some RFI on 40 Meters, but, since I don't operate 40 that much anyway, I've never tried to correct it.  I have a good grounding system and try to engineer the station to be as clean as possible.  Oh yes, and throw in a spoonful of LUCK!  73, Terry, WØFM  
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NB6Z
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« Reply #6 on: August 15, 2001, 12:24:05 PM »

Terry,

Designing homes that are "RF friendly" sounds like a good cottage industry. ;-) I assume that a wire in a tree is not an option for you.

To be on the safe side, as you increase the power to your antenna system for the first time, monitor the SWR carefully. Watch for any sudden changes in SWR as you increase power. The SWR will probable change with more power, but it should still be "reasonable" by the time you reach full power.
I agree with the other post that you should be concerned about the RF levels you expose yourself and others in the house to...

GL, Griff
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W0FM
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« Reply #7 on: August 15, 2001, 03:27:56 PM »

Hi Griff,  
Yep.  Wish I had the time and knowledge to start that little business.  Maybe, with the help of others, I could write a little "handbook" on building a ham radio home :-}  As far as wires in trees, I have several.  I have experimented with most everything over the years.  Have loaded the gutters, flagpoles, window screens, "back-to-back" Hamstick dipoles, inverted L's and simple wires tossed over the peak of the roof.  They ALL work, but nothing as well as the Alpha Delta DX-EE multi band dipole in the attic.  As far as RF exposure, as I said, I am normally home alone during the times I operate and my shack is in the basement, some 50 feet below the antenna.  Forgot to mention that I had the builder include a 4" PVC pipe direct from the attic to the ham shack.  Running or changing coax is a breeze!  73, Terry
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WB2WIK
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« Reply #8 on: August 16, 2001, 02:12:26 PM »

I wouldn't be quite so cavalier about the possible fire hazard issue.  As one who started a fire, myself, when operating with a kW on 80 meters, it's pretty scary stuff.

In my case, the antenna was installed completely outdoors, an inverted vee supported by my 55' tower in the center, with two drooping ends coming down to about 15' above ground on each end.  One end was tied off with a long rope to a tree trunk.  The other end was tied off with a long rope to a fencepost; however, that end of the antenna wire itself came within about 4"-5" of the side of my home, which was composite (looks just like wood) shingle construction.  

Operating on 75m SSB one day with the kW, I did note that my SWR was "bouncing around," but figured it was just the effect of wind blowing the antenna around a bit.  I didn't think much of it.  Later, upon walking outside to take in the mail, I noticed a huge, black char mark on the side of the house, close to where the end of the 75m inverted vee was.  Upon closer inspection, I could tell the side of the house literally caught fire but had extinguished itself, probably due to new, flame-retardant construction materials used.  The SWR changes noted were likely the arcing from the antenna tip to the house siding, which was changing the antenna impedance as it arced.

Arcing need not be to anything metallic.  Wood, being organic, is semi-conductive, and conductive enough; however, arcing to a nail head is even more likely.  If the nail head is attached to a nail holding wooden rafters or beams in place, you've got potential for a fire.

Obviously, the highest voltage point of any dipole or doublet antenna is right at its tips (ends); the highest current point is at its feedpoint.  The tips are what you need to be careful about: With 1500W PEP output power, the peak voltage at the element tips of a doublet are likely to exceed 2000V (E = square root of P x R) because "R" is likely to be 2000 Ohms or more.  If "R" were 2000 Ohms, E would be 1732Vrms, which, x 1.414 (square root of 2) becomes 2500Vpk.  That will travel a distance through dry air, for sure.  But if you have a really high-Q dipole, "R" could be much higher, possibly 20k or so, which makes Vpk = 7745V.  Quite a lot of RF voltage.  Probably that was the case in my own example, since the end of the wire was inches away from the house siding, and it arced, anyway.

73 de Steve WB2WIK/6

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AB8BC
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« Reply #9 on: August 17, 2001, 03:51:03 PM »

I agree with Steve!!  I had a similar experience only my dipole was in a tree (both legs).  It was near a metal staple (about 4") and arced several times.  Although no harm done, it could have been more scary inside a home!  Take great care and good luck!
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