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Author Topic: Fire Hazards with Attic Antennas  (Read 27917 times)
KT0DD
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Posts: 277




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« on: May 01, 2012, 07:38:23 AM »

Just what are the fire dangers with arcing and attic antennas? I am putting up a 140' seriously folded around doublet. I am making sure none of the wire comes within 1 foot of any metal objects, and I am able to get "fair" separation between the legs. I am feeding it with 450u ladder line. I will be using a Kenwood TS-480 HX 200 watt rig and an MFJ 949E tuner. Thanks for any advice. 73. Todd - KT0DD
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N4UM
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Posts: 457




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« Reply #1 on: May 01, 2012, 07:56:31 AM »

I have an almost identical setup...  TS-480HX (plus an Alpha 76CA 1.5 KW amp) using 450 ohm ladder line to feed a dipole about 140 feet long that's wound around all over my attic.  I use 12 inch standoff insulators made from cut up Lexan cutting boards.  Maximum power I've put to the setup is about 900 watts on 80 meters.  No apparent problems, no signs of arcing  and no fires yet.  Normally I run about 100 wats.  I doubt that you have nmuch to worry about with 200 watts. The thing makes a decent NVIS transmitting antenna on 80 but there's so much electrical noise in the attic that it's a lousy receiving antenna.
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KQ4AR
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« Reply #2 on: May 09, 2012, 06:30:38 AM »

I have used a fan dipole and lots of verticals in my attic for years with no problems.

73 de KQ4AR
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W1AJO
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Posts: 108




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« Reply #3 on: May 23, 2012, 12:27:50 PM »

I have used a fan dipole and lots of verticals in my attic for years with no problems.

73 de KQ4AR

With how much power?
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KG4NEL
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Posts: 373




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« Reply #4 on: May 23, 2012, 12:40:34 PM »

I have used a fan dipole and lots of verticals in my attic for years with no problems.

73 de KQ4AR

With how much power?

I've been using 100W to a horizontal loop indoors and I haven't noticed any problems other than the 706's LCD getting a little wonky on 6 and 10 meters. But it's temporary until I can get an outdoor loop up, so I haven't worried about it.

Insulators on the ends of the loop are standard ceramic dog bones.
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KB2FCV
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« Reply #5 on: June 20, 2012, 08:08:30 AM »

I ran 100w to a loop in the attic for about 5 years with absolutely zero problems. I also had a 6m dipole and 2m beam up there. I would not thing 200w would be a problem either.
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WX7G
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« Reply #6 on: June 20, 2012, 11:04:19 AM »

Food grade Lexan (polycarbonate) might not be fire retardant. If the antenna were to start it on fire it might continue to burn on its own. Common plumbing PVC pipe is fire retardant.

I had an antenna ignite a sheet of polycarbonate and it continued to burn after the RF was shut off. 
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W4CNG
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« Reply #7 on: June 28, 2012, 11:53:56 AM »

I have had wire antennas in my attic for the last almost 12 years with no issues.  I have been running 800 watts out into them for the last 11 years with no sparking or arching.  I also wrote an article here on Attic Antennas back in 2000 when I got back on HF.  Lots of fun and all are resonant with no tuners used.
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K4FMH
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« Reply #8 on: November 04, 2013, 05:24:28 PM »

Here's a belated follow-up...

None who've posted here have experienced sparking issues in attic based wire antennas, even when running several hundred watts of power. Yet there are many out there who note the very high currents at the ends of such antennas.

In order to go an extra mike here, I wonder if adding an insulation "barrier" at the ends of, say, a standard dipole would be useful? For instance, the lampshade style dog collars are usually plastic and if a similar shape were placed just before the usual dipole-end insulator, would that be additional protection if using QRO power?

[/url][img][http://ep.yimg.com/ay/healthypetscom/pet-botanics-e-collars-assorted-colors-large-neck-size-9-1-2-15-1-2-4.jpg/img]
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N4JTE
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« Reply #9 on: November 04, 2013, 05:52:37 PM »

The current max is at feedpoint, high voltage at ends.
Bob
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K4FMH
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« Reply #10 on: November 04, 2013, 07:05:45 PM »

Bob,

Thanks for the correction. Thoughts on the idea?

Frank

The current max is at feedpoint, high voltage at ends.
Bob
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N4JTE
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Posts: 1154




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« Reply #11 on: November 04, 2013, 07:11:37 PM »

Todd as far as fire hazard I see nothing that's gonna be a problem as long as feed point and ends are not laying in christmas decorations!
Bob
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W1VT
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Posts: 811




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« Reply #12 on: November 05, 2013, 12:57:36 PM »

One way to lower the voltage is to lower the impedance by making the effective diameter of the element(s) larger.  This also has the benefit of making the antenna easier to match--perhaps with an autotuner in the attic.  Instead of winding a thin wire around--I'd use "fat" dipole fed in the center with an autotuner.  I'd look into aluminum wire to keep the cost down.

Zack W1VT
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K4FMH
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Posts: 254




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« Reply #13 on: November 05, 2013, 04:38:09 PM »

Thanks Zack! I already have a 80/40 dipole and a phased set of 20M dipoles installed and working fairly well with no observable issues. But I've not used my amplifier with my attic antennae yet. After reading various websites mentioning the dangers of arc-caused fire potential, I just wondered if putting an insulation barrier at the point of greatest odds of arcing (the ends)...sort of like the  plastic lampshade dog collars:



One way to lower the voltage is to lower the impedance by making the effective diameter of the element(s) larger.  This also has the benefit of making the antenna easier to match--perhaps with an autotuner in the attic.  Instead of winding a thin wire around--I'd use "fat" dipole fed in the center with an autotuner.  I'd look into aluminum wire to keep the cost down.

Zack W1VT
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W5WSS
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Posts: 1657




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« Reply #14 on: November 07, 2013, 05:41:38 AM »

Although I realize that nobody has recommended or mentioned the following but just to communicate to whom it may concern, Never use an metal staple out near the end of any indoor transmitting antenna. Always use a ceramic insulator when one needs to secure out near the ends.

The unique mutual coupling of nearby conductors CAN redistribute the voltage to current locations that normally are known. Into being somewhat different but as a rule of thumb it is always a good idea to use the best insulators for this application.

It is possible that a nearby conductor with insulation such as an electrical wire can couple enough RF to get hot
behind the walls etc.

Keep the RF exposure within specifications and one should be safe from fire when all the proper techniques are combined.

73 

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