Antennas in attics and fire risk

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eswnl
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Antennas in attics and fire risk

Post by eswnl »

I know that voltages in antennas can sometimes arc across to metal objects or across the antenna itself. This may cause a fire risk as attics are full of combustibles.

However, I have not heard of any case involving fires and antennas nor does there seem to be any rule about it. Is there a power level at which arcing becomes a risk? eg. 50W

We have lots of cardboard boxes in the attic.
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26CAB40
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Re: Antennas in attics and fire risk

Post by 26CAB40 »

Depends on the antenna.

A magnetic loop probably generates the highest nearfield voltages, I'd wouldn't have one inside myself.
Anything electrically short with loading coils is likely to generate large electrical fields.
The safest bet is a full wave loop, very very low voltages generally.
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Re: Antennas in attics and fire risk

Post by eswnl »

thanks for the reply.

How about a half size G5RV running around the edges of the attic in a loop?
A G5RV doesn't have loading coils or anything.
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26CAB40
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Re: Antennas in attics and fire risk

Post by 26CAB40 »

A G5RV is not so bad within it's proper operational range.
So 20m up to 10m should present no serious issues.
If however, you decide to try and 'tune' it down to 40m or even god forbid, 80m
using an ATU, you'll soon start to see high voltages and common mode currents.

I have in the past tried a half-size G5RV in the loft of my old QTH, and even on it's
nominal bands it coupled into the mains wiring and caused the lights to flicker on 100w.

What bands do you want to operate ?
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Re: Antennas in attics and fire risk

Post by eswnl »

26CAB40 wrote:A G5RV is not so bad within it's proper operational range.
So 20m up to 10m should present no serious issues.
If however, you decide to try and 'tune' it down to 40m or even god forbid, 80m
using an ATU, you'll soon start to see high voltages and common mode currents.

I have in the past tried a half-size G5RV in the loft of my old QTH, and even on it's
nominal bands it coupled into the mains wiring and caused the lights to flicker on 100w.

What bands do you want to operate ?
I have no preference for bands, so I bought the half g5rv + tuner. The attic is about 7m by 3m.
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26CAB40
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Re: Antennas in attics and fire risk

Post by 26CAB40 »

Does your tuner have a 4:1 balun on the back ?
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Re: Antennas in attics and fire risk

Post by eswnl »

26CAB40 wrote:Does your tuner have a 4:1 balun on the back ?
My tuner is an automatic LDG-897. This has only the 2 coax connectors on the back and nothing else(so it does not have a 4:1 balun, if my understanding of a balun is correct).

The G5RV has a coax connector also, so this links to the tuner by a length of coax.

The LDG manual does say that with an optional balun(presumably the one you mention) connected, the tuner can feed a long-wire or ladderline.

Perhaps, I could have one half of the g5rv in the attic and the other half coming out the roof and into the garden. Or put the whole g5rv outside, but attic seems a lot easier and less messy.
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Re: Antennas in attics and fire risk

Post by JamesA »

eswnl wrote: The attic is about 7m by 3m.
That would give you room to put a full wave 20M loop up. Just run 20 meters of some reasonable thickness wire right round the attic and connect both ends to a 4:1 balun, then coax back to your tuner. Should tune 20 17 15 12 and 10 no trouble. You may also be able to get it to tune up as a half wave loop on 40 too, depending on your tuner. My LDG z100 will tune my full wave 40m loop on 80 ok.
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Re: Antennas in attics and fire risk

Post by 26CAB40 »

Actually, you can run a loop several times around the loft with little change in radiation pattern.
So you could run an 80m full wave loop around the loft connected to a 4:1 guanella/current balun and
work 80-10m easily.
Horizontal loops have a high angle of radiation on the lower bands, so effective for inter-g and Europe,
but a lower angle above 30m so not bad for DX on 20m and above.

For some good tips, take a look at this.

http://www.sgcworld.com/Publications/Bo ... thbook.pdf
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Re: Antennas in attics and fire risk

Post by JamesA »

Was thinking of trying that with my 40M loop to get better efficiency on 80. Nice PDF that some useful stuff in there, even if you dont have an SGC Smarttuner :)
M6JCA -> 2E0JCA

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Re: Antennas in attics and fire risk

Post by eswnl »

In the attic, do you rest the wire on the wooden beams or is the wire suspended from the wood using insulators?
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Re: Antennas in attics and fire risk

Post by 26CAB40 »

I used electric fence insulators.
You can buy them dirt cheap on ebay and they come in handy for all sorts of antenna projects.
Some wag has even started selling them in pairs in the radio section for crazy money !!
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Re: Antennas in attics and fire risk

Post by eswnl »

26CAB40 wrote:I used electric fence insulators.
You can buy them dirt cheap on ebay and they come in handy for all sorts of antenna projects.
Some wag has even started selling them in pairs in the radio section for crazy money !!
Managed to get 25 for £9 from a shop down the road.
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Re: Antennas in attics and fire risk

Post by scooterman »

speaking as a former firefighter with 22+ years experiance - your gretest "risk" is filling your loft with flammable materials - - i would not worry too much about a properly insulated antenna -unless you are running power - but more about having a smoke alarm up there ( wired to one down where you can hear it ) - a fire extinguisher suitable for domestic risks - and ensuring you have safe and rapid method of egress from the building for your family ;)

also IF you have any LIVE chimneys - getting them checked internally and externally - especially in the roof space and NOT piling stuff against them !!

the safest loft is an EMPTY LOFT - been to a few fires where contents have exacerbated situations that would otherwise been easy to deal with or would not have happened at all if simple precautions had been taken

if you need further advice your local fire prevention dept will oblige - they are in the phone book

stay safe out there :D
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Re: Antennas in attics and fire risk

Post by eswnl »

scooterman wrote:

the safest loft is an EMPTY LOFT - been to a few fires where contents have exacerbated situations that would otherwise been easy to deal with or would not have happened at all if simple precautions had been taken

:D
Oh dear, we have loads of cardboard boxes up there.
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