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Old 8th Jul 2021, 8:12 pm   #1
Radio Wrangler
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Default RF interference from a filament bulb

This is split from a thread on interference from LED lamps. It had been mentioned that an interference problem affecting VHF airband comms had been traced to a light bulb... an incandescent filament bulb. People were wondering how this could be.

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Old 8th Jul 2021, 8:42 pm   #2
Lloyd 1985
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Default Re: RF interference from a filament bulb

Maybe something else on the mains was causing the interference, and the bulb made a very good antenna for it, but only when switched on?! I've heard filament bulbs 'sing' before, normally shortly before they die, but never had any RF interference from them. Could it have been a lower voltage bulb, but had a diode fitted in the base to drop the voltage a bit? It would be nice to be able to tear down the bulb to find out why!

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..maybe I'll buy one and see!
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Old 8th Jul 2021, 8:59 pm   #3
Graham G3ZVT
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Default Re: RF interference from a filament bulb

OK, I'll bite.
This is the context we have to go on.
https://www.ofcom.org.uk/about-ofcom...rference-issue

That style of writing is something I expect from the Daily Mail, not a government-approved regulatory body.

Given the non-academic style, is there any evidence the image of the tungsten filament lamp is the actual lamp in the case, and not a random stock image?

In the absence of a proper report, all bets are off.
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Old 8th Jul 2021, 9:05 pm   #4
kellymarie
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Default Re: RF interference from a filament bulb

I've heard of this before apparently its to do with the length of filament and the fact its in a vacuum causes electron emission from the filament this makes it oscillate at a high frequency this effect was first mentioned in wireless world in the 1950s where the issue was affecting band 1 TV chanel 3 around 67 megacycles it showed up as black bars on the screen caused by the 50 cycle hum
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Old 8th Jul 2021, 9:18 pm   #5
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Default Re: RF interference from a filament bulb

Yes, I'm sure this was mentioned in a forum thread some time back, a Barkhausen-Kurz type mechanism involving boiled-off electrons being accelerated and decelerated by the filament potential gradient. A filament light bulb seems so innocuous at first glance!
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Old 8th Jul 2021, 9:23 pm   #6
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Default Re: RF interference from a filament bulb

Quote:
Originally Posted by turretslug View Post
A filament light bulb seems so innocuous at first glance!
I had tended to think of filament lamps as being essentially resistive, like a heating element, but the equivalent circuit must be considerably more coplex for it to emit RF radiation.

PMM
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Old 8th Jul 2021, 9:44 pm   #7
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Default Re: RF interference from a filament bulb

Yes, I've definitely heard of similar problems before. There exists the (maybe apocryphal) tale of the military shoe cleaning kit which had to be sent for EMC testing. Rules is rules, and all that. It was duly tested - and failed! It contained a lamp, tungsten filament, which turned out to be emitting RF.

At the time I heard the tale, the mechanism was explained to me as something to do with plasma forming in the gas-filled bulb close to the very hot filament, but a Barkhausen-Kurz type mechanism sounds also plausible. With a very long filament (such as the one in the picture on the Ofcom page) I can imagine that there'd be enough of a potential gradient between one end of the filament and the other for some portion of each mains cycle to make oscillation possible.

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Old 8th Jul 2021, 9:46 pm   #8
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Default Re: RF interference from a filament bulb

In vacuum and hot, not all electrons travel the entire length of the 'resistor' some are thermally emitted from the filament and travel alongside it, attracted to the far end when it's positive. Magnetic fields can cause spiralling as well. Then there is the Barkhausen-Kurz effect on top.

You know the old saying that every amplifier wants to oscillate? Well, every damned thing wants to oscillate.

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Old 8th Jul 2021, 10:11 pm   #9
Mr 1936
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Default Re: Severe MW interference solved

Hi

There is a discussion on the "RFI from Incandescent Bulb" effect here:

http://bulbcollector.com/forum/index.php?topic=1101.0

I suspect that the filament layout of the "vintage style" bulb may be a contributor, as it has several points with both close spacing and a significant potential difference between them. There may also be issues with the vacuum maybe not being quite as "hard" as it should be.
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Old 8th Jul 2021, 10:54 pm   #10
kellymarie
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Default Re: RF interference from a filament bulb

Hello radio wrangler thats the effect the wireless world report i had forgotten some of it but your post reminded me
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Old 8th Jul 2021, 11:17 pm   #11
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Default Re: RF interference from a filament bulb

I have seen this when an infinitesimal break in the filament effectively creates an arc transmitter. It is usually a sign that it will be blowing very soon!
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Old 9th Jul 2021, 12:23 am   #12
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Default Re: RF interference from a filament bulb

Was it a purely filament bulb though? I would suspect a modern variation with some nasty circuitry to drive it.
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Old 9th Jul 2021, 1:26 am   #13
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Default Re: RF interference from a filament bulb

A filament lamp can certainly produce RF interference. Parts of the lamp function as a very crude thermionic valve that can self oscillate.

Most lamps do not produce this interference, but the odd one can, it depends on the exact spacings between parts of the filament and the supports.
Supply wires of exactly the "right" length and orientation will assist in radiating the interference.

The problem largely went away when coiled filament lamps replaced the older squirrel cage filament lamps. Squirrel cage filament lamps are now being mass produced again for the retro look.
Some types use LED arrays with a switched mode driver, this can produce RFI.
But the genuine incandescent lamps with no electronics can do this due to thermionic action.

The risks may be reduced by use of lower voltage lamps supplied from a transformer or from accumulators.
Wiring to the lamps in earthed steel conduit helps reduce radiated interference because the capacitance between the supply wires and the conduit is virtually a short circuit at radio frequencies.

If interference is detected, simply replace the lamp, another one of similar design may be free from interference due to manufacturing tolerances.
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Old 9th Jul 2021, 8:20 am   #14
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Default Re: RF interference from a filament bulb

I assume a vacuum bulb would be needed to work, those decorative tungsten lamps with the long filaments are vacuum ones, the rest argon (or similar) filled.
 
Old 9th Jul 2021, 8:48 am   #15
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Default Re: RF interference from a filament bulb

I have come across similar noise. In this case it sounds like the explanation is slightly different though. In this case generated by a heating element. The heater involved was printed on glass, and had multiple elements. One of these had a scratch so it wasn't conducting normally. This caused interference on a nearby radio whenever the heater was on.
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Old 9th Jul 2021, 9:27 am   #16
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Default Re: RF interference from a filament bulb

Yes, vacuum filament lamps are more liable to self oscillation than are gas filled lamps.
Modern coiled filament lamps for 220/240 volt circuits are usually vacuum up to 25 watts, and gas filled for higher wattage lamps. 40 watt vacuum lamps exist mainly for outdoor use.

Squirrel cage filament lamps are generally vacuum in any wattage.
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Old 9th Jul 2021, 10:04 am   #17
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Default Re: RF interference from a filament bulb

I have used a 60W bulb as an RF dummy load, but not like this :

https://qrper.com/2018/11/the-2019-lightbulb-qso-party/
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Old 10th Jul 2021, 9:27 am   #18
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Default Re: RF interference from a filament bulb

I have a few squirrel cage lamps so it's worth an experiment to see if anything can be detected from them. They look like they are made with great precision so as far as filament layout is concerned I should think they should be very similar. Hardness of vacuum etc of course is impossible to know.

Thanks to David for moving/starting the thread. It has turned up some interesting comments. Until today I missed what you posted at the end of the MW interference one - so thanks once again.
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Old 10th Jul 2021, 11:16 am   #19
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Default Re: RF interference from a filament bulb

Quote:
Originally Posted by GMB View Post
I have seen this when an infinitesimal break in the filament effectively creates an arc transmitter. It is usually a sign that it will be blowing very soon!
I have come across this causing interference too.
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Old 10th Jul 2021, 5:52 pm   #20
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Default Re: RF interference from a filament bulb

I've not come across oscillations from an ordinary incandescent light-bulb but do remember something about unscheduled Barkhausen-Kurz oscillations being an issue in some TV line-output stages, and how these could be suppressed by strapping a small magnet to the side of the valve 'bottle'ยท
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