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Topic Title: High Frequency Fluorescent light
Topic Summary: and interference
Created On: 15 April 2015 09:06 PM
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 05 May 2015 11:30 AM
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John Peckham

Posts: 8793
Joined: 23 April 2005

Mike

I am still toying with my choice of aerials. My current lash up is a diplole in the loft for cut 14MHz, no balun and a mismatched 75 ohm coax. Very poor SWR but the transceiver AMU matches it and no problem with QSLs in to Eastern Europe and the Mediterranean with 50W (1/2 power) SSB. I have a big bag of bits in preparation for the aerial choice when I make it. I have 12:1 UNEN for an end fed and also a 1:1 balun if I go for the dipole. If I go for dipole one leg will pass at an oblique angle to my and my neighbors overhead phone line hence my indecision.

In the interim I might strip out the 75 ohm coax and replace it with some 50 ohm terminated in a balun. This might keep some RF away from the computer which plays up when I am transmitting.

Sorry to pirate your thread Zs but a golden opportunity to explore an RF topic.

-------------------------
John Peckham

http://www.astutetechnicalservices.co.uk/
 05 May 2015 12:06 PM
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AJJewsbury

Posts: 16117
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You said you have Celotex insulation. If this is the foil faced stuff then you have constructed a sort of Faraday cage

But maybe not a very effective one - as part of my refurbishment I've been installing two 75mm layers of celotex (150mm total) on the outside walls - so 4 layers of foil in total - with joints on the innermost surface taped with foil tape, although nothing deliberately earthed - and there's been no noticeable effect on radio or mobile phone reception.
- Andy.
 05 May 2015 05:08 PM
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weirdbeard

Posts: 3116
Joined: 26 September 2011

Originally posted by: Zs


Maybe FM radio isn't a guide for interference



Hi Zs, I'd suggest a temporary hook -up and make a recording, see if theres any problem at the main point, then work out from there to try and solve any problems, if it's just a problem tuning the radio in maybe an external aerial or use the tv's radio function.

-------------------------
:beer)
 05 May 2015 07:38 PM
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Fm

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Joined: 24 August 2011

External wall insulation A house with low energy demands,

What type of tape do you use Andy. Tescon or an alternative.
 05 May 2015 07:53 PM
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AJJewsbury

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External wall insulation A house with low energy demands,

That's the idea!
What type of tape do you use Andy. Tescon or an alternative.

Just unbranded Al foil tape I'm afraid - (like http://www.toolstation.com/sho...Insulation+Tape/p32233 but without the words) - It's going on the inside of an established masonry house, so movement shouldn't be too much of a problem and if it did fail locally it shouldn't be a complete disaster vapour-wise unlike say a timber frame situation.

- Andy.
 05 May 2015 07:59 PM
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Fm

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And you have no concerns of the tape adhesive drying out in a few years?
What's the external finish and how do you deal with them window fills?
just waiting on the structural engineer completing my drawings and 160m2 extension to start, not looking forward to it, would rather do it at work all day but not for myself!
 05 May 2015 08:20 PM
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AJJewsbury

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And you have no concerns of the tape adhesive drying out in a few years?

Yes, and the gas escaping from the bubbles in the celotex degrading it to nothing better than 'fluffy' insulation with twice the lamba value, and the overhanging trees crashing through the roof in a storm (they're effectively protected trees so we can't do more than trim them back to 2m away) and lots of other things besides! It's all been compromise from day 1 - and still is. It's just a case of doing what I can with the skills and money available - so limited on both counts. It's been a case of trying to see the risks and mitigate them the best I can, rather than look for a perfect solution for everything. As far as the tape goes it's sandwiched between the celotex/timber studs and the plasterboard, so wouldn't go far even if the adhesive failed completely - and as a DIY job, I've perhaps been able to put a bit more effort into detailing than you might have to presume from a jobbing building with little idea of air tightness etc.

I was quite lucky with the windows - despite being a 1910 build, the frames were set 'in check' (i.e. fixed behind the outer leaf of the wall, with the reveal wider on the inside - German style these days I gather) so it was easy to bring the insulation to the edge of the frames (if with copious amounts of expending foam). Of course the insulation had to be thinner in the reveals, but I generally got 50 or 75mm to the sides/bottom and at least 25mm to the top. It should be much easier with a new build if you've got an insulated cavity and insulated cavity closers.

- Andy.
 05 May 2015 09:50 PM
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mapj1

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From an RF point of view unless the foil sheets are very large cf the wavelength, they are unlikely to provide much shielding - yes at GHz perhaps, where its more optical ray like, and the waves are less than a foot long, but not at broadcast radio frequencies. The tape is a good conductor, but the adhesive is not, and so the connection is more of a spotty occasional affair than a continuous seam and while it may look connected on the ohm meter, its likely to be full of holes at high frequencies.

@ JP - happy to digress, not sure its the right thread really, so what..
I'd be wary of anything unbalanced, and by the sound of it also within a few feet of the house wiring. A simple test would be to reverse the coax inner and outer at the dipole center -I bet there is a difference - though maybe not a good one.
An outdoor anything will be better - but of course keep the ends (which are where the volts all pile up if the current is more or less in the middle) away from the building and the phone line.
Actually despite the text book sketches, even quite a kinked dipole will fare only a dB or 2 worse than a perfect in-line one, so don't be afraid to be fairy cavalier with the geometry, and then claw back a match by trimming the lengths. HF is likely local noise limited for you, rather than ultimate sensitivity, so losing a bit off the signal and interference together may be preferable.
End fed requires you to have an earth path you trust - which at the house end may not be ideal in terms of the risk of induced problems if the house wiring and its rather distant earth (distant in wavelengths, not ohms on the meter) ends up acting as a sort of parallel counterpoise antenna element.
Equally, I cant see it from here, so everything I say should be treated with caution, as it may well not be relevant at all.

73 de G7VZY



Have you got something like the Moxon's "HF antennas for all locations" or the RSGB handbook - some nice sketches of all this.

-------------------------
regards Mike
 05 May 2015 11:02 PM
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Zs

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Mike, thank you so much for the pm and I will do justice to a decent reply to you soon.

On a friends of the forum excuse for not doing it now; I've been over to see our pal Martyn for the evening so the Zs head is in a spin but he's OK today. We're going to be seeing him off to Australia in a few weeks. New beginnings and all that. I've been there - that's Australia and broken heart (multiple) so we do what we can don't we? But I'd take that broken heart from him and have it myself if I could, yet they were the worst times ever. Anyway, I'm keeping Mart busy and despite not being able to afford him the new bathroom starts on Monday. Mum, can you help me out if I spend my tax savings and don't manage to make them up in time? I am going to be soo broke.

Back to the fab shed though. Mike has explained the difference between the interference from the antenna of a radio and that caused by the mains into it. It looks as though I am indeed going to need some filtering and whilst it might end up waiting a while (unplanned new bathroom/occupied friend you see) I'm rather hoping to get on to it very soon so I'll be asking you what you recommend.

Take it as a learning curve for all of us non radio-heads. I'll log it with the Chauvin Arnoux thingummy first because we need to know what is going on and let you know the steps to clear it. Thank you Mike.

And of course thank you all. this issue is close to my heart so I'm glad of the advice.

I need to play a guitar for half an hour before bed. Thinking out Loud, Ed Sheeran I think.

Zs
 05 May 2015 11:35 PM
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mapj1

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Mike has explained the difference between the interference from the antenna of a radio and that caused by the mains into it.

Not quite. What I was hoping to explain is that interference detected by the radio, may either be plucked from the air by the antenna (radiated interference) or it may sneak in up the wires (conducted interference).
The former is stopped by shielding, or better by strangling it at the source, the latter is more easily stopped by suitable filters on the affected cables.
However one sometimes gets converted to the other by random bits of wire or other metal (mis-) behaving as incidental antennas. In that case carefully placed lumps of magnetic material on the wires can often reduce the problem from 'disaster' to 'undetectable'. These are the magic grey ferrite rings sometimes seen on computer cables and so on - a sure sign of a design with problems that had to be fixed at a late stage - if you get it early in the design its much easier to put all the filters on the inside.


foto shows a rather undersize makers original filter (the black over molded lump) and then a simple home brew ferrite ring filter backing it up to give more lower frequency rejection.

edit
Having googled Chauvin Arnoux Thingumy, I am strugging a bit, as I don't know what gear you have - please by all means make measurements, but you will have to explain what has been done. In general what we are after is a nice sinewave supply that is not too distorted, and to minimise any little sproggles riding on the back of waves like chairs on a chair lift, at least on the supply to the sensitive audio gear. Generally the faster the event the more likely it is to cause trouble - after all a few hundred volts at 50Hz is quite normal. This all comes from the fact that we are trying to move charge around. If you like, consider trying to get a given voltage strep on a small capacitance - the the voltage is current times time, so to charge to 1volt in a microsecond is the same peak current and charging to 1000volts in a millisecond - so a volt or two of RF at a frequency of megahertz is like a few kV at audio frequencies, in terms of the magnetic fields those currents can generate.
Similarly the voltages all pile up across surprisingly short lengths of wire at higher frequencies, as you can have perhaps a microhenry per meter of wire. Now an amp of AC flowing though a microhenry drops six microvolts at one Hz, and only 300 microvolts at 50Hz, and is likely not to be noticed.
But again at our arbitrary test freq. of one MHz, it becomes half a dozen volts of drop per amp, and as noted above, that is enough voltage gradient to drive serious trouble in some circumstances.
Actually near serious RF sources (kilowatts) it is quite possible (and painful) to get an RF burn by holding onto a solid metal bar in two places, even though to low frequencies it is all interconnected and grounded, if it just happens to be the right length and an orientation to be excited into a(n un)lucky resonance.
In summary it all gets harder and less well connected as the frequencies go up - in some ways more like underwater electrics where currents sort of leave out through the side of the wires and end up inducing unwanted effects in ones nearby, though its not an analogy that can be pushed for any great insight.

PS chaps and chapesses - now you see how the longer version became a PM

-------------------------
regards Mike


Edited: 06 May 2015 at 08:19 AM by mapj1
 06 May 2015 07:14 AM
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rogerbryant

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Originally posted by: John Peckham

By the way the laws of nature determine that men have sheds and workshops, women have studios! If a women ever put a Myford lathe or a pillar drill in to her studio the world would tilt on it's axis and permanent darkness would descend on the earth at it would be the end of mankind.


The lady I purchased my Hobbymat lathe from has at least four other lathes, three millings machines and is currently sorting out the three phase problems to install a Hardinge HLV (this apparently requires proper sinewaves, not something synthesized by a static converter). The world is still turning :-)

Best regards

Roger
 06 May 2015 07:40 AM
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mapj1

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Ahhh. Perhaps the earth's orbit is nutating slightly as a consequence, and this explains global warming and the rise of UKIP - could it all be your fault Roger ?
Joking aside I've always felt the planet never needed saving, what needs saving is humanity from its own short-sightedness.

As regards the Hardinge, one of these ? I think the problems relate to the sharp edges from some designs of inverter, rather than the static converter per-se. Some L-C filtering load side to round things off to a near sine may be enough.
Part of the problem is that the variable speed carriage motors are DC, and the AC is rectified to produce this, and the old selenium rectifiers are not sympathetic to the wrong sort of waveform - the magic smoke escapes after a few seconds (although modern silicon diodes and a some series aluclad ohms are infinitely better anyway). In addition there are various parts like the sump pump and various end stop reversal relays that are all AC.
Personally I'd be trying to only invert the main motor part, and keep the rest running direct on the single phase supply as far as possible -just because the single phase parts are split accross more than one originally does not always mean they can't simply be paralleled.
If that is not practical not then there are inverter designs that synthesise sinewaves in the manner of an audio power amplifier although these are both more expensive and not as efficient as the block waveform types.

-------------------------
regards Mike
 06 May 2015 08:04 AM
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rogerbryant

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It's always been my fault ;-)

Saving humanity from itself is another topic, but I won't hijack Zs's thread.

The electronics (I suppose a selenium rectifier is electronics) is part of the problem. A poor 3 phase waveform apparently induces vibration in the main motor which can be transmitted to the work piece. Her planned solution is a free running 3hp motor to act as an ersatz rotary converter. The cheapskate solution is to leave the motor on another 3 phase machine running ;-)

As to Zs's problem, I said at the beginning that candles produce very little problematic EMI.

Best regards

Roger

Edited: 06 May 2015 at 08:29 AM by rogerbryant
 06 May 2015 08:24 AM
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mapj1

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The Pony Motor as an electromechanical flywheel is certainly a well established and successful technique if a bit big, noisy and 1950s - and there is no question about the transient behaviour or loop instability. It may be the simplest way out, especially for someone with a spare motor and some big cogs already lying about..

-------------------------
regards Mike
 08 May 2015 10:36 AM
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Zs

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Roger, I am an avid candle burner. However, I have banned them from the wooden construction at the bottom of the garden...

Royal visit imminent. JP is coming this way soon to collect a tool from Mart and I'm going with him from here. So, he gets to meet Gladys (the shed). He's going to love the armoured cable installation method Method Zs - bunged across the gravel behind the cement mixer and youngmans boards left behind by the builder.

I'm looking forward to it actually and will get to demonstrate the radio issue to a fellow nerd. I shall open up all the trunking in anticipation JP.

Mike, the kit mostly has those black lumps on the cables...I'll pull it apart as soon as I get time though, and look for the culprits.

It is a PEL 103, I'll lend it to you one day, you'd be in seventh heaven because you can watch it from your desk from afar and see every light that is being switched/left on in your house while you are at work.

Zs
IET » Wiring and the regulations » High Frequency Fluorescent light

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