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Old 2nd Jun 2020, 9:36 am   #21
Craig Sawyers
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Default Re: Faraday chamber.

I've caught people pulling a BNC-BNC off the wire rack, discovering it was flaky, and putting back on the wire rack. For someone else to use...

I have had a number of "discussions" with people who I've caught at that game.

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Old 2nd Jun 2020, 10:26 am   #22
Wendymott
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Default Re: Faraday chamber.

Interesting comments re RF Test rooms...and cables.... When I worked for UK Tv manufacturer NEI, EMC testing was becoming mandatory, and we had to find a suitable EMC test house... the one we found was in the centre of england "ish" in a forest near Henley in Arden..far down a narrow track....there were various buildings well separated and a big tower. Each building was charged with specific jobs.. RF Radiation or Not..RF susceptiblity or Not..and other tests.. one building was an anachoic chamber big enough to fit an army tank in....I have left that place at 1am...after getting the results required.
I have no idea if that facility is still there, but I guess it will be, as EMC testing is mandatory for just about everything electrical.
Haa haa Jeremy... just the sort of thing I would do.... not just me then
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Old 2nd Jun 2020, 12:25 pm   #23
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Default Re: Faraday chamber.

Quote:
Originally Posted by G0HZU_JMR View Post
The solution for this was for me to find a suitable place to hide whilst the goods in team temporarily removed the door hinges and the rear plate.
No doubt spending the time devising solid engineering reasons why it had to be that particular size, and any smaller either wouldn't have worked, or involved a big jump in cost...

Good engineers always have fall-back positions prepared.

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Old 4th Jun 2020, 9:56 am   #24
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Default Re: Faraday chamber.

I won one, ex Ofcom, at a good price, in an auction.
This was supposed to be for pretesting before committing to a full test suite at the test house.

It was a good size and we had the manpower and vehicles to collect it as required.

Management never granted it the floor space needed so it sat in a heap and finally went for scrap. 6 letter word beginning with M and ending in s.
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Old 4th Jun 2020, 10:49 am   #25
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Default Re: Faraday chamber.

I was looking on ebay last night in the vane hope that there may be some small EMC type cases around.. but no...
I have finally achieved - 30db with the addition of copper braid around both seal surfaces..whether that is considered good/bad or ok ish I have no idea but I am not going any further...see photo...
But thanks all for your interest and comments.
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Old 4th Jun 2020, 2:32 pm   #26
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Default Re: Faraday chamber.

Some of the equipment for the contract I was on in Saudi Arabia was installed in screened rooms the size of large sheds. Built on site by Belling Lee.
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Old 4th Jun 2020, 5:23 pm   #27
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Default Re: Faraday chamber.

The biggest one I've been inside was at Fort Monmouth in New Jersey. It was called building 600 and the screened test chamber was absolutely huge. The chamber is the section of the building circled in yellow below. That really is a huge test chamber! Inside, there were suspended walkways above pyramid absorber cones pointing up from the floor and there was a space by the door to allow vehicles to be driven in and out. At the time it was owned by the US military but they vacated the whole site a few years ago.

There is/was a fairly large one at Great Baddow near Chelmsford and I have been inside that one a few times. At work here we had our own EMC division for quite a few years and we had three formal EMC test chambers that could be hired by other companies. Each was about the size of a garage. We sold it all off a while back.
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Old 4th Jun 2020, 5:44 pm   #28
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Default Re: Faraday chamber.

Quote:
have finally achieved - 30db with the addition of copper braid around both seal surfaces..whether that is considered good/bad or ok ish I have no idea but I am not going any further...see photo...
I found that it was much harder than I expected to achieve broadband screening when I tried to make boxes like this in the past. I ended up making them smaller and smaller with a box inside a box and it became harder to make any use of them because there was very little space inside the enclosure and it was a real chore to unscrew the lid each time I wanted access. If you can get a measurable improvement like 30dB inside your large 'suitcase' enclosure then that is pretty good and it will be very easy to gain access inside. I don't think I managed to get results that good.

I gave up trying to make something that large and bought one. I got mine from MCSTest in Wales. It was well used and the door hinge and lock needed adjusting to maintain a stiff seal into the braid. Once I did that it was as good as new. I think it cost me about £100.

http://www.ramseyelectronics.com/

I'm not sure how much it cost new but it was made (marketed?) by Ramsey and I'm very pleased with it.
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Old 4th Jun 2020, 7:04 pm   #29
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Default Re: Faraday chamber.

All this talk of screened rooms takes me back. My background is automotive, and even as a trainee back in the eighties, I was involved with one of the first EMC chambers for testing vehicles, in the world. One of the important features as well as those mentioned, was RAM: Radio Absortion Material. This is foam impregnated with lamp black and has a similar effect to in an acoustic anechoic chamber. Acoustically very 'dead'. This was a commercially built chamber that at the time cost £1million, and in those days that was a LOT of money. It's still running today.

It wasn't our own own facility, and we couldn't afford anything like that money, so for basic experimentation we built a room using standard mild steel wall panels but then bonding them using braid. It was quite effective at lower frequencies (as you'd expect being a magnetic material) but you could still see a decent signature from the Sutton Coldfield FM transmitter that was not so far away.

It's been a few years since I was at 3Ctest. I've also used MIRA in Nuneaton, and a place near Titchfield that I can't remember the name of as well as too many hours at Gaydon.

Immunity testing, especially in the early days showed some interesting results. There's one in particular I remember. During a test, at a fairly low frequency using an overhead parallel line, we noticed a warning light was coming on when it shouldn't. Of course you would expect that some part of the electronics was being affected switching on the light but when we investigated it was an electronics free circuit, simply wires and a bulb. It was just the length of wire involved acting as a big long wire antenna and powering the bulb directly from the RF. I also saw examples of pretty poor design being shown up in a chamber. One particular controller would always go 'mad' around 88-90 MHz which is far from ideal if you want to drive anywhere near Sutton Coldfield. Repeated retests of new designs barely made any difference. Getting fed up with this I had a look at the schematics and realised there were absolutely no capacitors (low or high value) anywhere near the voltage regulator. 5 minutes and a soldering iron later, problem solved, all done inside this first EMC chamber!

Sorry for the reminiscence, but it does kind of apply to the topic (I hope).
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Old 4th Jun 2020, 10:32 pm   #30
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Default Re: Faraday chamber.

Do not apologise Duncan... all grist to the mill as they say...EMC is a subject all of its own.. as Jeremy will attest. I was glad to be part of it in my time at NEI... I used some of my meagre knowledge to get our products through EMC testing....... I did hope to buy some equipment to pre test, but it was deemed too expensive....BEAN counters didnt approve.
With Jeremy's comments.. I can be quite happy to continue with my VHF transceiver now.
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Old 5th Jun 2020, 4:59 pm   #31
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Default Re: Faraday chamber.

Final post..ish... Looks like the hard work has paid off...-40db screening / isolation at Band 2 and 2 metres..those are the areas I was interested in...when I started the project I really didnt know how it would go, as the basic box was very poor material, but its size and lid opening were ideal. The hard part was the lining and sealing...fortunately the hinges and fasteners had so much play in them I could mount copper braid as gaskets, that now it is a nice firm fit. Slightly opening the lid totally destroys the isolation.
The gaskets are also grounded every 50mm....
The DC supply was also modified to a 2.1mm DC Jack which allowed me to use RF coax from the psu...... If necessary I can also fit a dry 12V Battery internally.... so much space.
If any one is interested.. I can post more detailed photo's and dimensions.. 73's and QRT.
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Old 25th Aug 2020, 6:36 am   #32
M0AFJ, Tim
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Default Re: Faraday chamber.

Just spotted this thread, I’m sure I will have crossed paths with some on here doing my day job, was involved with the first large chambers at Gaydon and MIRA back in the day. I’m actually going up to Manchester to SE test a chamber and run a training course tomorrow. Someone mentioned using shipping containers, there was a company in the Cambridge area tried that, whatever absorber they put in it they could never get the field uniformity to come into spec as defined in EN61000-4-3..
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Old 25th Aug 2020, 8:03 am   #33
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Default Re: Faraday chamber.

We used to install euipment into MRI scanning rooms in hospitals, these were fully screened rooms with the metal fingers on the doors as mentioned in a previous post. The window between the control room and the scanning room had a mesh in it which reduced the light quite considerably.
Where technicians needed access between the control and scanning rooms they passed fibre optic cables through bit of tube about 1 inch diameter and about a foot long. This puzzled me until "waveguides below cutoff" were explained to me.

Peter
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Old 25th Aug 2020, 12:13 pm   #34
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Default Re: Faraday chamber.

At HP we used to get in a robotically positioned field sensor - all plastic and pneumatics, which would wave a sensor around and plot the deviation from uniformity in the intended DUT location volume. This got done whenever there were changes inside the room.

Being me, I thought it was a good starting point, and it kept some awkward people quiet, but I did wonder about the effect of the DUT itself on field uniformity and interaction of that with the room. They're artificial tests anyway considering the variety of environments the finished DUT will have to work in.

Combining radio, EMC and the need for vehicle size test rooms:
One car usually in the same car park I used was a bit sensitive. I used to be chatting on 2 metres over the local repeater in a morning as I swung into the car park, just saying 73. As I dropped the carrier, if I was close to this other car, it would flash its indicators, beep and unlock its doors. No problemo. Another blip of the PTT and it would lock itself!

So much for a security system and alarm. RF was getting picked up all over the wiring and overloading things, randomising the I am locked/I am unlocked memory. It was of a brand once associated with Gaydon.

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Old 25th Aug 2020, 3:30 pm   #35
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Default Re: Faraday chamber.

Quote:
Originally Posted by Jon_G4MDC View Post
Re : BNC and N types compression and crimp. Many don't make the compression joints properly.
There is rubber sleeve and an open ended metal top hat.

You put the nut and rubber sleeve on the jacket you strip the braid and fan it out evenly in a circle. You put the top hat between inner insulation and braid and slide it up inside as far as it will go. Now you slide down the rubber sleeve trapping the fan of braid against the top hat rim. Then trim back the wire fan to the edge of the rim and rubber. Now fit the centre pin and assemble.

The point here is that the nut applies force to the rubber which transmits it on to the fan of braid, then to the top hat and the body of the connector.

Many don't make the braid fan and then the only braid grounding is by "hopeful" swelling up of the rubber sleeve. You soon know you are up against one made like this when you can rotate the connector on the end of the cable and that is a test I make on every cable out of instinct. Not only have they no screening they are intermod generators.

I cut them off every time I find one.
Agree 100% with this - I've had to replace dozens of BNCs that have been ripped off the cables by clumsy colleagues - all clamped badly in the first place. The fix (IMHO) is to fan out the braid - then solder it to the "Top hat". Need some care to avoid distorting the coax insulation though.
There were no subsequent return loss problems, and the cables met their specs up to high VHF. As I remember, the connector leakage was reduced as well.
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Old 28th Aug 2020, 9:16 pm   #36
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Default Re: Faraday chamber.

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Originally Posted by M0AFJ, Tim View Post
Just spotted this thread, I’m sure I will have crossed paths with some on here doing my day job, was involved with the first large chambers at Gaydon and MIRA back in the day.
We must have crossed sometimes.

Gaydon: did you ever see the giant Dymo label and blue spot on the control room wall? I put them there as a trainee, but then was involved in the testing over many years in my real job.

Mira: I was involved in one of the very first tests in the first chamber, though it was a bit of a dodgy affair. But then on and off have been involved with tests there again, even last year doing some investigation on a sub assembly that was doing some weird stuff.

Essentially have been involved with both facilities my entire working life since the late eighties.
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