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Old 20th September 2012, 02:23 AM   #1
Bando is offline Bando  Canada
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Default RF interference and general help for a Newbie

Hello all, I am asking for some help understanding why things are:

I work for a company that does PA system installs and maintaince. I havenít got an answer to how does a wire become an antenna and other wires in the same snake/trunk do not. I am new and I do not know all the proper standard industry terms, so I apologize. I want to do my homework and I am looking for some direction, for instance:

Such as
situation 1: retrofitting a old building with unshielded solid core 22 gauge 4 conductor per room ( ground announce speaker speaker ) going into an old trunk/snake whatever name you give it, one wire picks up a radio station and others donít. Run a new wire 22 gauge 4 conductor braided core shield wires with the shield attached to nothing, though that solves the issue. Which leaves me wondering, if the shield is not "earthed" than whatís the point of using it? A wire has to be connected to something to work. Doesnít any shield "earthed" at one side make it an antenna?

Situation 2: new build, new wiring independent home runs with double shielded 22 / 4 wire ( one shield on speakers and the other shield on call ) still getting background noise. Even with the shield wires connected and "earthed".

In both cases may or may not be in conduit, or just run may or may not be intersecting AC lines, some at 120 others higher.


We try to run our wires away from these obvious sources of interference, but our electrical contractor ( since we are subs ) or other wiring guys ( security, automation, fire alarm, data ) seem to run willy-nilly with no regard for interference whatsoever.

If there is a book or something I should read please do not hesitate! I want to know why this is instead of doing what works and not caring.

I know some electromagnetic theory, like inductance, resistance, capacitance, and what they do on paper but not in real life dealing with a 25 Constant Volt system ( 25volt is code in Ontario ).

Thank you for any help and thoughts,

James
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Old 20th September 2012, 04:15 AM   #2
DUG is offline DUG  Canada
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Radio Amateur Handbook at your local library.

I think chapters 2, 3, 4, and maybe 5 make a very good start for basic electronics as they relate to the RF world.

EMI / EMC has been called Black Magic in the electronics world.

Some generalizations:

anything with area has capacitance to something else

anything with length has inductance

everything has some resistance

So some of the results of the above are:

The higher the frequency the smaller your effective ground is.

Capacitive coupling (or bypassing) will eventually appear electrically inductive (higher impedance) at some higher frequency

Inductors for isolation will eventually appear electrically as capacitors at some higher frequency. (and then back to inductance again)

A wire, generally, will pick up signal in proportion to its' length and the height above "ground"

So,

If you have two wires running together they may receive different amount of signal based on their position and what is hooked up at each end. A twisted pair help the two wires pick up the same amount of signal - up to the limit of the twist length vs wavelength of the RF.

There are different scenarios for twisted shielded pairs in that sometimes grounding one end, neither end or both ends give different results. Sometimes a double isolated shield, one grounded at each end would be required.

So if you have done all you can to decrease the "pickup" of interference then it comes time to find the susceptible circuitry and EMI harden that circuit.

Hope that gets you started.

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Old 20th September 2012, 04:17 AM   #3
DUG is offline DUG  Canada
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By the way:

Howdy neighbor.
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Old 20th September 2012, 07:25 PM   #4
AuroraB is offline AuroraB  Norway
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Some more generalisation - high load impedances, 10's of kohms or more, are worse than low, say 600 ohms.........

A more detailes description of the probelms are needed....
RF interference...? Radio signals, cell phones etc?

Noise - what kind of noise? Just High freq. ( treble hiss ) ? Radio interference?

How does ths system operate? One announce line, 2 speaker....
Active speakers? 100V system? Lots of 8 ohms speakers in parallell?
Keyed system? How does the announce line work - audio feed or keyer line?
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Old 21st September 2012, 12:33 AM   #5
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Jim Brown is both the AES, EMI/RFI committee chairman and a HAM. He has several papers on the subject.

Audio Systems Group, Inc. Publications
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Old 21st September 2012, 01:11 AM   #6
Enzo is offline Enzo  United States
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And most any wire can be an antenna, what makes it so is what it connects to. If you have multiple section cables and one shielded wire acts like an antenna, but a seemingly identical one does not, Then either they are connected to different things, or the bad one has a break in the shield or some other malady.
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Old 21st September 2012, 01:16 AM   #7
DUG is offline DUG  Canada
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Speedskater

Excellent page, I now have it bookmarked.

Thank you.
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Old 21st September 2012, 11:21 PM   #8
Bando is offline Bando  Canada
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The system is a bogen multicomm so it can be small 20 rooms, maybe 3 outside horns and maybe 8 hall speakers. Sometimes quite large over 200 rooms, maybe 12 horns and 50 some odd hall speakers.

It is a 25 volt constant volt system ( 25 volt is code in my area for schools, 70 volt for industrial applications )

The interference I have the most difficulty with is radio stations. I can deal with 60 hz hum and usually no other interference is present. How a single wire is picking up a radio station perplexes me.

I am going to be reading the link tonight after my little one goes to bed.

James
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Old 23rd September 2012, 01:43 AM   #9
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Jim Brown just added this one:

The EMI - RFI page
EMI - RFI page
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Old 23rd September 2012, 12:01 PM   #10
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Quote:
Originally Posted by Bando View Post
The system is a bogen multicomm so it can be small 20 rooms, maybe 3 outside horns and maybe 8 hall speakers. Sometimes quite large over 200 rooms, maybe 12 horns and 50 some odd hall speakers.

It is a 25 volt constant volt system ( 25 volt is code in my area for schools, 70 volt for industrial applications )

The interference I have the most difficulty with is radio stations. I can deal with 60 hz hum and usually no other interference is present. How a single wire is picking up a radio station perplexes me.

I am going to be reading the link tonight after my little one goes to bed.

James
Generally we run a cable with a shielded pair and a pair with an unshielded pair when doing intercom systems. Yes, I have done numerous multicom systems. Just like using a shield when using audio gear in a commercial system you do not terminate both ends of the shield. The 25 volt systems are common for schools and all Bogan intercoms are 25v. The 25 volt is not code for the school but it is common for intercom systems. Some of the newer schools are dropping the typical intercoms systems for a simple 70 volt all call systems that consists of 70 volt speakers, one microphone in the office, and a simple 70 volt commercial mixer amp. The down side is the schools cannot talk back. Check the ground at the head end. If memory is correct the power cable plugs into a filter type receptacle that has RFI.

Are you close to a radio station? Sound like you might be. Try some ferrite beads and see if that helps you out.
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