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 Post subject: Re: Electromagnetic Compatability
PostPosted: Dec Sun 15, 2019 3:04 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 129
Location: Millis, MA
wazz wrote:
The most effective suppression of EMI is at the source before it can be conducted thru the house wiring. Even if you put a filter on the power lead of the radio, the house wiring can act as a transmitting antenna to send the noise all over the house. I have suspicions that manufacturers nowadays don't really care much about any EMI below 30 MHz as most popular broadcast media is VHF or above, and TVI is a non issue with cable and satellite being predominate, and there are no giveaway herringbones or hash on digital TV screens in OTA interference, just a loss of coverage that can be dismissed as something else. "No one listens to AM radio anymore" etc. And any "wireless" devices operate in spectrum that is not very susceptible to common sources of interference way down low. IF nothing else, safety rated bypass caps in the offending appliances at the line input might have benefits, as the typical L/C line filters that are rated for enough current to use in large appliances are bulky and possibly not practical to install. Maybe in a fridge, there is alot of room below.


Well, they still need to care about EMI below 30MHz because the Conducted Emissions standards haven't changed between 450kHz and 30MHz, and if they want to sell to the rest of the world they may have to worry down to 150kHz (and for luminaries down to 9kHz).


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 Post subject: Re: Electromagnetic Compatability
PostPosted: Dec Sun 15, 2019 4:07 pm 
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Joined: Jun Mon 20, 2011 6:09 pm
Posts: 1170
Location: 56401
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Actually not really; since the lead-in has braided shield around it, it couldn't possibly receive RF signals. The suspended horizontal lines are therefore the only actual antennae...

Sorry but I don't believe that statement is true. Coax appears to have a shield around the center conductor, but that outer braid only shields providing there are equal and opposite currents existing everywhere. Notice at the junction of the coax to the antenna, that situation does not exist, which means the braid is also part of the antenna. And at the station end, grounding doesn't help much(define "ground").

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 Post subject: Re: Electromagnetic Compatability
PostPosted: Dec Thu 26, 2019 4:24 am 
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Joined: Jun Mon 24, 2013 3:00 pm
Posts: 1562
Location: Champaign IL 61822
I highly second the BOG. Its what I use, and it is entremely quiet
compared to anything else I've tried. Its not perfect, but gets more
nearly so the farther it is from the house. I have tried 30 and 100 foot
lengths. The 100 foot is better only below 600 kHz.

I attach it to a transistor antenna preamp that has a 6 kOhm impedance
and 50 ohm output. I have found that it also helps to have RF transformers
at both ends of the feedline which have less than 5 pF across them.

I have posted FFT spectrograms of the LF/VLF regions using this and
people at first did not believe that all those big spikes every kHZ were real ..
but they are. Tonight is especially nice.


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 Post subject: Re: Electromagnetic Compatability
PostPosted: Dec Thu 26, 2019 4:51 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 22762
Location: Haledon, NJ, usa
I was just testing my Detrola 436 AA5 radio last night

viewtopic.php?f=2&t=369549

and found out what a strong performer it was... but ,yes, I did notice some static buzzing between stations. I figured it had to be caused by something outside of my house because ALL my bulbs are new 13w (60w equiv) LED bulbs.

But a quick test determined that THAT buzzing was from the (somewhat new) LED bench light directly above the radio. So I used a different LED from the same box another spot in the room... and it worked silently directly above the radio under test... and the noisy bulb in a different socket 15ft away showed no interference.. so go figure.

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 Post subject: Re: Electromagnetic Compatability
PostPosted: Jan Thu 09, 2020 11:38 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 20791
Location: Warner Robins, GA
fifties wrote:
Actually not really; since the lead-in has braided shield around it, it couldn't possibly receive RF signals. The suspended horizontal lines are therefore the only actual antennae...


If the shield isn't grounded to anything, it would capacitor couple anything on it to the inner conductor and at what frequency it does that is determined by the antenna input impedance of the radio it is connected to.


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 Post subject: Re: Electromagnetic Compatability
PostPosted: Feb Sat 01, 2020 3:32 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 1014
Location: St. Louis, MO, USA
One interesting test I did was to tune in a weak am station on my car radio while on the street and pull in to my garage. Minimal static on the street but lots of interference in the garage. I’ll try throwing circuit breakers as noted above and see if I can localize the source assuming it’s not actually in the power lines.

Denis


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 Post subject: Re: Electromagnetic Compatability
PostPosted: Apr Fri 10, 2020 11:35 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 844
Location: dayton oh usa
i go on a search and destroy to keep noise generating crap out of here.
switchmode wallwarts are the worst offenders.
esp the noname cant pronounce it stuff.
the rfi emitting ones go to the trash replaced with old iron ones.
or better yet i run the item off my solar.
direct 12v or a 78xx reg.


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 Post subject: Re: Electromagnetic Compatability
PostPosted: Apr Sat 11, 2020 1:23 am 
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Joined: Sep Thu 23, 2010 6:37 am
Posts: 12437
Location: Powell River BC Canada
Can you take a 36 inch length of your twisted speaker, un terminated,
and measure what the capacity is with a small LC meter?

Telephone indoor wire wire is twisted to avoid 60 cycle hum induction.

Inside a cable, each pair is twisted to avoid cross talk.

It has been years, since I heard of an AM radio station interfering
with a land line telephone.

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VE7ASO VE7ZSO
Amateur Radio Literacy Club. May we help you read better.
Steve Dow
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