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 Post subject: Electromagnetic Compatability
PostPosted: Oct Thu 11, 2018 6:32 pm 
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Joined: Jun Wed 13, 2018 9:18 pm
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One thing I don't like about antique radios is that these days, there is a lot of electronic equipment which emits radiated or conducted interference. I recently got my electrical engineering degree and I had to take a class called Applied Electromagnetics and one of the topics we studied was electromagnetic compatability. We studied use of capacitors and ferites in circuits to reduce conducted and radiated emissions and we got to use a spectrum analyzer and sniffer probe across a transmission ribbon cable with a high-speed switching signal to study ways of reducing radiated interference. Something I've noticed since then is that a lot of devices are very cheap and un-optimized and give off a lot of interference.

In my house, something I've noticed is that specifically the refrigerator and air conditioner create lots of interference in my antique radios, in all of them, across most of the dial, at any location in the house, near or far from these appliances. Are there any EMC engineers here who can give tips to reducing received interference in the radio or reduce emitted interference in large power appliances like the refrigerator and air conditioner?

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 Post subject: Re: Electromagnetic Compatability
PostPosted: Oct Thu 11, 2018 7:55 pm 
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Location: Wayside, NJ Monmouth
All radio transmitting equipment has to meet spectrum Purity requirements. However, Home Electronic( power supplies, laptops, home computers, stereos, etc.) do not. If you took a spectrum analyzer into your home and did a sweep from 100 KHz to 500 MHz to you be shocked to see how much generated Noise is out there. With all the WY-Fi connected devises, cell phones, tablets, smart watches, and such generating RF. It's a wonder we can even think.


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 Post subject: Re: Electromagnetic Compatability
PostPosted: Oct Thu 11, 2018 8:04 pm 
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There are at least two ways to minimize the interference. One, use an outdoor long wire antenna, and the other, use battery powered sets, such as non-Zenith farm radios (Zeniths use a vibrator, and require heavy current).

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 Post subject: Re: Electromagnetic Compatability
PostPosted: Oct Thu 11, 2018 8:28 pm 
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Joined: Jun Wed 13, 2018 9:18 pm
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fifties wrote:
There are at least two ways to minimize the interference. One, use an outdoor long wire antenna, and the other, use battery powered sets, such as non-Zenith farm radios (Zeniths use a vibrator, and require heavy current).


That would be a pretty good way to figure out whether interference is conducted or not, by using a 100V battery on a transformerless AC/DC AA5 radio.

However, a lot of interference isn't conducted. My dad has a very heavy Furman rack-mount power filter for his TV room and I took my radio over there to see how the power filter would work, but there was still plenty of interference.


For the long wire antenna solution, does any old outdoor TV antenna work? I remember years ago we had a rotor to point our antenna for the TV, so I know they are directional at least for analog TV. Are they directional for AM/shortwave radio as well?

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 Post subject: Re: Electromagnetic Compatability
PostPosted: Oct Thu 11, 2018 8:40 pm 
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SixFiveZeroTwo wrote:
fifties wrote:
There are at least two ways to minimize the interference. One, use an outdoor long wire antenna, and the other, use battery powered sets, such as non-Zenith farm radios (Zeniths use a vibrator, and require heavy current).


That would be a pretty good way to figure out whether interference is conducted or not, by using a 100V battery on a transformerless AC/DC AA5 radio.

Other maker farm sets, Philco, Motorola, etc., used the same type of battery power as tube portables, i.e., 1-1/2 volts for the heaters and 67-1/2 - 90 volts for the plate circuits.

SixFiveZeroTwo wrote:
For the long wire antenna solution, does any old outdoor TV antenna work?

You can try it, but doubtful it'll work as well as a simple single long wire, as high off the ground horizontally and as long as possible. If the TV antenna's lead in wire is unshielded flat 300 ohm, there'll be no protection from indoor AC interference.

My outdoor long wire uses the center lead of RG-59 coax for the lead in, which has shielding around it.

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 Post subject: Re: Electromagnetic Compatability
PostPosted: Oct Thu 11, 2018 9:42 pm 
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fifties wrote:
My outdoor long wire uses the center lead of RG-59 coax for the lead in, which has shielding around it.


How does a shielded cable work as an antenna? I haven't done much research on antenna theory.

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 Post subject: Re: Electromagnetic Compatability
PostPosted: Oct Thu 11, 2018 10:43 pm 
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SixFiveZeroTwo wrote:
fifties wrote:
My outdoor long wire uses the center lead of RG-59 coax for the lead in, which has shielding around it.


How does a shielded cable work as an antenna? I haven't done much research on antenna theory.

It wouldn't work very well in that configuration. I use stranded copper wire for the antenna itself, although almost any kind of wire would work. It's suspended between two points so that it's not grounded, but instead electrically floating in the air. The lead in, which is the wire that connects between the suspended antenna and goes into my radio room, is the one that has the shielding,ostensibly to shield the inner copper lead inside of it, which is the one connected to the antenna. Here's a similar diagram; the length is not critical.

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 Post subject: Re: Electromagnetic Compatability
PostPosted: Oct Fri 12, 2018 8:02 pm 
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Joined: Jun Wed 13, 2018 9:18 pm
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In the radio I usually use (Philco PT-95), the diagram has an antenna lead that says "attach to chassis for loop operation."

Is the radio using the chassis itself as a "long wire" style antenna? I assume the electromagnetic radio waves pass through the loop antenna to get into the receiver rather than being picked up by the chassis. How does the antenna selection electrically work based on whether the long-wire lug is connected to the chassis or not?

I like asking questions here, it's a lot easier than pouring over EMC textbooks and trying to connect the dots myself :D

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