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 Post subject: Electromagnetic Compatability
PostPosted: Oct Thu 11, 2018 6:32 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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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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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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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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 Post subject: Re: Electromagnetic Compatability
PostPosted: Jan Thu 03, 2019 7:29 pm 
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I admit surprise at the appliances that are giving you major interference; fridge and air cond sound like mainly an ac motor type load. Are these new high-end models using 'inverter technology' ? The inverter (like a switching power supply) may be the source for either or both conducted and radiated RFI.
Is the a/c a split, window, central, or what?
Could it be that the computerized control boards (variable speed drives for fans?) are the problem source, rather than the motors? My central air has a bunch of processors; in and for the: outdoor compressor, furnace air handler, system controls interface, thermostat, remote control/display, wireless internet link, and maybe even the wireless outdoor temp/hum sensor.
(My 1971 fridge has NO electronics, what does yours have?)
Suppression at the source is the place to start.
Adding ferrite cores on wiring should be safe in almost all cases, to cut 'hash'.

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 Post subject: Re: Electromagnetic Compatability
PostPosted: Jan Fri 04, 2019 4:14 am 
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Adding ferrite cores on wiring should be safe in almost all cases, to cut 'hash'.

These are available in clip-on form so you don't have to cut the wire. They are available optimized for various frequency ranges; get ones that are effective at the frequencies you are interested in.

It is a good idea to find out what is actually causing the interference. This is usually done by unplugging everything or turning off all the circuit breakers and then turning things on again one by one (obviously a battery powered radio is required). Don't be surprised to find multiple sources. Some of them may not be under your control so other measures (like a different antenna) may be needed. Noting the time of day when noise appears may help localize the source, for example, if it is coming from street lights.

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 Post subject: Re: Electromagnetic Compatability
PostPosted: Jan Mon 07, 2019 2:50 am 
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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.


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 Post subject: Re: Electromagnetic Compatability
PostPosted: Jan Mon 07, 2019 1:35 pm 
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Location: Potomac, Md.
fifties wrote:
SixFiveZeroTwo wrote:
fifties wrote:
Here's a similar diagram; the length is not critical.

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Not to hijack the topic, but I have to ask: An inverted-L, as I understand it, uses both legs--the suspended horizontal portion and the vertical portion--as active elements. What I got from your post is that you're using coax for the entire length of wire that extends from the horizontal portion all the way to the house.

If the shield is grounded, as I assume it is, then only the horizontal wire is functioning as an antenna.

You've been messing with this stuff long enough to know what you're doing, so I must be missing something. I'm betting it's that you know very well you don't have a true inverted-L but what you have works fine for you and the configuration made sense to post.

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 Post subject: Re: Electromagnetic Compatability
PostPosted: Jan Mon 07, 2019 9:08 pm 
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Avery wrote:

Not to hijack the topic, but I have to ask: An inverted-L, as I understand it, uses both legs--the suspended horizontal portion and the vertical portion--as active elements. What I got from your post is that you're using coax for the entire length of wire that extends from the horizontal portion all the way to the house.

If the shield is grounded, as I assume it is, then only the horizontal wire is functioning as an antenna.

You've been messing with this stuff long enough to know what you're doing, so I must be missing something. I'm betting it's that you know very well you don't have a true inverted-L but what you have works fine for you and the configuration made sense to post.

Avery old boy, I've never claimed to know what I'm doing, I just kinda fake my way through things and sometimes get away with it! :wink:

As I'm sure you realize, that diagram was taken from the net, simply to illustrate a suspended horizontal antenna, W/ the lead-in connected at one end. Why it's termed "inverted L", IDK, since I only see one line being used as an antenna. Maybe that's confusing.

In my own setup, looking at it from above, it would appear as a "t" shape, with about 140 feet going north, and another hundred feet tied into that, going east and west. No particular reason for the shape other than that's what would fit on my back acre, and be suspended by trees.

I used a length of RG59 coax for the lead-in, but the outer braid is not connected to anything. The idea is that it hopefully shields household electrical RF interference from the inner lead. Of course I did try grounding it, but saw, or I should say heard, no difference in reception. As you recognize, it's what works for me.

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 Post subject: Re: Electromagnetic Compatability
PostPosted: Jan Tue 08, 2019 3:16 am 
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fifties wrote:
Avery old boy, I've never claimed to know what I'm doing, I just kinda fake my way through things and sometimes get away with it! :wink:

As I'm sure you realize, that diagram was taken from the net, simply to illustrate a suspended horizontal antenna, W/ the lead-in connected at one end. Why it's termed "inverted L", IDK, since I only see one line being used as an antenna. Maybe that's confusing.

In my own setup, looking at it from above, it would appear as a "t" shape, with about 140 feet going north, and another hundred feet tied into that, going east and west. No particular reason for the shape other than that's what would fit on my back acre, and be suspended by trees.

I used a length of RG59 coax for the lead-in, but the outer braid is not connected to anything. The idea is that it hopefully shields household electrical RF interference from the inner lead. Of course I did try grounding it, but saw, or I should say heard, no difference in reception. As you recognize, it's what works for me.


Well, it's called an inverted L because, yes, both legs are part of the antenna--the vertical and the horizontal portions. And since you're not grounding the shield of your coax, the center conductor is the vertical leg of your inverted L.

I have no idea whether an ungrounded shield has any effect on reception in a case like this.

Congratulate yourself. You had an inverted L all this time and didn't know it. :D

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 Post subject: Re: Electromagnetic Compatability
PostPosted: Jan Fri 11, 2019 5:07 am 
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Avery wrote:
fifties wrote:
Avery old boy, I've never claimed to know what I'm doing, I just kinda fake my way through things and sometimes get away with it! :wink:

As I'm sure you realize, that diagram was taken from the net, simply to illustrate a suspended horizontal antenna, W/ the lead-in connected at one end. Why it's termed "inverted L", IDK, since I only see one line being used as an antenna. Maybe that's confusing.

In my own setup, looking at it from above, it would appear as a "t" shape, with about 140 feet going north, and another hundred feet tied into that, going east and west. No particular reason for the shape other than that's what would fit on my back acre, and be suspended by trees.

I used a length of RG59 coax for the lead-in, but the outer braid is not connected to anything. The idea is that it hopefully shields household electrical RF interference from the inner lead. Of course I did try grounding it, but saw, or I should say heard, no difference in reception. As you recognize, it's what works for me.


Well, it's called an inverted L because, yes, both legs are part of the antenna--the vertical and the horizontal portions. And since you're not grounding the shield of your coax, the center conductor is the vertical leg of your inverted L.

I have no idea whether an ungrounded shield has any effect on reception in a case like this.

Congratulate yourself. You had an inverted L all this time and didn't know it. :D

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...

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 Post subject: Re: Electromagnetic Compatability
PostPosted: Feb Sun 24, 2019 9:25 am 
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The first thing to do with a specific source of EMI is to find out how the interference
is radiating in the building.

As described as a 'high powered' appliance , perhaps it has a dedicated power
source (home run) back to the panel board, and has a dedicated circuit breaker.

A method would be to have an electrician go over the circuit, checking for loose
or improperly bonded ground conductors.

Another thing that could be checked is the line disturbance that is created when
that appliance starts. (http://www.dranetz.com/)

Remedial action could be the installation of filters at the outlet that the air conditioner
is connected to.

As an aside, as a ham radio operator, i own a 12 volt power supply to operate radio
transceivers. It is not the most costly of units.

It is a switch mode type. The supply is capable of radiating a spectrum of interference
that can hetrodyne with weak incoming signals on the 80 and 40 meter band.

The manufacturers of this supply know this. There is tuning knob on the supply
that can slightly alter the switch mode frequency, to push it off the annoying place
on my radio dial.

Your house is probably full of switchmode powered things. When a 'high powered'
appliance starts, the change in line voltage, might shift their oscillation frequency to
bother your radios.

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