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Author Topic: Mains Filters??  (Read 7993 times)
W2MV
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Posts: 207




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« on: August 21, 2012, 06:42:46 AM »

I recently received a copy of the RSGB book "Elimination of Electrical Noise." The author recommends that a/c mains filters, consisting of three a/c-rated capacitors across the hot to ground, neutral to ground, and hot to neutral (translating from UK terminology) should be installed both at your and your neighbors' QTHs. Interesting. This will help to bypass any RFI to ground (earth) that might be present on the line.

I've never seen this recommendation in any other publication, i.e., installation of bypass circuitry on the power lines to your and your neighbors' houses.

Any comments?
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KC9V0
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Posts: 7




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« Reply #1 on: September 20, 2012, 11:05:09 AM »

I use a Tripp-Lite ISOBAR ULTRA 8 to protect my equipment in the house. Each pair of outlet jacks has EMI filtering of 40dB - 80dB, and it is equipped with MOV's that have a clamping voltage around 140 volts. I'm sure there are other products around that are as capable. Try looking in some of the larger computer outlet stores.

73
KC9V)
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W2MV
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Posts: 207




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« Reply #2 on: September 20, 2012, 11:54:37 AM »

Actually, I also use the same 8-outlet Tripp-Lite unit, but I was referring to RSGB book which recommended a "whole-house" mains filter. Three ac-rated bypass capacitors are housed into a box which is the plugged in to one of the ac outlets on the premises, thereby bypassing hot and neutral to ground. They also recommend installing this type of filter in your neighbor's house!
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ZENKI
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« Reply #3 on: September 21, 2012, 04:03:50 AM »

The fix for this is to use  or buy a filter with a earth line choke. Just do a search for earth line  choke filters. You can take a  norm  mains filter and just wind the earth wre through a #43 toroid.
Earth line choke filters are considered to be best practice these days for EMC suppression.

You have to very careful  because the USA and UK mains systems are very different. You have to very careful about adopting there wiring practices in the USA and vice versa. If you dont know these difference you are best
advised to buy a standard EMC filter with earth line choke in it designed for the USA. Homebrew filters can be very dangerous if you size the capacitors wrongly or use the wrong kind of practice. These filters are so cheap there is no need to risk your life trying to be cheap.



I recently received a copy of the RSGB book "Elimination of Electrical Noise." The author recommends that a/c mains filters, consisting of three a/c-rated capacitors across the hot to ground, neutral to ground, and hot to neutral (translating from UK terminology) should be installed both at your and your neighbors' QTHs. Interesting. This will help to bypass any RFI to ground (earth) that might be present on the line.

I've never seen this recommendation in any other publication, i.e., installation of bypass circuitry on the power lines to your and your neighbors' houses.

Any comments?
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WB4BYQ
Member

Posts: 179




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« Reply #4 on: September 21, 2012, 11:41:34 AM »

the best filter for rfi is a common-mode choke using the ft-240-31 or ft-240-43 material winding
about 10 turns thru the core with all three wires.  this includes the green wire ground.  in common-mode all the rfi flows thru all the power wires.

since your have read in the RSGB book, this goes along with the AC Power Handbook, where the author states to the effect that you should wind the green wire ground with its own choke, i.e.
ft-240 31 material.  looking at the filters from a google search, i see can chokes that now include a third choke for the ground, i have not seen this. 

this looks like solving the problem of rfi should be less trouble.

look at this group for rfi help as well.

rfi@contesting.com or www.contesting.com  very good place to read about rfi and post messages.

you will need to register to be a member.

richard
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WX7G
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Posts: 5918




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« Reply #5 on: September 21, 2012, 02:00:00 PM »

Not all RFI from a switching power supply makes it to the AC line as common-mode. Some is differential-mode and a ferrite configured as a CM choke will do nothing. After getting into the AC power system the differential-mode noise can be partially converted to common-mode noise by asymmetries in the line-to-GND and neutral-to-GND paths.

I prefer an EMI filter topology where the DM filtering nearest to the noise source and the CM filtering is nearest the AC power.
« Last Edit: September 21, 2012, 02:04:35 PM by WX7G » Logged
AA4HA
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Posts: 1378




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« Reply #6 on: September 23, 2012, 06:25:48 AM »

I recently received a copy of the RSGB book "Elimination of Electrical Noise." The author recommends that a/c mains filters, consisting of three a/c-rated capacitors across the hot to ground, neutral to ground, and hot to neutral (translating from UK terminology) should be installed both at your and your neighbors' QTHs. Interesting. This will help to bypass any RFI to ground (earth) that might be present on the line.

I ran across this type of installation at a decommissioned commercial FM radio transmitter site. It had to be at least 30 years old and utilized very large 500 pF, 15 KV doorknob capacitors. The setup was a bit more complicated as they were powering the site off of 240 volt three phase power. Part of my project work was to tear out the existing surge protection and to convert things over to a newer technology for surge protection, filtering, UPS, transfer switch and backup generator. I kept the old parts because it was a pretty cool looking arrangement they had in place.
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Ms. Tisha Hayes, AA4HA
Lookout Mountain, Alabama
W8JI
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« Reply #7 on: September 23, 2012, 08:45:42 AM »

I recently received a copy of the RSGB book "Elimination of Electrical Noise." The author recommends that a/c mains filters, consisting of three a/c-rated capacitors across the hot to ground, neutral to ground, and hot to neutral (translating from UK terminology) should be installed both at your and your neighbors' QTHs. Interesting. This will help to bypass any RFI to ground (earth) that might be present on the line.

I've never seen this recommendation in any other publication, i.e., installation of bypass circuitry on the power lines to your and your neighbors' houses.

Any comments?

Think about the system.and how it really works. There are two paths for noise:


1.) There are power lines and other wiring that act like an antenna, and couple noise through space to our antennas.

2.) There are conducted emissions that reach the shack from noise sources. Based on my measurements, the typical modern radio has about 75 dB minimum common or differential mode rejection.

The worse radio I have is an ICOM 751A, and it was  -75 dB rejection on 30 MHz, based on injection into the power leads compared to a matched injection into the antenna port for the same S-meter scale reading on any radio. That same ICOM has about 100 dB common or differential mode rejection from power leads on 1.8 MHz. My other radios are all better, except for old tube type radios. This is based on injected RF current levels. In other words I injected enough power supply RF current to read something like S-5 on the meter, and then moved the generator to the antenna port and found the current level required to obtain the exact same S meter reading. 

Several samples of 50 and 75 ohm cable show in excess of 80 dB CM rejection on 160 meters, and they get better going up in frequency.

It looks like, if we have no cable defects or connection issues, noise current on the power mains ***if it 100% coupled into the radio's power supply connection***,  would be at least 80 dB down and probably a whole lot more. Most of my radios were well over 100 dB down worse case!

So now we have to ask ourselves two questions:

1.) Would noise following the lines couple into our radios more through coupling to our antennas, or through power wiring?

2.) If coupling is through wires, and not by radiation, would a bypass capacitor or mains filter actually reduce the power mains signal level significantly enough to make a change?

The test for number 1 is very simple.

a.) We have to go out to our antenna and replace the antenna with a load resistor, but still have the antenna and any ground connected to the coax shield without altering that setup. This shows us any common mode response into the radio, including via feedlines.

b.) We have to disconnect the antenna from the radio and terminate the antenna port. This shows us radio shielding issues from noise ingress.

In almost all cases, except those where we have bad antenna designs or poor feeder connections, noise ingress will be through radiation and not conducted currents. Since the noise transmitting antenna is all the wire from the house to the noise source, and all down the street, how much are we altering that system by bypassing only the house entrance?

I think it is a pretty safe bet the reason mains filters are not popular at all, except right at a known noise source or at a screen room entrance, is that in nearly all cases noise ingress is almost always by radiation coupling.

The reason we do not see many bypass or line filters used, almost never outside of at noise sources or screen room entrances, is because noise or EMI almost never leaks into equipment connected to outdoor antennas through the mains.

In my 40-50 years of working 160 meters and working weak signals, I have ONE case of power line conducted noise where bypassing at my house changed things. In that case my neighbor had a battery charger that generated hash. The hash followed the mains, and the noise radiated from my house wiring to my antennas that were near my house. In that case I could move capacitors on a plug around my house. By trying different outlets, I could find "sweet spots" in my house wiring where charger noise would almost go away on any given band. This is because I was either changing the radiation pattern of my house wiring, or using the power line wiring as a tuned stub.

I've personally never had a case where noise came into the mains and into a radio.

My opinion is installing a filter as prevention is probably pretty silly, because my only experience where bypassing at my house made a difference in external noise required I move the filter around! It would not even work at the mains box unless I used a series L/C circuit and tuned it for a certain band, and then not nearly as well as a "sweet outlet bypass"!

Making things worse, if someone plugged in something new in my house, the sweet outlet for any band would move, and sometimes not even work.

The solution for me was to buy them a new battery charger.

My opinion is, if there ever is a need for a line filter, it really belongs at the noise source.

73 Tom
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WX7G
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Posts: 5918




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« Reply #8 on: October 12, 2012, 08:31:33 AM »

The oft mentioned test for RF noise on the AC line is to disconnect the antenna. Hams do this and of course the noise goes away and they conclude they have no RF noise on the AC line. This is incorrect. The ingress path for line noise is not via the radio power leads but via the antenna.

Example: A radio is connected to a dipole via a length of coax. A wall wart in the house is injecting common-mode RF noise current onto the AC wiring. The noise current flows along the AC wiring, onto the radio chassis, and along the coax shield to the antenna. The coax shield is "hot" with RF noise and is acting as a third antenna wire and noise is received.

So, we might place a ferrite current balun at the antenna to reduce the noise. But, the coax shield is still hot with RF noise, radiates RF noise, and although possibly weaker, is received by the antenna.

The best place to take care of RF noise is at the noise source. This is the place to add ferrites or Line Filters.

And sometimes the best solution is to replace the offending RF noise generator with a device having no RF noise.

« Last Edit: October 12, 2012, 08:34:31 AM by WX7G » Logged
K1CJS
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Posts: 5871




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« Reply #9 on: October 13, 2012, 06:18:49 AM »

People who think that a whole house filter will eliminate noise from the mains coming into the house don't stop to realize that the electrical mains are just one source of such interference coming into the typical house, nor do they stop to think that there are receivers--and generators--of RFI and EMI interference in their house as well.

Think of the little Dutch boy holding his finger in the dike.  He may be stopping one leak--but how many other leaks are there in that dike???
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