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Author Topic: Isolation Transformer for RFI/EMI Reduction  (Read 5711 times)
AK4SK
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Posts: 150




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« on: January 03, 2013, 11:14:59 AM »

I was wondering if anybody has experience using an isolation transformer to reduce RFI/EMI comming into the shack from the AC supply? Tripp Lite and others make several models designed to reduce RFI/EMI (see links below).

http://www.tripplite.com/en/products/model.cfm?txtModelID=226

http://www.tripplite.com/en/products/model.cfm?txtModelID=227
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KB1NXE
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« Reply #1 on: January 04, 2013, 06:39:42 PM »

My question would be how would this help in the case of the power cord to the rig is what is picking up the RF?  Or the Mic/Key cable, etc.

I guess it could be part of a whole station solution, but there are many cheaper and possibly more effective RFI solutions.  A good station ground, chokes where needed - to include ferrites of the correct type and size at the right places, etc would come to mind before shelling out $100 to $200 for an isolation transformer.  An isolation transformer is more effective in reducing RFI getting OUT of your station (Tempest 101 in the USAF).  But if you have RFI causing issues IN your station, an Isolation transformer is not going to be much help. 

Do you have a specific issue, or was this just a 'What If' scenario?
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AK4SK
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« Reply #2 on: January 05, 2013, 07:11:32 PM »

Thanks for the info. You ask some good questions. I know that I have some RFI/EMI problems causing noise with receiving but that is about all I know. Some of the sources are from inside the house and are known (power supplies for other electronics). Some are external, there is a welding shop right across the street and other power line issues that I am pretty sure are entering the shack from the AC supply. I have a good station ground, or at least I'm pretty sure that I do. I am not using chokes anywhere so that may be a solution to look at. I don't think I'm having issues with RFI getting out of my station but I'm not certain. I do know that I have RFI issues in my station that are not always present.
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WX7G
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« Reply #3 on: January 06, 2013, 12:47:49 PM »

It is common-mode and not differential-mode noise that can be a noise problem on AC power. The isolation transformer offers galvanic isolation but not much isolation of RF frequencies. The primary-to-secondary capacitance will be over 100 pF which is 113 ohms at 14 MHz. Much higher common-mode isolation can be achieved with ferrite cores.

G3TQX has information on 1:1 baluns that apply to wrapping the AC cord through ferrite.

To see if the AC line is your noise problem power the radio off of a battery and shut off the AC power to the house.
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AA4PB
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« Reply #4 on: January 06, 2013, 01:21:15 PM »

Even if the noise is being conducted into your shack via the power line, it is probably not being conducted through your power supply into the radio so an isolation transformer isn't going to be of any benefit in that case. More than likely the RFI is getting into the receiver via the antenna or feed line. A simple test is to disconnect the feed line at the receiver. If the noise goes away then that's how its getting in.
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WX7G
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« Reply #5 on: January 06, 2013, 03:13:50 PM »

AC power line noise always enters a radio via the antenna. The common-mode mode current travels up the AC power cord, along the radio chassis and up the antenna.

Disconnecting the antenna is not a valid test for AC power common-mode noise.

A valid test is disconnecting the radio from AC power and powering the radio off of a battery.

The Tripplite isolation transformers mentioned do have Faraday shields and this does decrease common-mode coupling from the transformer primary to secondary. But again, you may obtain higher common-mode noise attenuation with an inexpensive type 43 core.
« Last Edit: January 06, 2013, 03:18:15 PM by WX7G » Logged
AA4PB
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« Reply #6 on: January 06, 2013, 06:00:26 PM »

In every case I've had experience with the power line noise was being radiated from power conductors outside the house directly to the antenna so disconnecting the antenna DID cause the noise to stop. Running from a battery is a good test to see if the noise is really being conducted down your power cord, across the DC power supply, and across the chassis into the antenna input circuit. An isolation transformer won't do much of anything if the noise is being radiated into the antenna from an outside power line.
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AK4SK
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« Reply #7 on: January 07, 2013, 07:45:19 AM »

Thanks for all of the replies. It sounds like an isolation transformer may help, depending on the noise source (AC line vs antenna), but that a type 43 choke on the AC line would provide better results and be cheaper. Do I have that straight?
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