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RFI from Plasma TV

Sep 1st 2011, 17:20

W1MG

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
A member asks...

My neighbor has a large 50" Plasma TV aprox 50 feet from my house. When it is on I have a 10 over S-9 noise level on my Icom 756 Pro. When he turns it off my noise goes away, so we are sure that is the source. I had the cable Co, Verizon fios, come over and check both of our entrance panels for correct grounds etc. The neighbor is cooperative and allowed me to put toroids on the HDMI cable from his cable box to the TV, this did not help at all. At that time I was not able to place any toroids on the AC line from the TV, but today I hope to do that. Have you had any other reports of RFI from a plasma TV?

Thanks for any advice you can offer.
Sep 1st 2011, 17:27

W1MG

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
There may be a cure in some (but not all) cases for this problem. Plasma TVs in particular may not have a practical cure in cases involving direct radiation from the plasma display.

I would first suggest contacting the retailer for help. The retailer in this case may be willing to swap an LCD TV for the plasma. Next, contact the manufacturer for help. If that doesn’t work, I would then suggest the usual cures involving brute-force ac line filters and common mode chokes.

I would start by disconnecting the coax from the TV, along with any other wires other than just the ac power cord. I would then start adding common mode chokes and a brute force ac line filter. If the frequencies involved are at HF, I would further suggest toroid cores in this case. Wrap ten to fifteen turns of the power cord onto an FT-240-31 ferrite core. You can also use a FT-140-31 core if the plug or wires are small enough.

Note: Toroid cores with type -43 material (such as an FT-240-43) are also relatively common and should also work for most applications. You can also use type -75 ferrite material if the problem is primarily at 160 and 80 meters.

The common-mode chokes should be installed right at the source device, i.e., the TV.

Note: Split ferrites and toroid cores are not as effective as toroids at HF. Do not assume that a ferrite common mode choke will not work unless you've tried a toroid core with a suggested ferrite mix. In some cases, additional chokes may be necessary. Add chokes, one at a time as necessary.

The ARRL "RFI Web Site" contains a list of EMI/RFI materials suppliers for ferrite chokes. You can also refer to the advertisements in QST -- there are a few advertisers offering ferrite materials and chokes.

If that doesn't work, one of the "brute-force" AC line filters might help. Some of the Corcom filters sold by DigiKey are examples. You can contact Corcom at www.corcom.com for engineering help in the selection of a filter. Also see the Short Takes review on page 48 of the March 2005 issue of QST for a review of an ICE filter.

Next, reconnect the coax to the TV. If the noise returns, add common mode chokes to the coax as required. Add any additional wires (such as speaker wires) and add chokes on a case by case basis as necessary.

If these suggestions don’t help, there may not be much you can do without adding shielding to the TV, which is obviously not practical in most cases. The problem is direct radiation from the TV, as opposed to conducted emissions from the TV. Increasing the distance of the antenna from the TV is one possible solution. Obviously, this isn’t practical in many neighborhoods with small building lots.

Good luck and hope this helps.
Sep 13th 2011, 16:28

N0IVN

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
My experience with plasma TVs is that they can be notorious sources of RFI, both radiated and conducted. I suspect a combination of the two interference mechanisms is at work here. It sounds like the interference is typical plasma TV noise rather than, say, cable TV signal leakage by the plasma TV, considering the connection to the TV has been described as an HDMI cable to the set-top box.

The plasma TV in question may have multiple wired connections, any of which can be susceptible to common mode currents and contribute to the observed interference: AC power lead, HDMI cable(s), baseband audio/video cables, Ethernet cable for Internet connectivity, etc.

As recommended by W1MG, try the usual approaches with common mode chokes on the interconnecting leads, brute force AC filter, etc., as close to the TV as possible. Keep in mind that the problem may well be radiated interference, or at least have a significant radiated component. Chokes and filters won't do any good against interference radiated directly by the plasma TV.

73,
Ron Hranac, N0IVN
ARRL EMC Committee Member
ARRL Colorado Section Technical Specialist

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