I’m having a problem with my TVs. I will call them TV1 and TV2. TV1 is Asanzo brand, and TV2 is LG 4K, which is much more expensive.

After replacing the antenna wall socket of TV2 (The TV is near to me in the video), the silly thing happens. Every time I turn off the TV1, TV2 has very weak or no signal. To watch in TV2, I have to turn on TV1 (even when the two TV have different channels) . But when I turn off TV2, TV1 still works normally.

(update: disconnect TV1 from power supply and antenna wire, TV2 still won't work.)

USB Power Schematic

Video: https://www.youtube.com/watch?v=COGERGH66oU

The two TVs have integrated DVB decoder, use the same antenna with USB power for each one. The antenna has amplifier circuit. Antenna info enter image description here

I’m not sure if it’s caused by the new wall socket, because I had replaced it very carefully. Hope to hear your explanation about this and of course the solution. I don’t want to have both TVs on all the time just to watch one. Thank you!

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    \$\begingroup\$ what happens if tv 1 is disconnected? \$\endgroup\$ – Passerby Sep 8 '18 at 2:55
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    \$\begingroup\$ Did you change anything else? I find the USB antenna device you show here suspect. I believe it probably uses power from the TV to run an amplifier to repeat the signal from the first TV to the second TV, so it doesn't work when the first TV is off. Did you change from switching the first TV off with the remote, to switching it off with a power strip, or something like that? (Actually, what is that USB device you have a picture of? Can you clarify that please? And how exactly things are connected?) \$\endgroup\$ – Glenn Willen Sep 8 '18 at 2:58
  • \$\begingroup\$ Is there a splitter involved or simple parallel RF connector. Are the two TV's different? are the two adapters the same? Does the TV have a polarized AC plug? if not does that affect the Tuner noise? The splitter should have 30dB isolation unless 75 Ohm Z changes with power off on 1 set. Then verify by disconnecting Coax when Off \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Sep 8 '18 at 3:14
  • \$\begingroup\$ I have tried with TV1 is completely disconnected from the power supply and the antenna wire, TV2 has no signal. And yes, there is a splitter with antenna input and output to the two TVs. \$\endgroup\$ – Feliks Sep 8 '18 at 3:24
  • \$\begingroup\$ I also update image of USB connection. \$\endgroup\$ – Feliks Sep 8 '18 at 3:30

It seems to me the problem is your antenna is only ever powered through the USB adapter at TV1, which is plugged into TV1.

If you turn off TV1, the USB port has no power and neither your antenna gets any power. The power adapter at TV2 is redundant. It can't power the antenna because there's no power pass through the power adapter plugged into TV1.

Power the antenna through a mains-USB adapter instead.

  • \$\begingroup\$ I believe this is called "Phantom power". \$\endgroup\$ – Sparky256 Sep 8 '18 at 3:42
  • \$\begingroup\$ that doesn't make sense, even if your advice at the end makes sense. The signal is not in series it's in parallel. \$\endgroup\$ – Passerby Sep 8 '18 at 3:45
  • \$\begingroup\$ @Passerby I agree \$\endgroup\$ – Old_Fossil Sep 8 '18 at 4:34
  • \$\begingroup\$ USB port might be a good place to think. I guess USB port of TV2 should have turned off for some moments (maybe due to saving mode or anything related to power settings of TV) . An hour ago, I tried to plug TV1 to antenna wire of TV2, then TV1 works normally, strong signal. I then plugged the wire back to TV2 and TV2 works fine, too, without TV1 being on. I should check USB port voltage when TV2 doesn't have signal again. \$\endgroup\$ – Feliks Sep 8 '18 at 5:31

Something that you mentioned caught my eye. When both TV's are on same channel one TV doesn't work. Both TV's on different channels both TV's work. That in itself rules out any antenna issue. It could be a simple RF interference issue. It could be the IF frequency. Google superheterodyne radio design . When a TV or radio receives the stations frequency it is mixed with the local oscillator to generate the intermediate frequency. Same channel = same IF , different IF channel = different. If the TV's are in relative close proximity this could be the issue. If the antenna amplifiers are phantom powered you should have a DC block on the input of each TV.

  • \$\begingroup\$ I think you misread that. To me it reads that it works when both the Tv's are on the same or different channels \$\endgroup\$ – Passerby Sep 8 '18 at 4:50
  • \$\begingroup\$ I must be going blind or just very tired...LOL. But the DC block might actually be part of the solution. \$\endgroup\$ – Old_Fossil Sep 8 '18 at 4:56

It's quite possible is that you need to correctly split the antenna signal into two TV feeds and do so without disrupting the antenna impedance and/or creating standing waves. This is what the problem sounds like. For instance doing this: -

enter image description here


Dong this on the other hand: -

enter image description here

Is much more likely to work because it splits the power to both TVs correctly and provides an impedance to the antenna that remains at 75 ohm. If you want the theory read this website.

If you think you can treat an antenna feedline like you can (say) an AC power line and plug in appliances all to one circuit then think again.

We're talking about transmission line theory and impedance matching when we are dealing with high frequency signals on a line that is many times the wavelength of the signal.

  • \$\begingroup\$ I think that antenna has amplifier built in, and USB part is used to supply power over coax for it. In which case the splitter should be connected after USB, and the second USB probably removed altogether \$\endgroup\$ – Maple Sep 8 '18 at 8:43

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