I'm trying to understand the circuitry of TV signal splitters and associated boosters/amplifiers, and I have two questions. My situation is as follows. I have a TV aerial (antenna) in my loft. It connects to a non-powered box, which I assume must be a passive splitter. From there cables run to sockets in a total of 6 rooms. However, only in two or three of those rooms does a TV connected to the socket show any signal; and even in those rooms, there is no signal unless a booster, located in the room nearest to the aerial, is connected and powered up. That all sounds sensible, but then I stop being able to understand.

  1. Here's the odd thing, and the first question. To make any TV on the system work, the booster has to be connected to the aerial socket, but nothing needs to be connected to the booster output; so long as the booster is connected and switched on, the TV in a neighbouring room will work. It's as though it is somehow sending the amplified signal back up its input cable. But I've looked at circuit diagrams for boosters, and that doesn't look possible. Can anyone explain what is going on?

  2. I am trying to find out what is wrong in the rooms where no signal is ever reported. I read somewhere that if I look across the terminals of the TV socket with an ohmmeter, I should see effectively zero resistance, since there is continuity through the aerial. However, this is not true for any of my sockets. With all devices disconnected, if I look at the resistance at the socket the booster normally connects to, I see about 4k. If I look across any of the other sockets (including those where a signal is successfully received), I see no continuity at all. So I suppose that the passive splitter must have a capacitor or transformer somewhere in its circuitry, but I can't find a circuit diagram anywhere that would show whether this is true. Can anyone say whether this is the case, i.e. whether I should be able to see continuity when looking into a socket?

Background information:

  • Until recently we have never tried to use TVs in the rooms where we now find they don't work, so this is probably not a new problem.
  • In particular, the non-functioning sockets have not been used since the analogue era.
  • The wiring is probably at least 30 years old, and certainly pre-digital.
  • I'm in the UK, and the TV signal is digital terrestrial.
  • Fitting an outdoor aerial to get a better original signal is not an option in our neighbourhood.
  • The passive splitter and aerial connections are all screw-downs, and they are not conveniently located, so swapping cables around for test purposes is slow and painful.
  • \$\begingroup\$ I don't think you can get sensible readings when poking an ohmmeter into antenna sockets that are being driven with RF energy signal. One thing might be that the signal is too strong so it kind of blindfolds the TV analog RF frontend so it can't see the signal. Then when you have some loads connected like the booster, it is a load that reduces power to other device that can then see the signal better. So maybe you just need more attenuation between TV and antenna socket. \$\endgroup\$ – Justme Apr 7 at 23:20
  • \$\begingroup\$ If cables are unterminated they can represent harmonic 1/4 wave short circuits at those wavelengths to some rooms on some channels. But poor cable shields nicked can also degrade signals. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Apr 7 at 23:30

You've made some wrong assumptions about what the parts of the system are. The part you're describing as a non-powered passive splitter is actually a powered active splitter. Splitting one aerial signal into six with a passive splitter is unlikely to give you sufficient signal on any of the six outputs, especially if the aerial is in the loft.

The part you're describing as the booster is just the power supply to the active splitter. It sends a DC voltage up the cable running to the active splitter, and filters it out of the cable running to the TV. That's why it has to be powered up for any of the TVs to work.

The rooms where TVs don't work are either down to a faulty output from the splitter or a defective cable. To do a basic test on each cable, disconnect it from the splitter and check that it's open circuit, then short one end together and check that it now shows a short circuit at the other end.

  • \$\begingroup\$ Thank you very much, Finbarr (and apologies for a delay in responding, I have been away from computers for a week). That makes perfect sense, and explains what is going on. I'd read about these power supplies, but in the context of masthead amplifiers, so I hadn't put two and two together. So now a jolly session of lying flat out scrabbling at the cables going into the splitter looms, to find out where the breaks are... And thank you, too, to others who have responded and helped my understanding. \$\endgroup\$ – seglea Apr 19 at 19:24

It is desirable to have the amplifier be as close to the aerial as possible and certainly prior to the signal being split, however getting mains power to said locations is often problematic.

The solution to this is amplifiers that are powered via one of the output coax lines. The amplifier is sited close to the aerial while a power injection unit is sited close to the TV, These are often sold as "masthead" amplifiers. I believe this is the setup you have.

I don't think you can read much into whether or not there is DC continuity on the output of an amplifier, whether you see it or not depends entirely on the details of the amplifiers internal circuitry.

I would start with end to end continuity and short-circuit tests on the cable runs (note: some sockets have isolation capacitors, so you may need to test from the terminals on the back of the sockets rather than the connections on the face). If you find any opens or shorts then you obviously need to fix them.

Failing that it may be worth swapping around connections to see if the problem follows the cable or follows the connection on the amplifier, but honestly given the age of the amplifier and the difficulty to access i'd be more inclined towards replacing the amplifier at that point.

  • \$\begingroup\$ Can an active/powered TV splitter be used to split a coaxial SPDIF digital audio signal into two outputs with the same signal? With the appropriate physical connector adaption and on the basis that TV splitters handle high frequency signals, I would have thought this would be possible. \$\endgroup\$ – therobyouknow Apr 8 at 10:08
  • \$\begingroup\$ @therobyouknow sounds like a different question - it could go either way, the powered splitter might filter out signals that aren't in the TV band. \$\endgroup\$ – pjc50 Apr 8 at 13:19
  • \$\begingroup\$ I think it is pretty unlikely that a TV amplifier would work for s/pdif. Firstly because I doubt the frequency range would reach down low enough, secondly because the voltage levels are very different, thirdly because radio receivers are designed to tolerate a wide range of signal levels while systems like s/pdif are not. From some googling it seems that normal TV signal levels are about 45-70 dBuV which works out to about about 200uV to 3mV, wheras S/PDIF is up in the hundreds of millivolts. \$\endgroup\$ – Peter Green Apr 8 at 15:47

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