|TV cards are often less tolerant of weaker video signals than TVs are. The TV card often can't amplify the incoming video signal -TVs can. |
You often need to use a video signal amplifier between the source cable and the TV card, and an additional cable if you don't have one.
RCA 10dB Video Signal Amplifier (for coax cables)
If you need something to fasten RCA male plugs to, you could buy adapters for that and use those with the video signal amplifier.
If you're using (a) cable splitter(s)....
.....to connect to more than one device that requires the video signal, the splitter(s) also weaken the signal to their outputs. E.g. for a 1 input to 2 output splitter, each output has half the strength of the input video signal.
If you're using more than one cable splitter, you'll get the strongest signal for the TV card from the ouput from the first splitter.
If that's not strong enough, connect a video signal amplifier directly to the TV card for the best result.
Or get a a video amplifier with more outputs
RCA 10dB Video Signal Amplifier (1 input 4 outputs)
...although, in the case of those two RCA brand ones, the total amplification is the same - the signal will probably be stronger from the output of the one input one output model.