TV Antennas: Active or passive? Also, splitters. - Page 1
Forum section: electronics / projects

author: rs20
author_rank: Super Contributor
timestamp: March 27, 2014, 04:20:58 am
content: Does a standard, normal, bare-bones TV set up have any amplification/buffering/silicon up by the antenna? I can't find any reference to such doing Google searches, but I feel like Dave mentioned something about this in a recent video (the one with an amplifier chip with only 3 pins), and I've also seen tiny little sparks and even got a little jolt once off an antenna jack on the back of a TV (two separate TVs, so I doubt it's a fault), making me think that TVs provide a fair bit of phantom voltage to the antenna by default. I should probably just get a voltmeter and stick in the antenna jack, but I don't have a TV anymore...Also, a more fundamental question: a TV splitter takes a signal off a 75 ohm cable, and splits it into two 75 ohm cables. Now, my impression is that a properly terminated cable with a 75 ohm impedance behaves likes a perfect 75 ohm resistor. If you connect two of those in parallel, you've got 37.5 ohms. Is the input cable from the antenna therefore improperly terminated, leading to reflections, or do splitters include resistance to match the parallel outputs to the input? Or is there some other clever option?

author: ludzinc
author_rank: Unknown
timestamp: March 27, 2014, 05:22:33 am
content: Quote from: rs20 on March 27, 2014, 04:20:58 amAntennas can, unsurprisingly, pick up a lot of electric charge when not plugged into anything. I once was installing a 2MHz weather radar in a field in Indonesia (using 100m long dipoles. Yes. One Hundred Meters Long.) and when I picked up the feed to one - while leading against the Rx rack - it bit me. HARD!Measuring the volts between the grounded chassis of the Rx and the outer conductor of the coaxial feeder, I was seeing over 100V AC. It probably had something to do with the thunderstorm rolling in, and the fact the Radar field was a big old swamp....Quote from: rs20 on March 27, 2014, 04:20:58 amYes, if you connect two 75 ohm *impedance* cables together you end up with a 37.5 ohm *impedance* load. The splitter can be designed to match your 75 ohm system to the two 75 ohm cables - you can get lossy resistive dividers that achieve this, or you can use other matching techniques (such as a 2:1 balun - it transforms impedances in a ration of 2:1 stepping up your 37.5 ohm load back to 75 ohms).Lots of good stuff about this here:http://en.wikipedia.org/wiki/Power_dividers_and_directional_couplers

author: cravenhaven
author_rank: Contributor
timestamp: March 27, 2014, 06:34:04 am
content: Well having just installed a new aerial and cable and a new TV, I can say positively that the basic aerial system has no active parts whatsoever. However, I did note that my TV has an option to provide 5volt power up the antenna lead, presumably to power a masthead antenna.I did actually add a masthead amp to my antenna and the power supply for it is 28Volts DC.If you had an earthing problem on your TV then you could easily get sparks when bringing the antenna lead nearby the connector as the outer shield on the cable is possibly earthed.