Why 300 ohm (twin lead)/pre-amp

I've never seen that thread before. I only look at my own local thread in the "Local HDTV" forum. Frankly, I'm surprised that it is located in that forum. I suspect that more members will see SD4934's post here.


Most common antenna designs tend to naturally have an impedance of 300 ohms. Antennas with 75 ohm terminals are generally antennas with 300 ohm active elements that have a built-in 300 ohm to 75 ohm adaptor.


I haven't taken apart any preamps in a couple of decades, but I'd guess that preamps with 300 ohm inputs don't have any input adaptors in them. I'm guessing that the 300 ohm input mates directly to the inputs of an op-amp's inverting and non-inverting terminals. If that is the case, then one probably would enjoy a half dB reduction in signal loss by eliminating the 300 to 75 ohm converter cartridge on the antenna, and another half dB by not having to convert the 75 ohm unbalanced input to a form compatible with the input of the pre-amps amplification component. Also, whenever you make a broadband impedance conversion device, it never is flat across the band. Therefore, I suspect that connecting a 300 ohm antenna output directly to a 300 ohm preamplifier input will tend to facilitate the development of a slightly better quality preamplified output signal.