What does an antenna "matching transformer labeled 75 ohm to 300 ohm" really mean?
That's a lot of questions.


  Shouldn't it be the other way around? (300 ohms to 75 ohms) 300 ohms coming out from the antenna wires to 75 ohms coaxial cable?


Transformers are bidirectional; they don't care which way you hook them up. You could use one at each end of a piece of 75 &Omega; coax to connect a 300 &Omega; antenna to a 300 &Omega; TV. In fact, they first became common when people wanted to connect their 75 &Omega; CATV outlet to their 300 &Omega; TV input.


  Where does the 300 ohms comes from? is it because most antennas use wires that have 300 ohms impedance by default and you need this matching transformer to bring it down to 75 ohms.


The main element in a typical TV antenna is a "folded dipole" &mdash; a flattened loop of wire or tubing. Its natural impedance is close to 300 &Omega;.


  What if the antenna is labeled 50 ohms? can we still use this matching transformer? or it is not necessary in such case.


No, if you have a 50 &Omega; antenna, it will typically have a coaxial connector, and you wouldn't use a transformer. Although such an antenna would not be a good match for 75 &Omega; coax; you'd normally use 50 &Omega; coax with it.