Impedance

Impedance 

Impedance is simply the ratio of Voltage to current. That ratio (impedance ) changes from very high at the open end of any antenna wire to low ( let’s just say around 50 or 75)when you move 1/4 wave back from that end. So take a half wave piece of wire, at the frequency for which it is a half wave, the impedance will be about low at the center. (About 50 ohms)

That point is 1/4 wave from both ends. It is a good match to 50 ohm coax.  SWR will be low. In reality the impedance will vary with the height above ground. Never too far from the coax impedance be it 50 or 75 ohm cable. At this point there will be very little reactance. We call that resonant.

Some antennas have different impedances at the feed point and then you need to “match” them to the coax. Depending on the situation a 2:1 transformer can match 25 or 100 ohms to 50 ohm coax. 4:1 transformers can match 12.5 ohms or 200 ohms to 50 ohms. 

Baluns are really not necessary with dipoles. However there is a chance that the outside of the coax could become a conductor or part of the antenna. Think of it this way. The current that came up the inside of the shield has a decision to make as to if it will go out on the antenna wire or down the outside of the shield. The current coming up the center conductor has no choice it’s only path is out on the antenna wire. But the current coming out the shield will ALL go out on the wire if and only if the path down the outside has a lot more impedance than the antenna wire. It may divide evenly and half go on the antenna and half on the coax. That really only happens in the case that the feedline length is resonant. Is it a problem? Probably not. Do I care with a dipole? No.   Do I care with a highly directional Yagi antenna? Yes.