HamTrain Foundation Online: Feeders and Antennas 

A Good video; I was impressed that a field diagram was shown for the dipole, and was then disappointed that these diagrams didn't continue on the next slides. You also failed to point out that the electromagnetic field doesn't actually touch the antenna, and will travel further away as the height of the antenna above ground increases.

Using a single antenna on a band its doesn't resonate on may be possible with a ATU to convince the radio you have a resonant antenna connected to it, but you will receive very little signal and will transmit even less because the antenna resonance is required for the creation of an Electro Magnetic Field which will not happen on a non-resonant antenna, worst still like the G5RV you have the feeder line becoming part of the antenna and start transmitting energy where you don't want it transmitted.

Actually, it is not how an antenna gain works. It is not increasing the power, because it would contradict the conservation of energy. So if you feed 5 watts you will not get 10 watts but, of course, in the main direction the power density will be 2 times higher than from the reference antenna (isotropic or dipole).

I was surprised you didn't say what an impedance is, you also said that wavelength affects Impedance, sorry but wavelength has no effect at all on impeadance,if it did we would have coax cable for use on each band which we do not.