Firstly A Longwire is classically a multiple λ antenna, however the term has been hi-jacked to mean anything that is end fed!
However, many amateurs just stick as much wire as they can in the air and hope for the best using either a 4.1 or 9.1 unun. For using on more than one band all longwires require a Antenna Tuning Unit (ATU). Also it pays to remember that the longwire is Omni Directional!
What the diagram doesn’t make clear is that you need an insulator before the main elevated mast, and an insulator between the secondary mast otherwise the masts will become live, causing a serious health and safety issue!
Note that it requires an UnUn not a Balun and also that you need to feed it at the far end not the near end!
Improvements can be made to reception by using an equal length wire as a counterpoise BEWARE however, that this should not EVER be connected to the Station Ground!