How exactly do the elements "connect" to each other to make an antenna directional? I see some Yagi's have a reflector but most do not.
A Yagi antenna is an end fire array of dipoles. Energy is supplied to a driven element via a transmission line, and the remaining dipoles are excited by mutual coupling rather than by the use of a transmission line. Each element has a fairly high Q factor, and so the coupling can be sufficient even when the mutual impedance is small. K=MQ where K is the coupling coefficient and M is the mutual impedance. The mutual impedance is a mixture of reactive and radiative coupling. The required phasing to obtain end-fire action is obtained by de-tuning the elements.How exactly do the elements "connect" to each other to make an antenna directional? I see some Yagi's have a reflector but most do not.
I see some Yagi's have a reflector but most do not.
What you mean is that the reflector often has a large area. There is always at least a dipole 'reflector'. It reflects because its length is greater than the driven element and the phase of the re-radiated wave from it is opposite (approx) to the phase of the wave from the driven element so there is (partial) cancellation in the rear direction. As has been hinted at above, the simplest form of Yagi is an H antenna which has a roughly cardioid pattern.I see some Yagi's have a reflector but most do not.
Early work on some very successful Yagi arrays was done decades before the dreaded simulations were available.For more elements it looks like simulations are used to optimize the gain and directivity..