The properties of a receive-mode Yagi are relatively uncritical. The bandwidth and VSWR preformance matters less than the gain of the antenna and its discrimination against unwanted signals. However, for a transmit Yagi such as is commonly used by Hams and short-wave broadcasters, the accepted power depends critically on getting a good match to the feed. This will vary across the band, and is susceptible to variations in local environment and geometry distortions.
The lore of the Yagi designer has it that the gain of a Yagi is governed more by the overall boom length than by the number of elements. For an HF Yagi, the boom length can be a critical design factor, and the Ham is usually seeking to optimise the forward gain, the front-to-back ratio, and the construction techniques required. Yagis having thick rod elements (in terms of a wavelength) are better-behaved than those made from thin wires.
The gain of a Yagi-Uda is only moderate, but for the frequency range given above it is cheap and relatively simple to build. It is reasonably tolerant to variations in construction, and indeed, many Yagi-Uda designs have been arrived at by cut and try empirical methods. This is why antenna design is often seen as a black art. With proper numerical simulation, useful improvements have been made to the empirical designs. Trade-offs may be made between the various factors, such as bandwidth, impedance, front-to-back ratio, gain, sidelobe performance, and ease of mounting. A vertically polarised Yagi-Uda often is mounted on the top of a vertical conducting mast which, being in the near field, and also polarisation- matched, will modify the electrical properties. There is less of a problem with mounting a horizontally polarised Yagi-Uda.
For moderately long Yagis with several directors, the reflector spacing and size has little effect on the forward gain, providing that there IS a reflector, but being close to the driving element it has a strong effect on the front-to-back ratio and on the driving point impedance of the antenna. The driving element has of course a big effect on the impedance of the structure and it can be tuned to make this impedance nearly real. The directors form the majority of the travelling wave structure and amply repay care in design.