The "basic" formula for a Yagi design is that the driven element is tuned to resonance in the center of the desired frequency range. The reflector is then tuned appr 5% longer than the driven element and placed behind it. Varying the space will affect the front to back ratio and forward gain. This is the most basic Yagi that one can build and have it work well.
To increase the gain, a director is added in front of the driven element. This is appr 5% shorter than the length of the driven element, making it 10% shorter than the reflector. If one wanted to increase gain further, you can add another director, which in turn would be another 5% shorter in length. This works well until you hit about 5 - 6 elements total ( 1 reflector, 1 driven element and 3 or 4 directors ). After that point, the boom becomes longer, wind load is increased and the increases in gain are not as sizeable.
As far as bandwidth goes, 20 MHz is actually quite wide in spectrum at that frequency range. As one goes up in frequency it is FAR easier to make an antenna that covers a a wide range and maintains good tuning / good gain. At the frequency range that we are at with FM, obtaining a 5 MHz bandspread with excellent tuning would be a very reasonable goal. As one strayed further away from the center of this band, antenna impedance and gain would begin to vary. As such, most antennas are tuned with a center frequency of appr 98 MHz ( middle of the FM band ) and performance is relatively lacking at the low end ( 88 MHz ) and top end (108 MHz ).
As such, if one really wanted to pull in specific stations within a certain frequency range, having an antenna made ( or making one yourself ) for that specific center frequency could result in noticeable improvements. The fact that many people are happy with indoor dipoles or simple "stick" antennas like the Magnum / Fanfare / Metz design ( that are not very broadband at all ) shows that a simple antenna can work quite well. More advanced designs simply give you even greater range with less noise and interference.
Other than that, some of the older, well designed receivers ( HF, shortwave, broadcast band, etc.. ) can work phenomenally well. Depending on how involved one wanted to get in terms of user adjustable controls, some of the "fossils" that are out there are actually "diamonds in the rough". The biggest advantage that i see to a well designed and highly tuned tube front end is that they are less prone to front end overload and adjacent channel interference. If one lived in the city near a lot of strong stations, that is probably what i would be looking for. Sean
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