The 50 Cl SWR for the example three

The 50 Cl SWR for the example three element Yagi is shown in Fig 19-4. Note that while the frequency of lowest SWR is somewhat below the design frequency of 145 MHz, the actual resonant frequency, the frequency with Cl reactance, is at 145 MHz. The impedance at resonance is about 26 O, which is not a very convenient impedance for use with common transmission lines. This is fairly typical of Yagi antennas and I will discuss some of the techniques used to match transmission line to the generally low impedance of Yagi driven elements a bit later.
In general, this is a solved problem and I will not dwell on it as I talk about the Yagi properties in this section. For example, in this case, a W4 section of 36 Cl transmission line (which is available, but two common 75 Q lines in parallel could also be used) will transform the 26 Cl to 50 Cl. The SWR plot transformed to 26 Cl, under the assumption that some sort of matching arrangement is provided is shown in Fig 19-5.