The developers at Diamond thought that for the BB6W, or the BB7V for good measures, a 6:1 transformation would be the way to go. This aerial design assumes a 50Ohms feedline. The aerial's feedpoint is therefore considered having an impedance of 300Ohms. Remember, in this design, there is 600Ohms of termination resistor to ensure a maximum SWR of 2:1.
Assuming that the average auto-ATU will match a transmitter's 50Ohms antenna connector to a 75Ohms feedline easily, a 4:1 transformation will do in order to match a feedpoint impedance of 300Ohms when using 75Ohms feedline.
This actually brings together good news from two worlds:
- 75Ohms coax cable has got less losses than 50Ohms coax (it is cheaper too)
- a 4:1 transformer is easier to make than a 6:1 transformer (QRP: old TV-xformers)
As a side remark, the feedpoint impedance at the voltage maximum is considered to be 5kOhms. Neither window line nor a 9:1 transformer gets us there. This can only be matched using ladder line (or open wire line) with a symmetric coupler.