A standard dipole has 1/4 wavelength at each side relative to the center feed point. The standing wave ratio relative to a 50 Ohm transmission line that we can expect for such a dipole depends on the height above the ground, and it is found to change between 1.4 and 2.0, the first optimum is obtained around 11.5 meter above the ground. All calculations assumed an antenna length of 10.35 meter and a frequency of 14.2 MHz.
Oftentimes it is inconvenient to feed the dipole from the center, one would need two masts and the coax cable attaching in the center. More practical is to put the feed point closer to one side, this idea leads to the so-called windom antenna. I assumed a version that had 1/6 of a wavelength on one side, for instance the side running to your house, and another 1/3 running into the garden to a pole. This saves at least one mast to worry about, the antenna was discussed by Loren G Windom, W8GZ written in the September, 1929 issue of the QST Magazine.
The half-wavelength dipole has a theoretical impedance of 73 Ohm that results in a SWR of 1.46 for a 50 Ohm transmission line. The antenna tuning unit in the transceiver can usually match this, as long as you stay under 3.0 when the FT-991 is used. But as said before, the actual value one obtains for the SWR depends on the height of the dipole above the ground which we assumed to be an “average ground” in the NEC-2 software.
For the windom antenna discussed before you find a greater impedance that also depends on the height, with NEC-2 I found a minimum value of 1.7 at 12 meter above the ground. Point is however that the windom antenna has a larger range of SWR values when we vary the height, I found with NEC SWRs between 1.7 and 2.5 which is still acceptable for most transceivers.
With the help of my autotuner (an YT-1200 from LDG) one can tune over a much wider range, namely from 4 Ohm to 1000 Ohm, but the maximum power it can handle is limited. A balun is always required for both the dipole and the off-centered dipole (the windom antenna), a 1 to 4 ratio is sufficient but it is already an overkill. A 1 to 2 balun would be good enough, possibly a 1 to 1 balun would also do the job in combination with the autotuner or the ATU in the transceiver.
Attached is the predicted plot of the SWRs of both antenna’s as a function of the height above to the ground, the values were estimated with the NEC-2 software. There is no reason to assume that SWR’s larger than 2.5 will occur for the windom antenna discussed here.