This is a standard calculation method that can help you while tuning dipole antennas, by adjusting wire lengths.
For my opportunity I will use the metric unit in this article, but you can easily convert to inches.
If you wan to tune a dipole on 7.070 kHz, while the lower SWR is at 6.950 kHz, you need to short the wires, since it is obviously too long. You need therefore to change the wire total lenght in order to move the center band of 7070-6950 and therefore of 120 kHz.
You need to multiply the difference in kHz (120)by 100 and dividing by the desired frequency (7070) in kHz. You will obtain the percentage of the total lenght to be cut.
So, assuming your antenna for 40 meters band, is 21.60 m you will have
21,60 x 1,69 = 36,5 cm
The dipole will have to be shortened of 36,5 cm and therefore 17,75 cm each leg.
This method can be used also when you need to add lenght to your wires, and can be additionally used to quarter waves vertical antennas, dividing by 2 the final result. Note that on vertical antennas also ground radials need to be adjusted.
If you want to move by 100 kHz
|80 meters band||110 cm||55 cm|
|40 meters band||28 cm||14 cm|
|20 meters band||7 cm||3,5 cm|
|15 meters band||3 cm||1,5 cm|
|10 meters band||1,8 cm||0,9 cm|