Half-wave dipole antennas are great but can be a pain with the initial tuning; up, down, trim, up, down, trim, up, down, trim, etc. There is a way to make a half-wave dipole and only cut it two times, initial cut and final tuned cut. In this example we want the length to have the lowest SWR at 7.2 MHz, here’s how to make it.
First, cut the dipole slightly longer to 7.1 MHz.
L(feet) = 468 / F(MHz) or L = 468 / 7.1 = 65.9155 feet for our example.
Raise the antenna up in the air to its final position. Next, with a MFJ-269 antenna analyzer, find the frequency with the lowest SWR. For this example say the frequency is 7.044 MHz which is way below the 7.2 MHz we would like it to be.
Now, use the same formula L = k1 / F1, but this time replace 468 with k1 and solve for k1. To solve to k1, use the measured frequency at the lowest SWR for F1, which in our example is 7.044 MHz times the calculated length for L, which in our example is 65.9155 feet; k1 = L x F1. So, k1 = 65.9155 x 7.044 = 464.3088.
Now calculate the corrected length for the frequency of 7.2 MHz using the new value fork1. L = 464.3088 / 7.2 = 64.4873 feet.
Last, you need to cut off a total of 65.9155 – 64.4873 = 1.4282 feet or 17.1384 inches from the total length of the dipole or 8.5692 inches from each end.
The SWR should be the lowest close to 7.2 MHz.