On the HamRadioHelpGroup mailing list, Tim, N9PUZ, pointed a link to the eHam article, “Your First HF Dipole.” As Tim points out, the article “describes a good technique for zeroing in on a low SWR,” Here’s that portion of the article:
- Put the thing in the air as high as you can. Then find the frequency where the SWR is lowest. This might be at the bottom of the CW band or at the top of the phone portion. It doesn’t matter. RECORD that frequency.
- Then take the actual length of the antenna (you wrote it down, remember?) and multiply it by the frequency (in MHz) of the lowest SWR. That number will be your new constant, to replace 468.
- Divide the new constant by the frequency you want to have in the middle of your preferred range. This is the length the antenna should be. Now you need to adjust the one you have in the air to this length. You might find it’s easier to simply add or take away equal lengths on either side near the center insulator rather than on either end.
- After doing this haul the antenna back up into position. It should now give you the lowest SWR at the desired frequency.
If for some reason you later want to trim an HF wire antenna (say, you decide to move to a different band segment), don’t waste your time cutting a half-inch at a whack. You can estimate how much to cut or add based on the band and how far you have to move it.
For example, compare 468/ 14.0 = 33.42 ft with 468 / 14.35 = 32.61 ft , so only about 10 inches to move the width of the entire band on 20 meters.
On 75/80 meters, the difference between the band extreme edges is better than sixteen feet.