Introduction
In this appendix we introduce a process for efficiently tuning an antenna using a 40-meter dipole as an example. We know that tuning must be done at the height of deployment, but we need to make adjustments with the antenna lowered so we can reach it. Thus, we need to raise the antenna, measure the frequency of minimum SWR, then lower it to adjust it or adjust it to a lower frequency of Minimum SWR while only a few feet off the ground. This is repeated until the correct length is found. The purpose of these instructions is to minimize the number of times we have to “rinse and repeat.”
Building the Antenna
For a 40-meter dipole, we calculate the length using the familiar formula, L = 468/F. Assuming we want to tune to the middle of the 40-meter band, we use 7.15 MHz. In this case, L = 65.45 ft. Each dipole leg will be half that length, so each leg is 32.73 ft long. We need to add about 8 inches to each end for attaching the wire to insulators and Balun and we want to cut the wire a little long to make sure we have enough wire to make adjustments during tuning. So, we add 16 inches (1.33 ft) to tie to insulators and increase the length by about 5% (1.6 ft) to allow for tuning. So, we cut the wire for each dipole leg to 32.73 + 1.33 + 1.6 = 35.66 ft.
Now to build the antenna. We attach each leg to the center insulator or 1:1 Balun/Choke or balanced feeders and its end insulator. The amount of spare wire for each leg is 1.33 + 1.6 = 2.93 ft. So, we have about 1.5 ft of spare wire for each end. Wrap the excess wire around the end of the dipole to prevent it from radiating as shown in Figure 1.
Figure 1: Wrap spare wire around radiating wire
The length from center insulator to each end insulator should be the design length, 32.73 ft.
Tuning
We know that the antenna will resonate at a lower frequency when close to the ground than at deployment height. Therefore, we will do the initial tuning close to the ground, 4 to 5 ft, so we can adjust it. But we tune it to about 1% lower frequency than our design frequency. For our 40-meter antenna this is about, 7.08 MHz. Hopefully it will increase to precisely 7.15 MHZ when we raise it to deployment height.
So, with the antenna at 4 to 5 ft, measure the frequency of minimum SWR. Let’s say we measure 6.9 MHz for minimum SWR. It’s too long for 7.08, so we need to shorten it. But, by how much? We can approximate that with the following formula:
New_Length = Old_Length * Current_Frequency / Target_Frequency
The total length is 65.45 ft:
New_Length = 65.45 * 6.9 / 7.08 = 63.79 ft.
The difference is 65.45 – 63.79 ft = 1.69 ft = 20 inches. If working with a center fed dipole, shorten each leg by 10 inches. If working with an off center fed dipole with the feed-point at 20% of total length, shorten the long leg by 16 inches and the short leg by 4 inches. If working with an End Fed antenna, shorten the far end by 20 inches. Measure the frequency of Minimum SWR. If it is off by more than 1%, repeat the tuning procedure.
Now we are ready to raise the antenna and measure the frequency of Minimum SWR. If necessary, lower it, repeat the tuning calculation, and adjust the length as above. Raise the antenna again and measure the min SWR frequency. With any luck, it will be close enough. If not, take it down and repeat the tuning exercise.
The approximation leaves the antenna a little bit long, so if we start with the antenna a little long each iteration should result in further shortening. It should converge quickly, within one or two iterations of tuning at deployment height. Once it is tuned, we can trim the extra length wrapped around the end of the wire or tape it down so it won’t unwrap. We now have a well-tuned antenna and have eliminated the electrical mayhem and associated losses a poorly matched antenna causes on our feedline and to our rig.