You’ve gone to great lengths to make sure your antenna is cut for the proper frequency using the 468/MHz formula. You’ve measured twice and cut once, yet the SWR readings are higher than you think they should be.
Although it’s not exactly a crucial life skill, learning to tune an antenna is important to ham radio operation. It can save you a lot of frustration and get you on the air in a short amount of time.
Just a Trim, Please
Trimming a dipole antenna means adjusting antenna length to the operating frequency you plan to use. When the dipole is properly trimmed, the antenna feed point will present an impedance that is closely matched to the feed line impedance. When feed line and antenna feed point impedances match, your antenna system will radiate efficiently. If not, some of your transmitter’s power will be reflected back down the feed line instead of radiated as RF energy.
Nearly all antenna systems will have at least a little power reflection due to some impedance mismatch at the antenna feed point. The standing wave ratio (SWR) is a comparison of the forward power in your antenna system with the reflected power. A low SWR indicates minimal power reflection and efficient power transfer to the antenna, while higher SWR values indicate greater reflection and less efficient power transfer. Generally, you should adjust for a low SWR in your antenna system.
The most common tool used to help you adjust the antenna is an antenna analyzer. It connects to the feed line, generates its own RF signals for the antenna system, computes SWR, and displays it along with the frequency. Using the built-in menus and controls allows the user to move through a range of frequencies while observing the SWR readout. You can watch for the SWR value to dip to a minimum value, and see the precise frequency for which the antenna is currently trimmed. Some models allow a wider scan of a single band or multiple bands.
Step by Step
Now let’s look at the steps for tuning a dipole antenna. You’ll need an antenna analyzer, coax connection to the antenna, pencil/paper/calculator to do the math, and wire cutters to trim the antenna.
Determine the band and frequency range you want the antenna to cover. For example, you may want to trim a 40 meter dipole for the General Class phone frequencies of 7.175 MHz to 7.300 MHz. Compute the approximate antenna length for the center frequency of the range for which you are trimming–about 7.238 MHz. Using the formula to calculate dipole length (468/frequency in MHz), the total length of the dipole would be about 64.66 feet (64 feet, 8 inches).
Cut the dipole wire to be a little longer than the computed length. It’s easier to cut wire than to extend it, so let’s cut your total dipole length to be about 66 feet long, with each of the two segments at 33 feet. (33 x 2= 66).
Move the completed dipole into its approximate operating position and height above ground–this will provide the most accurate SWR measurements. If you cannot erect the dipole near its final operating position, approximate it as closely as possible. As the antenna height increases, so does the resonant frequency. Be sure to take a look at the section “Factors That Affect Dipoles” later in this article.
Use the antenna analyzer to determine the frequency at which the lowest SWR occurs.
When adjusting the frequency of the antenna, remember the following:
- To raise the frequency, shorten the antenna
- To lower the frequency, lengthen the antenna
- Remove or add the same amount to each leg of the dipole. Both halves should measure the same after you adjust them
For example, let’s suppose you measured a minimum SWR of 1.2:1 at 7.150 MHz. To raise the minimum SWR to match our target of 7.238 MHz, we’ll need to shorten the antenna. Reduce the length for each of the dipole’s segments by equal amounts. You may trim in one of two ways: either cut the wire or wrap the wire back over itself toward the center. I prefer the wrapping method during the initial stages of tuning because you can cut any excess wire once the tuning process is completed. Be sure the wire is pulled through the insulator before wrapping.
Reposition the dipole and make another SWR measurement to see what effect your trim has had. Likely you’ll find the frequency of lowest SWR has been raised closer to your desired target frequency. Repeat the trim action in small increments until you achieve lowest SWR near the desired frequency.
When you’ve finished trimming your dipole, you’ll probably end up with an SWR of 1.5:1 or less at the center frequency, rising to 2:1 or somewhat higher at either end of the band. Don’t expect 1:1 SWR across the entire band, especially on 80, 10, and 6 meters.
A One Trim Method
So far, we’ve discussed a trial and error method. However, there is a way you can trim the dipole at the desired frequency while adjusting the length only once.
First, use the standard 468/MHz formula. For example, for a center frequency of 14.2 MHz, the length comes out to 32.96 feet (468/14.2 Mhz) or about 16.5 feet per side of the half wave dipole. Then, add about 6 inches to that length on each side and raise the antenna to its permanent location.
Attach the antenna analyzer to the end of the coax run and find the lowest SWR. Now, solve for theconstant (Constant = Frequency x Length) by multiplying the total length you initially used (in this example, case 34 feet) times the resonant frequency you found on the antenna analyzer.
For example, if the antenna analyzer showed resonance at 14.4 MHz, the real constant of your antenna would be 34 x 14.4= 489.6
Now, replace the 468 in the standard formula with the antenna’s constant we just calculated:
489.6 / 14.2=34.48 feet (17.25 feet on each side, 17 feet 3 inches).
Since you used 17 feet 6 inches to start with, just trim 3 inches off each end. The antenna analyzer should now show a resonant frequency of 14.2 MHz.
Final Check
Once your dipole is safely in the air, fire up your transmitter and check the SWR at many points throughout the band–it helps if you can plot the results on graph paper. If you see that the SWR is getting lower as you move lower in frequency, your antenna is too long. Trim a couple of inches from each end and try again. On the other hand, if you see that the SWR is getting higher as you go lower in frequency, your antenna is too short. You’ll need to add wire to both ends and make another series of measurements.
Close is good enough–don’t become obsessive-compulsive about SWR readings. If your SWR is under 1.5:1, you’re in good shape. SWR readings from 1.5- 1.9:1 have some room for improvement, but should still provide adequate performance.
Factors That Affect Dipoles
Height above ground matters! Here are the results from an EZNEC simulation of a simple 20 meter dipole resonance above average ground at three different heights:
- 8 ft., 13.975 MHz
- 20 ft., 14.050MHz
- 35 ft., 14.200 MHz
A simple dipole for HF will have more capacitance to ground in a low position and less capacitance higher up. Generally, the frequency of resonance will go higher when raised. You can see that height above ground affects antenna resonance–the resonant frequency increases.
Dipoles will provide low take-off angles for good over-the-horizon skip propagation when it is about one-half wavelength above the ground. At lower heights the radiation pattern will become more vertical and more omnidirectional.
Keep it in the clear. Try to isolate the dipole from other conductors, especially long, linear ones like rain gutters or aluminum siding–at least try to avoid aligning the dipole parallel with them.
Radiation pattern. The strongest signals radiate broadside to the antenna, or at right angles to the orientation of the dipole’s wire. You may want to install your dipole so that those strongest signals are pointed in desired propagation directions.
Weather resistance. Be sure to seal up any connectors that will be exposed to the elements to avoid water penetration into your coaxial feed line. Waterlogged coax can cause high SWR readings.
