Ever need a way to estimate the amount of wire to add to or remove from a center-fed wire dipole antenna to achieve resonance at a desired frequency? If your antenna analyzer indicates a resonant frequency below the desired frequency, the antenna is too long and needs to be shortened. If the analyzer indicates a resonant frequency above the desired frequency, the antenna is too short and needs to be lengthened. A good rule of the thumb when constructing center-fed wire dipole antennas is to cut the radiating arms a little long so they can be trimmed for resonance at the desired frequency. It is easier to remove wire than to add it.
How do you estimate the amount of wire to add or remove to achieve resonance at a desired frequency? Follow the steps listed below.
- Calculate the radiator length for the upper limit of the band using the formula 468/upper limit frequency in MHz.
- Calculate the radiator length for the lower limit of the band using the formula 468/lower limit frequency in MHz.
- Calculate the difference in lengths between the upper and lower limits of the band by subtracting the upper limit frequency length from the lower frequency length.
- Calculate the frequency width of the band by subtracting the lower frequency limit from the upper frequency limit.
- Divide the frequency width by the length difference yielding kilohertz /unit of length.
- Calculate the difference between the desired resonant frequency and the observed resonant frequency by subtracting the desired resonant frequency from the observed resonant frequency.
- Divide the frequency difference by the kilohertz/unit of length yielding the estimated amount of adjustment needed to achieve resonance at the desired frequency. A positive value indicates the amount to add and a negative value indicates the amount to remove. Be sure to divide the estimated value in half and make the adjustment to both radiators.
Note: A diagram of the amateur radio bands (courtesey of ARRL) is available at the link below.
Example: You want your antenna to be resonant at a frequency of 7.2 MHz (40 meter band). You observe a resonant frequency of 7.05 MHz on your antenna analyzer. The observed resonant frequency is below the desired frequency which indicates the antenna radiators are too long. The upper limit for the 40 meter band is 7.3 MHz and the lower limit is 7.0 MHz. The calculated radiator length at the upper limit of the 40 meter band is 468 / 7.3 or 64.110 feet. The calculated radiator length at the lower limit of the 40 meter band is 468 / 7.0 or 66.857 feet. The difference in lengths is 2.747 feet or 32.964 inches. The frequency width of the 40 meter band (7.3 MHz – 7.0 MHz) is .3 MHz or 300 KHz. The kilohertz per unit of length (inches) is 300/32.964 or approximately 9.101. The difference between the desired frequency and the observed frequency (7.2 – 7.05) is .15 MHz or 150 KHz. The estimated amount of adjustment needed is (150 / 9.101) or 16.482 inches. Divide 16.482 inches in half (8.241 inches) and adjust each radiator by that amount. Note: You should remove at least 5% less than the estimated amount from the radiators and check the antenna again with your analyzer. It is easier to remove wire than to add it. Continue incremental adjustment of the radiators until resonance is achieved at or near the desired frequency.
Use the tuning calculator at the link below to compute the estimated amount of adjustment needed to achieve resonance at a desire frequency.
Good luck with your antenna projects.