Dipole Resonance Tuning Steps
Finding the Point of Minimum SWR
1. Determine your target frequency for resonance tuning
(the frequency that is roughly in the middle of the desired band segment you wish to use)
2. Use appropriate mathematical formula for determining an estimate of the length in feet for each half of the dipole
- 246 / target freq (MHz)
(if is impractical to add length to antenna elements during tuning)
- 234 / target freq (MHz)
(if you can add or subtract length during tuning)
3. Raise the dipole to the height it will be used for
(when SWR is checked while antenna is closer to ground the resonant frequency will appear to be lower).
4. Connect an SWR meter to the radio end of the feed line.
Find the frequency at which the SWR is the lowest (closest to 1:1).
Note the impedance also.
5. If the frequency in the previous step needs to be raised to reach the target frequency, shorten each half of the dipole by equal amounts. If the frequency in the previous step needs to be lowered to reach the target frequency, lengthen each half of the dipole by equal amounts. Make adjustments a few inches at a time. Repeat adjustments and measuring until SWR at target frequency is minimal.
(ideal SWR= 1:1, anything under 2:1 is adequate, could damage radio if over 3:1)
Note #1: If lengthening is needed and it is not possible to physically lengthen the halves of the antenna, electrical length can be added with a coil, adjusted for the necessary number of turns. This is called “loading”. The less loading the more efficient the antenna.
Note #2: Impedance of antenna should be around 50 ohms. Some increases can be achieved by slightly off-center feeding of the two dipole halves (different lengths). Some reduction can be achieved by reducing height of antenna off ground. Major adjustments require an impedance matching device.
The information on this page is provided
courtesy of Pete Harris, KE6ZIW.
Images on this page are courtesy of Wikimedia.