dipole length tables
last updated 18 August 2023.
I haven’t been entirely happy with the conclusions in my analysis of the length of a half wave dipole. It basically said that there are a lot of variables, so cut the wire somewhat longer than the standard formulas and trim it to resonance. I wanted to come up with some usable numbers that readers could use.
Some of the variables include:
- wire thickness
- wire insulation material and thickness
- height above ground
- angle of the wires in an inverted vee
- how the wire is tied to the insulators
- coax length and termination, if not used with an effective balun
- the sag in the wires
And one critical realization was that, when making antennas using small insulated wire, the measured wire length to use was a different matter from the length of the dipole itself, which is what the standard formulas provide. I never measure the final length of a wire antenna – just how much wire I have to pull off the spool to make it.
So I set out to take actual measurements of real antennas using different types of wire from the junkbox, to develop recommended dimensions for each band. This has taken a lot longer than I had anticipated: so far I’ve measured over 50 wire dipoles at 3 different locations. And the study isn’t complete yet, either: I still need to complete the data for 80m (at a site with sufficient space), and further investigate the changes as the antenna is installed in different configurations. But I have enough data to start with.
In order for my results to be repeatable by others, the first step was to standardize my antenna. The test setup intended to be more or less typical of a portable or temporary antenna: an inverted vee with a center height of 6 m (20 feet), with the wires sloping down at an included angle of 120 degrees (each wire about 60 degrees from vertical). Longer wires were made flatter to try to keep the ends at least 2.5 m (8 feet) off the ground, although that wasn’t always convenient. A description of the test range will be added in a separate article.
The wires were tested with my standard dipole kit hardware. I allowed 20cm of wire to tie the wire to the center insulator and connect to the coax. The loop at the far end was about 2 cm (1 inch) long, with a hanging tail for adjustment that varied by band. Wire sizes varied from 1 mm ( AWG #18 ) to 0.4 mm ( AWG #26 ), mostly stranded, with various types and colors of insulation. (Some of these were cut for antennas over 40 years ago, some cut specifically for this test, and some were just random pieces that I pressed into service.) Construction details are shown here. Each set of wires is stored in its own marked plastic bag so I can go back and repeat the measurements as needed.
When I plotted the length vs. frequency of minimum SWR, the points didn’t fall on a nice neat curve. Sometimes a longer wire would have a higher resonant frequency. That may have been because the measurement conditions shifted over the course of the study, or because the ends of the wires were closer to the ground in some sites. (Antenna center height was standardized by marking the coax with a piece of tape where it should reach the ground.)
So, yes, there is still some variation in the results. I still recommend you start long and adjust the antenna for your specific “normal” conditions. But I’ve tried to provide not only the length that I came up with in my “standard conditions”, but a reasonable range of what to expect based on the variations that I have encountered in the process. And a suggested starting point for the wire length, so you don’t cut it too short, but don’t waste too much wire, either.
using these tables
It is quite possible to build a dipole that doesn’t require any field adjustment over a wide range of installations conditions. To achieve that, I recommend the following steps:
- Tune the antenna in a configuration typical of how you will use it. If you expect to use it with a particular support, tune at that height, with the ends tied off at a typical distance. Sure, the conditions in the field may not always be the same, but the closer you can get initially, the more likely that the antenna will still work.
- On 20m and higher frequencies, the operating bandwidth is wide enough that there probably is no need to leave a tail for field tuning. If so, tune it up that way.
- When you tie knots for the support ropes, leave them tied in the wire when you roll it up. That way they will be in the same location each time you use it. Standardizing how the wire is tied to the center insulator will also help.
- If you do leave a tail for tuning, a Cord Lock is convenient for adjusting the antenna.
- The change in length for a 100 kHz shift in frequency is only approximate – it varies with the length of the tail, among other factors, so don’t try to cut the antenna exactly to length in one try: that tends to overshoot and result in a wire that is too short.
- The ideal target is when the SWR is low enough to be usable over the desired frequency range. When the SWR is under 1.5 : 1 across the whole band, there is nothing to be gained by adjusting the resonant frequency any further, even if it isn’t exactly where you wanted it.
- If the SWR is too high at the minimum point, try loosening the end ropes. That lets the wires sag a little, and lowers the center angle.
In practice, I have used my backpack dipole kit in many different situations, at various heights, without any field adjustment or using a tuner. I only remember one case where the SWR was so high that it didn’t work, but that was with the antenna running along a barbed wire fence.
IMPORTANT NOTE: All lengths given here are the length of one side of a dipole, not the total length as given by the traditional formulas.
For each band, I will provide a typical wire length for one side of a dipole for the target frequency, which is usually the bottom edge of the band, along with expected variance, recommended starting wire length, expected SWR bandwidths, length of the hanging tails, and an estimate of the change in wire length to shift the frequency by 100 kHz, in case you want to resonate the antenna at some other frequency.
The 80m results are still incomplete.
- a 20 m ( 65.6 ft ) wire measured 3.475 MHz
- another 20 m wire measured 3.575 MHz
- a 18.23 m ( 59.8 ft ) measured 3.835 MHz
- a 16.77 m ( 55 ft ) wire measured 4.12 MHz
- a 17 m ( 55.75 ft ) wire measured 4.22 MHz
Those will, at least, give you some ideas of the expected lengths, even if they are not entirely consistent.
It is important to remember that the end heights will make more difference on 80m than on higher bands, as the antenna is closer to the ground (in wavelengths), as well as the need for more distant supports for the ends to keep them up at a reasonable height.
I should also note that, while it is tempting to try to make an antenna cover the whole IARU Region II band from 3.5 to 4.0 MHz by folding back the ends, preliminary indications are that would require folding roughly 1/3 of the total wire length. That will likely lower the feedpoint impedance at the high frequency end of the range. This will be evaluated in future testing.
- target frequency: 5.300 MHz
- Typical length of each wire with 50 cm ( 12 inch ) tail: 13.22 m ( 43.5 feet )
- Likely range of lengths: 13 to 13.6 m ( 42.5 to 44.5 feet )
- Suggested starting length: 14 m ( 46 feet )
- Wire length change for 100 kHz: 24 cm ( 10 inches )
- Tuning range with 50 cm tail: 5.270 to 5.415 MHz
- SWR bandwidth at 1.5 : 1 : 140 kHz
- SWR bandwidth at 2 : 1 : 250 kHz
- target frequency: 7.000 MHz
- Typical length of each wire with 30 cm tail: 10.15 m ( 33.3 feet )
- Likely range of lengths: 9.66 to 10.36 m ( 31.7 to 34.0 feet )
- Suggested starting length: 10.4 m ( 34 feet )
- Wire length change for 100 kHz: 14 cm ( 5 inches )
- Tuning range with 30 cm tail: 6.975 to 7.115 MHz
- SWR bandwidth at 1.5 : 1 : 200 kHz
- SWR bandwidth at 2 : 1 : 400 kHz
- target frequency: 10.100 MHz
- Typical length of each wire with 12 cm ( 5 inch ) tail: 7.22 m ( 23.67 feet )
- Likely range of lengths: 7.1 to 7.3 m ( 23.25 to 24.0 feet )
- Suggested starting length: 7.3 m ( 24 feet )
- Wire length change for 100 kHz: 4 cm ( 1.5 inches )
- Tuning range with 12 cm ( 5 inch ) tail: 10.075 to 10.300 MHz
- SWR bandwidth at 1.5 : 1 : 250 kHz
- SWR bandwidth at 2 : 1 : 500 kHz
- target frequency: 14.00 MHz
- Typical length of each wire with 30 cm ( 1 foot ) tail: 5.2 m ( 17 feet )
- Likely range of lengths: 5.1 to 5.3 m ( 16.75 to 17.4 feet )
- Suggested starting length: 5.35 m ( 17.5 feet )
- Wire length change for 100 kHz: 3 to 4 cm ( 1.5 inches )
- Tuning range with 30 cm tail: 13.90 to 14.40 MHz
- SWR bandwidth at 1.5 : 1 : 350 kHz
- SWR bandwidth at 2 : 1 : 800 kHz
At 20m and higher frequencies, the SWR bandwidth is wide enough that using the folded end for field tuning the antenna probably isn’t needed.
- target frequency: 18.100 MHz
- Typical length of each wire with 20 cm ( 8 inch ) tail: 4.12 m ( 13.5 feet )
- Likely range of lengths: 4.0 to 4.2 m ( 13 to 13.75 feet )
- Suggested starting length: 4.25 m ( 14 feet )
- Wire length change for 100 kHz: 2 cm ( 1 inch )
- Tuning range with 20 cm ( 8 inch ) tail: 18.05 to 18.70 MHz
- SWR bandwidth at 1.5 : 1 : 400+ kHz
- SWR bandwidth at 2 : 1 : 1000 kHz
- target frequency: 21.000 MHz
- Typical length of each wire with 13 cm tail: 3.58 m ( 11.75 feet )
- Likely range of lengths: 3.50 to 3.65 m ( 11.5 to 12 feet )
- Suggested starting length: 3.75 m ( 12.3 feet )
- Wire length change for 100 kHz: 1.5 cm ( 5/8 inch )
- Tuning range with 13 cm tail: 20.95 to 21.45 kHz
- SWR bandwidth at 1.5 : 1 : 600 kHz
- SWR bandwidth at 2 : 1 : 1000 kHz
- target frequency: 24.9 MHz
- Typical length of each wire with 10 cm ( 4 inch ) tail: 3.0 m ( 9.8 feet )
- Likely range of lengths: 2.9 to 3.1 m ( 9.5 to 10.2 feet )
- Suggested starting length: 3.2 m ( 10.5 feet )
- Wire length change for 100 kHz: 1.25 cm ( 1.2 inch )
- SWR bandwidth at 1.5 : 1 : 800 kHz
- SWR bandwidth at 2 : 1 : 1300 kHz
- target frequency: 28.4 MHz
- Typical length of each wire with 20 cm ( 8 inch ) tail: 2.65 m ( 8.7 feet )
- Likely range of lengths: 2.6 to 2.74 m ( 8.5 to 9 feet )
- Suggested starting length: 2.75 m ( 9 feet )
- Wire length change for 100 kHz: 1 cm ( 3/8 inch )
- Tuning range with 20 cm tail: 28.15 to 29.8 MHz
- SWR bandwidth at 1.5 : 1 : 1000 kHz
- SWR bandwidth at 2 : 1 : 1600 kHz
Along with further measurements for 80m (and perhaps 160m when I find a suitable site), I will experiment with different ways of installing the antenna to try to quantify how much variation to expect.