dipole length tables

last updated 31 January 2025.

Updated January 2025 with more 80m data.

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 actual 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 75 wire dipoles set up at 3 different locations. And the study isn’t complete yet, either: I still want to make more experiments with the changes as the antenna is installed in different configurations. But I have enough data to make a good start.

approach

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 (8 inches) 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.

results

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.

80m

Even with additional measurements, the 80m results are the most variable. And, because the SWR bandwidth in kHz is narrower than on the higher bands, this may require field adjustment. For example:

one 20 m ( 65.6 ft ) wire measured 3.575 MHz
another 20 m wire measured 3.475 MHz and 3.540 MHz on different days
a 19.36m ( 65 ft ) wire measured twice gave 3.650 MHz both times.

The biggest variation on 80m seems to be the height of the ends above the ground: the capacitance between the the wire ends and ground lowers the resonant frequency. The longer wire lengths, the flatter angle required to accommodate them when the center is not very high (often the case for portable operation), the low height in wavelengths and the sag due to the weight of the wire and rope (especially when the supports can’t hold a lot of tension) all contribute to the variation of resonant frequency, even with the same antenna wire. How and where knots are tied in the wire still have an effect, as they do on other bands, with an additional twist that a hanging end may change the capacitance to ground. Getting the center (and the ends) of the antenna higher in the air will reduce the uncertainty.

For this band we have data for 3.500 and 3.900 MHz:

target frequency: 3.500 MHz
Typical length of each wire with 50 cm ( 20 inch ) tail: 20.3 m ( 66.6 feet )
Likely range of lengths: 19.5 to 20.5 m ( 64 to 67.3 feet )
Suggested starting length: 20.5 m ( 67.3 feet )

target frequency: 3.900 MHz
Typical length of each wire with 50 cm ( 20 inch ) tail: 18.25 m ( 60 feet )
Likely range of lengths: 17.5 to 18.8 m ( 57.5 to 61.7 feet )
Suggested starting length: 19 m ( 62.3 feet )

Wire length change for 100 kHz: 50 cm ( 20 inches )
Tuning range with 50 cm tail: 50 kHz
SWR bandwidth at 1.5 : 1 : 100 kHz
SWR bandwidth at 2 : 1 : 180 kHz

Note: in practice, because this antenna is relatively flat, the feedpoint impedance is higher than for the other inverted vee antennas in this study. Minimum SWR(50) was typically 1.5 : 1, and the 2 : 1 SWR bandwidth was about 100 kHz.

The question often comes up (at least in North America), whether it is possible to make an antenna cover the whole IARU Region II band from 3.5 to 4.0 MHz by folding back the ends. So, as a test, I started with an antenna using 21 m ( 68.9 feet ) of wire on each side (resonant frequency about 3.340 MHz) and folded the ends back in increments. With 4 m ( 13 feet ) of wire folded back, the resonant frequency was 3.975 MHz. The minimum SWR remained at about 1.5 : 1. For an antenna that is resonant around 3.5 MHz at full length, a 3 m ( 10 ft ) tail is probably enough to reach 4.0 MHz.

So, yes, this seems practical, although such a tail is too long to hang down when the ends are this close to the ground. It was easier to attach the support rope using a knot that would slip along the wire (such as shown in Figure 11 of this article on loop antenna construction) to make adjustment easier. The free end can then be secured to the standing wire using a Prusik Hitch, Cord Lock or plastic clothes pin, and approximate frequency settings marked on the wire using different colors of heat-shrink tubing.

60m

target frequency: 5.300 MHz
Typical length of each wire with 50 cm ( 20 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

40m

target frequency: 7.000 MHz
Typical length of each wire with 30 cm ( 12 inch ) 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 (12 inch) tail: 6.975 to 7.115 MHz
SWR bandwidth at 1.5 : 1 : 200 kHz
SWR bandwidth at 2 : 1 : 400 kHz

30m

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

20m

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, once it is tuned up initially.

17m

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

15m

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

12m

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

10m

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

Still more experimenting to do, especially to determine how much the frequency shifts with different ways of installing the antenna to.

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half-wave dipole lengths

simple construction of wire antennas

Practical Antennas