Half-wave Dipole Antenna Calculator

 

The measurements below are for building a simple Dipole Antenna. The Wire Size can range from 18 AWG to 12 AWG. The larger the wire, the wider the bandwidth. The antenna is designed to be fed with 50 or 75 Ohm Coax Cable of most any length with a Balun. Place a 1:1 Balun on the Antenna end of the Feedline. The Balun matches the Balanced Antenna into the Unbalanced Coax Cable. The balun will keep stray antenna currents off of the Feedline and help to keep stray RF out of the Radio Shack. If a Ferrite Balun is used, the balun needs to be rated at least twice the operating wattage to prevent the balun from overheating. This will help mitigate mismatches and stray RF due to using the antenna at frequencies other than what the antenna was specifically designed for.

For a dipole antenna constructed to resonate at a desired center frequency (cf) calculate the antenna length by using the formula: L = 468 / cf (cf is in MHz). To determine the two element lengths (E), divide L / 2= E.

Please note: The data in the following tables were calculated using the above formulas. Many of the numbers have been rounded off to the nearest 100^th, so some final length adjustment may be necessary. Dimensions will also slightly change due to antenna height and ground conductivity variations.

Antenna Diagram

Amateur Bands 160 Meters MHz "L" (Feet) "E" (Feet) 1.800 260.00 130.00 1.900 246.32 123.16 2.000 234.00 117.00 80 Meters 3.500 133.71 66.86 3.600 130.00 65.00 3.700 126.49 63.24 3.800 123.16 61.58 3.900 120.00 60.00 4.000 117.00 58.50 60 Meters 5.331 87.80 43.90 5.347 87.53 43.77 5.367 87.21 43.60 5.371 87.13 43.56 5.404 86.61 43.31 40 Meters 7.000 66.86 33.43 7.100 65.92 32.96 7.150 65.45 32.73 7.200 65.00 32.50 7.300 64.11 32.05 30 Meters 10.100 46.34 23.17 10.150 46.11 23.05

20 Meters 14.000 33.43 16.71 14.100 33.19 16.60 14.200 32.96 16.48 14.220 32.91 16.46 14.300 32.73 16.36 14.313 32.70 16.35 17 Meters 18.068 25.90 12.95 18.130 25.81 12.91 15 Meters 21.000 22.29 11.14 21.100 22.18 11.09 21.200 22.08 11.04 21.300 21.97 10.99 12 Meters 24.890 18.80 9.40 24.950 18.76 9.38 10 Meters 28.000 16.71 8.36 28.500 16.42 8.21 29.000 16.14 8.07 29.500 15.86 7.93

NOTE: The halfwave dipole antenna element lengths are measured differently when building an inverted V dipole.

You usually make an inverted-V dipole slightly shorter than a flat horizontal half-wave dipole because the downward-sloping legs change the antenna's capacitance and feed point impedance.

A common rule of thumb is:

• Horizontal half-wave dipole:

Each leg:

• Inverted-V dipole:
  Start with 2–5% shorter than the horizontal calculation, then trim while checking resonance.

A practical formula is:

or

Example: 20 meters (14.2 MHz)

Horizontal dipole:

• Total:

• Each leg:

Inverted V:

• Total:

• Each leg:

That's about 3.4 inches shorter per leg.

Why?

When the legs slope downward:

• The ends are closer to ground.
• The antenna has greater end capacitance.
• It becomes electrically longer than the same physical wire length.
• Therefore, less wire is needed to resonate at the same frequency.

How much shorter?

The exact amount depends on:

• Included angle (typically 90–120°)
• Height above ground
• Ground conductivity
• Wire diameter
• Nearby objects

Typical reductions are:

Included Angle	Length Change
120°	1–2% shorter
90°	2–4% shorter
60°	4–6% shorter

Best practice

Most amateur operators:

1. Cut each leg about 2–3% long initially.
2. Raise the antenna to its final height.
3. Measure SWR or resonance with an antenna analyzer.
4. Trim both legs equally until the desired resonant frequency is reached.

This approach accounts for your actual installation, which has a much larger effect on resonance than the theoretical correction for the inverted-V geometry alone.