The Dipole Antenna
The half-wave dipole is the most popular first antenna for HF, and for good reason: it's cheap, easy to build, and it works well.
How a Dipole Works
A dipole is simply two wires, each about 1/4 wavelength long, fed in the center with coax. Radio energy from your transmitter causes current to flow in the wires, which creates radio waves.
The total length of a half-wave dipole is approximately:
Length (feet) = 468 / Frequency (MHz)
For 20 meters (14.2 MHz): 468 / 14.2 = 33 feet total, or about 16.5 feet per leg.
Building a Dipole
Materials Needed
- Wire (about 10-15% longer than calculated length)
- Center insulator (commercial or homemade)
- Two end insulators
- Coax feedline (RG-8X or RG-213)
- Coax connector (PL-259)
- Rope for support
The Build
- Cut two pieces of wire, each 5% longer than the calculated quarter-wave length
- Strip about 1/2 inch of insulation from the center end of each wire
- Attach wires to center insulator
- Connect coax: center conductor to one wire, shield to the other
- Attach end insulators
- Add rope for hanging
Tuning
- Hang the antenna
- Check SWR with an antenna analyzer or your radio's SWR meter
- If SWR is too high, trim small amounts from each end equally
- Repeat until SWR is acceptable (under 2:1 ideally)
Installation Options
Flat Top (Horizontal)
The classic configuration. Run it as high as you can, ideally at least 1/4 wavelength above ground.- Good all-around radiation pattern
- Lower angle radiation = better for DX when high
Inverted V
Hang the center high with ends sloping down at 90-120 degrees.- Only needs one high support point
- Slightly different radiation pattern
- Feed impedance is lower (closer to 50 ohms)
Sloper
One end high, the other end low.- Can be somewhat directional
- Good when you only have one tall support
Common Questions
How high should my dipole be? Higher is generally better for DX. At 1/2 wavelength high, you get good low-angle radiation. But even a dipole at 25-30 feet will work contacts worldwide.
Does it need to be perfectly straight? No. Bends, sags, and imperfect installations still work. Get it up and make contacts!
Can I use a dipole on multiple bands? A dipole is resonant on one band (and its odd harmonics). For 40m dipole, you'll also have usable SWR on 15m. For true multi-band use, consider a fan dipole or off-center-fed dipole.
What if my yard is too small? You can bend the ends down or in other directions. It won't be quite as efficient but it will work.
Dipole vs Other Antennas
Dipole Pros
- Cheap to build
- Easy to construct
- Predictable performance
- No radials needed
Dipole Cons
- Only resonant on one band (usually)
- Requires two support points
- Can be long for low bands
A dipole is an excellent reference antenna. Many hams keep one up even after getting more complex antennas just to have a reliable comparison.