Building Your First Antenna: A Practical Guide

The first antenna most operators build is a half-wave dipole, and for good reason. It costs very little, it needs no special tools, and when it is up at a reasonable height it will outperform most commercial antennas sold for the same money.

The construction is simple. The part that catches people out is the tuning, because a real antenna is not in free space and its resonant frequency depends on things you cannot easily calculate.

Materials

• Stranded copper wire, 1.5 to 2.5 square millimetres. Stranded survives flexing far better than solid.
• Two end insulators and one centre insulator or feed point assembly.
• A 1:1 current balun at the feed point to stop shield current.
• Coax of a size suited to the length of the run and the highest frequency you will use.
• Support rope that will not stretch badly when wet, and pulleys if you can arrange them.
• Self-amalgamating tape for every outdoor connection.

Calculating the length

A half wavelength in free space is 150 divided by the frequency in megahertz, in metres. The resonant length of a real dipole is about five percent shorter because the wire has thickness and the ends are not perfect radiators. The practical formula is 143 divided by the frequency in megahertz for the total length.

Building it

Tuning

If the antenna resonates below the frequency you want, it is too long — shorten both legs equally. If it resonates above, it is too short and you need to add wire. Trim in steps of about two percent of the total length and re-measure each time.

The ratio of length change to frequency change is roughly constant, so after one or two trims you can calculate the final adjustment rather than guessing. Note the figures as you go; they will be useful next time.

What to expect

A dipole at a half wavelength above ground on 20 metres will make contacts across continents at 100 W when the band is open. At a quarter wavelength it will work the same continents with a higher angle and less reliability. The antenna is not the limitation — the ionosphere is, and the antenna only decides how much of your power gets into it at a useful angle.