T9A: Wavelength vs. antenna length; 1/2 wavelength dipole and 1/4 wavelength vertical antennas; multiband antennas


You calculate the length (in feet) of a half-wavelength dipole antenna using the formula:

Divide 468 by the antenna's operating frequency (in MHz)  [468/f (in MHz)]

You calculate the length (in feet) of a quarter-wavelength vertical antenna using the formula:

Divide 234 by the antenna's operating frequency (in MHz)  [234/f (in MHz)]

A quarter-wavelength vertical antenna for 146 MHz (measured to the nearest inch) is 19 inches.

A quarter-wavelength vertical antenna for 440 MHz (measured to the nearest inch) is 6 inches.

A quarter-wavelength vertical antenna for 28.450 MHz (measured to the nearest foot) is 8 feet.


If an antenna is made longer, its resonant frequency decreases.

If an antenna is made shorter, its resonant frequency increases.

You could decrease the resonant frequency of a dipole antenna by lengthening the antenna.

You could increase the resonant frequency of a dipole antenna by shortening the antenna.


One advantage to using a multiband antenna is that you can operate on several bands with a single feed line.

One disadvantage to using a multiband antenna is that it can radiate unwanted harmonics.