Military utility tools / Antenna length
Antenna length
Frequency in megahertz. Every common antenna length in metres, corrected for end effect.
0.95 for bare wire. Around 0.66 for coax elements.
Set the inputs, then work it out.
Wavelength--m in free space
Half wave dipole--m end to end
Each leg--m
Quarter wave whip--m
Five eighths wave--m
Full wave loop--m perimeter
Ground plane radial--m, use four
Half wave spacing--m between elements
Why is the dipole shorter than half a wavelength?
End effect. Capacitance at the tips makes the element behave electrically longer than it is physically, so the physical length must be cut back by roughly five percent. That is what the velocity factor accounts for.
Why four ground plane radials?
Four is the practical minimum for a usable artificial ground, set slightly longer than the driven element. More radials improve the ground plane, with diminishing returns past about eight.