[Antennas] 468/Freq in Mhz

Mon Aug 24 08:07:24 EDT 2009

de WB2CPN
Jerry, the length of a conductor, wire or tubing, for
a half-wave or whatever, depends on the velocity
that the radio wave travels along the conductor. 
Radio waves are said to travel at such-and-such
speed through free space, as light is supposed to do,
but everywhere else they travel somewhat slower.
Also, the speed of radio waves are influenced by
everything in the neighborhood of where they are
traveling. So, after much experimentation it was
agreed that the formula for Feet of a half-wave,
468 / F(Mc), was close for frequencies below
about 30 MHz, the formula for Inches for frequencies
above 30 MHz approximates 5540 / F(MHz).
Free space half-wave in space is about 492 / F(MHz).
The other input is the ratio of Length to Diameter of
the conductor. Hit the Books for that. TRIVIA:
Thin conductors have higher "Q" but lower bandwidth.
Thick tubing would be the opposite, but there is a
limit to how thick a conductor can be and still radiate
very much. Therefore, you can cut once, and trim
twice. Note that charts for coax usually give the
velocity, .66 being for RG-8 types I think.
73 Clete