First of all, we need to ask "what kind of antenna"? The presumption is a
half-wave dipole fed in the middle with 50/75 ohm coax.

Since 315 is the frequency of choice, we must presume glideslope, which IS
probably a dipole.

So let's go the long way around the maypole...

A wavelength is (3eE^8 / frequency in Hertz) (expressed in meters)

or 300 / frequency in MEGAhertz.

So a wavelength at 315 MHz is 300 / 315 = .915 meters


But each "ear" of a dipole (think of a rabbit ear dipole on top of a 1950s
tv set) is a quarter wave long, so the ear is .915 /4 or .23 meters

Since a meter is 39.37 inches, .23 meters is 9.37 inches.

Now comes the art instead of the science.

How "fat" are the dipole elements? An infinitely thin wire would be 9.37
inches less 5% for "end effect".

Thicker elements? Like brazing rod or copper tape? Measurement and
instruments. A good first cut for antennas in this domain is to subtract 5%
for end effect and then 2% for every "fatness" of the length of the dipole.
A dipole 9.37" theoretical should be cut 8.9" for infinitely fine wire. You
get wire .1" in diameter? Take off another 0.18".

The equation is transcendental, sloppy, and I'm not going to go into it
here. Get wire (or tape) greater than a quarter inch and you put it on an
antenna bridge and measure it.

Cut it out, bend it up, kick in the sides. weld it up, file it down, paint
it to match...FLY IT

Jim

Jim





So the theoretical length (end to end) if a half wave dipole
"Ken" wrote in message
m...
what is the formula for calculating the length of an antenna based on
the frequency to be recieved- I am interested in 315mz ?