antenna length

[Ken]

what is the formula for calculating the length of an antenna based on
the frequency to be recieved- I am interested in 315mz ?

[RST Engineering]

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 ?

[Smitty]

In article >,
(Ken) wrote:

> what is the formula for calculating the length of an antenna based on
> the frequency to be recieved- I am interested in 315mz ?

The wavelength times the frequency equals the speed of light. (radio
frequency waves travel at the speed of light.) Make the antenna
proportional to 1 wavelength. 1/4 wave antennas are common, but you can
make them half wave or full wave or 1/8 wave or whatever you have room
for.

[UltraJohn]

Hey Jim
I ordered one of your gps antenna (the one with more filtering) the other
day. Just received it on Monday and hooked it up today to a GPS at work
that had it's front end go essentially dead (couldn't pick up a satellite
if it landed next to it!) and lo and behold 8 channels of full strength
satellite signal. Works great! Thanks.
John
Wakefield, Va

[UltraJohn]

Smitty wrote:
>
> The wavelength times the frequency equals the speed of light. (radio
> frequency waves travel at the speed of light.) Make the antenna
> proportional to 1 wavelength. 1/4 wave antennas are common, but you can
> make them half wave or full wave or 1/8 wave or whatever you have room
> for.

Sure you can make your antenna any length you want. . . It's just that some
work great and others don't work worth a d#$n. I don't think your 1/8
wavelength antenna will do all that well. Use the K.I.S.S formula and make
either a 1/4 groundplane or 1/2 wave dipole. Jim's formula's work great,
figure out what length you need from what he said then cut it a bit long
hook up a SWR bridge to it. Check SWR at low end and high end of band trim
the antenna a little and when the high end starts to get a smidgeon better
than the low end stop, your there.
John
wd8ope, wa4fms, kg4jp, kg6jp and now kc5vb.