Here are some calculations for CB and HAM radio:



How do I find the wavelength of a frequency ?

In meters: 300 divided by frequency in MHz
In feet: 984 divided by frequency in MHz
In inches: 11,811 divided by frequency in MHz.

How do I find the frequency of a wave length ?

300 divided by wavelength in meters equals frequency in MHz.
Example, What frequency has a 6 meter wave length ?
300 divided by 6 equals 50 MHz.

How do I calculate the length of wire I need to build a wire antenna. ? (Also see comments from Dave below!)

Full wave: 984 divided by frequency in MHz equals the length in feet
Three-quarter wave: 738 divided by frequency in MHz equals the length in feet
Five-eighths wave: 615 divided by frequency in MHz equals the length in feet
One-half wave: 492 divided by the frequency in MHz equals the length in feet
One-quarter wave: 246 divided by the frequency in MHz equals the length in feet

How do I convert meters to feet ?

Meters multiplied by 3.281 equals an equivalent length in feet.

How do I convert feet to meters ?

Feet multiplied by 0.3048 equals an equivalent length in meters.


This additioanl information was sent to me by Dave, it clarifies details about calculating the correct antenna wavelength.

Hi Scott:

You have a big error on your page.

These are your figures....

---------------------------------------------------
How do I calculate the length of wire I need to build a wire antenna. ?

Full wave: 984 divided by frequency in MHz equals the length in feet
Three-quarter wave: 738 divided by frequency in MHz equals the length in feet
Five-eighths wave: 615 divided by frequency in MHz equals the length in feet
One-half wave: 492 divided by the frequency in MHz equals the length in feet
One-quarter wave: 246 divided by the frequency in MHz equals the length in feet
-------------------------------------------------

These figures are correct only if you just want to get the correct wavelength equivalent. But if you use these formulas to cut an antenna, they are totaly wrong,and will not work.

Your math does not take into consideration the VELOCITY factor of coax. Cutting a 1/2 wave by using 492 divided by Freq. will be too long for the frequency. The velocity of coax, (any length) electrically adds length to the antenna. So you must compensate for that Velocity.

You must use the following math to correctly cut an antenna.

Full wave = 936 divided by frequency.
5/8 wave = 585 divided by frequency. But there must be a matching device, or 5/8 will not work at all.
1/2 wave = 468 divided by frequency.
1/4 wave = 234 divided by frequency.
1/8 wave = 117 divided by frequency

3/4 wave does not work under any circumstances. Any wavelength must always be able to divide into itself by 2.
A 5/8 is an exception, and only because you MUST have some kind of matching device on the antenna like , a gamma match, or a tuning coil. A length cut to 5/8 wavelength alone will not work at the desired frequency. It would become a 1/2 wave at a lower frequency, and mismatch.

Also. Your math says a 1/2 wave antenna for CB is 18 feet. That is an error. The CB band in the USA is 26.965 to 27.405 MHz . The center is ch.20 or 27.205 MHz. Cutting an antenna to 18 feet would be a perfect 1/2 wavelength at 26.000 MHz. 1.205 MHz lower than it should be.

To cut a 1/2 wave antenna for 27.205 MHz., take 468 divided by 27.205 = 17.2 feet. or 17 feet and 4 inches.

Your math only tells you what an exact measurement is in wavelength. But you can not cut an antenna that way. It will be too long.

Dave


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