Calculate radial lenght

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bill gatewood:
Can anyone tell me how to calculate radial lenght for a 1/4 wavelenght,and a full wavelenght, for 160 meters and so on? Thanks Bill

Pete Allen:
An accurate calculation of a buried radial would require an equally accurate determination of the velocity of propagation IN YOUR SOIL. And that's not only a tough number to come by - it can change within inches.

Practically speaking, the old 234/f calc for 1/4 wave radials will err on the safe side. Probably very safe! Also practically speaking most hams get very satisfactory results with radials that are a bit longer than the vertical is tall. Provided of course, they put enough down to make a fairly dense radial field.

73  Pete Allen AC5E

Bob Lewis:
The basic formula for a 1/2-wavelength wire is L=468/F where L is length in feet and F is frequency in Megahertz. Calculate the length of 1/2-wavelenght and then double it for 1-wavelength or cut it in half for 1/4-wavelength. Multiply the result by 12 if you want it in inches instead of feet.

Allen C. Ward:
Consider that the speed of light is 300,000,000 meters per second and divide that number by the frequency in Hertz (cycles per second) and you get the wavelength of that frequency in free space.  The usual calculation for a half wave dipole 468/f (in MHz) gives the wavelength in feet with the average velocity factor of wire figured in (light or rf doesn't travel as fast in wire as it does in free space).  We use a shorthand method of describing bands so that the wavelength of a "20 meter" signal is greater than 20 meters because the actual frequency is lower than 15 MHz.  Just remember the speed of light, carry a calculator to go from one measurement system to another and that the velocity factor of wire is about 95% and you won't go far wrong.

Pete Allen:
It's not the propagation velocity in wire that's the problem. Most RF is propagated as a field AROUND the wire and the VP of a buried radial in contact with soil is essentially the propagation velocity of the surrounding earth. In some heavily mineralized soils that can be as low as 30,000 miles per second, or about one sixth the free space speed of light.

A free space half wave is close to 493/f. End effects, the field extends beyond the wire, plus propagation effects brings that to a nominal 468/f, and the result is almost always long. A quarter wave is nominally half that, 234/f.

73  Pete Allen  AC5E


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