I don't know of any such instrument.

One problem is the very definition of far field. It's a region in which
the properties of the radiated wave approach their final values. Some of
the characteristics of these final values (that is, the values the wave
approaches at very great distances) are (for propagation in air):

-- The field strength decreases inversely with distance
-- The ratio of E/H = the Z of free space (about 377 ohms, purely real)
-- The field has no component in the direction of travel

All these characteristics are approached as a limit, and the distance at
which one or more are "close enough" is strictly a matter of definition
and the particular application. It's at that somewhat arbitrary point
that you're said to be in the far field.

You can devise instruments to measure any or all of these properties,
although I'd think doing it with any sort of accuracy would be beyond
the reach of most amateurs. But if you did, you'd still have to decide,
on the basis of the measurements and your own criteria, where you'll
draw your own particular line.

Roy Lewallen, W7EL

Henry Kolesnik wrote:

Is there a homebrew instrument that I can make that will let me know when
I'm out of the near field and in the far field? I'd like it to work with
mobile whips as well as dipoles on HF. The ARRL Ant HBK formula for the
boundary D between the two fields is an approximation where D = 2L^2/w where
L is ant length in same units as w, the wavelength.
tnx