Richard Harrison wrote:
Steve, W9HJW wrote:
"Something like .05 wavelength (between ends of buried radials) rings a
bell, but I`m not sure."

That would depend on the length and number of radials. But, the .05
number may be significant in the case of 120 radials. 120 radials is an
FCC standard. Spacing is 3-degrees.

In a right triangle, the side opposite an angle is equal to the tanget
times the adjacent side length. For a 3-degree angle, difference between
the adjacent side and hypotenuse lengths is insignificant.

The tangent of 3-degrees is .05. The distance between radials spaced by
3-degrees is therefore very nearly .05 times the length of the radial.
That`s my guess as to the source of Steve`s .05 number.

It also works, roughly, for shorter radials.

Jerry Sevick, W2FMI, published data on the effects of shorter radials on
feedpoint impedance, and came to the conclusion that if you use shorter
radials, there is little advantage in using very large numbers. If you
re-plot Sevick's data, it turns out that, for a given length of radial,
you only need about enough to ensure that the ends are about 0.05
wavelengths apart. Adding more radials of the same length will bring
little further reduction in feedpoint impedance.

However, this is a "soft" limit, so others may interpret the same data
to say that the limiting tip-to-tip spacing is different, even as little
as 0.02 wavelengths.

The "tip-to-tip rule" is a useful guideline, but it isn't helpful to
argue over the detail, because feed impedance isn't the only important
criterion. Longer radials will also reduce losses due to ground
reflection, and improve your low-angle signal in particular... but in
order to make a difference at very low angles, the radials need to be
very long indeed.

All the "tip-to-tip rule" is saying is: if the radials are as long as
you can make them, and you already used enough to make the tips
0.05...0.02 wl apart, then you've done all you can. Go look for
something else to improve.



--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek