Here is the data from the original definitive study on Field Strength
at 1 mile versus Number and Length of Radials as described by
Brown, Lewis, and Epstein in the June 1937 Proceedings of the IRE
(Institute of Radio Engineers). Measurements were taken using
0.2 W at a distance of 0.3 Mile and converted to an equivalent
number for 1000 Watts at 1 Mile.
This data was measured at 3 MHz. using a vertical mast of 2.5 in.
O.D. up to 90 ft in height in 10 ft increments. Number 8 copper
wire radials were buried 6? (typo) inches in the ground. I did not
find the ground conductivity numbers. The Field Strength numbers
below were interpolated from the original graphs for an antenna
height of 75 degrees.
Length of Radials in ft and Wavelengths
Number 45 ft 90 ft 135 ft
of Radials 0.137 WL 0.274WL 0.411WL
2 115 115 119 uV / M
15 144 152 157 uV / M
30 145 161 170 uV / M
60 150 176 181 uV / M
113 150 179 190 uV / M
The following table converts the Field Strength numbers
to dB referenced to 190 uV/M which corresponds to 113
radials 0.411 Wavelengths (WL) Long.
Number 45 ft 90 ft 135 ft
of Radials 0.137 WL 0.274 WL 0.411 WL
2 4.36 4.36 4.05 dB
15 2.4 1.93 1.65 dB
30 2.4 1.44 0.97 dB
60 2.0 0.66 0.42 dB
113 2.0 0.51 0 Ref dB
Observations:
1 It is clear that for Short Radials (0.137 WL), there is
negligible benefit from more than 15 radials. This could
be considered a 'Minimal Radial Field'. It would have
been useful to have data for 8 radials for a more complete
picture.
2 For radial lengths of 0.274 WL and greater, continuous
improvement is seen up to 60 radials. Note that doubling
the number and doubling the length of radials over the
'Minimal Radial Field' yields 1 dB greater Field Strength
with 4 times the amount of wire.
3 Lengthening radials 50% over 0.274 WL to 0.411 WL
provides only a small benefit compared to the increase
from 0.137 to 0.274 WL. Note that 113 Radials 0.411 WL
long is only 0.66 dB stronger than 60 radials 0.274 WL Long
but uses nearly 3 times as much wire. For Amateur
purposes, 60 radials 0.25 WL is a cost effective 'optimal'
solution as has been suggested by several proponents.
4 It would be interesting to repeat these measurements with
2, 4, 8, 15, 30, 60, 120 radials on the surface of the ground
with radial lengths of 0.125, 0.250, 0.375, and 0.500 WL.
I doubt anyone is willing to go to that expense and effort.
Tom N4KG
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