In view of all the discussion on verticals and radials, I thought it would
be useful to reproduce some of the radial data reported in the Brown, Lewis,
and Epstein paper which Tom W8JI has cited. For those who have access to
old engineering journals, the paper is "Ground Systems as a Factor in Antenna
Efficiency", by G.H. Brown, R.F. Lewis, and J. Epstein, Proceedings of the
Institute of Radio Engineers, Vol. 25, No. 6, June 1937, pp. 753-787. Among
other things, the authors made field strength measurements at 3 MHz for
verticals of different heights and with different radial systems. As Tom
has said, this paper is considered the definitive work on the subject.
Although there have been other studies, I know of none as thorough and
comprehensive as this one.
I have transcribed some of their data onto the table below. The numbers in
the table represent the difference in dB between the measured system and
the theoretical limit for a vertical over perfect ground. In some cases
where there were missing data points in the original paper, I did an
an extrapolation to fill in the data. Three different radial lengths
were considered: .41, .27, and .14 wavelengths long. The number of radials
used were 2, 15, 30, 60, and 113. I picked off data points for three
different vertical antenna lengths--1/4, 1/8, and 1/16 wavelengths.
VERTICAL ANTENNA FIELD STRENGTH MEASUREMENTS REFERENCED TO IDEAL (dB)
radial no. 1/4-wave 1/8-wave 1/16-wave
length radials vertical vertical vertical
******** ******* ******** ******** *********
.41-wave 2 -3.9 -5.6 -10.0
15 -1.8 -2.2 -3.8
30 -1.0 -1.6 -3.3
60 -0.5 -1.0 -2.3
113 -0.2 -0.3 -0.6
.27-wave 2 -4.2 -6.1 -10.7
15 -2.1 -2.4 -4.7
30 -1.6 -1.8 -2.9
60 -0.8 -1.2 -2.7
113 -0.7 -0.9 -2.5
.14-wave 2 -4.2 -6.3 -10.6
15 -2.3 -3.6 -5.5
30 -2.2 -3.5 -5.3
60 -2.1 -3.1 -4.8
113 -2.1 -2.9 -4.4
As mentioned in the paper, on the order of 120 radials approaching 1/2
wave in length is considered nearly perfect for a 1/4-wave vertical.
As the vertical is shortened in length, the losses increase, particularly
for antennas shorter than 1/8 wave. The gain in going much beyond 60
radials is small unless the antenna is electrically very short. For
short radials (.14 wave), a few radials is almost as good as many.
Keep in mind that this data was measured for a specific site with
particular ground conductivity. How that generalizes to other locations
is a bit of an open question, but I think the trends, if not the
exact numbers, will hold up.
Whether you think differences of 1 dB or so are worth the trouble of
installing much more extensive radial systems is a matter of debate.
For ragchewing across town, I think we would agree it's not. On the
other hand, when trying to raise 9M2AX or 9V1XQ on Top Band long path,
I know some of us would kill for that extra dB.
I hope this information is useful to all.
73, John W1FV
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