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Fw: [Towertalk] Field Strength vs. Radial Field

To: <towertalk@contesting.com>
Subject: Fw: [Towertalk] Field Strength vs. Radial Field
From: n4kg@juno.com (n4kg@juno.com)
Date: Sat, 28 Sep 2002 21:59:27 -0600
Here is the 1937 Field Intensity Data vs Radial Field
followed by a conversion to dB relative to 113 radials
0.411 WL long.  This was originally posted 3 / 2002.

Tom  N4KG

--------- Forwarded message ----------
From: n4kg@juno.com
To: TOWERTALK@contesting.com
To: <towertalk@contesting.com>
Date: Tue, 12 Mar 2002 11:58:42 -0600
Subject: [Towertalk] Field Strength vs. Radial Field

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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