A simplified circuit diagram of the measure-

BNC Connector

ment electronics is shown in Figure 2. As can be

seen, three measurement voltages are required:

The applied voltage, *V*ref

Base

The voltage drop across the probe circuit,

The phase voltage between *V*ref and *V*div,

In this simplified example, the capacitance of the

probe is calculated using

Tine

Diameter

(1)

Probe

Length

where *R *= 500 ohms for this circuit

ω = 62.8 106 Hz.

Short

Inner Tine

The total probe circuit actually consists of the

probe capacitance in parallel with the capacitance

of the coaxial cable that connects the probe to the

readout box. To provide optimum sensitivity to

the measurement of the capacitance of the soil, a

Probe Radius

the probe cable input connector so that the ca-

pacitance of the cable itself could be nulled out of

the measurement process. By using this variable

inductor, the individual cable's capacitance may

be eliminated from the measurement so that only

U1

the probe's capacitance is "seen" by the measure-

ment electronics, thus simplifying the calculations

required to determine the probe capacitance us-

ing *V*ref, *V*div, and *V*ph. In addition, as long as a

standard cable length is used for each field mea-

U2

surement, the process of nulling out the cable

capacitance is required only once for each of the

measurement circuits. Since the capacitance per

Vref

unit length of coaxial cable is very uniform, the

error caused by using different cables should be

500 ohms

very small as long as the cables are all of the same

length and have the same capacitance per unit

Soil Probe

length.

Vdiv

While the correction for various cable lengths

may be reduced to a small error or actually mea-

sured and accounted for in the calculations, there

Csoil

Rsoil

are two problems with this method that need

careful consideration and that can cause consid-

erable errors if not accounted for.

First, the dielectric constant *K *of a material is

defined as the ratio of the permittivity of the me-

dium ε to the permittivity of free space (a vacuum

or air as a very close approximation); ε0 = 8.85

pF/m in the MKS system, for instance:

2