C O A X I A L P R O B E S

The Coaxial probe is the most efficient of all probe configurations

and should be the first consideration in all applications. Analogous

to the efficiency of modern, coaxial cable, coaxial probes allow

almost

unimpeded

movement

of

the

high

frequency

pulses

throughout its length.

The electromagnetic field that develops between the inner rod and

outer tube is completely contained. See Figure 1. The efficiency

and sensitivity of a coaxial configuration yields robust signal

strength even in extremely low dielectric (

ε

r

≥1.4) applications.

The sensitivity of this “closed” design, however, also makes it more

susceptible to measurement error in applications of coating and

buildup.

TWIN ROD PROBES

The relationship of the Twin Rod probe to a Coaxial is similar to

that of older, twin-lead, antenna lead-in to modern, coaxial cable.

300 ohm twin-lead cable simply does not have the efficiency of

75-ohm coax. The parallel conductor design is less sensitive than

the concentric coaxial. See Figure 2. This translates to Twin Rod

GWR probes measuring dielectrics of only

ε

r

≥1.9.

The “open” design also allows more accurate measurement where

coating/buildup are possible. A film coating has little effect on per-

formance. However, bridging of material between the rods or

buildup on the spacers can cause improper measurement and

should be avoided. Figure 2 also shows that the electromagnetic

field develops not only between the rods, it also expands outward

making it more sensitive to proximity effects of objects located

immediately around it.

P R O B E

O V E R V I E W

Figure 1

Coaxial Probe

Figure 2

Twin Rod Probe

Choosing the proper Guided Wave Radar (GWR) probe is the most important decision in the application process. The

probe configuration establishes fundamental performance characteristics. Coaxial, twin element (rod or cable) and sin-

gle element (rod or cable) are the three basic configurations used today; each with specific strengths and weaknesses.

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