705-5100-111 Datasheet, PDF (3/28 Pages) Magnetrol International, Inc. – Eclipse® Enhanced Model 705 Guided Wave Radar Level Transmitter

PROBE OVERVIEW
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.
Figure 1
Coaxial Probe
Figure 2
Twin Rod Probe
COAXIAL PROBES
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.
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