I n s t r u m e n t z s

a n d

M e t h o d s

Sensordesign andinstrumentation depends on the actual
frequency ofoperation.Inthe range of20-40MHzflat
plate capacitive sensors are used. The monopole-antenna
is used for snow wetness detection in the frequencyrange
of100 MHz up to 3 GHz. Primary measurement quantity
using thesesensors is thesensor impedancewhichdepends
on the dielectric properties of the surroundingmaterial. In
the microwave X and K bands (8 to 16 GHz) reflection and
transmissionmeasurementshavebeenperformedusing
high-gain horn antennas. From the measured reflection /
transmissioncoefficientssnowdielectric(andalso
magnetic) properties can be derived.

20-40 MHZRANGE

In the frequencyrangeof100 MHz up to 3GHz a monopole
antennaisusedasdielectricsensorforsnowwetness
measurements.Primary measurement quantity is the sen-
sordriving-point impedance. This impedance can easily
be measured by a simple vector voltmeter. A scetch of sen-
sordesign(withdimensionsinmillimeter)andablock
diagram of the instru-mentation is given in Fig.3.

8 - 16 GH ZRANGE

The dielectric properties of snow in the microwave X and
K bands have been derived from an analysis of the phase
and magnitude of the reflection and/or transmission coef-
ficients of the air/snow boundary by applying a free-space
measurement technique. The magnitude and phase of the
reflectedor transmitted signals are detectedby a standard
grade microwave vector-volt-meter and a reflection/trans-
mission adapter port.A sketch of the experimental setup

Fig.2. Sensordesign (left) and block diagram of instrumentation (right) of the 20-40MHz measurement setup. O: oscillator; TT: twin-T-
bridge; DC: down-converter; SP: sensor plate B: battery; 1: Option formanual tuning; 2: Option for automatic tuning

The flat-plate capacitive sensorisconnected to a twin-T-
bridge which canbe tuned manually or automatically by a
microprocessor; so continuousrecordings of snow wetness
are possible.A sketch of sensordesign (with dimensions
in millimeter) and a block diagram of the instru-mentation
is given in Fig.2.

100 - 3000 MH

ZRANGE

forreflectionmeasurementsand ablockdiagram ofthe
instrumentation is given in Fig.4.Details of the measure-
mentproceduresaregivenbyAchammerandDenoth
(1994).

MEASUREMENTS

cable

Fig.3.Sensor design (left) and block diagram of instrumentation (right) of the 100-3000MHz measurement setup.P: Power splitter,
VM: vector voltmeter, ref: reference signal, r: reflected signal.

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