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upper frequency limit 1.75 GHz decreased to 1.4 GHz and 1.5 GHz after passing
the reed-relay and GaAs switches, respectively.
Fig. 47.
Input pulse and output pulses from the tested SP16T switches
The comparison of reflectograms showing the reflections of the initial pulse
from TDR probe in the soil, in the cases then there was no and with the switches
connected, are presented in Fig. 48.
Fig. 48.
The reflected signal attenuation introduced by the prototype reed relay and MMIC GaAs
switches when the TDR probe was inserted into dry (left picture) and wet (right picture)
sand
The reflections are more sharp and higher amplitude when there is no switch
along the signal propagation path. The upper limit of the frequency range is lower
for the reed relay switch because the reflected signals are less sharp than in the
MMIC GaAs switch. However in both cases the reflections from the TDR probes
are easily distinguishable even in dry sand.
It should be stressed that before the reflected signal produced by the generator
is sampled, it must travel through the switch two times, towards the TDR probe
and back to the sampling head. Therefore in the case of materials that attenuate
the TDR signal (saline soils) the reflection from the TDR probe end will not be
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