filter the problem is aggravated as the measurement height h increases because the signals reflected on the surface 20 become much weaker and can be masked by the parasitic signals on the other hand for short distances the reflected signals r are much larger and interference with the parasitic signals has less effect fig2 shows a so called bistatic radar probe a frequency modulated signal is generated by a high frequency oscillator 30 and separated in the divider 32 into a reference signal ref and a transmitted signal t the latter is sent through a waveguide 34 and a transmitting antenna 26 on to the surface 20 whose distance is required to be measured and the signal r reflected by he latter is picked up by a separate receiving antenna 38 the reflected signal r is sent through a waveguide 40 and is identified in the mixer 42 by comparison with the reference signal before being processed in a data processing unit in order to compute in a manner which is known per se the height h this system has the advantage that there is no crossing between the transmitted signals and the reflected signals such that there is a better detection of the reflected signal which makes this system advantageous when the reflected signal is weak i e for relatively large heights h on the other hand this system suffers from a handicap with respect to monostatic antennas when the measurements are made at a short distance from the reflection surface 20 in effect given that the two antennas 36 and 38 are placed side by side the reflected signal r is picked up at a certain reception angle u03b8 and this angle must be.