Jouni Peltonen

Millimeter wave backscattering experiments in arctic winter

Millimeter wave sky backscattering in artic winter conditions close to ground level is not dominated by snowfall. Preliminary measurements at Ka and V-bands suggest that average equivalent backscattering areas stay around 1-3 m2 also during wet snowing for both linear polarizations. Sudden high values of backscattering are observed in temperatures below -30 degrees. Successive meteorological inversion layers seem to cause backscattering areas above 10 m2 in otherwise similar test conditions. The process can last 3-4 hours. Requirements for such phenomena are steep vertical temperature and humidity profiles, which may exceed 20 degrees and 20 per cent for 200 meters, respectively. Some of these measurements have been performed with a portable 10 mW bistatic system. The receiver noise floor is about ¿90 dBm. Different antennas are used according to frequency, spatial resolution and desired illumination, including horns supplemented by dielectric lenses. Their gain varies from 19 to 40 dBi. The current synchronization scheme relies on a common X-band IF that is connected through coaxial cable.

A simple device for long-term radar cross section recordings

A sample and hold circuit with settable delay can be used for recording of radar echo amplitude variations having time scales up to 100 s at the selected range bin in systems utilizing short rf pulses. The design is based on two integrated circuits and gives 1% uncertainty for 70 ns pulses. The key benefit is a real-time display of lengthy amplitude variations because the sample rate is defined by the radar pulse repetition frequency. Additionally we get a reduction in file size at least by the inverse of the radars duty cycle. Examples of 10 and 100 s recordings with a Ka-band short pulse radar are described.

Microcontroller-based binary integrator for millimeter-wave radar experiments

An easily on-site reconfigurable multiple binary integrator for millimeter radar experiments has been constructed of static random access memories, an eight bit microcontroller, and high speed video operational amplifiers. The design uses a raw comparator path and two adjustable m-out-of-n chains in a wired-OR configuration. Standard high speed memories allow the use of pulse widths below 100 ns. For eight pulse repetition intervals it gives a maximum improvement of 6.6 dB for stationary low-level target echoes. The doubled configuration enhances the capability against fluctuating targets. Because of the raw comparator path, also single return pulses of relatively high amplitude are processed.

A High-Isolation S-Band Ring Hybrid Combiner

Through careful dimensioning, avoidance of line transition discontinuities and by using special blocking elements to reduce stray couplings across the internal airgap together with inductive fine-tuning posts, a conventional ring hybrid can be an attractive RF power combiner for L- or S-band transceiver applications, where a high isolation is desired between the transmitter output arrangement and the receiving front-end, or for antenna arrays. When typical commercial units hardly exceed 40 dB, an improved narrow-band isolation up to 90 dB and more is demonstrated here while keeping the residual attenuation for desired coupling below 0.15 dB. The required milling and mounting uncertainty for a successful and repeatable result in a combined metallic / PTFE stripline form was found to be 0.01 mm.

Experimental Results of Maritime Target RCS Fluctuation

Fluctuations in apparent radar cross section of various maritime radar targets have been measured at Ka-band. A dedicated sample&hold circuit has been utilized for the long data series in addition to the more conventional time-gated data acquisition equipment. Peak variations exceed 20 dB also for anchored buoys in 5 m/s wind. Results suggest that observation lengths as long as 500 seconds may be necessary to cover typical statistical characteristics of target radar backscattering behaviour, of course depending on the type of target and sea state. Index Terms – Radar measurements, radar targets, radar cross section, sampling circuit, data acquisition