Experiment 1: Propagation of waves on coax cables and waveguides

In this introductory experiment you'll make yourself familiar with the propagation of waves on double lines and waveguides, utilizing coxial and waveguide transmission lines as well as bi-directional couplers. The determination of mismatches and their effects are being investigated. Special emphasis is put on measuring absolute and relative powers (the former with thermystors, the latter with detector diodes.). This experiment is familiarize the students with HF equipment such as wobblers, oscilloscopes, power and frequency meters. Handling such devices is an indispensable prerequisite for all subsequent experiments.

Experiment 2: Heterodyne receiver

The goal of this experiment is to set up a working radio-astronomical heterodyne receiver. To this end, the necessary active and passive components are studied in the first part of the experiment (amplifiers: gain, bandwidth; mixer: mixing losses; filters: bandwidth, sideband rejection). Having determined the components' characteristics, the next task is to construct an overall concept for the receiver, including a level diagramme, aiming at highest sensitivity and stability. The receiver is then built up using the tested components. Its characteristics (i.e. sensitivity, bandwidth, gain, linearity, stability) are determined. The experiment is complemented by studying the properties of partially polarized waves, describing them via the Stokes parameters. The analysis of such waves can be carried out using an IF polarimeter as is generally done in radio astronomy. The methods with which such a polarimter is calibrated are shown. Both, CW as well
as coherent and incoherent noise signals are then analyzed with the balanced polarimeter.

Experiment 3: Cooled Dicke system

This one is more like a demonstration experiment in which you have ample opportunity to interact though. The aim is to make you familiar with the performance a continuum receiver, whereby in particular the tremendous effect on sensitivity by cooling active and passive components is shown. The experiment teaches how the Dicke principle, i.e. the continuous calibration of the amplification to improve the stability, is realized by a phase-stable control of the Dicke switch, by a phase- selective demodulation of the measured signal and by computer supported combi- nation of individual measuring periods. Since the backend used here can be found at most modern single-dish telescopes this experiment is a good preparation for practical continuum measurements.

Experiment 4: Observations with the Effelsberg 100-m telescope (special arrangement required)

This, too, is more like a demonstration experiment, with active participation of the students. The main telescope characteristics of the 100-m telescope are to be de- termined. Using radio sources with known flux densities and the temperature of an internal calibration signal, the aperture efficiency is derived. By cross-scanning a point source, the telescope half-power beam width and the main-beam efficiency are subsequently measured. Both, a point source as well as an extended target are mapped thereafter (with all Stokes parameters), and maps of the total and polarized radiation are produced. The results are discussed in terms of angular resolution, telescope gain and sensitivity. Finally, spectroscopic measurements are performed by observing radio recombination lines in the Orion nebula. We usually carry out this experiment at 5 GHz, but there is flexibility since we have a choice of receivers in the secondary focus.

Experiment 5: Twin-element interferometer (currently out of order)

Using a twin interferometer on top of the Astronomical Institute, the basic problems and performance of aperture synthesis are demonstrated. The target of these observations, which are carried out at 11.5 GHz, is the (quiet) sun. After measuring the characteristics (HPBW) of the individual dishes we determine the fringe sepa- ration and fringe frequency as a function of baseline (transit mode). The source visibility is then measured as a function of hour angle (tracking mode), from which the (one-dimensional) source structure can be computed via an FFT. The effect of fringe washing is shown and discussed.