ASSA Radio Astronomy Group

Radio interferometry is a powerful tool that can be used for a number of diverse applications. A radio interferometer consists of more than one antenna tuned to receive radio emissions at a selected frequency.

The antenna separation along the baseline (distance A-B) is typically 10 or more wavelengths (lambda) and the baseline is oriented East to West to follow the natural rotation of the earth. The signals from the antennae are cross-correlated in a Summing Amplifier at the input of the main radio receiver.

As the radio emitting object (source) passes overhead the angle of arrival (theta), and hence the difference (C-B) in signal travel distance to each antenna changes and therefore the relative phase of the two received signals alternates between in-phase (add) and anti-phase (subtract). Over a period of a few hours the receiver output (measured correlated signal strength) follows a "fringe pattern" as shown below.

From the Hans Michlmayr - Amateur Radio Astronomy Website

Such fringe patterns can be analysed to determine the approximate location of a nearby extra-terrestrial radio emitter. For a larger linear array containing many antennae it is possible to combine all the cross correlations to generate a (radio) image of more distant celestial objects.

The main limitation for the Amateur Radio Astronomer is not really the electronics but more the available space on your property. This is because in order to obtain a suitable fringe pattern the two antenna must be spaced at least 10 wave lengths (or 10 lambdas) apart. For example, at 74Mhz VHF, the distance between the receiving antenna must be more than 40 metres.

Further reading:

1. Interferometry (Wikipedia)
2. Radio Interferometry (Wikipedia)
3. Phase Interferometry (Wikipedia)