First tests at 8.4 GHz – Dwingeloo Radio Telescope

Auteur(s): Tammo Jan Dijkema, Thomas Telkamp, Jan van Muijlwijk, Ed Dusschoten, Cees Bassa et al

Photos: CAMRAS Tammo Jan Dijkema

On Saturday 9 November, we conducted first tests using an 8.4 GHz feed (“X-band”, 3.5cm wavelength). To our knowledge, this is the highest frequency which for which the Dwingeloo telescope was used so far.

The 8.4 GHz band is commonly used for deep space communications, such as for satellites exploring the solar system.

The feed itself was made by Dutch radio amateur Bert Modderman PE1RKI, the LNA/downconverter was manufactured by Kuhne electronic. The feed was mounted inside the 21 cm horn, rendering the 21 cm horn unusable for the duration of the tests. In the morning, we mounted the feed using the newly renovated elevator, and in the evening we removed it again.

The tests showed that the 8mm mesh still reflects sufficiently for the dish to function effectively, and the surface accuracy is ‘good enough’. Of course, it helps that the dish is quite large. We are working on quantifying the performance using scans of the Sun and comparing our results with those of other amateur X-band observers.

A concern was the pointing accuracy, as the beam width at higher frequencies is very narrow. We calibrated the pointing using the solar observation spacecraft Stereo-A, a convenient point source. We found the pointing error to be only about 0.02 degree in azimuth and 0.01 degree in elevation (see figure). The beam width was 0.1 degree, almost exactly what is expected for a 25m dish.

We also calibrated the axial focus position, moving the focus box backward and forward. Interestingly, the optimal axial focus differed between observations of the Sun and Stereo-A. This phenomenon was also noted in Figure 5.13 of Baars (2007), where it is explained as ‘probably a result of a somewhat asymmetrical sidelobe structure of the defocused beam’.

Given that 8.4 GHz used by many spacecraft, we attempted to receive signals from some. We successfully detected the carrier signal of three Mars orbiters: Tianwen-1, MAVEN and MRO. We also received signals from Europa Clipper and HERA. For HERA, the signal was strong enough to see the modulation (see figure). The same was true for Stereo-A, from which we also decoded some data (see figure).

With these encouraging results, we are planning further experiments.