Cutting-edge diy projects
Today I tweaked the position of the feed and the VE4MA ring on the 3.5m dish. Signals improved quite a bit, with STEREO A at 9dB above noise, and BepiColombo at 15dB above noise. However, there is still quite a bit of room for improvement. I had to take down my measurement setup when the “severe thunderstorm” alert came in. Will try again tomorrow.
CLICK HERE for high-resolution image.
Lately I’ve been asked about the downconverter that I built to receive X-band DSN signals. Because of time limitations, I took some shortcuts, namely using commercial connectorized modules and a high-stability signal generator with integrated mixer (DS Instruments MX12000 Integrated-LO Mixer) instead of designing the circuit from scratch. The cost of the unit is thus relatively high, but it was either that or not being able to receive X-band spacecraft at all in the foreseeable future.
We tested the downconverter (without LNA) at MUD2019, and measured a 4.58 dB noise figure and a downconversion gain of 24.4 dB.
Yesterday I mounted the VE4MA with squeezed-tube depolarizer made by M0EYT (uhf-satcom) on the 3.5m dish using a bracket designed and fabricated by KC2TDS. The tube holder is 3D-printed nylon, while the bracket itself is aluminum cut with water jet.
It got late by the time I finished mounting the feed, so I didn’t have time to optimize the position of the feed and super Kumar ring relative to the focal point. Nevertheless, measured 11dB of Sun noise vs. cold sky.
This morning, before Mars would set, I was able to receive the Mars Express spacecraft. I could see on DSN Now that it was locked to Goldstone.
Fading in the signal shown below was because I still have to tweak PstRotor and the rotator controller (Green Heron) driving the BIG-RAS Az/El to deal with the very narrow beamwidth (0.69 degrees) produced by the 3.5m dish at X-band.
Feeds with squeezed-tube depolarizers are popular among amateur DSN enthusiasts because of their straightforward, no-tune construction. Luis Cupido CT1DMK published a design that is often followed, and has been used successfully by Paul Marsh M0EYT, Micha? Zawada SQ5KTM and other hams interested in DSN.
Signals from planetary probes are weak, so as many variables as possible need to be eliminated when first setting up and debugging an X-band system. One of them is the polarization of the antenna. I found myself in that situation, and after some experimentation plus helpful answers from hams on the Amateur-DSN groups.io board, I’m now certain that I have the probe correctly oriented for DSN DXing.
As shown in the figure, an LHCP feed is needed to receive the RHCP signals transmitted by the vast majority of DSN spacecraft (when operating nominally). This is because when the feed is mounted on a parabolic antenna, the feed’s LHCP signal becomes RHCP after being reflected by the dish. The exception to this is for Cassegrain or Gregorian dishes, where a a secondary reflector is used to direct the energy into the parabolic reflector from a feed antenna located away from the primary focal point. In this case, the RHCP signal becomes LHCP at the secondary reflector, and then back to RHCP again when reflected by the dish.