X BAND Weather SATELLITE RECEPTION

An excellent GEO-Quarterly article describing Jean-Luc Milette’s pioneering work on X band weather satellite reception can be found following this link .

See also @Aaang254 excellent work on Github  !

For the direct reception of weather satellites on 8 GHz typically a ~1.5 m or bigger dish is needed, a AZ/EL or better X/Y rotor, a suitable feed and low noise amplifier, a downconverter and SDR with the appropriate software.

SDR used is a LimeSDR-USB , the whole pass is recorded at 15 MSPS or at higher rate depending on the satellite and then offline demodulated -decoded.

Below are some X band LEO weather/earth observation satellite frequencies as of March 2021:

SatelliteFreq (MHz)
FY3B 7775 (MPT) / 8146 (DPT)
NOAA207812
SUOMMI NPP7812
FY3D7820
AQUA8160
AURA8160
TERRA8212.5
OCEANSAT-28300.0

ANTENNA / TRACKING

I was lucky enough to find a 1.4 m prime focus dish with f/D 0.38 (Andersen True Focus) . It is not easy to find prime focus dishes nowadays…

For tracking I use an SPID BIG RAS rotor and my own software ( I call it Satellite FUN) which calculates passes/tracks satellites, programs the local oscillator and starts recording.

When using a small dish there is not much margin in the link budget so besides antenna G/T etc accurate tracking is also very important. I initially used HamLib but position update wasn’t fast enough so decided to switch to directly writing to the controller serial port and updating position every 0.5s .

So far I have had good results with most: AQUA / TERRA / FY3D / FY3B /OCEANSAT-2/AURA.

RECORDING A PASS

Recording is done with HDSDR . I had also tried GNU Radio but the resulting file had gaps and wasn’t useable.

GNU Radio Recording
HDSDR Recording

DEMODULATION / DECODING SOFTWARE

The resulting RF wav file of a satellite pass is further postprocessed:

See Jean-Luc’s article (page 11) mentioned above and @Aaang254 work on Github especially SatDump .

8 GHz LNA

I designed my own LNA and am still using the very first prototype consisting of 2 stages with some spare MGF4919Gs . This is still work in progress as I am going to try more modern devices soon.

NF achieved without tuning was around 1 dB. See Noise Figure measurement below:

DOWNCONVERTER

The LNA is followed by a VBF-8000+ Mini-Circuits 8 GHz band pass filter which is connected to a 20 dB gain block. Then comes the ZX05-153-S+ Mini-Circuits mixer . I use Kuhne’s programmable MKU LO 54 to 13600 PLL Oscillator. Frequency can be altered over the serial port in 1 Hz steps and it accepts a 10 MHz external reference.

In the future, as time permits, all these may be replaced by a single PCB featuring the LNA, mixer, filters , PLL etc.

ANTENNA G/T

With the 1.4 m dish , the 0.9 dB NF LNA (see measurement above) and the 3D printed /copper plated feed I am getting about 6 dB sun noise over cold sky (SFU 73) . Need to check what the maximum expected is.

Possible further improvement is to get rid of the SMA connectors in front of the LNA , use a modern device to maybe reach 0.7 dB noise figure .

Spectra – View Sun Noise measurement

Approximate radiation pattern using the sun as a signal source just taking noise power measurements at a few points around the maximum

Rough radiation pattern
Sun Noise/Cold Sky – Note that there is a band pass filter at the output of the LNA

DUAL MODE FEED FOR OPTIMUM ILLUMINATION OF A PRIME FOCUS DISH WITH F/D~=0.4

See below some pictures and details about the 3D printed / copper plated feed I currently use:

LINK TO STL FILES ON THINGIVERSE

LINK TO STL FILES ON DROPBOX

Version 1.0 !

This is a low noise dual mode prime focus parabolic dish feed for 8 GHz weather satellite reception.
Suitable f/D is around 0.4 for optimum reflector illumination and best overall system G/T.
The 3D print consists of the base, the waveguide and the dual mode section which are connected with screws.
Two waveguide sections have been included: one for linear polarization and another one for both RHCP and LHCP polarizations on separate SMA connectors.
This work has been based on the W2IMU/N2UO 23cm design found here:
http://ok1dfc.com/EME/technic/septum/N2UO%20opt.pdf.
After printing, it has to be painted with a conductive paint and then electroplated in a bath.
It is absolutely important the resulting surface to be as smooth as possible.
Roughness will result in losses so patience when sanding/smoothing will pay off !

I used PLA and smoothed it with sandpaper starting from grit 500 up to 2000.
Conductive paint and plating solution used were bought from  https://www.tifoo.de/
Painting has to be carefully done paying attention not to leave any blank areas.

If ABS/ASA are used I guess the smoothing process can be expedited in acetone vapors or the like.
The final efficiency of the feed depends on surface roughness and the presence of any non-plated gaps.

Notes:
1) You could try scaling it in the slicer to make e.g a feed for 10 GHz!

2) For 8 GHz SMA probe length inside the waveguide is 7mm .
The dimensions of the block where the SMA sits had been adapted so that no tuning is necessary when these connectors from Kuhne are used or the equivalent Amphenol 132147 e.g from Mouser leaving just 7 mm inside the waveguide. These connectors are about 17.8mm long and the Teflon section is about 15mm.

3) The Teflon section in the waveguide needs to be cutoff.

4) The Teflon section in the SMA block can be painted and plated to avoid having to rely on the plating inside the hole.

5) The SMA connector itself can even be soldered onto the plated feed !!!
Obviously, it is not as robust as on a feed of metal but should be good enough for fixing it once with an SMA torque wrench. Don’t push it too much though 🙂

6) In the 7.75-8.25 range simulated sidelobes are lower than 25 dB, simulated axial ratio less than 3dB in the entire 100 degrees radiation angle and measured isolation better than 25dB.

If you build one, please let me know how it goes !

73 de SV1CAL Michael

Copper-plated 8 GHz dual mode feed
Measured S11/S21/S22/S12 – Isolation better than 25 dB
Printing the waveguide section
PLA / ABS prints
Copper plating in a bath
Dual mode section plated
Copper plating the PTFE section
Feed in action
AQUA 8160 MHz
TERRA 8212.5 MHz
FY3D 7820 MHz
FY3B morning light
FY3B crop
FY3D 7820 max EL 34.5
Oceansat-2
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