Observations with no signal

SatNOGS Hardware
1

Hi everyone!

We have set up a new Ground Station and we are facing some challenges getting a clear signal, and we were hoping the community might give us some advice.
Our equipment, located at the roof of our building in Athens and set up in series in this order, is:

  1. Antenna: Kx137 [wraase de] hardware/antennas/
  2. LNA: NooElec SAWbird+ NOAA [amazon com] Nooelec-SAWbird-NOAA-Ultra-Low-Applications/dp/B07TWPR871
  3. 20 meters low loss coax cable [[amazon de]] gp/product/B0995QX5FV
  4. SDR: Noeelec SMART SDR v5 [nooelec com] store/sdr/sdr-receivers/nesdr-smart-sdr.html
  5. Ubuntu server with SatNOGS running in a docker container

For example in SatNOGS we get waterfalls
with no visible signal.

We also run a test with CubicSDR for NOAA 19 at 137.1 MHz (our antenna center freq) and although the signal in our opinion is clearly visible as seen here:

signal
signal935×711 106 KB

the .wav demodulated with wxtoimg is pure noise.

We also tested the configuration of [antenna, coax cable, LNA, SDR, server] but saw higher noise levels.

We would greatly appreciate any guidance.

2

Hey @ohb_hellas, and welcome in the community!

From the metadata of the observation you linked, it looks like you haven’t set the gain of the SDR.

gain: null,

You can set it in Advanced --> Radio --> SATNOGS_RF_GAIN in sudo satnogs_setup.
You can start with the value of 32.8 and adjust after checking the performance in the observations.

2 Likes
3

thank you @fredy ! The gain variable really improved the signal (NOAA19 NOAA15). Unfortunately though the demodulated data is still noise. Is there something else that you can think of that we can tune? Thanks again for your help!

4

The LNA should be mounted as close to the antenna as possible to be of any use.

5

Our current setup has the LNA on the antenna so it is [antenna, LNA, coax cable, SDR, server].

6

Allright. Are you running DC (bias) over the coax ? In that case we can probably rule out the cable as an issue. What’s supplying the DC/bias btw ?
The signal is still very weak for such a setup. Do you have the possibility to hook a VNA or some antenna analyzer up to the antenna ?

Also, you should probably change the antenna config for that station to 135-140MHz as the SAW filters the rest.
bild

7

There is no DC bias over the coax. The LNA gets power from a external power source. Unfortunately since there is no DC bias over the cable, the preamplifier embedded in the antenna is off. We don’t have an antenna analyzer. We also just changed the antenna config as you suggested.

We just had a slightly better observation now SatNOGS Network - Observation 7929913 by changing the RF_GAIN to 29.7 but still no luck.

8

Err, is it a preamp with bypass relays ? A plain LNA will absolutely block most signal if not powered.
I’m looking at the specs now, why do you have a sawbird lna there at all ? using a pure SAW filter would make more sense. Having a 17dB gain LNA screaming in to a 41dB gain LNA at VHF seems waay overkill if not just making it into squarewave. Hook it up to a 100MHz+ oscilloscope and find out (:

Kx-137:

Internal low-noise preamplifier with typ. 17 dB gain.
High selectivity due to high-Q-filter in the 137 MHz range.

SAWbird:

41dB of RF gain at 137.5MHz
2.6MHz 3dB bandwidth
Other satellite signals within 136-138MHz

9

Thank you for the reply! We have added a 12V DC power supply through a Bias T to the antenna’s preamplifier and now we are getting nice images from different NOAA satellites.

1 Like