Request for Assistance with UHF Signal Issues

SatNOGS
1

Hello everybody,

I’m an engineering student currently developing and implementing a ground station… While I am relatively new to radio technology, I am eager to expand my knowledge and skills in this field.

I am in the final stages of my project but encountering persistent signal issues, particularly in the UHF band, and I would greatly appreciate your expertise. Below is an overview of my setup:

Equipment Configuration:

  • Antennas:
    • UHF: Wimo WX-7036 (2×18 crossed Yagi)
    • VHF: Wimo WX-220 (2×10 crossed Yagi)
  • LNA Pre-amplifier: MVV-432VOX
  • Rotors: Azimuth/elevation rotor system with controller
  • Diplexer
  • Bias-tee FSP 435 12V
  • Airspy R2 SDR
  • On satnogs network
    • sation ID : 1062
    • sation name : CCERES Ground Station

Here is an extract of my satnogs configuration file :

  • soapy-rx-device: “driver=airspy,serial=26a464dc282a4093,biastee=true”,
  • samp-rate-rx: “2.5e+06”,
  • doppler-correction-per-sec: null,
  • lo-offset: null,
  • ppm: null,
  • rigctl-host: “satnogs_rigctld”,
  • rigctl-port: “4532”,
  • gain-mode: “Settings Field”,
  • gain: null,
  • antenna: “RX”,
  • dev-args: null,
  • stream-args: null,
  • tune-args: null,
  • other-settings: “VGA=11,MIX=7,LNA=10”,
  • dc-removal: null,
  • bb-freq: null,
  • bw: null,
  • enable-iq-dump: “0”,
  • iq-file-path: null,
  • udp-dump-host: null,
  • udp-dump-port: 57356,
  • wpm: null,
  • baudrate: “9600”,
  • framing: “ax25”

Issue Description : My main problem is that I manage to get signal in VHF but not in UHF

Troubleshooting steps taken:

  • Verified antenna alignment
  • Adjusted LNA/MIX/VGA settings values without significant improvement.
  • Verified LNA continuity and power supply (no apparent issues).

Questions:

  • Have you ever experienced such a problem?
  • Do you have any ideas about possible causes for the issues I’m encountering?
  • Are there any relevant tests I could perform to identify the problem more precisely?

Thanks you very much for your time and consideration :slight_smile:

1 Like
2

Good afternoon,

I had a quick look at the meta data of this observation SatNOGS Network - Observation 11458727

And something is puzzling me, in the list of details you mention a external BiasT but you also enable the BiasT that is part of the Airspy R2 SDR functionality. Without any details on the exact RF path it could be that you are shorting the BiasT dc option off the Airspy.

This can’t be good for the SDR.

My suggestion would be to go back to the bare minimum and add functions step by step, I hope there is no permanent damage due to possible short.

In the meantime, can you add some details on the RF path?

Computer/Pi → → SDR → <connection details, coax, connectors, place of filters, biasT, length, diplexer> → Antenna.

The more the better.

Jan - PE0SAT

3

Good afternoon Jan, thank you very much for your quick reply!

Oh no… I thought using an external Bias-T required activating the “biastee=true” option in the configuration file. I really hope I haven’t damaged my SDR… I removed the option and I will try to go back to minimum settings to do my tests.

Here is my complete RF path for UHF :

Computer (Debian) → SDR Airspy R2 → Diplexer → Manual switch → External Bias-T (powered separately) → Coaxial cable → Surge arrester → Pre-amplifier → Coaxial cable → Antenna

For more details:

  • I’m running the SatNOGS client in a Docker container on the computer connected to the SDR. I’ve made sure to give the container the necessary access to the SDR device.
  • The diplexer combines the VHF coaxial cable (2 m) and the UHF coaxial cable (70 cm).
  • The manual switch is currently unused, but it’s in place to allow for a future upgrade of the setup to include a “transmit mode”. I have verified that the correct path is selected on the switch.
  • The Bias-T is powered by an external 12 V power supply. I’ve confirmed that 12 V is still present just before the pre-amplifier.
  • I placed the pre-amplifier as close as possible to the UHF antenna to minimize coaxial cable loss.

Thanks again for your help!

4

Based on your information I get the following:

image
image741×467 48.4 KB

Some remarks:

The Airspy is a great SDR but picky when it comes to USB connections, make sure you connect it to an USB 2.0 port.

You can check the sample rate performance with airspy_rx -r /dev/null -t 0 this should stabilize at 10.000 MSPS. If this is a docker setup, you have to runsudo docker exec <container-name> airspy_rx -r /dev/null -t 0 (sudo is only necessary if the user running the command isn’t a member of the local docker group.)

It is possible that the High Pass part of the diplexer shorted the dc, have a look at the details of your diplexer.

Based on practical experience that after a fully disconnect and reconnect from the Airspy an error/fault situation will be mitigated, give that a try.

As mentioned before, step by step. Direct connection from the antenna to the sdr, first VHF and then UHF, if you receive signals (use a sdr program like GQRX, SDR++, SDR# etc) the use the diplexer and try again.

My guess is that the LNA and BiasT are only in one path but that is clear to me, but that will be the next add-on and see if you reveive signals also check the influence on the LNA, this gives you an idea on the amount of gain that needs to be configured.

Here the meta information off a station that is using a Yagi antenna, R2 and a LNA, this will give you an idea of the gain settings.

Good luck and let me/us know how things are progressing.

Jan | PE0SAT

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5

Any progress on your station ?

6

Good morning Jan,

Thank you for your message.

Based on your recommendations, I performed the following tests and verifications:

  • My Airspy is connected to my computer via a USB 2.0 port.
  • I ran the following command in my Docker container: airspy_rx -r /dev/null -t 0, and it did stabilize at 10.000 MSPS.
  • I fully disconnected and reconnected the Airspy.
  • I tried to find documentation about my diplexer but couldn’t find anything relevant regarding a potential high-pass section that could be shorting the DC.

Additionally, I’m a bit concerned because the station I’m working on used to function properly back in 2020–2021 with the same hardware setup. It was left unused for a while, and my current mission is to bring it back online using Docker. Unfortunately, I’m unable to contact the people who previously worked on the station, but from the reports I’ve reviewed, the hardware configuration was the same:

computer → SDR (Airspy R2) → diplexer → manual switch → bias tee → LNA →UHF antenna  
                                                   ↘︎ manual switch → VHF antenna

Further observations:

  • I tested the setup without the diplexer, both in VHF and UHF. The VHF results are still good, and UHF results remain unchanged.
  • I checked the coaxial cables used for the UHF path; everything seems to be functioning properly.
  • I’ve just implemented direct data streaming from my container to the host machine, so I can now visualize observations in real time using GNU Radio.
  • I’m beginning to investigate the impact of the LNA.

Regarding my results: here is a waterfall from an ISS pass — it looks promising, doesn’t it? However, I can’t seem to get reliable results for other UHF satellites — the waterfall remains either blank or shows only wide bands.

ISS record
ISS record832×1603 76.2 KB

One area I’m unfamiliar with is the gain settings. As I understand it, LNA, MIX, and VGA correspond to different amplification stages in the Airspy, correct? How should I choose appropriate values for these parameters? Is there any detailed documentation available? I also don’t fully understand the difference between linear and sensitive gain modes.

In any case, thank you very much for your help — I’d really appreciate any suggestions, recommendations, improvements, or tests you think I should try!

7

Thanks for the update and good to see you are able to receive data on the VHF.

This also tells us that the Airspy R2 is still functional and that there is another issue somewhere in the UHF RF path.

A lot of good questions and points of attention, some of them I will answer on another time.
Regarding the gain values, let me point you to this source code on github.

For now I would connect the UHF antenna directly to the R2 and check with a SDR program if you are able to receive some satellites. On Linux try sdr++ and on Windows SDR#, here you also see the gain options: sensitivity, linearity and free.

SDR++

image
image1193×397 33.8 KB

SDR#

image
image1145×577 20 KB

To have an idea what satellites are active, have a look at this link, this station has a 4 element Yagi, LNA and connected to a R2

8

Hi Jan, hi everyone,

I hope you’re all doing well, and thank you for your response. Here’s an update on my work and investigations.

As you recommended, I connected the UHF antenna directly to the Airspy (without the diplexer and the VHF path), and I installed SDR++ to visualize the antenna’s signal. It’s been very interesting to observe the impact of gain settings on the signal, as well as the different modes (linear, sensitive, and free). This helped me identify a problem in my setup: my LNA wasn’t working.

I tested the LNA by switching it off and on, but the signal’s baseline didn’t change at all.

So, I started by investigating the hardware and simplified the setup even further. I discovered that there was a surge arrester and a connector adapter between the antenna and the LNA. I removed both components and tried powering the LNA again — and it worked! I observed a small increase in the signal’s baseline (around 10 dB, depending on the frequency).

Then I did some research on my LNA. I couldn’t find the official datasheet, but I managed to gather the following information:

frequency range 430 - 440 MHz
noise factor 0,8 - 1,2 dB
gain ca. 22 dB, max. 26 dB
transmit-receive-switching PTT-controlled or HF-VOX
certified transmitting power

  • VOX-mode : 350 Watt SSB or 200 Watt FM
  • PTT-mode 500 Watt SSB or 250 Watt FM

sensitivity HF-VOX ca. 100 mW
insertion loss < 0,5 dB
current supply 13,5 Volt, ca. 200 mA
supply remote supply over coaxial cable or separate line
connection standard N - standard sockets

If I’m not mistaken, since the LNA operates in VOX mode, it means that a signal with sufficient power is required to trigger the amplification — is that correct?

If so, could it be that the LNA doesn’t activate simply because the UHF signals from the satellites I’m trying to observe are too weak to trigger it? That said, I do notice a drop or increase in the signal level when I switch the LNA on or off. This effect is even more noticeable when I set the gain to its maximum value. For instance, with LNA ON :

Capture d’écran_2025-05-07_10-25-22
Capture d’écran_2025-05-07_10-25-221757×494 116 KB

and with LNA OFF :

Capture d’écran_2025-05-07_10-25-59
Capture d’écran_2025-05-07_10-25-591756×538 198 KB

I’m also trying to target the satellites provided in your last link (to be able to compare results) and for some of them it seems that I’m starting to obtain some signal but it is still very weak :

waterfall_11548370_2025-05-06T22-12-23
waterfall_11548370_2025-05-06T22-12-23832×1603 70.6 KB

Again, thank you very much for your help and hope to read from you again !

Have a nice day !

9

Thanks for the update, sharing your analyses and great to see that things are moving the right way

Based on your information shared, I guess you using a SSB SP2000 (vhf) and or SP7000 (uhf) LNA, below the schematic. Also keep in mind that with these LNA’s there is also a bandpass filter inside the LNA that will influence the signals you can receive.

sp2000-7000
sp2000-70002276×1544 62.3 KB

No, VOX is a technology that switches the LNA from receive to send mode. So in normal RX mode the LNA should amplify the received signals, but when the transmitter is activated the RX path is bypassed and disabled.

Good luck and please keep us updated.

Jan | PE0SAT

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