West Virginia University has recently installed a SatNOGS system using a G5500 rotator with VHF/UHF yagis. We are using an Airspy mini for reception with an FM block filter.
We have been experiencing intermittent interference during our observations an are unsure if it is a hardware issue, actual interference, or a software issue. Some observations with interference are shown below. Any input?
HERMES H5 401MHz
TLEVEL2-6 436MHz
Thank you,
W8CUL
Good evening and thanks for enabling a SatNOGS ground station.
I am running multiple airspy SDR device and you can compare this meta data with your settings
This is an AirSPY R2 running at 2.5MSPS and this is only possible with the R2 and not with the mini.
Further this system has a LNA and a 3 element Yagi pointed at Zenith.
Looking at your spectrum I get the impression that there are local strong UHF signals overloading the input.
Maybe you can use a SDR program like GQRX, SDR++ or similar to scan the local RF spectrum and try to find some strong sources.
Maybe you are also able to share some extra details on the hardware used, computer, usb connection, coax cable/length etc etc.
Jan | PE0SAT
Jan PE0SAT,
Thank you for the quick response!
We are running SatNOGS software on a Raspberry Pi 4 4GB with the airspy mini plugged directly in. There is approximately 100ft of RG-213 to M2 Antenna Systems VHF/UHF yagis. We realize this coax is suboptimal, but are still experiencing worse reception than expected with 10+ element RHCP yagis.
I suspected local interference from our UHF DMR repeater on 440MHz and tested disabling it, however the difference was inconclusive. I will try monitoring with SDR# for any interference. An example pass with the repeater disabled is linked below.
Hermes H6 401MHz
Thank you,
W8CUL
Hi,
as Jan said, use some live sdr sw (sdr#, sdr++, satdump, gqrx) to browse the spectrum, with very low gain, antennas pointing at zenith. if it’s showing up as the entire rx bw, either the gain is too high or the transmitter is too close/powerful.
if you have transmitters close to the band anywhere near this, it will mess it up pretty bad. narrow bandpass filters can help if it’s not too close, having to use notch or cavity filters sounds like a hassle for this.
for this kind of setup, I’d recommend some proper mast preamps.they usually have some sort of bandpass filter built in as well.
how are you combining the two antennas to the sdr ?
SA2KNG,
I will hook up to one of our omnidirectional antennas to monitor for UHF interference. Issues seem less pronounced on VHF.
Our station is located on top of an 11 story building in close proximity to several other radio systems. My test disabling the DMR repeater last week seemed to eliminate the regular interval interference in some of our receptions, but there were still other reception issues.
I have lightly researched designing a notch filter to reject our DMR repeater frequency (35W tx @ 440.6375MHz), as relocating the systems is not an option for us.
We have avoided preamps/LNAs so far because we are hoping to be able to switch the system offline occasionally to make sat contacts. Do you have any that you recommend?
The VHF/UHF antenna signals are being combined using a Comet CF-4160N duplexer.
Thank you,
W8CUL
I am suggesting the sort that has TX/RX capability (: for example these.
Unpowered they just bypass the signal straight through and can handle a few hundred watts or so. Also making them unfiltered if you want to listen way out of band.
These work very well with radios that have built in preamp control, like the IC-910 and FT-847 to name a few.
To use them with a SDR system you will need bias-tees to feed 12V from preferably a linear supply, at around 100-200mA (relays draws most of that).
Then combine with duplexer or coax relay etc.
The close proximity to transmitters sounds pretty bad, especially DMR that transmits periodically 24/7. Less than 10MHz at 70cm will be pretty hard, like SAW, possibly cavity filters (maybe the pass/notching ones for repeaters).
I would also consider power limiting before letting it through to the SDR. a 10el yagi point blank on a transmitting antenna could pick up a considerable amount of power.