I have an unwanted signal around 436.750 Mhz, I suppose is a harmonic / spurious radiation, but I dont find the source. It is present on same frequency when I am using RTL V4 and HackRF One.
For example,
HackRF, Observation #10152129, 2024-09-01 00:03:51, AMP=14,LNA=24,VGA=48
and
RTL V4, Observation #10023735, 2024-08-12 13:45:29, gain: “19.7”
Does any body has an idea about the source of this signal ?
Cristian - YO4DFT
station 3387 - YO4DFT-Yagi to North-East
P.S.
I attach here a comparison of the spectrum received by the 2 SDRs, for 3 different frequencies (436.750 Mhz with main spurios, 437.050, less spurious, and 436.8358 Mhz, only CW signal). HackRF vs RTL V4.pdf (688.3 KB)
Good morning Daniel,
The setup is: 5 elements Yagi, LNA (SPF5189Z with dedicated linear power supply), home made UHF cavity filter centered on 436.5 Mhz.
The main problem is the cavity filter, is too narrow, so the reception at 435 and 438 Mhz is poor, so I dont think is a broadcast transmiter or a Tetra base station.
I am working now for a interdigital filter with a proper bandwidth and a better attenuation.
I shall try a test, to replace the antenna with a 50 ohm load, to see if the signal is still present.
Or, to check again around me, the laptop power supply, etc.
You can try scanning the RF spectrum and plotting it, or use a proper spectrum analyzer and look for strong signals.
Having small filtes on hand is pretty nice when analyzing these kinds of issues.
Even a simple triplexer with HF, VHF, UHF can be a simple tool to reduce the available bands.
In my experience, a SMPS can modulate other CW-like sources. Other sources are processors and their clocks, ethernet and buses especially unshielded. Many of these are unstable and drifts a lot, so any signal that is predictable is probably from a stable source, not a free running oscillator.
All my equipment (HF and SATNOGS) is located in my shack, a separate small building from the house. If I need 5v dc, I am using an old 13.8 V Yaesu linear power supply from the old FT107M and another linear stabilizer form 13.8 to 5 Vdc.
Nothing else inside, no LED lights, no TV, nothing. The Internet connection is wifi with the main router located in my house, 15 meters away.
Is a good idea to put at work an analyzer, I have a RF Explorer unit, I shall try to check with him.
