KiwiSDR and noise floor.

I wonder what kind of noise floors people are getting with their installations when the antenna is detached and terminated with a 50 ohm load? Mine is around -115 dBm from 10 kHz to about 20 MHz rising to -110 dBm at 30 MHz.

Steve KD2OM

Comments

  • Accepted Answer
    Hi Steve,

    What mode and bandwidth are you using when you measure the noise floor ?

    It shouldn't change from when a load is connected directly to the KiWi and when connected at the far end of the coax in place of the antenna.

    If it does you have a problem, and probably need to add some local grounding at the antenna and some choke baluns along the coax.

    Regards,

    Martin - G8JNJ

     


  • Hi Martin, I am looking at the full range 0 to 30 MHz. You are correct, there should be no change or at least not much. Too much would suggest poor shielding of the coax or ground loops. I notice a small increase in the noise floor at higher frequencies and just wanted to see if others are seeing the same thing to determine if my installation needs improvement. I have been considering putting the receiver in a box, but if there won’t be an increase in sensitivity or decrease in ambient noise, I won’t bother.

    73
    Steve KD2OM
  • Accepted Answer
    Hi Steve,

    Mine's pretty flat from a few KHz up to 30MHz.

    The problems most folks have are 'hedgehogs' of spikes at regular intervals, usually peaking around specific frequencies. These are generally originating from specific bits of kit such as switched mode power supplies, Ethernet routers or other nearby interference sources.

    The KiWi does radiate some noise (especially around 144MHz) and also has some issues with Ethernet spurs around 23127KHz. The metal case may help reduce these further, but only if all the connecting cables are already free from noise.

    The easiest way to check is to make a small screened loop about 1" in diameter from some coax. Connect the inner and screen together at the end of the coax and connect this to the outer screen of the coax to form the loop. Half way around the loop cut the outer screen so that there is an electrical gap in it. Connect the other end of the coax to the KiWi RF input and set it to fully zoom out.

    You can now 'sniff' around various cables and other bits of kit to see where spurious signals are coming from and what frequencies they occupy. Adding ferrites and trying different screening and grounding arrangements should help you to gradually reduce the levels of the unwanted signals.

    Regards,

    Martin - G8JNJ


  • Accepted Answer
    emeraldsdr.ddns.net:8073/admin

    Check out the above link to my SDR, I think it's pretty quiet.

    The absolute greatest thing I did was to eliminate some filthy dirty switch mode power supplies such as the one powering the Kiwi, the Antenna is powered from the USB port.

    I replaced the KiWi PSU, Broadband Modem PSU and satellite receiver with Linear power supplies and the difference was tremendous. I also got rid of a few cheap LED light bulbs and replaced them with Philips, Iphone chargers are pretty filthy on the MW band and 49 Meters. I eliminated these and use Ipad chargers instead. 

    There is no regulation on the noise from these power supplies and it's been known that certain filtering components are eliminated from some Chinese made power supplies which send noise back down the mains wiring causing the mains wiring to act like a gigantic antenna emitting huge levels of QRM. 


  • I like your setup. I like the antenna as well.  I see the same thing as I do on mine, the noise floor rises as you go higher in frequency. I will start changing supplies, I hate switchers anyway.

    73
    Steve KD2OM

  • edited February 7 Accepted Answer
    Chasing the 60KHz spurs across the band on http://ka7u.no-ip.org:8073 and have installed shielded CAT6 network cables and a Netgear GS308 switch at a network jack. This dropped the 60KHz spurs by over 10dB across the band. The network jack I plugged the Netgear switch into is fed by standard CAT5 cable placed in grounded EMT conduit. The CAT5 cable is not grounded directly to this conduit however. and from there this cable is fed by another standard switch and then in an unshielded cable back to the router. I'm not sure what I will gain by replacing the cable all the way back to the router and upgrading the first switch to a Netgear GS308 switch, but I suspect it will reduce the spurs even more. Ferrite on the RG-6 coax to the active loop antenna also dropped the spurs by about 5 dB on average. So far my efforts have improved the interference on this KiwiSDR by about 15dB, which is quite an improvement. When I put the KiwiSDR in the metal case, I did not see much improvement as far as the 60 KHz spurs decreasing. Adding ferrite to the 5vdc power cable at the KiwiSDR did not seem to make a difference on interference reduction, so the power supply seems to be providing clean power as is.
    Ron
    KA7U
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