BLUF: I have been battling what I thought was some kind of power interference problem for about a year. This "interference" is primarily in the VHF band, from about 120-155 Mhz. I do not live anywhere near any FM broadcast towers. Nearest cell tower is a few miles away. I do not have any battery backup / APC units nearby. The computer used for running SDR in the below pictures is a 2018 Dell i5 desktop mini-tower (not the micro station with the notorious power supply)
Hardware: DpDproductions Omni-X, roof mounted, cable is LMR-400 (50 feet) professionally terminated. Feeds two inline PAR filters - one for 88-108 FM broadcast and one for NOAA 162.55 mhz weather. (more on this later) which feed a Stridsberg multicoupler. Receivers are several Airspy SDRs, iCOM 8600, and 4 scanners. For the purposes of the pictures below, all have the same bandwidth, same Airspy SDR, same gain levels, etc. Nothing electrically is changed in between configurations.
this is what that setup looks like: (sorry for the dust) LMR400 comes in, goes into FM Broadcast filter then the NOAA filter, then into the Stridsberg and out to the radios.
This is a normal spectrum sweep 120-ish MHz, 8Mhz range, mid-way gain, etc. with the LMR-400 coming down, going directly into the two PAR filters and into the multi-coupler (as shown above). Note the "spikes" of noise about every 250khz or so. These do have modulation, "bzzzeeeP" and pulsate at about 1 hz or so. Yes, I see this same effect on the spectrum analyzer of the iCOM 8600 and can hear it on VHF "pulsing" on the scanners.
Here is the setup with the antenna cable disconnected and a 50ohm load in place of the antenna feedline. Zero noise floor, as expected.
now here is a picture of the setup running with the NOAA filter installed --- but the FM Trap REMOVED: (Note the "spikes" are gone but the noise floor did go up about 10db)
Some of you may go down the path of "hey, it's fixed - move on" --- well, it's not. For some reason, even though I am no where near any high power FM broadcast stations, I must use an FM filter for the SDRs ---- I have horrible amounts of intermod and bleed that occur in the military aviation band (specifically in the 280-320 Mhz range) ... so I absolutely MUST use an FM broadcast filter if I want to listen to mil-air.
Next you might say "Contact PAR Electronics" - already have. Dale is INCREDIBLY responsive to emails.... and we've been working on this most of the day... we don't have any ideas yet....
Next you are going to say that I should swap out the PAR FM filter trap. Tried that too.... I did a test where I replaced the PAR FM trap filter with the little crappy RTL-SDR FM trap filter... guess what? Got the same interference/mix/high noise floor symptoms.
or you might say "the multi-coupler is bad" - nope, took that out of the antenna feed path = no change. still have the noise floor problem.
The only common denominator here --- is "A" FM filter/trap (doesn't matter if it's the $100 PAR or the $18 RTL-SDR one) - being in the antenna feed path --- causes the noise floor to go crazy........ and I have no idea how a passive circuit like an FM trap could possibly be introducing stray RF into a SDR receiver front end.
open to ideas.
Hardware: DpDproductions Omni-X, roof mounted, cable is LMR-400 (50 feet) professionally terminated. Feeds two inline PAR filters - one for 88-108 FM broadcast and one for NOAA 162.55 mhz weather. (more on this later) which feed a Stridsberg multicoupler. Receivers are several Airspy SDRs, iCOM 8600, and 4 scanners. For the purposes of the pictures below, all have the same bandwidth, same Airspy SDR, same gain levels, etc. Nothing electrically is changed in between configurations.
this is what that setup looks like: (sorry for the dust) LMR400 comes in, goes into FM Broadcast filter then the NOAA filter, then into the Stridsberg and out to the radios.
This is a normal spectrum sweep 120-ish MHz, 8Mhz range, mid-way gain, etc. with the LMR-400 coming down, going directly into the two PAR filters and into the multi-coupler (as shown above). Note the "spikes" of noise about every 250khz or so. These do have modulation, "bzzzeeeP" and pulsate at about 1 hz or so. Yes, I see this same effect on the spectrum analyzer of the iCOM 8600 and can hear it on VHF "pulsing" on the scanners.
Here is the setup with the antenna cable disconnected and a 50ohm load in place of the antenna feedline. Zero noise floor, as expected.
now here is a picture of the setup running with the NOAA filter installed --- but the FM Trap REMOVED: (Note the "spikes" are gone but the noise floor did go up about 10db)
Some of you may go down the path of "hey, it's fixed - move on" --- well, it's not. For some reason, even though I am no where near any high power FM broadcast stations, I must use an FM filter for the SDRs ---- I have horrible amounts of intermod and bleed that occur in the military aviation band (specifically in the 280-320 Mhz range) ... so I absolutely MUST use an FM broadcast filter if I want to listen to mil-air.
Next you might say "Contact PAR Electronics" - already have. Dale is INCREDIBLY responsive to emails.... and we've been working on this most of the day... we don't have any ideas yet....
Next you are going to say that I should swap out the PAR FM filter trap. Tried that too.... I did a test where I replaced the PAR FM trap filter with the little crappy RTL-SDR FM trap filter... guess what? Got the same interference/mix/high noise floor symptoms.
or you might say "the multi-coupler is bad" - nope, took that out of the antenna feed path = no change. still have the noise floor problem.
The only common denominator here --- is "A" FM filter/trap (doesn't matter if it's the $100 PAR or the $18 RTL-SDR one) - being in the antenna feed path --- causes the noise floor to go crazy........ and I have no idea how a passive circuit like an FM trap could possibly be introducing stray RF into a SDR receiver front end.
open to ideas.
