Frequency...deviation?

Iceman859

Newbie
Joined
Dec 8, 2022
Messages
3
Location
Kentucky
First off I would like to say Hi! I'm new here, and new to this radio thing in general so I have tons to learn. I recently got into SDR and have a setup that consists of an
RTL-SDR and a ground plane antenna and I was wondering what causes the deviation in the frequency of the stuff i'm picking up. To clarify and give examples. NOAA weather broadcasts on 7 frequencies and I pick up pretty much all of them, but also NOAA will show up somewhere it totally shouldn't be like the 2m Band I picked up a FM station in my area 101.5 which comes in crystal clear on that frequency and it also exists at 30.25 (Not crystal clear but still intelligible. If this is a Radio thing I would like to know more about it, maybe point me in the direction of the theory etc. If it's a hardware defect that would be nice to know as well since it will help me to not go chasing all the signals I'm receiving when they're all just duplicates of each other.

Thanks,

Ice
 

wgbecks

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Joined
Jan 17, 2005
Messages
903
Location
NE Wisconsin
Most of the RTL-SDR devices are actually very poor radio receivers having no tuned rf stages or filtering. In addition, the RF amplifier (LNA) and mixer stages are very prone to signal overload whereby they become nonlinear in the presence of stronger signals to the point that they'll self-generate a lot of signal artifacts and explains why you’re seeing/hearing NOAA weather signals down into the 2m ham band.


A good practice to get into is to learn to use less LNA gain and be able to recognize when your reach a point that further increases in gain do not improve the desire signal and only serve to raiser the noise floor, or where you reach a point where you begin to see the development of these unwanted spurious signals begin to appear in the FFT (spectrum) and waterfall displays.

In many cases, simply lowering LNA gain will usually suffice. However, there may be cases when spectrum adjacent to the desired signal(s)
will be very crowded with stronger signals and may require that you use an RF preselector ahead of the SDR to sharpen the response to that of the desired signal spectrum.
 

Iceman859

Newbie
Joined
Dec 8, 2022
Messages
3
Location
Kentucky
Thanks for your reply, I don't have an external LNA but I do use the auto gain control so maybe I should just disable that and tinker with my own settings and see what happens.


Thanks again,

Ice
 

dlwtrunked

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Dec 19, 2002
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2,109
As another post has noted, you probably have the gain set too high for that SDR--that will cause overload and intermodulation. Often people do that. Lower the gain until the erroneous reception disappears and if it does not, you will need a better SDR or filtering. I recommend a better SDR (Airspy of SDRPlay). (Also, note that "deviation" has a different meaning in the RF world which is not what you intended.) A better description for your other frequencies is "spurious reception". And I strongly recommend running with a filter to block the FM broadcast band as otherwise you are inviting trouble.
 

n5ims

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Joined
Jul 25, 2004
Messages
3,993
Often there's a radio link on a different frequency for the NOAA broadcasts that feed the signal that they broadcast. These frequencies are not made public but they are in use in many areas. They may be digital signals to allow the one feed to host multiple feeds on a signal link frequency but they may be feed in many ways, radio links, telephone links, or internet links.

On your other point, that 30.25 MHz feed from the FM broadcast station may be what is called IFB ( Interruptible foldback - Wikipedia ) that is a undelayed transmission of the station's main signal to allow remote DJs or some other feed that station broadcasts to hear what will be broadcast without any digital delay (typical the HD transmission has a 6 to 8 second delay by design to handle all of the analog to digital conversions and their analog signal is also delayed so when the signal is switched from "HD" to analog you don't skip or duplicate some of the transmission. This is especially important for programs that have the station inserts commercials so you don't get silence when they cut to commercial or return from a commercial break.
 

vagrant

ker-muhj-uhn
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Perhaps purchase the VHFSYM162 and the VHF-FM filters from this website. You may also want to read this page for an explanation. There are other FM broadcast filters out there as well. Check the URL in my signature.

I use those filters above and I am pleased with the result. My local NOAA transmitter works very well with its watt output and elevation, so the VHFSYM162 filter keeps things friendly. I can still easily receive my nearby NOAA transmitter even though the filter suppresses / attenuates the signal quite a bit.
 

Iceman859

Newbie
Joined
Dec 8, 2022
Messages
3
Location
Kentucky
Perhaps purchase the VHFSYM162 and the VHF-FM filters from this website. You may also want to read this page for an explanation. There are other FM broadcast filters out there as well. Check the URL in my signature.

I use those filters above and I am pleased with the result. My local NOAA transmitter works very well with its watt output and elevation, so the VHFSYM162 filter keeps things friendly. I can still easily receive my nearby NOAA transmitter even though the filter suppresses / attenuates the signal quite a bit.
I'll look into that, thanks
 

Ubbe

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Sep 8, 2006
Messages
8,944
Location
Stockholm, Sweden
Thanks for your reply, I don't have an external LNA but I do use the auto gain control so maybe I should just disable that and tinker with my own settings and see what happens.
Auto are a terrible setting and will usually get you into trouble and only works for monitoring FM broadcast and those type of signals. Disable both RF and IF auto gain and set RF gain manually to something like 35dB. You should look at the spectrum while tweaking the gain to see where it starts to produce a lot of unwanted frequency spikes and then reduce gain until they disappear.

/Ubbe
 

spongella

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Welcome to the group and to the SDR hobby. What you've experienced with hearing NOAA broadcasts on other frequencies is familiar to me. Generally they will all show up in a group, just like they do in the 162.4 - 162.55 segment. It can be confusing but you've been given excellent recommendations by the above commenters. Also, RTL-SDR dongles vary in quality. The RTL-SDR.com dongle is quite good in my experience. Have fun with your SDR.
 

nd5y

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Dec 19, 2002
Messages
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Location
Wichita Falls, TX
Often there's a radio link on a different frequency for the NOAA broadcasts that feed the signal that they broadcast.
Those won't be in an amateur band or other non-federal fequency range.
On your other point, that 30.25 MHz feed from the FM broadcast station may be what is called IFB
30.25 is a federal frequency in the US. It is highly unlikely somebody would be using it for IFB.
 

spongella

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You may also hear NOAA stations being rebroadcast on Emergency Alert/TIS stations in the 1600 - 1710 kHz band.
 
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