Reflected/Shifted VHF Signals

CircuitJunkie

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Jun 25, 2022
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Hello All. My name is Sal. I'm a new RR member and would like to thank the good peeps at RR for letting me in. The lunatics will surely take over the asylum.

That said, I've been doing a LOT of scanning lately, reading/watching everything I, as a former ham 30 years ago, didn't have access to, when I came across a (one of many) interesting phenomena I thought would be worthy of sharing and inquiring about here on the forums.

The mystery in question arose when I was manually scanning chunks of 3.2MHz spectrum, sitting patiently, watching each slow-moving waterfall, and moving to the next chunk of spectrum in turn, just for (sick) kicks. I happened to land on a NWS broadcast, and noticed that it wasn't the familiar frequency I thought I had memorized. So I dug a little deeper, trying to optimize my settings to get the audio to sound good, and quickly gave up and moved on to the next dribble on the screen. It was a brighter signal, and lo and behold, it was THE NWS broadcast again. It started, ran for a bit, then stopped as abruptly as it started. Was this just the same repeater transmitting on one of the other 6 channels of the NWS? I wondered. I let the screen fill with dribbles and then stopped the (SDR) radio to investigate.

The first (continuous) stream was at 163.06250. This second (discontinuous) chunk was at 163.56875, a delta of 506,250Hz. I'm not a radio engineer, but I wondered: Could it be an IF stage in my SDR, but then I thought no, that would be an odd IF.

I went back for another look at the waterfall and noticed that there were chunks of signal in other channels of the spectrum that were copies of the original continuous stream, but offset in frequency and varying in intensity, some chunks stronger, some weaker, sets of the same chunks appearing at the same time on different channels, but not consistently. So let's say if a reflection chunk happened on channel "A" it would happen at the same time as channel "B", but each channel would have a stronger or weaker signal than the original continuous signal. I didn't stream long enough to see all the possible permutations that could happen over a 3.2 Mhz block of spectrum, and it probably would help to show you a picture, so I'll attach one. I annotated the screenshot with horizontal marker lines on the waterfall where I'm seeing the "reflections." You'll note that these reflections vary in intensity (signal strength) and in character (Q? or coherence or acuity or precision). The signals in some channels are more or less defined than others, something caused by weather/topography/general propagation?

Here are some of the channels. "F" denotes full or continuous streams. The "A" sets of channels correlate with one another, as do the "B" sets of channels, as well as correlating to the "F" channels. Seems simple enough to be a harmonic or reflection at first glance but if you look closely, some signals stop and start in such a way that is either mechanically controlled or whatever natural phenomenon is causing that particular reflection is discontiguous.

163.06250F
163.51875A
163.56875A
164.06250B
164.08750B
164.09375B
164.13750B
164.18750F
164.63750A
165.25625A

In any case, I was hoping someone more experienced than I might be able to shed some light on this mystery. Can't wait to hear some answers/theories!

NWS Signal Reflections for Forum Post(web).png
 

kb5udf

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Unfortunately, you may encounter similar issues such as FM radio in the military aviation band and paging signals splattering across the VHF high, in addition to the NOAA radio you are seeing. These issues tend to worsen with higher power transmitters and of course your proximity to them.
I'm just a few miles from a WX radio station, broadcasting, I believe at 1kw and this produces many images.

Images in SDR receivers often worsen with increasing receiver gain, but of course lowering the gain can reduce ability to hear desired signals; a balancing act.
 

CircuitJunkie

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Jun 25, 2022
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Probably images generated inside your SDR hardware . . .
Yeah, probably so. What puzzles me though, is why these "images" are not consistent, but instead stop and start in chunks of time.
The radio was a NOOELEC NESDR. I just picked up an SDRPlay RSPdx, so I'm going to take a look at the same signal and report my findings.

Follow-up: There is absolutely nothing at those frequencies when I tune to them with SDRUno/RSPdx. Could my NOOElec have been that far off? I should check with the NOOElec again, but too busy enjoying the RSPdx at the moment. A HUGE difference (clarity and quality is perfect) and a pleasure to use, except for the buggy SDRUno software (or maybe something isn't configured correctly on my PC).
 
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N6ML

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Sep 26, 2008
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The SDRplay receivers are in a different league from the NESDR, which is just a slightly enhanced "RTL-SDR". Those were designed for use in PC-TV receivers (watch digital TV on your PC). To support that, they needed to cover a wide frequency range, which makes them convenient as a cheap general purpose SDR, but they expect a strong/solid TV signal. They do quite well for some other applications, in some environments. Prefiltering can help for some specific applications.

The downside of SDRplay is that "third party" software support is a bit lacking. I found SDRuno to be clunky and overly complicated. Depending on your application, https://www.sdr-radio.com/console might be worth checking out.
 

Ubbe

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If you in some way could look at a much wider spectrum +/-10MHz you would probably see the fundamental frequency of the transmitter that creates those intermittent mixing product signals. I made a video using a receiver that uses one of those $1 SDR receiver chip to show how poor its RF performance are and it receives multiple other frequencies than the one that the receiver are set to receive. On a spectrum it would show several images from that generated single frequency.


/Ubbe
 

dlwtrunked

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Your screen capture shows that your receiver is in overload from strong signals. With any receiver (not just SDR's), it is possible to set the gain too high if the receiver allows you to do that. You need to tell us what receiver and the gain setting (those are not in your screen capture).
 

CircuitJunkie

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If you in some way could look at a much wider spectrum...
Yep. Harm-onics looking for love in all the wrong places, hehe. I in all fairness, that could be your transmitter actually emitting those, especially if it is in close proximity to the receiver.
 

Ubbe

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Gain setting for RF LNA depends of the general local RF level. The more of stronger signals the less gain has to be used. If few transmitters and a narrow bandpass filter are in place then a much higher gain can be used until the SDR chip overloads. In a moderate populated RF band with medium signal strengths I use 36dB RF gain and for 1090Mhz ADB-S with a SAW filter I can use full gain, but at a high signal spectrum I use 30dB with a RTL-SDR. The SDR chip has a frequency tuned bandpass filter but are not very effective or narrow.

/Ubbe
 

merlin

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With all the low end SDRs, there is little or no IF filtering so you will get images with stronger signals.
Band pass filtering in front of the SDR helps a lot.
Adjust your IF gain for a 'sweet spot' with lowest noise and best signal.
Cheers
 

merlin

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Yeah, probably so. What puzzles me though, is why these "images" are not consistent, but instead stop and start in chunks of time.
The radio was a NOOELEC NESDR. I just picked up an SDRPlay RSPdx, so I'm going to take a look at the same signal and report my findings.

Follow-up: There is absolutely nothing at those frequencies when I tune to them with SDRUno/RSPdx. Could my NOOElec have been that far off? I should check with the NOOElec again, but too busy enjoying the RSPdx at the moment. A HUGE difference (clarity and quality is perfect) and a pleasure to use, except for the buggy SDRUno software (or maybe something isn't configured correctly on my PC).
Sounds like mixing images from LMR. I am literally next door to a repeater and when it keys up, I have hundreds of image peaks and noise. sometimes stronger than the desired signal.
 

Dirk_SDR

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Jan 3, 2022
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Germany
There are several "Nooelec NESDR SMArt" receivers.
If you have the V5 version, which is the last one and actually sells, then verify, in which mode you are: in "direct sampling mode" you can receive from 100 kHz to 25 MHz.
In "normal mode" the range goes from 25 to 1750 MHz.
If you look at frequencies above 25 MHz in direct sampling mode and your software doesn't switch automatically between the 2 modes, you'll see nonsense.
 
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