Is this interference i see at 137MHz?

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Is this interference i see at 137MHz?

Post by LAH » Wed May 27, 2020 8:02 am

Hello everyone, I am relatively new to SDR and have learnt a lot from you all reading here so thanks to all who share their valuble experince, i really appreciate it. This time i'm stuck and really need some advice.

I moved my antenna yesterday roughly 5 meters away and 2 meters higher from its previous location for a better view of the sky. Now the antenna also sees west as good as east. The new location is wide open and i can receive signals as early as 2 degrees elevation and register an image by 4 or 6 degrees. However with the move came these annoying signals, which i believe are interference but can not pinpoint.

1. Erratic, unpredictable 10db signal, seems to be centered at 137MHz shifts around all the time. Can shift in to 137.100MHz and ruin satellite reception.
2. Series of many weaker signals (seemingly mirrored) about 330kHz apart all moving together (somehow independent from the one described above) could act erratic, static or drift. Also causes me to loose pictures. They are strongest around 137MHz but present 10MHz up and down the frequency range getting weaker as they get further away from 137MHz.
3. Noise floor flooding 10dB for no apparent reason. I usually have a noise floor around -55 to -60dB with 44.5dB gain (seem to have the best SNR around here) when flooding happens usually adds up to 10dB, can be constant, pulsate or wave in and out.

i attached some pictures but can't find one of the #1 problem, so the pictures are for #2 and 3.

My setup is as simple as it gets. RTL-SDR blog kit with a powered 10m USB cable. Bunny ear kit antenna configured as a horizontal V dipole tuned for 137MHz for weather satellites, goes into dongle, and powered USB cable brings it all the way to my desktop, plugged in a USB 3.0 port. I believe the additional current from USB 3.0 helped power both the active cable and dongle also the voltage loss along the way. The only addition i made to this setup due to the move was the 3m coax included with the kit, since the USB cable could not reach the new location. Oh also the dongle changed orientation, it was velcroed against the antenna mast, sitting vertical, now it simply lays on the roof (concrete) tiles almost horizontally and still doesn't see west.

My take so far:
I believe the signal is local and probably an interference of some sort. My structure is all concrete, with the closest neighbor located on the level below me. Apart from that the antenna is a good 25-30m away from any other structure. FM stations are about 40km's away. I have a large airport nearby but it's not too active these days. Maybe i'm gettin some radar stuff?

I tried shutting down all electrical equipment in my place one by one in an attempt to isolate the source with no success. One thing i noticed though, i managed to loose some spiking signals when i unplugged the TPlink network switch i have. It is behind a concrete wall and around 6m away from the antenna but can still be seen on the waterfall when operating.
I also tried playing with the way the extension coax lays and the dongle itself with no luck.

The weather turned bad yesterday, lots of wind and probably rain, so i packed everything for now and only had the chance to troubleshoot thus far. The new antenna location do not allow me to temper with it easily but i intend to play with the location when i get it up next time, i don't have much space there but i can shift it about half a meter to west and north. Its resting next to a chimeny now, i also plan to try it on top of the chimney, the flat part. I believe this would also affect reception since it will be the new "ground" clerance for the antenna. I am planning on using the included kit tripod with some weights to hold it down. Probably try an old brake disk also and see if it reflects anything or changes the behaviour.

Another thing i am considering is internal interference from the dongle. I know this thing is noisy by itself but could it be picking up these? Or does the kit coax work as good as the antenna picking up signals?

At this point im pretty sure it is not coming from within the house. It's either coming from the neighbors or is a strange signal bouncing around the city. Can anyone please help me identify what this is and what can i do to get rid of it? Also what would you suggest trying to troubleshoot this next time i get the antenna up there?

Wow didn't realise i wrote a whole essey here :shock: so if you've read so far, thank you for your patience :D
radio interference5.png
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radio interference.png
interference about to ruin my Meteor signal
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flood.png (277.67 KiB) Viewed 4353 times

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Re: Is this interference i see at 137MHz?

Post by rtlsdrblog » Fri May 29, 2020 8:08 am

Is there any solar panels near by? Definitely looks like some electrical interference and not something produced by the dongle. Perhaps you could try SDRTouch on your mobile, and walking around to find the source.

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Re: Is this interference i see at 137MHz?

Post by LAH » Mon Jun 01, 2020 5:58 pm

Thanks for the input! No solar panels around that i can see. I did a quick trial just now and managed to replicate the interference with the SDRTouch setup. Took some screenshots. Looks like the pattern is there and seems to not change as i move around on my terrace. The interference frequencies roughly match. I also managed to pick up both NOAA 15 and Meteor, experimenting with the antenna oriention to see how reception changes was interesting. I can see a Mobile setup can be really nice. I will do a second more elaborate run when i have the opportunity.

The weather turned wet again so i will not be able to setup my regular rig for a while.
Meanwhile i am planning to try a 15m powered USB cable run. This will allow me to hide most of the USB cable along a concrete rain gutter which i hope will attenuate any interference it might possibly be picking up. It will also provide some protection from the sun. The gutter is lined with aluminum foil from the roof insulation material, i don't think it will act as a shield but will be interesting to see the gutter effect if any and if the setup works at all with a 15m long active USB cable.

I also have the RTL-SDR blog FM trap and wideband LNA on order. Will see what changes with them on the line. I will post the results when they arrive, could be a while though.
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receivign Meteor
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receiving NOAA
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Re: Is this interference i see at 137MHz?

Post by LAH » Sun Jun 14, 2020 4:42 pm

Just received the 15 meter USB extension. It's the cheapest generic one I could find locally sold online. It's the semi transparent brownish gray one that is widely available on AliExpress At first glance it's thinner than the black one I had which got me worried a little at first but surprisingly it worked. However the picture I was presented when purchasing showed a ferrite core on it but the one I received doesn't have any. I was too lazy to return it so I attached 2 ferrite cores I had laying around at each end (my older cable had a smaller one at the computer end only). I also managed to make one turn around the cores since the cable was barely thin enough to allow this.

So far I didn't have the opportunity to try it in action but briefly compared the noise I received on both cables with the dongle with no antenna attached. I think this should give me an idea how much of the noise/interference is coming from the cable. The answer was, very little. The only thing I noticed is how different noise/interference patterns 2 different cables could have even at USB frequency.

I also found a seller offering a reasonably priced 137Mhz bandpass filter on AliExpress. actually I was in the process of building one myself but thought I might as well get a nice enclosure for a little more money than what it would cost me to build mine so I ordered one to try. I wish rtlsdr blog made one of those. I always appreciate their design and optimized cost effective products.

Decided to wait for all the toys to arrive before trying anything. Will report when I got my antenna back up.

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Re: Is this interference i see at 137MHz?

Post by JEL » Mon Jun 15, 2020 10:34 pm

When I used a powered USB-cable it flooded the receiver with noise which looked exactly like your images.

If you can; maybe try to re-arrange the cable-layout or see if you can use an un-powered extension between the powered cable and the receiver-dongle.

Anything you can do to create some distance between the receiver and the powered USB-cable really.

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Re: Is this interference i see at 137MHz?

Post by LAH » Tue Jun 16, 2020 5:45 pm

Good point, I thought about that in the beginning but quickly dismissed the idea thinking if that was the source i should have seen these at the old antenna location as well. Still worth giving it a try, that USB cable is the closest active thing to the receiver after all. I will pick up a passive patch cable this week, 30 to 50 cm should do the trick i guess.

Also now that you mentioned it, the cable layout could be a contributer here, it's much more exposed compared to the old route. Haven't tried the new cable though, i'm hoping routing through the concrete rain gutter will help attenuate any possible interference through the USB cable if any. I'm not sure how the ground/shield is wired in these cheap active cables. If the shield is sharing the USB ground then any noise it picks up might leak into the dongle? I remember reading somewhere about the dongle signal side ground was not properly isolated (older version dongle?) from the USB shield ground or something like that. I might be completely wrong though.

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Re: Is this interference i see at 137MHz?

Post by JEL » Wed Jun 17, 2020 2:06 am

Through time and experiments I have found that sometimes even small changes (Especially to antenna placement, but also to antenna-cable placement) can make a huge difference, so experimentation is key :)

I once had some issues with an antenna-cable that I had laid on the floor up against a wall, and I could lower a lot of some noise-spikes I got (Again, similar spikes to those in your image) by moving it slightly away from the wall (I assume electrical wiring inside the wall caused this noise)

Another time I had a similar issue with an active USB-cable. I ended up having to simply ditch that cable and exchange it with a passive USB-cable instead.

Anyway, one thing I found that has so far been true in all experiments I have gone through myself, has been that longer antenna-cable is always better than longer USB-cable (Regardless if passive or active)
Every single time.

Or rather, to be more accurate, the greater the distance between the antenna itself and the nearest noise-source, the better.

I found this holds true as long as the receiver is at least a few feet (Half a meter to a meter) away from the computer itself.
So I use short USB-cables between computer and receiver (I get a ton of noise if I plug them directly into the computer's USB-port), and then antenna-cables as long as necessary, and then place the antenna itself as far from any electrical devices (Anything electrically active really) as I can get it.

So far that has always given me the best SNR, so perhaps do a few tests yourself and see if you get similar results :)

TVs and LED-lights have so far been some of the worst noise-offenders I have come across.
But also things such as wireless blutooth-speakers and even the AppleTV-box need to be OFF when I'm using the receiver.
And of course nighttime is always much more quiet, in terms of radio-noise, than daytime :)

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Re: Is this interference i see at 137MHz?

Post by hotpaw2 » Wed Jun 17, 2020 2:56 pm

One additional trick that can help is to use ferrite beads or toroids. I prefer toroids, as multiple turns through the donut seem to work better than a string of multiple cores/beads on the cables. Try 1 or 2 meter passive USB cable, but loop several turns of the cable through a ferrite toroid, maybe on both ends if the cable is long enough. Then put another ferrite toroid with 7 or 8 turns on the antenna coax on the other side of the RTL-SDR. Both ferrites will help suppress computer RFI noise from traveling up the cables and coax to the antenna.

I also put big wrap-around ferrite cores around the power cords to the computers and network routers (etc.) to keep them from sending RF noise into the AC outlets.

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Re: Is this interference i see at 137MHz?

Post by LAH » Wed Jun 17, 2020 5:42 pm

Thanks for sharing the experince, radio feels like magic to me evey time i read stuff like these :) there are too many variables to consider and they're sometimes not immediately evident like in my case.

Longer coax, shorter USB is a little counterintuitive to me. My logic was to always get the signal digitized asap thus minimizing signal loss and/or corruption due to EMI through the coax. I might be underestimating the possibility of EMI leaking through USB cable. There's of course the active circuitry right next to the dongle (hopefully some distance between the dongle and the USB active end can improve the effects) and the USB cable itself picking up EMI and feeding it to the dongle throug the shield/ground. If this is the cause, I will have to find a safe way to isolate the cable and dongle grounds from each other.

As for the antenna placement, if your station is fixed like mine, it is the most limiting factor to experiment with. I really don't have many options with that. I got the antenna to the place with the best view of the sky and the horizon. At least I can get radio silence at my side pretty quickly, unplugging anyhting within a nearly 10m radius is easy and everything is behind at least 2 concrete barriers which should attenuate any offenders a little. but of course this is all in theory :)

The weather forecast shows a full week of thunderstoms and rain ahead for the last 10 days or so with very little to no actual rain. I've been keeping everything inside in anticipation of rain but I'm sure once i get my antenna up there it will start raining immediately :lol: i will wait a little more to begin experimenting. Hopefully my orders will arrive by then.

@hotpaw2 thanks for the tips, i will try adding toroids if the beads fail to change anything. At least it will be easier to wrap around them, the oversized snap-on ferrites i have now was not easy to wrap around.
I wonder if a ferrite core at the transformer cable would help with the router spikes? The router is the most annoying thing to power off even for a short period of time as it cuts all internet around the house causing havoc among the residents :lol:

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Re: Is this interference i see at 137MHz?

Post by JEL » Thu Jun 18, 2020 6:49 am

LAH wrote:
Wed Jun 17, 2020 5:42 pm
Thanks for sharing the experince, radio feels like magic to me evey time i read stuff like these :)
You're welcome :)

Yes, electro-magnetism is fascinating.
I like to think of it as being as chaotic as ripples (And their interactions and reflections) on the surface of a pond... a pond that is cubed (At least)
So avoiding the 'dead-zones' (Where waves cancel), when placing your antenna, can be a bit of a tricky puzzle (But also part of the fun :) )

LAH wrote:
Wed Jun 17, 2020 5:42 pm
Longer coax, shorter USB is a little counterintuitive to me. My logic was to always get the signal digitized asap thus minimizing signal loss
Yes, and that logic certainly is still true.

But in my case (Where max distance between antenna and receiver is always under 30 meters (100 feet)), it turned out there's a sweet-spot where SNR reaches its max, and that sweet-spot always has a much longer antenna-cable relative to the USB-cable.

Perhaps think of it this way;

You have a microphone and want to record a speech (Or maybe yourself singing? :) ) with the least amount of noise on the recording.

You can have a short microphone-cable and stand just next to your computer (Which you record on), or you can have a long mic-cable and stand some distance away from your computer.

Your computer has a loud and noisy fan.

In this example; even though a longer mic-cable may be less efficient than a shorter cable, the noise from the computer-fan is far worse.
So basically you trade lower efficiency for increased SNR (This obviously only works as long as you don't drop below the noise-floor of the receiver, but with cable-runs below 30 meters (100 feet) I don't think that will happen. I've used a super-cheap plastic dongle myself with cable-lengths like that, and the RTL-SDR dongle is better)

If you swapped your noisy computer for a silent tablet, then you could get away with the shorter mic-cable and have the best of all worlds.

As mentioned; if you can test this in your own setting, you should, as you may get different results than I did (I'm not saying my findings are universally true for all people, so one should always do one's own empirical testing)

LAH wrote:
Wed Jun 17, 2020 5:42 pm
As for the antenna placement, if your station is fixed like mine, it is the most limiting factor to experiment with.
I experiment with multiple antennas, but the placements that make a difference can sometimes be as little as less than half a meter (1 or 2 feet) either horizontally or vertically (And it's not always apparent why it makes the difference it can make, hence why I call it a puzzle :) )

(The higher the frequency in question; the smaller the positional change of the antenna can sometimes be, before it may result in a significant SNR change (Again; all this obviously only applies to my own equipment and situation and you may or may not get quite the same results. But I really think it is worth taking the time to do small tests whenever you have time and opportunity for it. Sometimes doing a test, that you think can't possibly have any real impact, can yield surprising results (And sometimes the results are as expected of course. But one never really knows until after the test, so in my opinion I'd always rather risk doing a test that does not achieve better SNR than risk not doing a test that achieves better SNR ;) As time and opportunity permits of course)))

LAH wrote:
Wed Jun 17, 2020 5:42 pm
I'm sure once i get my antenna up there it will start raining immediately :lol:
Yes, that seems to happen to me too all the time :lol:
That part rarely feels very magical :lol:

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