Reception problem nearby signals

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kovac
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Joined: Sun Oct 21, 2018 5:16 am

Reception problem nearby signals

Post by kovac » Sun Oct 21, 2018 6:03 am

First: I'm sorry for my English because it's not my native language.

Hello.

I made an air traffic control receiving station using an RTL2832u SDR; one 1/4 antenna; and SDR # v1.0.0.1.1666 for the purpose of making the signal available to the LIVEATC site. I live 6 miles from the airport and can receive PERFECTLY the Control Tower and aircraft flying at high levels up to 200 miles.

The problem begins when aircraft are flying low and close to my house, which is on the approach route to the airport. The SDR # shows the signal "exploding" in several waves and you do not hear anything at all from the aircraft, but I still hear the signal of the Control Tower perfectly.

The RF GAIN setting of the sdr # is -4dB. If I change the value from "RF GAIN" to + 17.9dB, I can hear the aircraft coming in at low altitude PERFECTLY, but now I do not hear the Control Tower and neither the aircraft flying high and very far.

I'm a beginner in these radio affairs. I wonder if there is a solution to this problem, as it is totally impractical to change the value of RF GAIN at all times.

Thank you.

rtlsdrblog
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Re: Reception problem nearby signals

Post by rtlsdrblog » Sun Oct 21, 2018 6:24 am

Yes unfortunately this is one of the pitfalls of low cost SDRs, they have trouble with very strong signals. You could try using AGC, but I don't think it will work very well. The answer might be to design a power limiter, or a custom narrow band AGC algorithm.

The other option is to purchase a more expensive SDR with higher ADC bit rate.

kovac
Posts: 5
Joined: Sun Oct 21, 2018 5:16 am

Re: Reception problem nearby signals

Post by kovac » Mon Oct 22, 2018 3:19 am

Hello rtlsdrblog. Thanks for listening.

Yes, I tried to activate TUNER AGC, but unfortunately it did not have a positive effect as you predicted

Could you suggest an SDR with better ADC bit rate value enough to withstand the high signals from nearby aircraft.

JEL
Posts: 98
Joined: Wed Feb 07, 2018 2:08 am

Re: Reception problem nearby signals

Post by JEL » Mon Oct 22, 2018 6:53 pm

Do you have "filter audio" (in the audio-tab) ON ?

It may not be related to your issue, but I sometimes run into similar problems when I have "filter audio" OFF.

The problem for me is that the carrier (from nearby aircraft) can get so strong it overloads and kills the audio. With "filter audio" ON, this carrier is filtered out so only the actual talk comes through.

Just a thought :)

kovac
Posts: 5
Joined: Sun Oct 21, 2018 5:16 am

Re: Reception problem nearby signals

Post by kovac » Tue Oct 23, 2018 1:58 am

Hello, JEL.

Yes, I have the "filter audio" ON. Even so, the signals from nearby aircraft are so strong that my SDR can not handle and kills the audio as well. And increasing the RF GAIN to 17dB I solve the problem of nearby aircraft, but I do not listen to the distant aircraft and neither the Control Tower . :(

Thanks
Last edited by kovac on Tue Oct 23, 2018 10:28 pm, edited 1 time in total.

JEL
Posts: 98
Joined: Wed Feb 07, 2018 2:08 am

Re: Reception problem nearby signals

Post by JEL » Tue Oct 23, 2018 5:56 pm

kovac wrote:
Tue Oct 23, 2018 1:58 am
Yes, I have the "filter audio" ON.
Ok, too bad it wasn't as simple as that.
kovac wrote:
Tue Oct 23, 2018 1:58 am
increasing the RF GAIN to 17dB I solve the problem of nearby aircraft
I can't say that makes any sense to me (that an increase in gain let's the too-strong signals come through) but sometimes the world of radio-signals is mysterious.

I hope you find a useful solution (How about an antenna that is more 'deaf' to signals coming from above it? I don't know if that's really feasible though. It would probably have to be too precise in how its lobes are distributed to be practically possible)

kovac
Posts: 5
Joined: Sun Oct 21, 2018 5:16 am

Re: Reception problem nearby signals

Post by kovac » Tue Oct 23, 2018 10:28 pm

JEL, do you think if I could get a more robust SDR I would solve this problem?

Strong signals from nearby aircraft elevate the SNR to 60dB, "killing" the audio.(Audible SNR is at most 35dB, here)

In GOOGLE research, I read about how to make a signal reducer with electrical resistive components, but I have no idea how to put it together.

I also would not know how to make my antenna more "deaf". It is made of 4 strands of 6mm copper with the length of 59cm.

Thanks

JEL
Posts: 98
Joined: Wed Feb 07, 2018 2:08 am

Re: Reception problem nearby signals

Post by JEL » Thu Oct 25, 2018 4:54 am

kovac wrote:
Tue Oct 23, 2018 10:28 pm
JEL, do you think if I could get a more robust SDR I would solve this problem?
rtlsdrblog knows more about this than I do.

I'm not sure. I have no issues receiving with either a 'cheap' SDR or the AirspyR2 that I mainly use.

I don't have any airports within listening-distance though, so I have no actual experience that quite matches with the situation you are in.

The closest aircraft I have are light aircraft occasionally crossing the city I live in.
The larger airliners are never really very low over my area (Never lower than a few thousand feet, and normally much higher)





kovac wrote:
Tue Oct 23, 2018 10:28 pm
I also would not know how to make my antenna more "deaf". It is made of 4 strands of 6mm copper with the length of 59cm.
Here is a page talking about antenna reception-patterns: https://www.telewave.com/2017/05/21/antennas/

Somebody with more knowledge on this may be able to say more.
I only know about the simple basics, so can't really say too much.
But the point is to get an antenna-setup that does not receive very well from where the aircraft are closest to you, and receives the best from where they are the farthest from you (It may not be practically attainable, but at least that is the basic idea)

On the page I link to, there is (about halfway down) a schematic for a collinear antenna.
It is this image: Image

It shows the antenna-pattern from its side (the zero to zero line is the ground. The upper half is the sky and the lower half is below ground). As you can see; this antenna receives better along the ground than straight above it.

If you can get something like this you may (I don't know if the effect will be strong enough to work in your case though) get at least part of the way to where you can hear without cutouts.

Maybe a ½-wave dipole could be a first experiment (it would take only 2 of your copper-elements to create one to test, but I don't know if it has enough of a null-pattern to make any real difference (theory is one thing, but it does not always translate well into the practical world))

JEL
Posts: 98
Joined: Wed Feb 07, 2018 2:08 am

Re: Reception problem nearby signals

Post by JEL » Thu Oct 25, 2018 5:32 am

One thing that just dawned on me:

The AGC tab... (the one below Audio tab)

Do you have "Use AGC" switched ON?

If not; then try that.

Set the threshold to something lower than the noise-floor (or just turn it as low as it goes while testing; -160)

Decay and slope values are not really important during testing.

This may bring down the audio-levels into the non-clipping range (and may stop the carrier from killing the audio if you're lucky)

kovac
Posts: 5
Joined: Sun Oct 21, 2018 5:16 am

Re: Reception problem nearby signals

Post by kovac » Tue Oct 30, 2018 2:01 pm

Hi, Jell.

I solved the problem by buying AirSpy.

Thanks a lot for the help.

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