Airband background noise

[xantegh]

Hi,

I'm noticing that most of my recording of aircraft communications have some background noise, not just a usual AM white noise. can you help me identify what is it and how I can possibly eliminate. attached for your reference a sample recording.

System:

Antenna: Diamond Discone D3000N
Multicoupler: Stridsberg MCA204M/N
Receiver: NooElec NESDR SMArTee
Software: RTL SDR Airband (szpajder/RTLSDR-Airband) - i set the gain to a middle value of 22.9 since I live 30-40 miles of SEATAC .


--
Regards,
K7CSA

 

[Markb]

Listening to your recording, the noise isn't consistent. Sounds to me like some of the aircraft are transmitting slightly off-frequency. The varying pitch of the noise likely correlates to how far off they are. If you look at a waterfall of AM aircraft transmissions, you will see in quite a few cases that the signals fluctuate a bit off center.
I haven't used RTLSDR-airband, but if there's a filter setting you could try widening it a little. You might lose some volume, but it might help dampen the noise.

That's my best guess...

 

[RFI-EMI-GUY]

Listening to your recording, the noise isn't consistent. Sounds to me like some of the aircraft are transmitting slightly off-frequency. The varying pitch of the noise likely correlates to how far off they are. If you look at a waterfall of AM aircraft transmissions, you will see in quite a few cases that the signals fluctuate a bit off center.
I haven't used RTLSDR-airband, but if there's a filter setting you could try widening it a little. You might lose some volume, but it might help dampen the noise.

That's my best guess...

Doppler shift.

 

[Markb]

What he said....

 

[Ubbe]

Doppler shift on an amplitud modulated carrier that produce an interfering tone? Airpilots and ATC staff would go crazy if they had to listen to that tone.

You are using a software that produce several simultaneous audio channels from different frequencies. I would think that some sort of sampling are in use that jumps between frequencies and it is the sampling frequency that can be heard. If you can disable multichannel and only listen to one frequency you probably will not hear that interfering tone, or just confirm with another program.

/Ubbe

 

[RFI-EMI-GUY]

Doppler shift on an amplitud modulated carrier that produce an interfering tone? Airpilots and ATC staff would go crazy if they had to listen to that tone.

You are using a software that produce several simultaneous audio channels from different frequencies. I would think that some sort of sampling are in use that jumps between frequencies and it is the sampling frequency that can be heard. If you can disable multichannel and only listen to one frequency you probably will not hear that interfering tone, or just confirm with another program.

/Ubbe

The carrier of a subsonic aircraft at 122 MHz might vary from 10 to 130 Hz depending upon velocity and cosign angle. If beating against the carrier of another aircraft, the doppler might add or subtract as received by the observer. Not a lot of offset, but observable.

 

[ab3a]

Hi,

I'm noticing that most of my recording of aircraft communications have some background noise, not just a usual AM white noise. can you help me identify what is it and how I can possibly eliminate. attached for your reference a sample recording.

System:

Antenna: Diamond Discone D3000N
Multicoupler: Stridsberg MCA204M/N
Receiver: NooElec NESDR SMArTee
Software: RTL SDR Airband (szpajder/RTLSDR-Airband) - i set the gain to a middle value of 22.9 since I live 30-40 miles of SEATAC .

The RTL-SDR dynamic range is okay for what it is. However, in comparison to most scanning receivers, never mind an aviation radio, it stinks. The noises you hear are likely the result of Intermodulation Distortion (IMD). What you're hearing sounds like IMD to me. I say this as someone who has an instrument rating and as a degree in electrical engineering. You may be able to reduce the IMD by building or purchasing a narrowband filter that covers only the airband from 118 to 136 MHz.

I suggest using a better receiver if you're interested in this band exclusively.

As an aside, background noises such as 400 Hz whine are commonplace in jet aircraft. It's the generator speed on the turbine. It's no different than hearing the alternator whine in someone's car.

 

[N4GIX]

Oddly enough, that same 'whine' is also caused by the inverters for 24 vDC to 110 vAC.

 

[dlwtrunked]

Doppler shift.

One does not hear Doppler when AM monitoring an AM signal -- even if it is there. One hears noise change but not a tone. Questions that should be asked are: Is his receiver in AM mode? What software is he using (HDSDR or SDR# or ?)? and did he try any other?

 

[Ubbe]

The carrier of a subsonic aircraft at 122 MHz might vary from 10 to 130 Hz depending upon velocity and cosign angle. If beating against the carrier of another aircraft, the doppler might add or subtract as received by the observer. Not a lot of offset, but observable.

The examle have tones of several KHz and why would others aircraft transmit at the exact same time during the whole transmission?
No, it is something completly different that are not heterodyn frequencies and I've never heard anything remotly like in the samples and have not heard anyone else mentioned such a phenomen. Only parameter that are different from all other receivers are the special software used that receives several frequencies at the same time and produce several simultanious audio sources, one for each frequency.

/Ubbe

 

[RFI-EMI-GUY]

The examle have tones of several KHz and why would others aircraft transmit at the exact same time during the whole transmission?
No, it is something completly different that are not heterodyn frequencies and I've never heard anything remotly like in the samples and have not heard anyone else mentioned such a phenomen. Only parameter that are different from all other receivers are the special software used that receives several frequencies at the same time and produce several simultanious audio sources, one for each frequency.

/Ubbe

Listen again. You will not hear intermodulation nor 400 Hz tones. I am hearing a beat note like you would hear when two radios transmit at same time. The beat note is the difference between the carrier frequencies of each radio (oscillator error +/- doppler shift). The last transmission had no beat tone if I recall. The beat tone could be another pilot transmitting, or it could be some spurious carrier. Remember that AM is used by aircraft because the beat tone is easily heard by ATC and they can tell if two units are doubling becuase there is no capture effect as in FM. Listen again.

 

[majoco]

]quote]
I suggest using a better receiver if you're interested in this band exclusively.[/quote]

You could try reducing the RF gain - you don't need to hear all the background noise, jusr enough gain to make the voice intelligable.

 

[majoco]

I had another listen - IMHO I would say that you have a 'birdie' on that frequency (127.05MHz ?) - have you got a handheld receiver that you can listen to that frequency and find if there is a birdie coming from any receiver near you or look at it with another SDR with a bit of wire draped around the receiver for an antenna? SDR's aren't noted for the cleanliness of their oscillators and there's also all the cr@p coming out of your computer. Every aircraft that talks has a slight difference in beat frequency but not enough off-frequency to put him out of the bandwidth. It's not two aircraft transmitting at once - often the beat note between the two changes for a second after the PTT is pressed making a sort of "yeeeooooow" sound. Doppler shift? Never happen - not enough Hz difference - that's why the cops speed radar is way up in the SHF radar frquencies - there probably is a doppler change in frequency for an aircraft flying towards or away but it will be miniscule. There is the possibility that it could be a distant transmitter constantly radiating on that frequency or a harmonic but I'm sure the pilots would have complained about that long ago if that was the case.

 

[RFI-EMI-GUY]

I listened again. It could be a birdie. It starts out high in pitch and works its way to a few hundred cycles and then tens of cycles. On the second round it does same thing. When the signal is strong, there is no note.

 

[VK3RX]

After 36 years in ATC I can say I haven't heard that from aircraft.

At times, 400Hz whine, background noise, and a loud heterodyne when someone else steps on someone, yes.

I also suspect it is something to do with the SDR dongle & PC combination. Try listening at the same time using another receiver e.g. a scanner.

 

[chief21]

The RTL-SDR dynamic range is okay for what it is. However, in comparison to most scanning receivers, never mind an aviation radio, it stinks. The noises you hear are likely the result of Intermodulation Distortion (IMD). What you're hearing sounds like IMD to me. I say this as someone who has an instrument rating and as a degree in electrical engineering. You may be able to reduce the IMD by building or purchasing a narrowband filter that covers only the airband from 118 to 136 MHz.

I suggest using a better receiver if you're interested in this band exclusively.

I agree with the general IMD diagnosis. For my money, however, this sounds to me like the IMD is coming from a local non-intentional radiator... perhaps a nearby radio or similar device. The interfering signal may not be strong enough to break squelch on its own, but when a stronger signal is received, the combination of the two signals creates the noise that you're hearing. Turn off anything in the house that might create radio noise, and see if the noise goes away.

 

[majoco]

You may be able to reduce the IMD by building or purchasing a narrowband filter that covers only the airband from 118 to 136 MHz.

Trouble is that the interfering signal is in the airband, only a few Hz away from the nominal centre of the channel so a airband-only bandpass filter won't stop it. Like chief21 says, the interfering signal on it's own won't break the squelch but you should be able to see it on your spectrum or waterfall, you can't hear it as there's nothing to hear, just an unmodulated carrier. You won't hear it until an aircraft transmits to provide the 'beating' signal - the fact that the tone heard is different for each aircraft is of no consequence - you cannot set up any transmitter to be spot on frequency for even a short period of time. Every crystal oscillator will drift, even the best ones in an oven. Believe me, I was a aircraft equipment bench tech for a long time! I just dug back into a couple of manuals, the 1970's Collins 618M-1 Comms transmitter tolerance is +/- 0.004% which is 5kHz at 125MHz. Admittedly that is/was an old item but it's indicative of how far out you can be.

 

[RFI-EMI-GUY]

I think what you all are describing is actually not Intermodulation Distortion (IMD) but simple co channel (within the channel) interference from either another transmitter or some internal or external spurious signal. IMD is a wholly different phenomenon.

 

[xantegh]

Thank you All for your replies. After simultaneous listening to a BCT15X through a noise canceling headphones while the SDR on a raspberry pi is recording the same reception, I tend to believe that it is more related to the SDR itself and the way it is picking up noise. I have noticed as well that for closer aircraft (stronger signal) the whining noise is still there but in lower volume.. versus a pretty much clean reception on my uniden besides the regular AM static.

note: although i do own a couple of BCT15X and a couple of BC125AT, i tend to free them up for range scanning instead, and I thought of using SDRs to monitor and record specific frequencies. ( it make sense from price perspective, in my opinion. 30$ SDR versus the cheapest Uniden is for 120$)