iMONITOR
Silent Key
I was running Close Call and not scanning or anything else. It's supposed to pick up any signal within monitoring range isn't it? That as I understand it.
I sold my SDR recently.
I tried the attenuator, no difference.
BCT15X: SIGNAL TO NOISE RATIO on AM VHF/UHF AIR BANDS?
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I was running Close Call and not scanning or anything else. It's supposed to pick up any signal within monitoring range isn't it? That as I understand it.
I sold my SDR recently.
I tried the attenuator, no difference.
iMONITOR
Silent Key
Ubbe
Member
No, you are probably thinking of Whistlers signal stalker or spectrum sweeper. Close Call needs one single frequency to be some 20dB (100 times) stronger than any other signal to be able to detect it. And the signal needs to stay exactly in frequency for more than 1 second and not modulated with FM broadcast wide modulation.
A RTL-SDR are just $20 and are a necessary tool in many cases when trying to figure out interference problems and configurations of unknown systems and decode data using DSDplus and many other things like scanning 30MHz in one second and finding all active signals in the air that a scanner are too slow to do.
Everybody should have at least one SDR dongle. And a variable attenuator that can be adjusted between 0-20dB are extremly useful when testing antennas and finding out interference problems, if its a too strong signal or too weak or if its a receiver problem and so on.
/Ubbe
iMONITOR
Silent Key
Well I did have a SDRplay RSPdx running SDRuno software and would frequently scan the entire VHF AM aircraft band viewing the spectrum using a 22" vertical whip antenna and I did not have this problem. In fact I made mention of it in another post awhile back that it sounded like FM not AM with full quieting! Go figure? No static, no mystery signals.
iMONITOR
Silent Key
Tomorrow I'm going to swap out my BCD996P2 with the BCT15X and see if it makes any difference.
Ubbe
Member
Can you make some recordings of it? It's often possible to hear what's going on if someone else have experianced the same thing and found the cause. It would at least help to suggest how to proceed.
/Ubbe
iMONITOR
Silent Key
Not today, we're having rain and scattered thunderstorms. Maybe tomorrow.
Where I live TV channel 13 was causing my aircraft reception to tank. I made a notch filter (a coax stub) for 212 mHz and that fixed my airband problems. If your scanner shows any signal strength even when you are tuned to an inactive frequency, something like this could be a problem.
I found that channel 13 would not affect all scanners, but it affected an SDS200, 536, 996XT, and probably others. Didn't seeem to affect a 996T or 780XLT.
Am also using a PAR FM notch filter.
I found that channel 13 would not affect all scanners, but it affected an SDS200, 536, 996XT, and probably others. Didn't seeem to affect a 996T or 780XLT.
Am also using a PAR FM notch filter.
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iMONITOR
Silent Key
I don't think it's anything like that as the problem is not consistent. I'm using a Stridsberg FM-Notch filter. I've monitored the VHF air band on my SDR when I had it and never saw and heard anything other than aircraft traffic. The SDR didn't seem to ever have this problem, always very clear and no hiss along with it. The best way I can describe it is it just sometimes sounds like a really weak signal when according to the signal bars on the display are a solid 5. No signal bars ever show when not actually receiving aircraft transmissions. The audio sounds weak with a slight hiss along with it. No actual distortion.
See if your radio swap has any effect. Just because your SDR did not show any interference does not mean that an out of band strong signal could be causing a problem. The fact that the problem is intermittent just makes it much harder to diagnose.
iMONITOR
Silent Key
It's not intermittent in the sense that the noise will come and go while monitoring the same signal. With aircraft transmissions are very short normally. If I get someone that's a little chaty they sound the same throughout that conversation. I'm going on the pretence that if several different frequencies come in with 5 bars I would think they should sound similar in clairty and audio modulation. For instance when I monitor both the North and South towers at Selfridge Air National Guard Base, about ten miles from my as the crow files, their audio sounds the same most of the time and I don't experience this problem. Could it be related to abient noise in the cockpit? I wouldn't expect that as I know they use noise canceling microphones, sometimes throat mics.
Audio quality from aircraft varies considerably. Wide variation of radios, microphone technique, and other factors really affect the audio. I’ve heard planes that are so far off frequency that the voice is unintelligible, but the tower can understand them. I don’t monitor much milcom since they talk in such short clips that I can’t understand them anyway, even with a clear signal (unless it’s an emergency). I don’t think 5 bars of signal strength will ever guarantee good audio. The signal meters on scanners seem hypersensitive, so I don’t think they are a reliable indicator of signal strength.
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Ubbe
Member
The squelch will disable the signal strength indicators in Uniden scanners. Set the SQ to 0 and check a frequency in the lower, middle and upper VHF airband what the signal bars show. It's only the VHF airband that have this problem? I've never heard any type of overload or intermod issues with my BCT-15 scanners and I wouldn't expect the X model to be any worse. But the issue do seem to be intermod, that two transmissions mix and the AM signal then are reduced in strength and gets a much lower audio.
/Ubbe
iMONITOR
Silent Key
I was suspecting their microphones a few post back so it's possible.
As an example of what I am saying, I have a 2.5 watt airport ASOS transmitter about 4 miles away. On my BCD536, it shows 5 bars of signal, readable but noisy. My SDS200 shows an RSSI of -94 dBm, also with 5 bars of signal. My ICOM R8600 shows a -87 dBm RSSI, still noisy. My SDRPlay SDR Duo shows -92 dBm with a S/N ratio of 21 dB. All radios are fed from a Stridsberg active multicoupler. So how can a -94 dBm signal equate to 5 bars? You tell me.
iMONITOR
Silent Key
That could explain it. As I said inspite of showing 5 bars the sound is typical of a weak signal. That's why I was so puzzled.
Ubbe
Member
Because in FM mode it is a noise free signal. AM are a bit more complicated as there's no automatic noise reduction that the FM modulation provides and the receivers sensitivity has to be regulated by an AGC function to not overdrive the AM signal in any of the receiver stages.
But ideally, as AM lets you listen to the full noise level, the signal strength bars should automatically be compensated for in AM mode and show less signal bars. One thing that Whistler has are calibration of the signal bars, that you can set the level for each bar. I then can see the difference between a local transmitter and one far away when both have a noise free signal.
Uniden scanners always show full bars as soon as the noise are more or less gone. The dBm values are then a better signal strength indicator in SDS scanners, although it starts to break when you reach -60dBm and higher from signals several MHz away.
/Ubbe
Ubbe - that is not the case for the BCT15X. Out of interest, I set the squelch to 0 on my BCT15X and the signal strength meter continues to function. I tried an experiment in the Airband with various signal levels (one - five bars on the meter) and the meter still displayed the relative strength. Additionally, when I selected weaker AM signals from various aircraft, I watched the signal strength increase or decrease with the SQ on 0. I tried this experiment with a NFM signal (also in the VHF band) and the meter still showed the relative signal strength (five bars for the local EMS system).
Ubbe
Member
I explain badly. I meant to say just what you describe. As long as the squelch is open, and it will always be when set to 0, the signal strength bars will work, but as soon as the squelch close, it starts to activate, it will disable the signal strength bars.
/Ubbe
Ok got it. I misunderstood your post. Thanks for the feedback.
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