I would think so, but then your “Auto” is pushing the gain only to 20. That means there is enough “signal” (3dB headroom). If it was at 21, I would think yeah, maybe it’s too low and I am maxed out at the gain.
So that’s where I am confused.
I might be totally wrong but it wouldn’t surprise me if a dedicated fixed gain LNA was just better than a variable gain amplifier (or whatever is directly on the input of the airspy).
Also again, the device at the other port of the splitter might be putting noise into the system that isn’t affecting itself.
Whatever the issue, using an LNA before the splitter might fix it.
If it does … you can argue then why it fixes it …
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Well… we’ll see:
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(Eats popcorn and waits with anticipation.)
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The N2+ that i picked up a week before this project made it totally unnecessary (LMAO) is very happy with the latest release
Great work on all these updates!!
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Very Smooth!
Now i need to scroll back a couple thousand posts to better understand reading the RSSI and Noise level graphs…
I have been testing most versions during the past weeks and my best results when it comes to far distance was with RC17 (around 3000ish aircrafts) . I didn’t upgrade until RC21 but then I immediately saw a difference (below 1000). However with the recent versions I saw great improvement in terms of # of planes and #number of messages but not the distance (400km+). Gain (auto) = 18 / 19, -e (auto) stay at 4 (i didn’t see any improvement changing it). PI4 overclocked 1750. Happy overall anyway ! good job guys
Jumping into the conversation on gains and Low Noise Amplifiers (LNA):
Assuming your antenna is one of the usual ADS-B antennas with 50 ohm impedance, using RG6 75 ohm cable creates about a 1/4 db loss, if your splitter is 75 ohm like most TV splitters, no loss except the 3.5 db loss of the splitter itself. Every transition of impedance adds a small loss to the signal. This is one problem I ran into when using a similar configuration. Because the antenna is at 50 ohms and both receivers are 50 ohms, the splitter and the intermediate cables should also have the same 50 ohm characteristics.
Any loss of signal strength before amplifiers is critical as the signal strength is weakened relative to the noise level of the entire system. That is why most of us put a LNA as close to the antenna as possible, so that the signal strength will be much higher than the associated noise level. Then the losses from splitters and cables are do not drop the signals we want before the noise. In a externally noisy environment, we can use cavity filters ( 50 ohm matched) before the LNA so the LNA is not swamped by the interference.
The gain on the Airspy and gain on the rtlsdr are quite different. Yes, both amplify the signals, but airspy gain is significantly less than the rtlsdr gain. That is why the LNA is placed before the airspy and its internal lower gain LNA.
Not sure if the above helps anyone, but when you want to get the low level signals just above ambient noise, amplifying the desired signals before downline noise sources can make a huge difference. My background includes ham radio, military and satellite communications systems, and associated hardware and software.
Highest Aircraft Count: 782 Impressive! That level of planes surly tasks these small computer receivers. Great location.
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That’s very good info, thank you 
I can compare some samples specifically on the range aspect.
Sonic has an LNA up the tree where his antenna hangs out (literally).
It’s just the 3 km (figuratively) of RG6 cause the level to drop.
Now if you then put a splitter, putting another LNA before the splitter can make sense in my mind.
So … this reading is intended as a sanity check, if it keeps increasing it means you’ve configured airspy_adsb in a way that causes it to keep aircraft on the whiteliste too long, so you’ll get (with high -e) somewhat regular noise decode of non-CRC messages that are passed on because of the whitelist.
LOL, it’s just 45 meters.
Calculators, for 1100 MHz, say that’s about 10.3db attenuation. Splitters, miss-match impedances, gas surge suppressor, all add probably 6-8dB of attenuation.
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Do the math. 22 steps x 3 dB = 66 dB of gain range in the Airspy.
Now check the gain range of thr RTL.
Nice to see how the new Airspy software is trying to cope with an antenna that has a breakdown due to water ingress and what happens when this antenna is replaced by a backup antenna with a lower gain.
Don’t they have the same RF front-end (tuner) R820T?
Are you assuming all humans are the same because they share 99% of DNA ?
More specifically, these devices don’t use the same driver. RTL uses a reverse engineered driver, Airspy uses a drivee built from the ground up to have the right gain distribution, power detection, stage powering, filtering and output level for the high speed ADC in the next stage. Actually, we contributed back some of that code to Rafael Micro for their next gen tuners like the R850.
There are other minor differences in the implementation, but it’s a matter of taste and electronics culture.
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wow
These all together - without an LNA at the antenna end - they are able to spirit away distant planes like a magician does with cards.
RTL dongle (V.3.) has a DR of 50 dB below 500 MHz, 40 dB up to 1 GHz and about 30 dB above 1 GHz.
In practice, it can be even worse.
DR is associated with the ADC, not with the RF part (the actual tuner chip). I think what’s relevant is 3.5 dB @ RF_IN
At least with the samples i’ve tested RC17 and RC30 perform basically the same in regards to distant positions.
1.err:range buckets (50 km): 234 442 492 675 450 230 201 142 78 0 0 0 0 0 0 0
2.err:range buckets (50 km): 234 442 492 676 450 229 201 142 78 0 0 0 0 0 0 0
1.err:range buckets (50 km): 395 394 161 84 6 4 0 0 0 0 0 0 0 0 0 0
2.err:range buckets (50 km): 395 394 161 85 6 4 0 0 0 0 0 0 0 0 0 0
But to run RC17 at the same -e 60 you needed a very fast computer
When i compare RC30 to 2.2-RC0 you can see a clear improvement across the ranges:
1.err:range buckets (50 km): 236 438 487 675 451 219 204 140 59 0 0 0 0 0 0 0
2.err:range buckets (50 km): 234 442 492 676 450 229 201 142 78 0 0 0 0 0 0 0
Edit: All those numbers are the positions depending on distance from receiver … split in buckets to show a distribution over the distance, then comparing the older version to the newer version.
@SoNic67
I’ll message you in regards to a sample but i’m fairly certain it’s not the decoder performance causing the issue you’re seeing.
Also … i took a peak at your station and the airspy is once again in the lead i believe.
Is the LNA before or after the splitter currently?
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I mentioned not the chip alone. The way how this kind of dongle uses harmonics for tuning causes a higher and higher noise floor within the mentioned freq ranges.
https://www.qsl.net/z33t/dynamic_range_eng.html
An “old”, analog receiver has DR, too.
Without an ADC.
SDR is a mixed solution with analog and digital parts. We are interested in a good “system-wide” DR to get the possible best signal processing.
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