My installation and questions

If the signal is dropping to zero (@keithma) then I think it needs a better filter.

The RTL-SDR-LNA is unaffected, even with 50W on 2m very close to it. I’d expect a supposedly better (at least, significantly more expensive) product to cope without needing additional filtering.

One instance is near an SSTV repeater which transmits for a couple of minutes at a time and the other one is near a D-Star gateway. In both cases, the received number of aircraft drops to zero within ten seconds. They’re just getting overloaded.

/edit - you’ve seen the pictures of mine. I can squirt 50W through the X500 and it’s within a few metres of the feeder with no issues.

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Note that the RTL-SDR Blog LNA is triple filtered, not a single SAW filter. That was the genius. They designed a very good LNA, triple filtered, for less than the established competition.

I have corrected the antenna mounting position on the mast, and I took the opportunity to position it higher ;+)

Nest step the Airspy Mini who will replace the Nano3 !

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I added a ground wire on my antenna, just to prevent a lightning strike come inside.
I added a gas surge protector too.

I have installed my new Airspy Mini using this great tuto !
It works very good, big difference with my precedent key ;+)

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This morning showing my graphs, i see that with Airspy Mini I’m using more than 50% on CPU Power !
Is it normal, knowing i’m using the 12 MSPS decoding on a RPi3B+ ?

Perfectly normal.
If you want to use 20 MHz, get a Pi4.

dump1090 uses 2.4 MHz, so processing 12 MHz obviously needs more CPU.

Thanks !
I have many pi4, when i found the time i will replace my Pi3 ;+)

With my test and differents gains settings i obtain this maximum :

  • Uptronics Ceramic model powered by the AirSpy Mini : 1650 Messages / s
  • RTL-SDR LNA powered by the AirSpy Mini : 1100 Messages / s

Do really need a separated Bias-T for the RTL-SDR LNA ?
The Uptronic model will be better with an USB power supply instead the power supply of the AirSpy Mini ?

Near my location (30/50 Kms) i have many differents littles airports, with little trafic like Cesna 172 or Robin DR400, but at maximum i seen only 2 or 3 planes like this, is it beceause my gain is too high ?

Thanks.

The airspy will power the rtlsdr LNA - I ran it like that for a few weeks. You should be aware that the current draw of that LNA is higher than the stated maximum of 50mA for the airspy. It has been said though that the 50mA maximum is in order to fit within the USB power supply spec and that is will work OK. Personally, I use an external bias-t but it may well be fine to use the internal one. I wouldn’t want to recommend you do it long term unless prog appears and clarifies whether it’s likely to be a problem or not.

The uptronics LNA data sheet specifically says that it can be powered by the airspy’s internal bias-t. If you are supplying the 5v USB from the pi, then there’s probably not going to be any difference over using the dongle bias-t. Perhaps if you use a battery or high quality linear PSU you might see a difference because of lower noise, but I doubt it’s going to be significant enough to bother doing that.

I suggest that you try reducing the gain for the rtlsdr LNA - it has a higher gain than the uptronics LNA and although the airspy does handle strong signals much better than an rtl-sdr dongle, too much gain will reduce the message rate. Looks at other things as well however - number of aircraft and range are also useful metrics.

Thanks Caius for your reply !

I personally switched away from internal power for two reasons:

  1. Airspy mini is hot by itself. In my garage, summer temperatures are high enough to trigger Pi3 alerts, so short of active cooling that I hate, I am trying to minimize the temperature of Airspy. Passive coolers, diminishing the internal supply load…
  2. I have a very long coax cable, and I chosen to combat the voltage drop by raising slightly the voltage at the Bias-T injection point (7.5V).
    I have added an analog 5V voltage stabilizer in my preamp sealed box.

How do you guys actually power the Bias-tee? I am looking at getting one I just don’t know how to get the power connected to it… do you use some sort of DC power pack with the end chopped off or something? Also which Bias-tee are you all buying?

I’m using this one. Power is being supplied from the 5V rail on the pi.

Thanks for that, a quick look seems to show pins 2 and 4, I guess just 2 random bits of wire and job done then!

Pins 2 and 4 are both 5v - pins 4 and 6 are 5V and 0V. I used a 2 pin header socket to feed it.

This is how I powered LNA through Bias-T

My OrangepiPC died 2 months ago. I ordered from Aliexpress, a new OrangePiPC ($15) + power supply adaptor ($4). I then cut the barrel pin of old adapter and used it to feed the bias-t/lna. :slight_smile:

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Thanks folks. Looks simple enough! Sorry to hijack the thread somewhat

Today I received my new RTL-SDR LNA !
So I’m testing with him instead of the Uptronics LNA.

I already see that this new model, works better than the old one that seemed defective (it generated a carrier signal around 1080Mhz). I ordered a Bias Tee also (this one), because at the moment the RTL-SDR LNA is powered through the AirSpy.

I test with different gain settings, which gives for the moment :

  • Gain at 15 → -20 RSSI
  • Gain at 14 → -25 RSSI