Would it help to add an antenna side LNA with a bias Tee to gain range and then reduce the gain?
Hard to say how much improvement you will get.
The bias-t can be the dongle or at the dongle just to make that clear.
This is a very good combination:
Useful items for purchase · wiedehopf/adsb-wiki Wiki · GitHub
If you don’t mind enabling the bias-t in software, you can get the excellent rtl-sdr v3 instead of the smarTee.
Depending on the loss in your cable the external LNA could overwhelm the integrated LNA in the FA receivers.
So that’s a combination i wouldn’t really recommend.
If you add a good quality LNA, with better noise figure and inter-modulation than the one that you have in your dongle, then yes, you will have some benefit. Result also depends of the actual traffic around you. You will have to reduce the gain in your dongle.
A filtered premplifier (1090MHz) will be even more beneficial.
PS: LNA’s with 2dB or less noise factor are available today, examples:
https://www.qorvo.com/products/p/TQP3M9028
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That second one is pretty nice, numbers and price-wise.
The Uputronics filters use a Mini CircuitsPSA4-5043+ I believe, which has similar spec and lower current draw.
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Actually I had bite my own lure and purchased two of the SPF-5189Z chips (also pHEMT MMIC LNA), to replace my lower spec ones (TQP3M9008) in the amplifiers. It was $4.13 each, plus $0.25 tax and free shipping from their US distributors… Yes I know that on eBay they sell much cheaper from China, but I choose not to trust that. been burned before.
Need to take down the antenna and dust off the soldering tools.
I know that running a separate power line to an LNA is a hassle.
But I suggest that you do not use the bias tee from the dongle, rather, use a separate low noise linear regulator power supply for the LNA.
Power supply noise from the dongle SDR and the Raspberry Pi can and does degrade LNA performance.
I tested (1) with the LNA powered by the generic Pi 5.1 volt switching power supply and (2) with the LNA carefully isolated on its own low noise linear supply. The total aircraft count measured over a month improved about 3% with (2). No other system changes made during the two months of comparison.
I never found it a problem. I ran the RTLSDR v2 dongle (which has the bias-t enabled with a solder bridge) for a long time with the LNA4ALL, and more recently switched it for the rtlsdr.com preamp. It performed perfectly well with no obvious degradation from noise.
I also ran the rtlsdr.com amp with the airspy bias-t briefly, also with no problems, though I have since switched to a separate bias-t since the airspy’s built in one is only rated at 50mA. It did work OK though I’d rather keep it in spec for long term use.
If you are going to go to the trouble of setting up a nice quality linear power supply for the LNA, you might as well feed the pi and dongle with it as well which will give you less PSU noise overall. I’m not sure it’s worth the extra cost for a theoretical gain in this case however.
Red - Airspy mini gain 19 @ 20Mhz 2019-8-11 - 2019-8-16
Blue - Airspy mini gain 20 @ 20Mhz 2091-8-18 - 2019-8-23
Recently I found a way to put my FA antenna outside for 24/7 (using a small hole in a ventilation grid, so no drilling in walls). 
Here’s the difference(s). Messages & aircraft went up significantly (from 75 to 112), range… meh (avg. max. range went up though).
Red: antenna inside (behind glass & wood); gain 49.6
Blue: antenna outside (app. 2 meters further south); gain 42.1 / 40.2
Could you show the 2 week signal graph?
To be honest, i’d be guessing if one or the other was better.
That could just as well be the atmosphere changing 
But the higher gain definitely doesn’t hurt i think.
Red - Airspy mini gain 20 @ 20Mhz 2019-8-18 - 2019-8-23
Blue - Airspy mini gain 21 @ 20Mhz 2091-8-24 - 2019-8-31
Had a rain storm come in last night. Not sure if I have water intrusion again or not.
I made some tests today with several manual gain settings for some hours.
Wat i have realized:
Only very small difference between settings like 49.6, 42.1 and 40.2
It even looks like the typical deviation over the day. So almost no impact.to all values.
But there seem to be a big jump between 49.6 and -10 which switches the device to AGC mode
In this case the message rate overall drops by 10%, the message rate > -3db increases up to 30% and the aircraft seen also has a slight downtrend.
Looks like it’s a large step between the hightest manual value and -10/AGC
Just in the case somebody has made a similar experience
sure, but that’s 5.6 “over” the 49.6 and therefore a smaller gap as between 49.6 and 42.1/40.2 which i played around with.
So i would have expected a similar difference either one or the other way
That’s bad logic.
Messages stronger than a certain level can’t be received.
A good portion of message you receive are relatively strong, so those are no longer received when you increase the gain too much.
On the other hand the weakest messages you get might be just as well received at a gain or 49.6 or 40.1
Additionally quite few messages are that weak in the first place, so maybe not receiving some of them any more won’t be a big of a percentage.
But instead of seeing a reduction in message rate, you typically see a reduction in range first.
Because the range often depends on few relatively weak messages.
Still depending on your geography, reception an noise levels, there is often a range of gain settings which basically give the same reception.
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Good Explanation, now it makes sense to me
Thanks. I will leave it at the lower level of gain for several days and make a long term comparison.
I was just a bit confused on that huge difference between AGC and 49.6.
This is how it looks like on the six hour graph.
The area before was set to 49.6, the one afterwards to 42.1
And i think i do not need to explain what setting was used between 09:10 and 09:20
The upwards bump after the change was caused by a larger amount of aircraft at this time
FWIW. there are 3 gain stages: LNA, mixer, VGA.
Manual gain path: rtl-sdr/src/tuner_r82xx.c at master · osmocom/rtl-sdr · GitHub
(“set fixed VGA gain for now (16.3 dB)”; LNA and mixer gain are set to step 15, the maximum, if you specify max manual gain)
AGC path: rtl-sdr/src/tuner_r82xx.c at master · osmocom/rtl-sdr · GitHub
(“set fixed VGA gain for now (26.5 dB)”); LNA and mixer gain are controlled by AGC feedback and will tend towards the maximum setting AFAIK.
If you trust the comments (they are consistent with the gain table, but see below) then that means that the AGC path sets the VGA gain about 10.2dB higher.
The gain tables used are quite odd (notably the vga gain table is entirely unused and the apparent negative gain in the final mixer gain step, which makes no sense). I believe they were empirical (for one specific setup) rather than what the datasheet says.
I really need to revisit some work I did on the driver a long time ago to clean this up…
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