Well, I already have these 4 pole fillters:
Custom made by a Dutch radio amateur for my main feeder
Older version for my test feeder.
Without one of these my LNA is as deaf as a post!
Older version for my test feeder.
Without one of these my LNA is as deaf as a post!
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Andy, youâre about to lose your streak 
Tried to set up 20 MHz on my Airspy Mini for testing.
The Airspy is connected to a Raspberry 4 using one of the highspeed USB ports
The MLAT status on FA went immediately to red saying that the data is unreliable.
This was independent from any -e setting.
After 30 Minutes of testing i switched back to 12 MHz as no benefits were noticable on Map or Graphs
Hi Keith,
Yes, Iâm afraid the house move is nearing its completion.
Internet turned off and the FlightAware Raspberry Pi uninstalled.
End of an era- Iâve enjoyed the whole FlightAware experience!
Looks like your site is still doing well though Keith- keep up the great work!
All the best,
Andy.
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i have a mini connected to a 3B+. i could never get it to run 20MHz when using the USB port. when i use wifi all works very well with 20MHz and MLAT works great.
Donât understand your comment.
What are you using the USB port for other than the Airspy?
That is my setup as well
USB used for the Airspy only, LNA powered by seperate power device, WiFi networks only
sorry, i was confusedâŚmy mistake
Not a problem at all.
As i did not see any improvement on my setup using the 20MHz at all, i switched back again without further root cause investigation.
I was thinking about the out-of-band interference problem for a while and I think I found a solution that improves the response of the preamp without adding any insertion loss. Hereâs a simulation of the said preamp which has no filtering whatsoever. There is a natural notch around the Tetra(pol) band which extends to the other strong interference sources (FM BC, GSM/Cell phone, etc.) The in-band IIP3 is around +10 dBm. For the out-of-band IIP3, add the rejection of the green curve.
More technical sauce for the RF nerds: The overall idea is instead of having the LNA amplifying the interference THEN filter the result, we make sure the interference is not amplified as much as the signal of interest using a negative feedback loop, then we rely on the linearity of the transconductance (amplification) stage to behave adequately.
Thoughts?
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I think the first stage of a LNB has to be direct and not filtered to preserve the SNR. After that the insertion loss of the filter doesnât affect that much the resulting SNR.
Am I thinking of the wrong thing?
In an ideal world with perfect electronics and infinite linearity, yes. Unfortunately, exposing an amplifier to a soup of large signals degrades the performance (inter-modulation products.) Restricting the set of signals being amplified reduces these intermod products.
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That âlarge signalâ needs to be defined. Whatâs the level that will create inter-modulation products on a modern LNA chip? The antenna is supposed to be tuned on 1090MHz so that will also provide some out-of-band reduction.
I think the people that fall into this category are so few that doesnât justify a special development for them. There are cavity filters that can solve their problem, or even filters like the one made by FA that can be placed in front of a problematic LNA.
Again I might be wrong, but I donât see here a bunch of people complaining that their LNA are âblindedâ.
You can safely ignore what you know about conventional techniques. To summarize: Weâre discussing a new type of preamps that give the best of both worlds: Sensitivity and Linearity. When using a filter you are always sacrificing one of them.
That âlarge signalâ needs to be defined.
I wonât enter into the IMD math here, but thatâs something radio designers know how to handle.
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After a recent deep dive testing a few different filters and LNAâs⌠Iâd buy what you are proposing.
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Yes, it sounds really nice if itâs working in practice. One thing Iâve learned with this hobby is that theory and real world results not always match each other and that itâs way more complicated than my understanding of the subject of signal processing.
But I would be happy to try out this new preamp once itâs ready 
When they match, you get a real world radio that works.
The preamp will not be for sale for a long time if ever. Itâs just a curiosity for me. The effort to over-specialize a device for a single band is never offset by the size of its market.
3 Likes
Thatâs true. Itâs a small market and probably hard to build a business case for mass production. But I suspect that there exists a group of hobbyists that are willing to build one themselves if they have schematics and a component list. Perhaps it can be a feasible business model selling the PCB and let the users do the rest of the work?
Anyway, whatever you come up with I hope that you share the results, itâs interesting to see whatâs possible when someone thinks outside the box and challenge current knowledge with a new approach.
Not really.
Some things are what they are, âchallengingâ settled experience can be just wasted time. Apples always fall towards the ground, Earth is round, 2+2=4, thermal noise is evil, insertion loss is a darn thing. Whitepaper:
Signal Chain Noise Figure Analysis (ti.com)
Formulas at page 8 are most relevant. One can see that, if noise factor of the first stage is constant, to improve SNR of the chain, the input signal has to be kept as high as possible, to the margins allowed by the respective first stage element.
Hence my previous question âhow high is too highâ? I think that in 99.99% of cases here the antenna signal in all RF band is lower that that max limit.
I see no need for a new design either, because there are on market passive filters, that one can install in front of an existing design filtered LNA and make tests, if feels like experimenting:
Amazon.com : 1090MHz filter
My experience: The RTL Blog 1090MHz LNA has added a high-pass filter in front of first stage to cut off the low band TV/Radio signals. It night be useful if you actually install the 1090MHz antenna near one of those commercial antennas, otherwise is just an extra expense. A cheaper LNA, that has no filter in front of the first stage, performs better for me.
Donât underestimate the human ingenuity. Things have been spinning for decades until new algos were developed and many âimpossibleâ things made feasible.
Iâm discussing with Carl of RTL-SDR Blog about industrializing this design into something non-tech people can use.
This is what I see with cavity filter - Wide-band LNA â Bias-T â LMR-400 â Bias-T â RTL-SDR
The first scan is with 10dB gain
The second scan is 30dB gain
The comb of bright-spots (real or not) are hidden at a lower gain.
Same setup without filter. RTL-SDR gain 30dB
I donât have the same with 10db gain because I didnât think of that at the time.
Therefore no filter is not an option.
I live in a rural location with poor to zero mobile reception and the nearest TV transmitter is more than 20 miles away.