Yes, memory is cheap, but cache is not. It seems that 16-bit (2 x 8 bit) seems to be borderline. Stuff above gets expensive aka the cycles for a lookup take longer than actually doing the math on a RPi (maybe not on a desktop CPU). For 24 bits, this will not work out. It is not about the amount of RAM. Getting stuff from there takes too long.
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I installed a new antenna and my collection numbers went down a lot - about 400-500 aircraft per day and my range decreased by 25-30 miles. I switched from a FlightAware 26 inch 5.5 dBi gain antenna to a Signalplus 45 inch 12 dBi gain antenna. I also have a LNA with 20 dBi gain after the coax followed by a FlightAware blue filter and finally the Airspy Mini. My question is: Could the new antenna be overdriving the Airspy Mini.
Here’s the relevant data from ‘airspy_adsb’:
GAIN= auto
OPTIONS= -v -t 90 -f 1 -w 5 -P 5 -C 85 -E 60 -p
SAMPLE_RATE= 20
Graphs (green line indicates when the new antenna was installed):
I think, “after the coax”, an LNA can amplify only the “remaining” signals… LNA is for preventing the losses of your coax. (amplifies the signal at the antenna end, compensating the losses on the cable - calculated by its lenght)
Other: In case of adsb, high gain antennas are mostly unnecessary. Using a 5.5 dB antenna, you can already achieve the calculated max distance (given by earth surface’s curvature), without experiencing holes and strange lobes in antenna pattern…
Antenna “gain” is not for free. Extra gain in a direction means the same losses from others. A plate-like antenna pattern means the risk of “bump” into the ground (lost of the farthest planes in a direction) by operating the high-gain antenna in a “not perfectly vertical position”. The flattened pattern can cause losses also from the planes above your head… (see: sidelobes of dipoles)
High gain antennas tend to shift their patterns’ lower or higher on the horizon - depending on the slight difference between the tuned resonance frequency and the freq of the received signal. The higher the gain - the bigger the shift…
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I would, in your specific case, use the fa filter before the LNA so you only amplify the 1090 MHZ signals. Your LNA is broadband and that might overload your reciever. Just my two cents 
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Thanks @janoskonya and @tomvdhorst . I would rather not change back to the old antenna - yet. It was a big job putting it up. But I will move the filter before the amp.
If you put the filter before the amplifier, you are adding not only the cable loss, but also the insertion loss of the filter before the signal reaches the amplifier input. The smallest signals will suffer even more loss, while nearby strong signals can still overdrive the receiver input.
In my opinion, you would get the best results with your previous antenna and an adsb (filtered) LNA right after it, additional filtering at the other end of the cable is possible but not absolutely necessary. (I use a cavity filter at the receiver end because it has less than 1 dB of insertion loss.)
Try to choose an LNA with built-in bias circuit - to feed it with DC voltage from the other end of the coax cable.
Antenna –> LNA –> coax –> Bias-T –> filter –> receiver
Note:
10m (30feet) average coax 3dB loss + filter 3-4dB loss = 1/4 of original signal strenght at the LNA input
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I have this SAW filter. If I add it to the mix, could I add it before the amp? Trying to avoid replacing the antenna unless I have to.
In my experience, I would pull the LNA out of line all together with the new antenna setup.
Here’s my setup now. Isn’t the FlightAware filter in front of the amp the equivalent of a filtered LNA? The SAW filter is shown FYI.
What we know about this filter:
SAW (Surface Acoustic Wave) filter technology uses the piezoelectric effect on a substrate (like Lithium Niobate) to convert electrical signals into mechanical waves, filter specific frequencies through constructive/destructive interference, and then convert them back to electrical signals.
SAW filter has a bit higher insertion loss than e.g. an FA blue (LC filter) but also a better 40-50 dB, somewhat steeper filtering can be achiavable on out of band ranges.
Short answer: I wouldn’t put it right before the amplifier.
Why is this not comparable to a filtered LNA? (not equivalent)
Because a filtered LNA takes care of filtering and amplifying in smaller steps, keeping the signal level optimal and noise level as low as possible in each stage. This is the way how the dynamic range of the LNA is preserved - and the amplification and filtering objective is reached in steps. So, you can not experience significant loss because of filtering, and also there is no gain stage overloaded with unnecessary high amplitude signals from out of band ranges. Filtered LNA is not just a filter and an LNA built in the same box…
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OK, thank you. (20 char)
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Yeah agree, filter a little and amplify a little and then repeat. The RTL-SDR triple filtered LNA was ideal. The topology of that device was high-pass filter first to filter out strong FM stations, followed by an amp stage followed by a bandpass filter, followed by another amp and filter. Unfortunately, my RTL-SDR triple filtered device gave up the ghost a few weeks ago, and you can’t buy a new one.
However, if you just have a separate filter and LNA, I would put the filter first. Granted, that will increase the noise figure by an amount equal to the insertion loss of the filter. But noise figure won’t usually be the limiting factor for performance in most cases. Noise figure will probably be the limiting factor only when receiving a distant aircraft and there are very few other aircraft causing interference. In other words at night.
Pretty simple to test for each installation if filtering before or after the LNA gives better results.
Those antennas aren’t that great as far as i know or at least there is big variation from example to example.
Lothar’s ADS-B Cavity Filter 1090 Mhz – Airplanes.Live
Great product. Good read on the website regarding losses.
I have 1 of these Cavity Filters made little difference after taking off the loss
(Lothar's ADS-B Cavity Filter 1090 Mhz – Airplanes.Live)
Here’s another one, or the same product with another label …saving a few coins.
I replaced the FlightAware filter with the SAW filter. A little bit of improvement, but I guess I really need to put back my FlightAware antenna.
Go one better than the Flightaware antenna. Vinnant CC1090/8-P
8Dbd/10Dbi GAIN
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The Vinnant antenna is back in stock. 3 remaining at this time.
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