FlightAware Discussions

Do I Need A Filter?

How do the same scans look with some gain and autoscaled?

All these scans were done with Spektrum’s default gain setting of 50


That’s actually 5, which equates to 7̶.̶7̶ err 3.7. Use autoscale as well :slight_smile:

Edit: Use the mousewheel for easier adjustments assuming you’re running SV Mod.

Can you please explain further.

Since I was performing a comparison, a fixed and identical scale and gain setting for all 3 scans makes comparison more realistic.

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Sure. The gain scales from 0 to 496 (0 - 49.6) RTLSDR can only tune the following denominations:
0.0 0.9 1.4 2.7 3.7 7.7 8.7 12.5 14.4 15.7 16.6 19.7 20.7 22.9 25.4 28.0 29.7 32.8 33.8 36.4 37.2 38.6 40.2 42.1 43.4 43.9 44.5 48.0 49.6

If you are using v2+ (SV Mod, which it appears you are using), you can click in the gain box, use your mouse-wheel to fine tune. Default value on startup is 50, which is linear 5.0 and the radio automatically tunes to the nearest value from the above scale.

So far as auto-scale - that’s self explanatory - have it scale so you can actually see what’s going on, not flat-lined.

Rule of thumb (as your baseline, then dont touch again for the rest of the comparison) for this sort of test would be to take the base noise floor and gradually increase the gain until the noise floor increases maybe 6-9dB - click autoscale and you’re off to the races for better output.

Do scan #3 again, changing nothing but autoscale - what do you see?

Here is a sample shot I just took using an RTL-SDR v3 connected to the small PCB antenna, nothing else. My noise floor a 0 gain was about -53dB: Note I use solid and also check max so it keeps tabs on the highest value seen. Let her run for awhile to completely fill in: i ended up with gain of 19.7

EDIT #2, added FA light blue filter between the PCB antenna and radio, no other changes:

Swapped the FA Light Blue filter for a cavity filter, same settings:

EDIT One more time, I cranked to max gain, let it sweep for a couple minutes, then back the gain down again to show how the increased gain dramatically effects the SNR: This is the neat thing about running this tool and letting it plot max. Also note that increasing gain from 19.7 to 25.4 (SDR equivalent didn’t do much for our target 1090Mhz, but it increased surrounding noise quite a bit and this is without external amplification (FA Orange, Blue, etc)

Best results come after letting her run to collect max values for a time denoted by the spurs. Basically the same thing as the plots everyone uses, but much more graphical and clear in my opinion.

That spike at 960Mhz (also 480Mhz, 1440Mhz)…is USB 2.0 noise - even through the RF Choke that the RTL-SDR v3 radio has… Go figure.

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looking at the top two plots it would be hard to guess that 1090MHz was even there…your cavity filter is great!

My unit is a prototype of this, Bought from a fellow member who ordered a lot directly from the manufacturer - a couple years later, sysmocom popped up with them: http://shop.sysmocom.de/products/cf1090-kt30

Scans Re-done

Settings for all 3 scans:
Horizontal Scale =24 MHz ~ 1800 MHz
Vertical Scale = -53 dB ~ 0 dB
Gain = 25.5


Scan 1 of 3 - FA Antenna + ProStick (Orange) - No internal or external filter


Scan 2 of 3 - FA Antenna + ProStick Plus (Blue) - No external filter


Scan 3 of 3 - FA Antenna + ProStick Plus (Blue) + External FA Filter

I’m not sure if you caught anything I was attempting to explain? I hate forums sometimes, tough to convey.


I did catch something you tried to convey, but sure not everything, as my scans still look different from yours.

I have only recently started using this tool and used only twice. I think I will have to play with it for some more time to grasp what you have said in your post.

Ahh, OK. Same sort of deal if you were to explain how to use 4nec2 (antenna simulator) to me, zero time in the seat so to speak.

Sit there and punch away at the different controls - can’t hurt anything and this tool will probably end up being one of your most used as time goes on. Lots of nifty things you can do with it now as opposed to a few years ago. I think you will find it very valuable once you get comfortable with the bells and whistles.

Back to your images above. I cant believe the difference in the noise-floor between your FA Orange and Blue given the same gain. I fully realize the blue is filtered, but it’s almost like the amp has given up the ghost looking at the ~11dB attenuation (at least, and ~17dB at 1090Mhz) between the two. The SNR on the FA Orange sweep looks pretty gnarly and downright strange. I have both of those radios sitting here as well - I may run the same to see how they fare in my (very noisy) environment. I’d be interested in seeing another shot of the Orange without then with a filter in between the antenna, and not jump straight into testing the Blue since they are apples/oranges, so they sorta need to stay in their own lanes.

When I mention attenuation above, I mean that loosely, perhaps less gain would be more accurate in this case even with the same gain setting between the two? I don’t know, not sitting on your shoulder to see - you’ll get it sorted.

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Ok, I will scan orange stick with and without filter. I have two orange sticks, so I will scan both and post results, but not right now, within a day or two.

Which FA filter is it? The 978-1090 or the 1090 by itself?


The filter I used in scans is FA Light Blue which covers both 1090 & 978 MHz

I do not have FA Dark Blue which is 1090 MHz only

Just done set of my rtl dongles yellow, darkblue and smartee grr but no airspy
should i alter gain setting as i used 50 gain on all

I don’t know if it’s been noted on here, but I was browsing the rtl-sdr.com site earlier and saw that they have their filtered LNA back in stock. I’d thought it was discontinued, but they appear to have been able to source more of whatever it was they were missing.

MGA-13116 is the LNA chip they use and is EOL. I just received one from their new batch but havent had a chance to crack it open yet to see what they are using now, or if they found some old stock somewhere.

EDIT, well here we have it, just cracked it open and stuck it under the scope for a looksie:

I got a pair here always use them

I have purchased it last year, and found it performs very good.

A short while after I have purchased it, it was discontinued. Glad to know it is back again.

I have used it like this:

Antenna >> Triple-Filtered-LNA >> Bias-T >> Generic DVB-T Dongle.

Used a dedicated 5V dc power adaptor to power the LNA through Bias-T.

Amazon US just got them back in stock so I ordered a few. Been waiting a while.