FlightAware Discussions

Filter Advice

After feeding data for about 3 years I’m interested in spending a bit of time/$ upgrading my site. I’ve read through the Do I need a filter post and used Option 1 to scan. I’m in a US metropolitan area and have the blue ProStick Plus with the FA antenna mounted in the attic attached to a Pi 3+. I’ve uploaded to Dropbox the scan.png plus smaller snippets of three parts of the spectrum that seemed ‘busy’. The 850M-900M looks pretty busy to me (US cell carriers). There are signals in the 930-950 range as well (some GSM in the US or ?). And there is the expected cluster around 1090M but there is also a smattering of signals across 1060-1100M (I wonder what those are?).

Based on this it seems worth trying a filter. Does my conclusion make sense?

If so, what options are there in the US? I see the uputronics SAW mentioned quite a bit but they don’t seem to have a US store. Are there any good alternatives available in the US or are folks ordering these for overseas shipment to the US?

Here are the snippets in-line in case that is easier/better for viewing.


I would try the external FA dark blue filter. They are available in Europe only, that’s where I ordered mine.

But based on the scans the light blue, available in North America, may be enough.

As a baseline here is how my spectrum looked in the UK before and after adding the Uputronics SAW filtered preamp to the blue Pro Stick Plus.

UK GSM 900Mhz alongside 1090Mhz ADS-B – blue Pro Stick Plus only.

Same again with Uputronics SAW filtered preamp before the Pro Stick Plus

You can see what a nice job the Uputronics did of removing the GSM, and you can also see the effect of the ADS-B amplification and how they compare.

Since you have similar out-of-band signals, and more of them, I think you’ll benefit a lot from the Uputronics or a similar quality filtered preamp.

Their page is a bit out of date. Their delivery page says they use Royal Mail International Tracked but the link they provide there is out of date. The current link for that is here for the USA.

I’d drop Uputronics a mail and ask them to price it up. Others will likely have suggestions for other makes and models which may be easier to acquire and perform as well.


or my favorite: https://www.rtl-sdr.com/new-product-rtl-sdr-blog-1090-mhz-ads-b-lna/

But the uputronics is a bit easier to handle with it’s own power via USB.
The rtl-sdr LNA you need this:

Then you feed that with 5 V from the USB for example.

Mobile / Cell Phone Frequencies Close to 1090 MHz ES and 978 MHz UAT

Click on image to see larger size
Click again to see full size

Thanks for the replies so far. Below is a crude sketch of my current setup. The blue FA dongle is connected using a one piece cable (so SMA on one end but I don’t know the name of the large connector on the FA antenna end).

If I wanted to go with the upu-fp1090s does it need to connect into the antenna end of the cable or can it connect directly to the dongle? Whichever way is best, what sort of cable or adapters would be needed to hook it together? If using the upu is there any other component needed? I think from what you said I don’t need the bias tee component with this one?


The Uputronics sits between the antenna and the dongle. If you have a long run of coax (eg up a wall and the roof) then you want it at the antenna end for it to work effectively. If you have a short piece of coax then it doesn’t matter. It’s powered using either the USB mini connection (easiest with a short piece of coax like I think you have in your attic) or it can be powered using the Airspy bias-tee (this method is best when it needs to be near the antenna on the roof when you have long coax).

Here’s a photo of how mine is set up with no bias-tee.

The bottom of the photo shows the coax going to the FA antenna which is sitting nearby. That’s just a 1m piece of CLF200 low-loss coax with an N-type male connector on the antenna end (fits into the FA antenna’s N-type female socket) and a SMA male connector on the end you can see at the bottom of the photo (fits into the Uputronics’ SMA female connector which is the same type of connector the dongle has).

To connect the Uputronics SMA female to the dongle SMA female you need a SMA male-male coupler which you can see between them in the photo.

SMA male to male coupler

I’m powering the Uputronics with a 0.5m USB A male to mini USB B male cable. It’s plugged into the USB socket on the Pi for power. Keeping this cable short is a good idea for maximum power stability, but you may have a shortish one knocking about given how common they are.

USB connectors

If your reply was intended to suggest that the Pro Stick Plus was able to remove the GSM 900 by itself without the Uputronics, @abcd567, please refer back to the images in the post you replied to where you can see the effect the Uputronics had. If you meant something else, I missed it, sorry!

Thanks for taking to respond in details!

Yes, as you surmised I have a short run. The cable from the dongle to the antenna is no more than 2’. The Pi is plugged into an outlet in the attic, the dongle directly into the Pi and the antenna is mounted vertically to part of the roof support structure (wood). I’m sure I have a mini USB cable but I’ll check if I have a short one (most of mine are micro these days).

I’m going to look for parts sources tomorrow.

You’re welcome! I use The Pi Hut for all these parts and it looks like they do ship to the US. I’ve ordered these parts twice from them without problems, although I’m in the UK so it’s a bit simpler since they’re over here too. If you can find a US supplier who also has these parts then that may be easier. For reference the parts on there are the Uputronics preamp, the coupler and the USB cable if needed.

Edited to add: If you’re powering the Uputronics from the Pi’s USB ports, which is the neatest way when they’re close together like that, make sure you’re using a decent PSU for the Pi, ideally the official Pi PSU. If you’re using a lower power or lower quality PSU you’ll probably have problems with the dongle or the preamp intermittently losing power.

That’s where I bought the dark blue filter:

For the pre-amp/filter combo, the RTL-SDR Blog one is a better value IMHO.

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It’s definitely worth looking into, I’ve heard good reviews of it too.

My reply had nothing to do with uptronics LNA

The RF scan graphs highlight that ProStick Plus (blue) could not remove GSM900, but RTL-SDR Tripple Filtered LNA did.

I see, yes I see the same result with the Uputronics. Any decent filtered preamp should help cut out unwanted nearby signals. The Pro Stick Plus does a pretty good job more generally and especially for its price.

I dont have uptronics LNA, otherwise I will run it’s RF scan by noise -source, like I have done for rtl-sdr tripple filtered lna.

There’s a response graph in the datasheet for that model.

I ordered the parts I need via AirSpy US store electing to go with the uputronics filter. Thanks for all the advice. Now I’m impatiently awaiting the arrival. :grinning:

Nice one, it’ll be interesting to run those scans again and see how much the GSM and other unwanted stuff is reduced. You may need to reduce the gain from max to somewhere in the mid-to-high 40’s to get the best from the preamp, but you can play with that later on. Here’s hoping it arrives quickly!

I received the parts on Saturday and had time to install the additions this morning. I ran the ‘standard’ frequency scan from this thread. For reference it uses a gain of 29.7. Here are the main results.


You can see the filter/amplifier is working but the gain of 29.7 used for this scan test is probably too high. Before running FA I checked the strong message percent which was 48% at a gain of 49.6, up from 4% prior to the change. I immediately lowered the gain to 38.6 which still had a strong message rate of 24%. I’m letting it run at this gain for awhile to gather a bit of a baseline. Then I plan to lower it for further testing. The weather this week won’t have idea flying conditions so a final assessment on gain and effectiveness will take some time.