FlightAware Discussions

Do I Need A Filter?

I wrapped the orange FA ProStick with aluminum foil (about 6 layers on each side) made sure the foil was grounded to the 50 Ω terminator. And then placed it all in a heavy metal box (this box was not fully closed due to a 3 foot USB extension.)

Here is the result but with the gain set to 20. So I’m guessing the 960 MHz signal is more related to the orange FA ProStick then an outside signal.

rtl_power -d 00000978 -f 800M:1200M:100k -i 30 -c 50% -e 30m -g 20 -F 9 > scan.csv

Anthony wrote back. Uputronics will test the 978MHz UAT Filtered Preamp with a spectrum analyzer in the next two weeks. He included this message:

Please find attached datasheet. The SAW is fairly tight on this unit I don’t have the figure for -25dB but the SAW attenuation @ 950Mhz is -58dB. I don’t have one to test with at the moment as we haven’t traditionally sold that many but if you want an exact figure I’ll make one up and get it on the SA.

Unit Datasheet & SAW Datasheet attached for your reference.

Cheers, Anthony

Anthony also sent two PDFs that I cannot attach to the post. This is one of the PDFs:

I’ll see if I can locate the other online.

EDIT: added SAW filter PDF. This is not the exact document Anthony sent, but it is very, very similar:

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Ace, thanks for posting the update, handy as a reference for future travellers!

You can get one of these cavity filters off eBay and tune it to 978Mhz (or pay to have it tuned when you order):
High Power Low Loss UHF 900 MHz ISM BandPass Cavity Filter Repeater NEW

Here’s how mine is tuned:


I bought several of these filters from this vendor and tried retuning them. Two of 6 tuned to 978 MHz. Four wouldn’t tune that high until I took them apart and used a metal lathe to slightly shorten the resonator lengths.

I’m using one of these in my UAT FA receiver. It has very good selectivity and just under 1 dB insertion loss.

I also built a custom designed 978 MHz 3 resonator cavity filter. That filter was only slightly better in loss and selectivity than the repurposed ISM filter. And it was a lot of metal work.


I’d like to do the tuning - What equipment did you use to tune the filters?

I used an RF noise generator and an HP RF spectrum analyzer for the initial tuning.

If someone does not have spectrum analyser ($$$), why not use software like GUI for rtl_power. (Alternatively SDR# can also be used).


How to use GUI for rtl_power.

I had the seller tune mine, way less effort.

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For 978 MHz ?
I haven’t found a cavity filter for 978 yet

Yes, see the graph I posted.

Thanks. I am glad you got it to work at 978 Mhz. I am using a Uputronics LNA mounted up the mast. Would be nice to have a cavity filter in front of it.

@mikkyo & @Jerryh47

What are the dimensions High Power Low Loss UHF 900 MHz ISM BandPass Cavity Filter?

Are the connectors SMA?

They are about 4” square and an inch high. Connectors are SMA (F).

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Here is a comparison of the (more or less) new Radarbox blue 1090 filter and the 978&1090 FA filter (light blue) and a sysmocom cavity filter. I used an orange FA dongle and FA antenna, located indoors.

without any filter

FA light blue 978&1090

Radarbox 1090

sysmocom cavity filter

On a first glance it seems the Radarbox filter works better than the older light blue FA filter, at keeping out signals around 930 to 960. The sysmocom seems to work best, but it costs more than double the price.

Might be interesting to see how a dark blue fa filter works, unfortunately I haven’t got one.

Maybe I’ll test the rtlsdr-blog 1090 filter&amp, but one right now it is feeding an airspy mini, and the other is lost in a drawer or box somewhere, and I would have to set up it up with a bias-t somehow.