FlightAware Discussions

Cavity filter

Hi all,

refresh my memory, setting up some new gear. Using a Pi4, uptronics pre amp, 1090 cavity filter and Airspy R2. The order of connection from the PI4: > arispy > cavity filter > uptronics pre amp > antenna ?

Correct ?


That would be the right order indeed.

Thank you for confirming :grinning:

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Surely the cavity filter should be the closest to the aerial. You want the cleanest possible signal going into the amplifier otherwise you’re going to amplify unwanted (out of band) signals.


First thing, going from the pi to the antenna doesn’t make sense as you want to describe the way the signal travels.
At least that’s what seems most logical to me.

antenna → cavity filter → uputronics LNA → airspy

As you’re not asking about the software side, you’ve probably already seen this and that it comes after installing dump1090-fa or readsb: GitHub - wiedehopf/airspy-conf: Configure airspy_adsb for use with readsb or piaware.

And just in case you’re not starting with a piaware image, this a complete from the ground up build based on Raspbian (also works on pretty much any architecture with debian / ubuntu:
Raspbian Lite: ADS B receiver · wiedehopf/adsb-wiki Wiki · GitHub

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PI4: > airspy > uptronics pre amp > cavity filter > antenna
The point of the cavity filter is to stop as many out of band signals as possible from entering the receive chain.


I have several uputronics pre-amps and cavity filters. The pre-amps are prone to interference and do generally need the cavity filter to be between it and the antenna. Also, it is difficult to power them by bias-t from the airpsy if there is a cavity filter in the way.

I stand corrected then, Thank you for the insights gentlemen

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There are good arguments either way between filter-then-amp vs amp-then-filter depending on the local environment and amp performance.

Other things to consider:

  • The uptronics amp has a SAW filter built in, so do you need the cavity at all?
  • The insertion loss of a cavity should be very low, so most of the objections to ‘below the antenna’ are negated.
  • How are you powering the Uptronics amp? A cavity will probably look like a dead short, so you can’t use a Bias-T if you are putting the cavity between the receiver and amp.

The Uputronics amp is junk when it comes to strong signals nearby. It doesn’t take much to bring it to its knees and be completely overloaded. I’d do anything to stop that, even go as far as replacing it with one that doesn’t display the same symptoms.

The maker has confirmed this and said he’s working on a newer model but that was well over a year ago and we’ve heard nothing since.


Oh - in that case the cavity should definitely be between the amp and antenna
(or a different amp selected)

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Well most people don’t have transmitters on the same pole.
Mobile towers in some distance it seems to handle just fine.

True, very true.

However, 1W on 70cms (430MHz) from around 10m away was still enough to totally overload it.

“still enough”

That’s the same as 625 W from 250 m away.
Well ignoring plane of reception and all that.

But distance is so important with transmitter power :wink:

In theory, yes. I’m not sure the field strength would actually be strong enough.

But there’s the problem. There’s always a radio ham around when you don’t want one :smiley:
I’ve got two others within 200m from me, one is new and less than 100m from me (as the RF flies) and so all it would take would be one of them to decide to run a bit of squirt on 70cms and I could well be screwed.

At least, I would be if I still had one of those amplifiers.

My current setup is bomb proof, both to HF and VHF/UHF. The feeder for my HF aerial runs up the mast within a foot of the network cable to the Pi and the coax from my transmitter is routed around the rotator along with the coax to the ADS-B aerial which is, as you know, mounted right smack bang in the middle of the aerial. Even with lots of power, I don’t have any problems at all and neither do I get any overload when I use the VHF/UHF aerial which is just a few metres away, even at 100W.

More so than the internet. 400W on 10m kills it stone dead!

But yet again, we digress :smiley:

I still reckon the cavity filter should be betwixt the aerial and the amplifier.


Oh obviously it should be.
Not what i was debating :wink:


I just finished putting together my second feeder (for putting in a church tower) and the airspy, LNA and cavity filter connection order can be clearly seen.



What heatsinks are you using for the airspy mini? Mine is getting very warm as well
And what cavity filter, just curious☺️

The heat sink on the Airspy comes from an old printed circuit board, I use a plain rubber band folded underneath the dongle as a way of fixation.
I got the cavity filter from Sysmocom.de.
This feeder is now up and running since yesterday, the view from the church tower is very impressive!

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Thank you and yes very impressive. Guess your reception is great as well :wink:

The cavity filter from Sysmocom is sold out