FlightAware Discussions

Designing cavity filter, may I know more about yours?

I own the Flight aware cavity filter, and its great: amazon.com/ADS-B-1090MHz-Ba … B010GBQXK8

I just got a uKeg 128, and I have a bunch of 16gram Co2 cartridges laying around, and trying to think of a good use for them…

I’m thinking that I could make some cavity filters for ADSB or Hydrogen line radio astronomy, and publish design for anyone to use.
Would it be possible to get documentation on your current design? Curious what it looks like inside.


I don’t know much about filters, what are the Co2 cartridges for?

its also not a cavity filter, it a series of caps and inductors on a PCB, don’t have any pictures to hand.

Her is the cavity filter that I use
jetvision.de/epages/6480790 … ucts/71010

It has a much narrower (10mhz) filter compare to the FA filter (100s MHz)

I am sure that there are plenty of designs on the Internet for cavity filters.


For $95 USD, it must be good. :mrgreen:

FA Filter is enough for most situations.
Only locations with RF noise too close to 1090 Mhz, or too strong, this costly filter is really beneficial.

A DIY cavity filter is possible

keptenkurk.wordpress.com/2014/1 … -b-filter/

changpuak.ch/electronics/int … signer.php

I have two and they are well worth it for me.
I did a test recently.
FA 1.0 pro stick
2 aircraft without filter
200 aircraft with filter
All other settings were the same.
NYC is noisy. I have 3 major airports and several minor airports within 30NM.
I have a dozen cell towers within half a mile.

The FA external filter is a LC ladder circuit.

Cavity filters in China are in the same price range of the FA filter if they have your frequency.
Custom filters are much more expensive.

I have also seen DIY cavity filters on RTL-SDR.com including design files.
Some are just a plain metal box and others are machined out of solid metal.

Has any one used this software?
“Elsie (LC) - The Windows program for electrical filter design and ladder network analysis”


Thanks for the replies!
I’m really enjoying seeing other’s filters!

The co2 cartridges are for my mini keg/growler(uKeg 128), and are accumulating by 1 every 2-3 days.

Ladder filter makes much more sense to me for that size…I read somewhere it was cavity so thats why my wheels started spinning.
I think I’ll start tinkering around with some of those cartridges soon, and see what happens on the specan.

Cavity filters have 3 cylindrical sleeves. The small (16grams) empty CO2 cartridges can be ustilized there.


My first attempt at building an interdigital filter.

The construction techniques leave much to be desired, but the end result is adequate.

Currently, this is indoors, next to my oldest heliax-based collinear antenna, and feeds a Pro-Stick on a Beaglebone Black.
Regularly get 200-220nm range with it, over the generally-flat prairies.

1 Like

I looks a bit like a Rube Goldberg device. However, what people usually overlook, such devices actually work!
How high is your antenna?

Good job, congrats. :slight_smile: :wink:

The antenna with the interdigital filter is a 10 1/2 segment heliax collinear sitting on a tripod in our second floor guest bedroom.
(Fortunately no guests at the moment)
That would be about 12 feet above ground and about 2 feet below roof level, indoors.

Today bitter cold here in Toronto. You are nearer to North Pole than me, how is it in Saskatoon?

Currently, -4 degrees Celsius (24 Fahrenheit) with 95% humidity. Expecting -20 C (-4 F) next week, after which it will start getting REALLY cold. :wink:

So far, the orangepi pc outside on the roof with the ProStickPlus is keeping itself at a rock-steady +10 C (50 F)

Downloaded Elsie (Student Edition), which is FREE OF CHARGE.


Gave a try. Screenshots for first attempt to design a “1090 MHz Band-Pass LC Filter”:





Am I reading the chart correctly, is it 20db insertion loss?

Not sure. After installing, tried it for only 15 minutes, just to get a feel of it. Will study its Manual and try again to get more in-depth understanding.