Filtering RFI from 440 MHz UHF Transceiver--Radio Hams Help!

Looking for some help from any Amateur Radio operators who may have run into a similar problem. I have a small low power (5w) home built UHF repeater which is very active. When the repeater keys up, my ADS-B receiver signals drop out. I really want to keep both the repeater and the ADS-B setup running 24/7.

The repeater antenna and the ADS-B antenna are about 50’ apart. I am in the City, so relocating them farther apart isn’t really an easy option. I have put ferrite beads on wires connecting to the Pi, but they just aren’t cutting it.

Anyone run into a similar situation and figured out how to better filter out UHF/440MHz (or any other frequency) RFI out of their ADS-B setup. I am running a blue Pro Stick Plus with the built in filter. I really do want to keep both setups running simultaneously.


The Blue ProStrick Plus’s filter is located AFTER the front-end RF pre-amplifier. I had to Add an external FA filter between antenna and Blue ProStick Plus to prevent its front-end’s overloading by strong RF signals existing at my location.

Interesting… Anyone else running a FA filter with the Blue ProStick Plus? I read that it shouldn’t be needed, but in this case, maybe it will be?

There’s a few sites in sufficiently noisy areas that see an advantage to the external filter with the blue Pro Stick Plus.

I had the same problem with a nearby GSM900 transmitter saturating the LNA of my FA-prostick dongle, a simple 1090MHz 4 pole cavity filter in series with the ADS-B antenna solved all my problems.


Every site in close range of GSM900 (that means pretty much everywhere in cities here) needs a filter, and a better one than the FA filter which is wide open at 950 MHz.

I really think you should source a good cavity filter, with <1db insertion loss and max 50 MHz BW, at a reasonable price.


The effectiveness of FA external Filter varies with location, hence your experience and my experience may differ.

I live in an urban area, in a high rise apartment building, surrounded by many similar high rise buildings. All these buildings have Cell Phone antennas on their roof top. Still, adding a FA external filter to the Orange & Blue ProSticks substantially improved reception. It is likely that if I use a good cavity filter, performance may further increase, but I did not try. I am satisfied with what reception I get, and dont want to spend another 50 to 100 bucks on my setup.

You are missing the point. I am specifically talking abut the GSM900 downlink problem.

Unless your location is incorrect, You live in Canada. You have no GSM900. American GSM850, is not only lower in frequency, but the down- and uplink is reversed.

The BTS you see transmits (among other bands, but this is the closest and most troublesome) at 869.2 – 893.8. In this region the FA filter starts to work reasonably well.
GSM 850 has uplink (mobile transmit) at 824.2 – 848.8, but mobiles are never such a problem as the strong BTS signals with high-gain antennas.

GSM900 BTS antennas transmit at 921.0 – 960.0 which is a completely different ballgame. At 960 MHz, there is NO attenuation what so ever in the FA filter.

I’m not asking FA to replace their current filter, I’m suggesting to offer a better filter that actually works in the whole rest of the world outside of North America. Many posts from different users here suggests it would be welcome, and most importantly, no-one is forcing you to buy it.

(But reducing passband attenuation by about 2 db also makes a difference, for everyone).

Anyway, unless you actually have experience with interference around 950 MHz, your input in the filter problem has limited value.


My PiAware nodes are inside my attic, about 2 meters from my HF 6-band trap dipole which runs along the attic ridge. Without the FlightAware bandpass filter, transmitting WSPR at 5 Watts on any of the HF bands would overload my orange Pro Stick and cause ADS-B messages/sec to fall to almost zero. With the FlightAware bandpass filter between the antenna and Pro Stick my HF transmissions have no effect on ADS-B messages/sec.

I just climbed up into my attic with a 70cm HT. Transmitting 1 meter from my ADS-B antenna had no effect on ADS-B messages/sec. I also tried transmitting about 0.3 meters from the Pro Stick itself and that also had no effect on ADS-B messages/sec.

So if the Pro Stick Plus is as well-designed as the Pro Stick, I expect the FlightAware bandpass filter will eliminate your problem.

put a filter on the front of the ProStick Plus - the FA filter, a cavity, a Mini Circuits high pass such as the SHP1000.

That will keep RF outside the filter bandpass away from the wideband front end, which is at least desensing it, if not swapming it and causing all sorts of mixing products.

You might also put the whole lot in a shielded enclosure.

I run one of my ADS-B systems from a discone on the roof. I use a Comet 413B duplexer in the shack, with the 1200MHz side going to the ADS-B box (through a Mini Circuits SHP-1000 high pass filter) and the HF-440 side going to either an Icom dual band (2m/440) radio, or my Elecraft K3S for 6m and 2m. I don’t have any problems with the ADS-B side when I’m transmitting.

–bob k6rtm

Problem solved!!! My closest PiAware neighbor who I have never met before read this thread, stocked me, figured out where I live, knocked on my door and gave me his old external FA filter! A big thanks to chrisjohnston50, site #21145 who not only solved my problem, but gave me the parts to fix it. Already seeing a huge jump in my stats!

In summary, yes, in my case, adding the external FA filter to the blue Pro Stick Plus made a huge difference in reducing RFI…


I’ve just received a 1090Mhz cavity from Germany and looking at a plot on the SA it has totally removed all signals from a GSM mast about 200 yards away.

Pictures below show a) Bare antenna b) FA filter fitted in line and c) Cavity filter in line - you can see the FA filter attenuates some of the GSM interference, but not all.
Antenna used for these tests is just mounted in the attic space, and not my main antenna fitted outside above the house, but still gives a good indication of local conditions.




Which one?

Got it from here - quick delivery, and built like a tank :smiley:

Thanks for posting the comparison by Spectrum Analyzer. Very informative. Very few of us have access to a SA or VNA, and for most of us, your post is a very valuable source of information

Waiting to see how much improvement in stats and max range is achieved by swapping the FA filter with the Cavity Filter.

Don’t suppose you have a photograph of the sysmocon filter? There’s nothing on their website that I can see.
How many poles is it?


Don’t suppose you have a photograph of the sysmocon filter? There’s nothing on their website that I can see.
How many poles is it?


Believe it is 4 poles - Pictures below :slight_smile:
Very happy with it - just got to sort right gain level now :stuck_out_tongue:

Internal filter picture obtained from
Thanks for that :slight_smile:

I ordered a half dozen of filters like these from a company in Singapore.
I wanted the UAT 979Mhz model. They first sent me the 1090Mhz model. The sent the proper model a week later at no extra cost.
For 6 they cost about $US30 each. Both models work well.

I do need to replace my FA Dual band antenna with the DPD 978Mhz antenna soon. I’ll put a 978Mhz cavity filter in the box, on the mast with the Nevis/hab amp, when I do the swap.