FlightAware Discussions

Where to place filter

Hi all,

I want to try to get a better range. I have max 380 km with good weather conditions, but almost around max 250km.

The picture is my setup.
I want to try to put the dark blue bandpass filter in my setup. (I live in the middle of a city)
But where must i have to place it? Before or after my preamp?


I doubt that the dark blue filter will improve things at all.
Your Uputronics preamp already has a filter and it is a narrower bandwidth than the dark blue filter.
Have you tried changing the gain of your setup?
The blue dongle has a preamp built in and with your uputronics preamp as well you may need to reduce the gain for optimum performance.

As a general rule of thumb, the filters (insertion loss) should go after the preamp, to maintain a good signal/noise ratio.
It’s very unlikely that the preamp first stage would get saturated.

@LawrenceHill @SoNic67

My gain is 36.4 and above my graph1090 from today

The number of tracks with just a single message is really high. That would suggest you have a lot of noise on the signal into the dongle. What power supply are you using? Also how long is your antenna cable? You would achieve better results if you moved your preamp to the antenna end of your feeder cable.
Maybe the single message rate is not that high after all. My main receiver is an Airspy mini and that shows virtually no single messages. However I have just reinstalled graphs on my ProStick receiver and it shows a similar proportion of single messages to yours.

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6 meter aircell 7
And i use an original rpi3 power supply, the uptronic is powered by the rpi3 usb-port

So, about 1.2dB feeder loss. Nothing to really bother about then. I wonder though, if there is noise being picked up directly into the dongle from your cables that are almost touching the dongle. It might be worth trying to move the cables as far away from the dongle as you can, to see if that improves things.
Do you already have the dark blue filter?
If you do, as an experiment, try removing the preamp and put the filter in its place.

See edit:

This is where the general rule might not be applicable. It would appear that the preamp being used in this case, might be susceptible to overload, so placing the filter before the preamp might well be best.

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I just ordered the dark blue filter… i will give it a try :smiley:
But first i will try tomorrow to seperate the cables as much as it can…

In other posts I have commented that the FM/TV emitters are a special case, since they have higher emitting power.
My made-up “rule” is that the ADS-B filters go after the LNA, in all of the normal cases. There is no question.
The very special installations, where an emitter antenna is at meters away from the ADS-B antenna, would require a FM trap in front of them. Those installations are hopefully managed by people that won’t ask around “where is the filter going”.

The RTL-SDR LNA has the FM filter in front. That FM filter probably has less insertion loss in the 1GHz region, than a SAW 1090MHz filter, so I think is a very smart design, conserving the SNR.

It’s not an FM filter, it’s a high pass.
And yes the loss should be quite low.

Looking at it now, you are technically right. Two pole high-pass is, IMO, pretty effective for frequencies up to 200MHz. They claim over 60dB’s of out of band attenuation.

After couple of weeks testing this is the best setting for me…
The filter before the preamp did notbwork… (no better results)
The filter behind the preamp works fine, more planes and messages

And every week lowering my gain (this week 25.4 and about 5% of messages >3 db)