FlightAware Discussions

Home made interdigital filter

The first scan show multiple products of inter-modulation, probably because of too high level of signal and gain.
You still can see some of those spikes in the graphs below. What gain level did you use to make those graphs?

@SoNic67 the gain was set to 49.6.
When the weather allows I will do some new acquisitions using different gains, currently it is set to 44.5 and only having 1% of messages >-3dBFS.

@SoNic67 here are the latest spectrum acquisitions:

ProStick Plus

Jetvision antenna + FA filter (978MHz+1090MHz) + ProStick Plus

Jetvision antenna + home made filter + ProStick Plus

Very interesting. You have some strong interference close-by! Cell towers probably?
Looks like the Download frequency (tower sending) for LTE band 20 (Digital Divident EU), band 8 (Extended GSM) and/or standard GSM.

@SoNic67 Near my site and in line of view (500m) I have probably 8 cell towers, power lines and at 1.5km there’s a cluster of antennas.

And what looks like a FM or TV station repeater that probably is responsible for the repeated 98MHz intermodulation :slight_smile:
There are LNA that have FM traps that can help with that: https://www.rtl-sdr.com/new-product-rtl-sdr-blog-1090-mhz-ads-b-lna/
Looks like your home-made one is working pretty well too.

Having strong interference close-by isn’t expected that the LNA will be saturated the same way the ProStick Plus saturates if used without the filter?

If I buy this Aliexpress LNA and put it between the interdigital filter and the ProStick Plus do you think there will improve the reception?

Why I’m considering the Aliexpress LNA and not the RTL-SDR LNA:

  1. It’s cheaper
  2. I expect to compensate the lack of the 1st filter with the interdigital filter or the FA light blue filter
  3. Don’t need to buy a bias-t to power the LNA

A non-filtered LNA will overload just like the LNA in the FA Pro+.

The rtl-sdr LNA has a two stage LNA with filtering after each stage.
It also has a high pass filter on the input that already reduces FM / AM / 400 MHz stuff probably up to 950 MHz.
The two stage design is done exactly for the reason that it doesn’t overload as easily.
The high pass on the input also helps a lot.

It’s purpose built to handle lots of interference, that’s why it’s often recommended. (while handling interference it also performs very well)

Posting ADS-B signal graphs and message rate graphs for the interdigital vs. FA filter would be interesting.
You could even do a third run and chain them :slight_smile:

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I’m already acquiring data to compare the interdigital filter with the FA. I’m planing to do 24h acquisitions with different gains (40.2, 42.1, 43.4, 44.5, 48, 49.6 and Max) and then compare with the 3 sites that are closer. It will take some time :slight_smile:

Like @ wiedehopf explained, a filtered LNA will amplify only the useful ADS-B signals, while cutting out the unwanted ones. It will not overload the next stage input, due to the low level of the ADS-B signals at the antenna input. Reducing gain in the receiver stage has huge benefits.

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@SoNic67 probably I will go for the RTL-SDR triple filter. I’ve found a supplier in UK (TechoFix UK) and I think, by now, will not create extra import duties. Now have to find a proper bias Tee, probably in Aliexpress, do you know if I can use the RPi4 5V GPIO pin to power the bias tee (probably adding a diode will keep it below 5V).

I have used a separate bias tee and power supply. Used a 5.2V supply from my Amazon Kindle.
Didn’t want my $$$ receiver to be at the mercy of an electrostatic discharge or a short in the cable. I also added a gas discharge arrester inside the house.

After a while, because by coax cable is long, I was annoyed by the voltage drop on that cable and the fact that I needed to be careful of how much voltage I was sending on that cable. So I have modded the LNB with a 5.0V power regulator and I am feeding it from house end with 7.5V.

Currently what I have is a 12V power supply at house connected with a long cable to a 12V to USB converter that is the RPi4 power supply. Since my receiver is installed in a place that is accessible to my neighbors I didn’t want to run a 220VAC line, so the power options are limited, probably will power the LNA from the RPi4 5V GPIO or add another 12V/USB converter that I have sitting around.
After that probably will need another case to put the RPi, USB dongle, LNA and filter :slight_smile: or add another case just to the LNA and filter.
This never ends… probably that’s the best example of a hobby :joy:

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I don’t like my electronics outside, I have the antenna, LNA filter and a 45m long coax cable to indoors.




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@wiedehopf the interdigital_V2 vs FA filter is ready, the data can be found here, the graphs were done by comparing with the 3 nearest sites (one runs a FlightFeeder).

@SoNic67 already got my RTL-SDR triple filter and I’m expecting to get the bias tee tomorrow.
What configuration do you suggest to start the tests?

  1. Jetvision + LNA + Prostick Plus
  2. Jetvision + LNA + filter + Prostick Plus
  3. Jetvision + filter + LNA + Prostick Plus
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You don’t need any extra filtering for the rtl-sdr triple filtered LNA usually.
But as you’re running with the ProStick Plus you might as well put both filters between bias-t and ProStick Plus to attenuate the signal a bit :slight_smile:

Start with a gain of 15 or 18, that should put you in the ballpark (you’re adding 27 dB of gain with that LNA, so you need to reduce it at the receiver appropriately).

Putting an extra filter in front of the rtl-sdr LNA is something i wouldn’t recommend but i suppose it never hurts to try.

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I would start with #1 configuration. I don’t expect that the LNA will overload with the stray signals, after all the antenna is tuned too. Reduce the gain in the receiver and enjoy!
If you want to play you can try with the filter after antenna (#3), just to see if there are any changes but I doubt.
Attention at #2, I am not sure if the filter passes the Bias T voltage… never tried it personally. Always installed the filter downstream of the bias injector.

LNA installed (antenna + LNA + bias T + Prostick Plus), inicial gain set to 16.6, over the next days I will increase decrease the gain, with 16.6 I’m already having >-3dBFS higher than 10, and compare the results with the nearest sites.

@SoNic67 @wiedehopf finally I have the test results. Messages >-3dBFS are high but I’m OK with that, there is a flight school not far. With the LNA I’ve seen an increase in range and positions, as a next step I will put the filter between antenna and LNA.

Those with the Ident RTV are single engine planes from the school
Screenshot from 2020-11-12 13-06-55