FlightAware Discussions

Home made interdigital filter

I’ve already share some picture of my filter in another topic “Do I Need A Filter?” but I think this project deserve is own topic.

https://discussions.flightaware.com/t/do-i-need-a-filter/27833/743
https://discussions.flightaware.com/t/do-i-need-a-filter/27833/739

After some internet search and following some advises from this forum I’ve decided to build my interdigital filter. The filter was built using double clad PCB and some brass tubing. Tuning was done using the POOR MANS SPECTRUM ANALYZER WITH AN RTL-SDR AND NOISE SOURCE.

Finally I had the opportunity to test the 3 settings (no filter, FA filter and home made filter) and here is what I’ve found:

  • I believe the Home Made Filter has a high insertion loss and that’s the reason the messages >-3dBFS are kept low while gain isn’t at MAX, but still is better than not having a filter.

  • I did compared my FlightAware statistics with the nearest 3 sites (one of the sites is a FlightFeeder). Not a very accurate method since the receivers don’t cover the exact same area, but it was the close I could get. Compared with the other sites the number of flight doesn’t change that much with all combinations (filter/gain), however the number of positions does.

Besides those big antennas in line-of-sight I also have 8 cell towers in less than 1km radius, making the use of a filter mandatory.

You can download the file that contains information about gain, signal level and the comparison with the other sites from this link ADSB_Gain.xlsx

I’ve already plans to build a second filter, this time I will do it with more precise measurements and extra care to keep everything aligned. Hope that the next filter together with the “refurbished” Jetvision antenna will increase the number of positions.

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If you have a LNA in front of that filter it cancels the insertion loss and cable loss. The modern LNA have <1dB noise in the first stage.
The first stage in a normal R820T tuner (without amplifier) has a noise of about 3.5 dB per datasheet. R820T2 tuner supposedly offers lower noise, but I cannot find a datasheet for it.

I’m using a Pro Stick Plus and 1.5m of coax cable. If I decide to install a LNA in front of the filter do I have to change the dongle or multiple LNA will not have a negative effect?

I used that combo, with a 25dB gain LNA, and had to reduce the gain on the dongle at about 12dB (from 40dB previously). I have added a longer cable too and a surge suppressor.

Not sure what gain are you using now… It depends of how much attenuation the filer and cable adds.

There are LNAs with lower gain. Mine have two stages and two SAW filters. One stage LNA will add less gain and can compensate for the insertion loss alone, because your cable is very short (no significant loss there).

PS: There are hairpin filter PCBs with an amp chip for sale on eBay:

Downloaded “Spektrum” on Win 10 (spektrum-win64.zip), plugged ProStick (Orange) in Desktop, No internal or external filter, connected FlightAware Antenna and ran the scan from 24 MHz to 1800 MHz.

Wow, how much interference on frequencies below 800 MHz !

CLICK ON IMAGE TO SEE LARGER SIZE

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That link does not take to any of your posts, it takes to opening post of the thread. Following are the links to two of your several posts:

https://discussions.flightaware.com/t/do-i-need-a-filter/27833/739

https://discussions.flightaware.com/t/do-i-need-a-filter/27833/743

@abcd567 thank you for bringing to my attention the error. Correction done.

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OK, I was bored so decided to follow this tutorial RTL-SDR TUTORIAL: MEASURING FILTER CHARACTERISTICS and were are the results.

For the next filter probably I will try to increase the bandwidth and try to lower the insertion loss.

Not sure if the problem is from the noise source or the RTL-SDR dongle but the baseline is weird.

3 Likes

Home made interdigital filter “Take 2”
Just finished my second filter, this time I did put more attention in the alignment of the filter walls and the filter looks nicer, but far from being perfect.
While cutting the internal elements I’ve done a mistake and the center element is 3mm shorter, but I was able to tune the filter to 1090MHz.

I have used Panorama SW with gain set to 49.6 to get the following shots:

Noise Source with a 10db attenuator and a generic dongle:

New filter between the attenuator and the dongle:

Previous filter between the attenuator and the dongle:

Next step is to replace FA filter and see how it performs.

Any comments and suggestions?

3 Likes

Looks like an excellent build. Can you replace the centre element? It might make tuning a bit more precise. You do have a lid for the box?

@triggers
One of the case sides it isn’t fully closed and can be easily opened. I will replace the central element and see if it gets better.

Center element replaced and box closed.
During the tuning process I’ve noted that the values can be different according with the SW used.

Spektrum (Linux):

RtlPanorama (W10):


RTLSDR Scanner (Linux):

Just to compare this is the FA light blue filter:


All measurements were done using the “Noise Source” a 10db attenuator before the filter and a generic dongle.

2 Likes

@pretoescuro
Looks like you have a good filter and a good learning experience :smiley:

Good job.

Today I’ve done some spectrum acquisitions using the Jetvision antenna, it was interesting to do a “site spectrum” survey because I can’t use my ProStick Plus without a filter (messages and planes drop to 0).

For now I will leave the interdigital filter in place to see how it performs.

3 Likes

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.