FlightAware Discussions

Req. for comments on my filter/LNA attemps

I’m in an area close to (and line of sight to) several 4G masts and have a high 800MHz signal level here. I thought a filter and/or LNA might help. I’m using in indoor 1/4 wave spider.

These just arrived from ebay, which I think I’ve seen pictured here before:


My message rate was around 250/s. Adding the filter alone dropped that to 60/s. It only took a moment to add it in. Removing the filter immediately raised messages to 250/s again.

Next I added in the filter followed (on the receiver side) by the LNA. I got about 150/s messages. Remove both and back to 250/s.

With the LNA alone, I got about 120/s. Again, remove and messages go right back up to 250/s.

All of that was done quite quickly, while I had a fairly constant message rate. I’m a bit surprised at the results. I really thought the filter would make a positive difference, given the proximity of 800MHz towers. I certainly didn’t expect such a huge drop.

I’m going to continue experimenting, and I’ll connect my RSP1A to the antenna and try to get some meaningful numbers. (so there’ll be no FA feed from me for a while when I do that).

Any thoughts or suggestions would be welcome. I’m effectively new to RF as well as ADS-B tracking, despite having a full amateur license gained decades ago. It, and my once-held knowledge, has been unused for most of that time because I got distracted by computers.

Other info
I’m using a cheap RTL dongle on my Pi for PiAware. I’m also surrounded by hills slightly higher than me. I’m on the side of one of them, facing into the broad valley, at a good height but not quite at the top.

I made a basic 1/4 wave antenna with four 45° radials from a length of 1cm thick 75Ω coax, by just stripping it down to expose the centre and poking the radials into the screen between the dielectric and outer. It works well, better than my first cantenna, especially when I put it about a foot above a metal plate. That increased my message rate by 20% instantly. I guess it moved the radiation/gain pattern a bit higher, where it’s useful to me over the hills. I know I have some impedance mismatches in the system. My dongle has a 75Ω input but the cables are 50Ω. I’ll deal with that some time but I don’t think it’s a huge issue.

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You need to ‘play’ with the gain settings as well. That LNA is adding, if like most eBay LNAs, around 20dB. The dongle is being swamped. Do not use -10 (default). If using -10, the total gain (dongle+LNA) is around 75dB now. Try settings between 20 and 48.

How are you powering the LNA? Is it for the correct frequency range? Is the filter before or after the LNA, from the antenna point of view? You may need a filter at each end of the LNA, if the surrounding RF is too strong. Shielding everything may be needed as well.

I have no experience with message counts, only planes seen. Did the number of planes, and how long they are tracked, change?

As for the cable, consider switching to RG-6 quad shield, the cable/satellite TV type, and an outdoor antenna.

Lastly, you may already have a good setup for your location/installation, sans LNA and filter.


Might also be an LNA that is just crappy or for another frequency range.

I’ll just leave my usual recommendation as far as LNAs go:


I replaced a generic dongle with an RTLSDR V3 which showed immediate performance improvement.
I then added the matching filter/amp, but the increase wasn’t as great as expected until I added an 8dB attenuator between amp and receiver.
Too much gain is just as detrimental as not enough.

In amateur radio circles, that amp has been shown to be a poor performer.

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Although the OP has not given any details and links, a quick search gave these, which have exactly same photos as posted by OP.


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Thank you all for the replies. Yes, sorry, I should have given more detailed info. The filter is as above, and the LNA is described as 0.05-4GHz, gain=18dBm and NF=0.6dB at 0.9GHz.

The LNA is powered from a 5V wall wart (a proper PSU from an old bit of network kit) with the wire wrapped around a ferrite a few times. The filter is between the antenna and LNA so as to get rid of the 800MHz mobile noise first.

I’ll try playing with the gain on the dongle.

I did some checks with the RSP1A and could see the effects of the filter and LNA, which seemed to be more or less as expected.

The whole lot is now connected to my dongle again, and this time it is behaving better. Perhaps something went wrong the first time; maybe I didn’t have a connection made properly but I did check. Right now it seems to be behaving about the same as it did before the additions. I’m still getting local contacts and maybe, just maybe have a little more range. Time will tell.

I’ll continue to experiment after dinner.

ETA: My cables are very short (no more than a metre total) and mostly RG58 apart from the MCX-SMA adapter for the dongle.

I bet your set up is being over saturated from too much signals. I plugged in my FlightAware pro Stick Plus into my computer and ran SDR Console tuned to 1090 mhz to see what the signal quality looked like on the water fall display. My signals looked strong to almost over saturated. I left the gain alone because I know my 1090 side runs excellent… Now on the 978 side I had cell phone interference from a cell site about 200 yards from my house. I ran into problems and had no aircraft showing up so I adjusted my gain from auto to 25 then used the older FlightAware 1090 filter and was able to filter out some of my interference. Now I see UAT equip aircraft as far as 80 miles now. Its beneficial to plug the dongle into a computer and run a program like SDR Console to see the condition of the signals you are receiving. Another thing is I noticed that these devices you show are no enclosed into a metal enclosure. Shielding is very important and also grounding is another thing to keep into consideration.

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Thank you glennblum. My setup with the new bits is definitely outperforming the antenna alone now. My stats are up quite noticeably after a few hours. Unless there was a sudden increase in traffic this evening over the usual. I doubt it.

I’ve seen the effects of saturation when I tried another (old, huge, but decent and good up to 2GHz) preamp I had last week. It increased my range right away but lost all close contacts and dropped my message rate and number of planes significantly. So far, that doesn’t seem to be happening with the filter in front of this LNA. I’ll still play with the gain a little once I’ve run it like this for a few days next week, to get a baseline for comparison.

You’re absolutely right about the shielding, I was being lazy at first but I’ve now rectified that. Temporarily at least, with cardboard and kitchen foil :laughing:

Oh, and I made a new 1/4 wave antenna on top of a chassis-mount SMA socket, just a touch more carefully than my previous one and without the additional 75Ω to 50Ω mismatch. Every little helps.

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I use RG6 type CATV cable since its losses are much lower at 978 and 1090 MHz, plus its cheep (Free) and I have bags of connectors and the tools to strip and crimp the connectors together. When I replaced my LMR400 for 1090 I found a increase in actual aircraft seen and message rates much higher and that is with the miss match at 75 vrs 50 ohms.

Experimentation is the best part of the hobby, but the best affordable antenna out there is the FlightAware one.

I just installed a second one on my second receiver. I removed the LNA and filter, and the performance is even better.

The previous antenna was a QuickSpider. The receiver is a generic $8 dongle.

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