DVB-T ADS-B receiver and feeder problems; advice and assistance please

[kingqueen]

I have an existing antenna setup for scanning. It is a 2m discone approximately 15m above the ground. It is rated for frequencies between 25MHz and 1300MHz.

This is connected via low loss 50 ohm cable to a wideband signal pre-amplifier at the point it enters my flat. This is a 25-2000MHz 6-20db (non-adjustable) wideband pre-amplifier.

In turn, this is connected to a screened socket, to which I connect a wideband scanner. I use this system for listening to various broadcasts, including aviation radio etc. and ham radio, and it functions well. Signals come in strongly over a variety of different frequencies and modulations.

I thought I would give it a try with the DVB-T Stick ADS-B receiver, using zadig, rtl1090 and the instructions provided. I know that a discone is not the best antenna compared to a 1090MHz specific one, but thought it would work OK. I bought the stick and made a BNC to Belling-Lee converter, and I have tested it - the conductor and the screen have good integrity throughout:

All looked fine.
Here's the startup log from RTL1090:


Here's the output of the "max/min" button:

Here's a minute or two of log data:


I am however facing issues. Please see the screenshot below:


I've tried multiple times; re-booting, selecting correct ports, using tuner auto gain control or setting the gain manually, and so on, and have run the system for days. But I experience the same issues.

1) RTL1090 only ever "sees" a very limited number of aircraft.
2) the FlightRadar24 feeder software doesn't ever see more than one airplane.

I am at somewhat of a loss as to what is wrong or what to do about it. Any help or advice would be very gratefully received.

[Oblivian]

I can see already of those 4, by HEX lookup only 2 broadcast position. You need 3+

But seeing as the feed SW cant see any, there seems to be an output issue. Does the command line have the right 30003 trigger line?

Check the hardware section, there is a number of RTL helper threads there where people have sorted it out after the fact.

(going to http://localhost:30003 will show you if it is outputting correctly)

[fungus]

kingqueen,
Just an observation. If you've got both the scanner and the DVB-T stick coupled to the discone antenna (not that being a discone makes any difference) in parallel, it likely wont work because of the impedence miss-match (the scanner hogs all the signal). I cant see from your pics if that's the case, nor am I familiar with the impedence values of your scanner or stick but that might be an issue for you. I cant run my basestation and scanner off the one antenna for the same reason. Unfortunately a high impedence device and a low impedence device need to be on separate antennas.
Regards,
Gregg

[sinnet3000]

What I would probably do is remove the preamp from the equation. It is probably causing frontend overload and desensitization. Have you also tried disabling the RTL AGC, and just set the gain manually?? A preamp plus the RTL SDR set at 48dB + the RTL AGC and a discone that is going to receive a very wide frequency range sounds like it's going to cause this kind of problems you are having. What coax are you using exactly???

What fungus also says is true. We don't even know what is the nominal impedance in the antenna input of the RTL SDR, which is probably nowhere near 50 Ohms.

Hope that helps you out.

Edit: Also give it a try to ADS-B# just to see what kind of results you get there

[Kelvin]

Pre-amps can be a nightmare. I noticed it is connected at the point it enters the flat.
If a pre-amp is going to be used, it should be as close to the aerial as possible. Having it near to the receiver means that all the noise etc that enters the system along the cable route etc gets amplified just as much as the signal.
I have also seen a report that states the "gain" of this pre Moonraker pre-amp will probably give you a loss at 1090MHz. The gain was measured at 20dB at 25 to 35 MHz, 6 dB at 1000 MHz and tapering off to a loss of 11 dB at 2000 MHz.
Add that to the impedance matching issues and my verdict would be; "bin it".

[kingqueen]

Thank you for the advice, everybody.

I didn't have the scanner connected at the same time as the DVB-T stick. I concur that the pre-amp isn't great and that it should be as close to the signal as it can be, ideally mast head. I retro-fitted it as a trial, and it makes a minor improvement to the scanner signal on some frequencies. I put it in to improve SnR ratio before my internal wiring. I wanted to take it out of the equation last night before posting but couldn't find a BNC barrel connector. I've just taken it out of the feed using a passive BNC 3 way splitter (third output unterminated) and unfortunately I am still getting very little output on RTL1090.

The cable was sold to me as lower loss / better construction than RG58 (a few years ago) but on inspecting the sheath it is marked RG58 (and that alone, no quality marks or manufacturers marks) so I've clearly been scammed to some extent. I don't think it is that low quality, however, as it works well for scanning. I am aware that lower quality cable will cause greater attenuation at 1090MHz than at lower ones. There is perhaps perhaps 17m of it from discone to DVB stick.

I've just tried connecting the DVB stick to an extendable portable wideband aerial I use for listening at airports etc. - basically an extendable rubber duckie - out of a ground floor window, and instantly RTL1090 is seeing lots more airoplanes and I've been emailed my sharing key :-/ So it is definitely something with my external aerial / permanent cable setup, which works well for scanning but plainly not for ADS-B.

I wonder what it is that is so bad about my discone / internal wiring.

Thank you everybody for your help and thoughts...

Doug

[Oblivian]

potential for half a dB loss for every m of that at the high frequencies..

so could be as bad as 8dB out the window and then adaptors and connectors 1 to 3 dB each

[kingqueen]

Thank you Oblivian. You think it is the RG58 that is at issue? When I next have some spare cash I will replace it. Which cable would you best recommend for 1090 feeder use?

Thank you.

[North Borneo Radar]

for cable, have a look here:

http://forum.flightradar24.com/threads/4945-Coax

antenna: PCB antenna if you are looking for something else from the discone

http://www.1090mhz.com/

obviously there are other types of ads-b antenna, and you can look into it at the "technical hardware (hardware)" thread.

[Kemistry]

I'd also suggest that a 1090 filter is better than a wide-band pre-amp which I also tried on my journey.
Filter still has insertion loss but it does help attenuate those nasty mobile phone transmitters at close frequencies that are everywhere and get into your front end.
I had to get a second hand one but it works a treat.
I cannot believe the difference a 1090 antenna made - my discone is rubbish by comparison.
Good luck

[kingqueen]

Thanks ever so everybody for helping me troubleshoot and work out what's best.

I shall stick for the moment with my portable extendable antenna for the DVB dongle, and with my existing discone / nasty RG58 for scanning, as I have no money. But as and when I can afford it, I'll replace the RG58 with dual R8 with multistrand core, and also put up a separate 1090MHz antenna.

One final question: What's the easiest / cheapest way of putting up the 1090MHz antenna, given my existing situation? Can I have it on the same pole as the discone? I assume it would be bad to have it under the 45º ground plane elements of the discone?

I wish I could shimmy up and do it myself, but I'm a wheelchair user in a care home, so I'll have to pay somebody else to do it :-)

Cheers ever so once again.

Doug

[sinnet3000]

Thanks ever so everybody for helping me troubleshoot and work out what's best.

I shall stick for the moment with my portable extendable antenna for the DVB dongle, and with my existing discone / nasty RG58 for scanning, as I have no money. But as and when I can afford it, I'll replace the RG58 with dual R8 with multistrand core, and also put up a separate 1090MHz antenna.

One final question: What's the easiest / cheapest way of putting up the 1090MHz antenna, given my existing situation? Can I have it on the same pole as the discone? I assume it would be bad to have it under the 45º ground plane elements of the discone?

I wish I could shimmy up and do it myself, but I'm a wheelchair user in a care home, so I'll have to pay somebody else to do it :-)

Cheers ever so once again.

Doug

Hello, probably you could replace the coax with H155. It's around $1.5USD per meter.

I think you could put your new antenna on the same mast, I don't think it's going to have a lot of influence with your discone. Buy a quality coax, keep the connectors nice and I don't think is going to be a problem.

[Duncan T]

Some further items to consider. The broadband nature of your pre-amp is destroying any decent 1090 MHz signals. I suggest removing it from the system. RG58 is a very lossy cable and you will need to invest in a really decent quality feeder, such as Westlake 103, which is double-screened and approximately 12 mm OD. Use 'N' connectors for preference and certainly no three-way splitters of BNC style, which are not really intended for RF, but for Ethernet. Your antenna needs to be clear of TV or dish antennas and you could stand the 17 metre length OK if you use this really low-loss feeder as suggested here. Use an absolute minimum number of connectors or patched leads, for each coupling introduces a substantial signal loss at this frequency, which is over a GigaHertz! Forget all about trying to use a discone to achieve a signal for Mode-S/ADS-B - a total waste of time but use a dedicated 1090 MHz antenna, mounted as high as possible. I have 15 metres to the antenna from my SBS-3 and the antenna height is approximately 7 metres above ground and looks over my single story home. I receive aircraft signals up to 165 miles out, obviously subject to flight levels!

[kingqueen]

Thank you!

Funny how these things grow isn't it. I started by thinking, wouldn't it be interesting and easy to buy a cheap DVB-T stick and patch it in to my existing scanner setup? Next thing I know, I'm thinking of replacing my discone and whole scanner rig and adding a new separate system for 1090 lol.

It will all have to wait until I have sufficient dosh anyway.

Thanks all

[fungus]

Thank you!

Funny how these things grow isn't it. I started by thinking, wouldn't it be interesting and easy to buy a cheap DVB-T stick and patch it in to my existing scanner setup? Next thing I know, I'm thinking of replacing my discone and whole scanner rig and adding a new separate system for 1090 lol.

It will all have to wait until I have sufficient dosh anyway.

Thanks all

I know the feeling- I'm in (and have been in the past) much the same boat as you. Well said
Regards,
Gregg