FlightAware Discussions

A balance between distant and local

Yes, it’s the orange one. I don’t have a Pi, just a desktop PC running a Debian Buster-based distro, dump1090-fa and the Linux Mono version of VirtualRadarServer 3.3.0-6 beta on Mono 6.12. I’m not (yet) a FlightAware feeder, I’m just tracking planes for my own enjoyment at the moment.

I hooked the cantenna back up to the original 10 meters of RG6 and plugged the dongle directly into a powered USB hub on my computer and will test for a while. So there’s a total of roughly 13.8 meters of coax, all good quality meant for satellite dishes, not the cheap cable TV stuff.

Oh well, the USB extension cable was a nice idea. I thought it might be OK if I powered it from the externally-powered hub with nothing else connected to the hub. Maybe I can get a better cable. The one I got is shielded and with a clear plastic coating. It’s probably a cheap one. Or else I’ll just stick with the RG6. I just tested with a NanoVNA and it says I’m getting a 1:1 VSWR at 1090! That was unexpected due to the impedance mismatches. I thought 1.5:1 was the best I could hope for. I wonder if the 8cm or so of RG316 I have between the RG6 and the dongle is acting as a matching bridge or something? Weird.

I have a SAW filter on the way and should get it next week. I’m sure that will help. It will go upstairs near the antenna.

If i am looking on my two USB extension cables, you will not see a difference beside the different length.
But the performance clearly differs from each other.

While the shorter one is the faulty one, the 2.5 Meter extension is capable to deliver the full performance.
Also a USB-Hub can cause issues. Finally the USB connector itself

I made the experience the other day with my AirSpy Stick. After a few days it started to drop off and on which can be seen in the graphs, even if the stick was attached to a USB-Port directly.

After some suggestions from here, i checked the connector, attached it tighter again and the issues were gone.

I’ve ordered another externally-powered USB hub. When it arrives I’ll try connecting it to the extension cable upstairs to power the FlightAware dongle and see what happens. Meanwhile with the dongle downstairs connected entirely via coax I’m getting quite a bit of my range back, not as much as with the NESDR dongle upstairs but as much as I had with it downstairs connected via the RG6 coax, so my issues were probably caused by not enough juice to the dongle over the USB extension cable. Maximum range (so far) is up to 115-120nm. Also nearby planes are no longer dropping off of VirtualRadarServer like they were last night when I had gain set to -10.

When Dump1090-fa is released later this month or so, and with my FA Pro on a powered hub with shorter coax and with my SAW filter on the antenna, I’m hopeful that the performance will be what I was expecting. If there are still issues I’ll consider replacing the extension cable with a repeatered one.

(BTW, I apologize if I seem to have hijacked this thread. That wasn’t my intention, I only meant to mention my orange FA Pro stick performance issues in passing.)

That should be an indication that gain was too high ending up in a signal overload

Yes indeed, that appears to be what was happening. Probably doe to the FlightAware Pro having a built-in 19dB LNA in its front end, which I’m not used to.

You could put the stick in the powered hub and connect the hub with the long cable.
If you have electricity where that is.
You shouldn’t need two powered hubs …

Don’t expect improvement from that update … just set the gain to a suitable level:
Automatic gain optimization for readsb and dump1090 fa · wiedehopf/adsb-scripts Wiki · GitHub

I know, that’;s my plan. I need toe other powered hub, or some sort of hub anyway, for my computer. The motherboard didn’t come with enough USB ports for my needs.

A long cable can help lower a VSWR due to loss too. Any loss (in dB) would lower the return loss (another way to look at VSWR) x2. So if the antenna has a 10dB return loss and the cable loss is 1dB, then the overall return loss presented to the receiver would then be 12dB. (10dB +1dB +1dB).

Granted, RG6 is 75 ohms, not 50 ohms - so you could very well be correct about some matching accidently occuring. If you really want to know if that is happening, then add an additional 1/4 wavelength of coax (that would rotate you half around the Smith Chart). If the VSWR is still good, then it’s the coax loss. If it’s worse - then the cable was inadvertently matching it and by adding additional length it’s no longer matched. Or just look at the antenna by itself with your VNA. Plus there may be a little of both (loss/matching) happening.

I can explain in more detail if you want to know why or more about return loss (maybe you already know this stuff since you have a Nano VNA!). I’m a recently retired RF engineer. Specifically, transmitter design but I also did the receiver LNA’s since they were usually located on my PWB. Everyday for the last 35 years of work, I was involved with impedance matching. I can read a Smith Chart better than a map!! :slight_smile:

1 Like

I used to kind of know about this stuff. I once had a 2nd class radiotelephone license from the US FCC so I could legally work on commercial radio transmitters but I haven’t done so since the 1980s. I just got the NanoVNA at the same time as the FlightAware Pro and used it for the first time yesterday. I set it for one trace, VSWR. I’ve never seen a Smith chart before but I understand what an old-fashioned SWR meter’s telling me. Unfortunately I switched careers from radio and telephony to supporting computers so I’ve forgotten quite a bit about radio and antenna theory.

I have another 3 meter length of RG6 in my parts box, but what I don’t have is a spare female-female F adapter. I do however plan to take the Nano upstairs when I have time and check it when connected straight onto the 3m of coax that runs from the antenna itself into the upstairs bedroom. I’ll get back to y’all after I’ve done that.

1 Like

Don’t mean to derail the thread, but the NanoVNA is pretty slick isn’t it? I actually have a HP VNA (8753), but for $60 for the Nano is amazing. I’ve compared the NanoVNA (I have the newer version) to the HP and it’s actually pretty darn close even in the GHz range. I take it you are calibrating it w/a open/short/load?

A Smith Chart looks complicated, but it’s actually very easy and visually helpful for matching. It’s literally drawing lines from one point to another. Those lines represent the L’s and C’s needed for a match. Of course there’s a lot more to matching, but the Smith Chart provides a visual representation of how to go from one impedance to another.

Return loss is as simple as this: The VNA spits out a signal. If it was a perfect match, nothing would be reflected back (i.e. a perfect 1:1 VSWR). Everything is absorbed in the load (in your case, the antenna). If it’s not a perfect load (i.e. some sort of VSWR >1), some of it is reflected back. So for example, if the VNA outputs a 0dBm signal and the the reflected signal is -9.5dBm, the return loss 9.5dB. The 9.5dB example is about a 2.0:1 VSWR. To me, return loss is more meaningful since it deals w/dB’s - easier to do the math in my head (i.e. 10dB return loss would be 10% of power being reflected, 3dB is 50% power being reflected).

Sorry, tried to not go too far off topic w/antennas, VSWR, etc.

1 Like

It is that. It’s the first time I’ve been able to measure VSWR of a receive-only antenna. My other experience was with VHF public safety band and CB radios where you clicked the mic while getting a reading, then trimmed the antenna length slightly until you got a pretty good match, 1.1:1 or 1.2:1, and called it good. I’m going to have fun exploring what else it can do.

FYI I took my cheap externally-powered USB 3.0 hub upstairs and zip-tied it to the antenna support contraption that I cobbled together out of the stand from an industrial-style electric fan and 1" PVC pipe, ran an extension cord to it, plugged the hub’s USB cable into the extension cable, plugged the FA pro into the hub and connected the 3 meter length of coax to it. Wouldn’t you know it, there’s suddenly a dearth of airplanes within range of my antenna which is level with the gutter on this 2-storey house and about a foot or so away, so now I don’t even know if it’s working or not. I’m still limited in many directions by some nearby houses and trees. When more planes show up I may have to readjust the gain again with the autogain1090 app. I’m waiting to find out.

When I get my new USB hub it will be deployed on my PC for general use. They never give you enough USB ports.

1 Like

That experiment failed. Dump1090-fa couldn’t talk to the FA dongle in that configuration. I decided to put the NESDR SMArt back upstairs sans USB hub, dedicate it to ADS-B and just use the FlightAware Pro as a general-purpose SDR. :frowning: (I should have bought an RTL-SDR Blog stick instead.)

Edit: I think the problem is some flakey ports on my cheap hub, not the dongle. It wasn’t even showing up with lsusb until I switched ports. I’ll try it upstairs with ADS-B again after I get the other hub.

I think I’ve solved the problem with the FA Pro dongle. The SAW filter I ordered arrived today. I put it in front of my NESDR SMArt and it seemed to work so well that I decided to move the FA dongle back upstairs again instead of the NESDR, connected via the 10 meter USB 2.0 extension as before. What a difference! My message count has increased by at least 33%, I’m seeing twice as many airplanes at peak air traffic times, and some of the gaps in my receiver range are being filled in. :grin: This is with the same antenna (a homemade “coketenna” cantenna) , location, coax and USB extension cable. I think the FA Pro stick with its built-in LNA was getting and amplifying a lot of out of band interference that it was covering the ADS-B messages.

1 Like

Hey Obj,

I saw a post earlier indicating a couple of weeks. What will the adaptive gain work with, just SDR dongles or also Airspy??

Jeff

For 6.0, rtlsdr dongles only. The logic is SDR-agnostic, though, so it’s easy to expand to other dongle types supported by dump1090-fa in the future - all that is needed is a couple of gain control hooks. Once we have airspy support merged into dump1090-fa it’ll get adaptive gain support almost for free.

2 Likes

Sorry, just to be clear, that meaning FA Pro plus as well?? (getting re familiarised with this sites “ADS-B speak” …lol)

Jeff

So regarding the “almost for free”, should be expected that those that are using an airspy will have to pay to have the adaptive gain function? Something similar to a plugin or addon?

Emil

1 Like

For free as in for no extra work, not as a paid feature.

2 Likes

You don’t need that on an airspy.