NooElec with small antenna ---> FA Pro Stick + FA 1090 Antenna = worse results?

[klinquist]

You can utilize the existing cable also by the following very easy and simple workaround:

Thank you for that. When I posted earlier today, I was at work and Amazon had same-day delivery on the right cable. Plugged it in as soon as I got home and the # of aircraft my Pi saw with default gain settings went from 14 to 50 immediately .

Looking forward to what I'll be able to see tomorrow.

[abcd567]

.....well crap, glad I posted the link! Just ordered a new cable same-day delivery, will report back!

You can utilize the existing cable also by the following very easy and simple workaround:

TEMPORARY WORKAROUND
The SMA-male has center pin. The reverse SMA (RP-SMA) male does not have a center pin, it has a hole. The center wire connection is open because of missing pin. A temporary workaround is to cut a short piece of center wire of coax and insert in hole of RP-SMA male. This piece of wire will act as the missing pin, and connection will be established.





DIFFERENCE BETWEEN SMA AND RP-SMA

[klinquist]

That cable appears to have an RP-SMA connector. That's the reason for poor data. You need an SMA connector:

https://oscarliang.com/difference-sma-rpsma-connector-antenna-fpv-copter/

The pin is missing. You can try inserting a short piece of copper wire to test.

.....well crap, glad I posted the link! Just ordered a new cable same-day delivery, will report back!

[Anmer]

That cable appears to have an RP-SMA connector. That's the reason for poor data. You need an SMA connector:

https://oscarliang.com/difference-sma-rpsma-connector-antenna-fpv-copter/

The pin is missing. You can try inserting a short piece of copper wire to test.

[klinquist]

Try Gain 30 dB.
Users having FA Antenna and Pro Stick /Pro Stick Plus, generally use gain between 25 dB and 35 dB.
I use 28 dB with one receiver, and 30 dB with other one.

Understood, will try this.
I'm developer and/or devops guy by trade, so I have no issues figuring out linux stuff (but thank you, I'm sure that'll be beneficial to others reading the thread).

[klinquist]

What spec cable are you using with the FA external antenna? Did it come with pre-installed connectors?

I'm using this one:

https://www.amazon.com/gp/product/B007PQ0LPG/ref=oh_aui_detailpage_o06_s00?ie=UTF8&psc=1

So last night I connected the yellow FA stick directly to the antenna with NO filter and it seems that I'm getting significantly better results - even saw one aircraft 160nm away. This was using dump1090 with completely default settings.
I'm running the gain python test now to figure out the ideal gain and see how much better I can do.... but it seems that any of the filters I've tried so far (the one built into the blue stick and the external one) are making things worse. Odd, I know!

[abcd567]

Try Gain 30 dB.
Users having FA Antenna and Pro Stick /Pro Stick Plus, generally use gain between 25 dB and 35 dB.
I use 28 dB with one receiver, and 30 dB with other one.

Python Script to optimize gain is a help, but dont give any concrete value. Finally it is trial and error to find best gain.

pi@raspberrypi:~ $ sudo nano /etc/default/dump1090-mutability

Above command will open file in nano editor
Press Ctrl+W (i.e. Find), then type GAIN= and press Enter Key. The cursor will jump to GAIN=
Change the gain value (shown in red)
Save file (Ctrl+o), then close the file and exit editor (Ctrl+x)

.....................
.....................
.....................
# Receiver options
#

# RTLSDR device index or serial number to use
# If set to "none", dump1090 will be started in --net-only mode
DEVICE=""

# RTLSDR gain in dB.
# If set to "max" (the default) the maximum supported gain is used.
# If set to "agc", the tuner AGC is used to set the gain.
GAIN=30

# RTLSDR frequency correction in PPM
PPM="0"
.....................
.....................
.....................

Now restart dump1090 for new value of gain to take affect.

sudo systemctl restart dump1090-mutability

Now check the status. It will show the gain value in use. This value will be retained on reboots and restarts, unless you change it.

pi@raspberrypi:~ $ sudo systemctl status dump1090-mutability -l

[Strix technica]

Yeah, I think trying the old antenna with the FA stick and visa versa is definitely worth doing, as is giving some thought to the coax feeder. Golden rule of experimentation: change only one thing at a time.

Golden rule of debugging something: start with what once worked, and reintroduce elements until it breaks, don't try to go from what's broken to what works because if there's more than one thing wrong, a problem fixed can remain masked by others and you won't even know that you did, in fact, fix a problem.

Unless your coax run is very short, then consider that a third variable. Remember that different coax has different amounts of loss per metre, but I'd be surprised if the supplied antenna was fed with anything other than RG58 (or maybe RG59) which is about the worst coax commonly found. But even then, not all RG58 is created equal either. Some of it's rubbish, some of it's okay (depending on application).

In theory, impedance mismatches will make things worse even for rx-only, but there isn't much you can do about that except not make it worse. At least in the generic SDR case, the receiver is designed for a 75Ω feed, yet the connectors used are typically 50Ω. (In practice, that impedance mismatch loss is only about -1 dB, but you don't want to switch between impedances more often than you have to.) In the FA case, the LNA might actually be 50Ω in which case mismatch loss hardly matters because the signal will be plenty strong enough by the time it reaches the 75Ω R820T2 RF mixer, and low-loss 50Ω RG-8 to a 50Ω antenna would be ideal.

Your antenna has a characteristic impedance also. Whips range between 36–50Ω, dipoles are about 72Ω. In practice, dielectric loss is going to be more of a problem that mismatch loss, but it's as well to bear it in mind. Just don't constantly switch back and forth between impedances any more than you have to.

Tonight I installed the "regular" orange FA stick and the FA 1090mhz filter....aaaaaand similar results to the blue stick.

Which is no surprise if the problem is intermod noise. As I said, the FA BP filter's bandwidth is too wide to exclude all of GSM, especially in North America and the in-line filter is a crappy LC network rather than a SAW or cavity filter.

If you're running dump1090-mutability or equivalent, have a look at the range of signal strengths it reports. My Munin plugins are good for this, but most of those only work with -mutability. If your peak signal stays stuck at > -5 dBFS all day and night, and your mean signal is around -10 dBFS, your gain is probably too high.

[Anmer]

The test I suggested earlier is to try the FlightAware Pro Sticks with the antenna that cam- e with the generic stick and then the generic stick with the FA antenna and cable.

What spec cable are you using with the FA external antenna? Did it come with pre-installed connectors?

[klinquist]

Tonight I installed the "regular" orange FA stick and the FA 1090mhz filter....aaaaaand similar results to the blue stick. Still nowhere near as good as the little antenna with the generic stick! I'm thinking my next move might be to order an adapter to hook the generic stick to the FA antenna to see if the issue is with the antenna or not.

Here's the setup (note the "better" 1090mhz filter has not yet arrived)

IMG_9636.jpg

[Strix technica]

heh apparently I can't post links in the forum until I have at least 4 posts...

D'oh! Thought it was 3, but they may have changed that.

Note I *also* ordered the regular Pro stick w/o the filter.... for even more comparisons.

I look forward to hearing your results! Money is tight right now, but I'm awfully tempted to order one of those SAW filters because I'm nearly certain I've got intermod problems.

The filter bandwidth 8MHz, Cutoff characteristics steep.

Thanks, abcd! Looks like a pretty neat piece of kit, pretty decent insertion loss for a passive filter (< 3dB). If'n I had the money, I'd get one of them and the cavity filter I linked the other day and compare them.

YES, the ProStick+ (blue) uses a SAW filter, but at wrong location. The SAW filter chip is located between pre-amp and tuner.

Alas... What were they thinking, aside from "ooh, there's a largely empty spot on the PCB where we can glom on a SAW filter for extra mark-up!"?

I have found this one couple of months ago while browsing ebay. I was very much tempted to order it, but it's price prevented me. At US $28.99, it is almost double the price of Flightaware filter (US $15.50).

You get what you pay for. The FA stand-alone filter, as you pointed out upthread, is a passive LC filter, ie crap.

[abcd567]

Ok, I just *also* ordered a SAW filter from Hong Kong .

I have found this one couple of months ago while browsing ebay. I was very much tempted to order it, but it's price prevented me. At US $28.99, it is almost double the price of Flightaware filter (US $15.50).

[abcd567]

Excellent! Can you post a link to its specs? It'll be interesting to compare the two (you should be able to post URLs now, I think).

From ebay page http://www.ebay.com/itm/272374693052

Features:
The filter bandwidth 8MHz, Cutoff characteristics steep. 1090MHz performance for the ADS-B receiver Optimization,Can effectively filter out-of-band signals,Reduce noise,Increase the dynamic range

Modell: FBP-1090s
Impedanz: 50 Ohm
Mittenfrequenz : 1090 MHz
Passband : 1086 ~ 1094 MHz
In-Band-Einfügungsverlust : <3dB
Maximale Lastleistung : 10 mW (10 dBm)
Schnittstelle: SMA-M, SMA-F

If the Pro+'s BP filter is also an LC network that might explain why the filter is after the LNA — after all, one of the two principal reasons for using a bandpass filter is to avoid intermodulation in amplifiers — but from this image, I would guess that the Pro+ uses a SAW filter.

YES, the ProStick+ (blue) uses a SAW filter, but at wrong location. The SAW filter chip is located between pre-amp and tuner. If it was located between the antenna connector and pre-amp, an external filter will not be required with ProStick Plus (Blue one).

[klinquist]

heh apparently I can't post links in the forum until I have at least 4 posts... so it's ebay item ID 272374693052

Note I *also* ordered the regular Pro stick w/o the filter.... for even more comparisons.

Already got too much money on this project .

[Strix technica]

Ok, I just *also* ordered a SAW filter from Hong Kong .

Excellent! Can you post a link to its specs? It'll be interesting to compare the two (you should be able to post URLs now, I think).

Based on what abcd567 said, the FA BP filter is an LC network, and LC networks suck, both in terms of insertion loss and linearity and probably also in terms of phase/group velocity distortion. Not saying SAW or cavity filters are perfect, but both are probably way better than LC networks.

If the Pro+'s BP filter is also an LC network that might explain why the filter is after the LNA — after all, one of the two principal reasons for using a bandpass filter is to avoid intermodulation in amplifiers — but from this image, I would guess that the Pro+ uses a SAW filter.