RTLSDR calibration - which reading to believe?

Bermuda1

New Member
I have been trying to receive ADS-B traffic using a "cantenna" and an RTLSDR. I am currently getting around 60 miles coverage from the upstairs window. My setup is just the cantenna, RTLSDR and a Raspberry Pi. No pre-amp or special co-ax at this time, just a short length (about 70cm) of RG58U + a short length of wire making up the BNC to SMA adapter.

During my research I found a reference suggesting that calibrating the RTLSDR migth bring some improvement, so for this purpose I downloaded and compiled kalibrate-rtl. The version cloned directly from the Github respository did not run on the Pi, which is another story, but this was ultimately resolved by downloading the ZIP package from this Github page:


This compiled and ran fine. However, I tried to calibrate using tow options:

Option 1) use kalibrate to scan for GSM900 signals and use the strongest one to calibrate the ppm offset.
Option 2)use rtl_test -p

The first option returned a value of -38.299. The second one after a few minutes settled at around 78. I re-ran the first option again and got -38.287 so a very similar reading again. So why are these two readings so significantly different, and which one do I trust/use?

I did play with both, but I'm not sure that I am seeing much difference to be fair, perhaps only a subtle one. The same can be said for using --enable-agc or --gain -10 on the dump1090 command line. The difference seems minimal. I have yet to experiment with using positive values for gain.
 

wiedehopf

New Member
What is the make of your dongle?

With good dongles like the rtl-sdr v3 dongle you don't need any correction.

For the others the correction is also not too relevant according to tests done by some people.

Maybe try making a quick spider and see if it performs better than your cantenna:
https://discussions.flightaware.com/t/quick-spider-no-soldering-no-connector/19181

Having no other electronic devices in the same horizontal plane as the antenna can help as well.
But if the window is all you have to work with then you will be stuck with it.
 

Bermuda1

New Member
The one I am currently using is a v2. Not sure about the two that I have just ordered, but they are these:


Thanks for suggesting the quick spider antenna. I had a look and found a few bits that I could use to build a quick spider. I had a short 30cm approx piece of co-ax and an odd metre of twin and earth mains cable out of which I pinched the earth wire to make the radials. I also found a salvaged BNC socket. I reckon it took me about 30 mins to put together. I have attached a photo. The foil near be bottom is to shield the joint with the inverted BNC socket. I placed some insulating material around the core.

Performance seems to be at least as good as, but possibly slightly better than the cantenna. The difference is subjective but I think I am picking up aircraft from slightly further away. The difference, if any, is quite small, probably no more than 5-10 miles and I seem to be getting perhaps a couple more entries on the list than previously. Still, any improvement is welcome and one advantage of this antenna is that I don't have to come into close contact with the sharp edges of a metal can every time I attach or disconnect the co-ax! I did file down of course, but they do remain pretty sharp nonetheless and great care is needed if one is to avoid cut fingers.

I guess this kind of range is probably the best achievable without a pre-amp, so I guess that will be the next step although I will probably have a go at building a bow-tie antenna for airband first.
 

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wiedehopf

New Member
Yeah the rtl-sdr blog produced dongles are better than the one you linked, they have a metal case and reduced noise.
You can the the rtl-sdr v3 dongles here:
https://www.rtl-sdr.com/buy-rtl-sdr-dvb-t-dongles/

Those also have a bias-t that can be activated via software, which is necessary for example for this LNA (https://rtl-sdr.com/new-product-rtl-sdr-blog-1090-mhz-ads-b-lna/)

Works very well together and due to filtering and amplification normally greatly increase range.
(Limited by terrain of course: https://discussions.flightaware.com/t/what-is-the-maximum-range-i-can-get/17248)

The antenna looks very nice :)

As you have probably already read, with a can and by reducing the whip length the DVB-T whip antennas can be made into acceptable cantennas :)
 

Bermuda1

New Member
I am experimenting with this one at the moment:


However, I have seen those from RTLSDR.COM on eBay and also these which seem to be similar:



The UZEUNER one comes with the necessary co-ax adapters which would be useful as I don't have any here. Also the warehouse is in the UK and the wait time is 4 days. Would this be OK, or is the RTLSDR.COM one better?
 
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wiedehopf

New Member
I wouldn't buy those. If you want to buy via ebay, use their store:
https://www.ebay.com/str/rtlsdrblog

The rest of the offerings pretending to be rtl-sdr.com blog v3 dongles are fakes.
(Or one of the offerings doesn't even pretend that it is the rtl-sdr blog v3 design.
It's of course still rtl-sdr compatible but not the design from the blog)

If you have to buy adapters separately that's unfortunate but unavoidable.
And you'll have to wait for shipping of course but you really don't want the fake ones.
 

Stealth

New Member
Anyone know much about this one:


It claims to have both a pre-amp and filtering built-in.
It would be ideal and reduce the amount of clutter?
It does have a pre-amp and a filter, although at the receiver end of the cable is not the best place for a pre-amp. I used one for about a year and it was fine. One thing to be aware of is that an additional filter may almost certainly be required if you are in a strong cell phone signal environment.

wiedehopf is very supportive of the rtl-sdr.com dongles and he will have done a fair bit of testing with them. The aluminium case for screening is a strong plus.
 

wiedehopf

New Member
The external LNAs are better but the Flightaware Prostick does work.
Depending on the electrical noise though you may need the FA filter in addition.
(The LNA may get overloaded. The external rtl-sdr LNA has a high-pass filter on the input and lower gain on each stage to avoid overloading problems)

FA pro stick plus + filter is almost as much as just getting the rtl-sdr dongle + LNA, that's why i recommend the package that has the better performance.

You could also get just the rtl-sdr v3 dongle now and maybe buy an LNA later if you are short on cash or just want to experiment first :)
 

Bermuda1

New Member
FA pro stick plus + filter is almost as much as just getting the rtl-sdr dongle + LNA, that's why i recommend the package that has the better performance.
Having considered all of the helpful advice, I have decided to order the RTLSDR dongle + the ADS-B LNA as this does seem to be the best option. It will be interesting to compare their performance with just an ordinary RTL dongle.
 
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