Friday, 1 April 2016

Best RTL-SDR setup for $60

April 1, 2016
Update April 2, 2016 with Adam's insight.


For 60 dollars, the setup discussed here will be pretty much the next evolutionary step from a cheaper dongle, and will be an excellent platform from 24 MHz to 1700+ MHz for all signals.

An LNA can be good for you, and bias-T makes life easier. 
It's not widely known that the popular LNA4ALL can be ordered with bias-T enabled, which makes it a plug-and-play companion to rtl-sdr.com's Silver Dongle.
Bias-T means that the radio receiver is sending power down the coax cable, and that power makes pre-amplifiers work. No hassle with batteries or wires, LNA can be placed at the antenna.
The only RTL-SDR dongle right now which can do bias-T easily is rtl-sdr.com's excellent Silver dongle, latest R820T2 chipset with temperature controlled oscillator - TCXO for short.
"Standard" dongles can be also modded for bias-T, but the operation requires considerable soldering skill (link), and removes the static protection diode. I wouldn't do that.
Some SDRs, like the Airspy or HackRF, already come with bias-T from the factory, but cost over 200+ USD.
Some SDRs, like the SDRPlay RSP have a built-in LNA, but also cost significantly more (SDRPlay 150+ USD).

Money, money, money


The dongle in this review costs $25 (link) or $20 without antennas, and the preamplifier (small golden square in images) costs $30 from Adam (link).
That's close to sixty dollars, which is a lot, you get what you pay for.
It's the same dongle (link) and LNA4ALL (link) as I tested earlier, the only difference is bias-T.
Disclaimer: I did not pay for equipment in this review. That doesn't make a difference, neither maker said a word what to write, but I think it's fair to let you know.

Why use a pre-amplifier / LNA?


A low-noise preamplifier (that's the "LNA" in "LNA4ALL") will add 20-30 dB gain, depending on frequency.
That's a lot, 20 dB will be 100 times stronger, 30 dB means signal will be 1000 times more.
If you wanted to listen to something, and you had to move the slider midway in SDRSharp, now moving just a little bit from the left will have the same effect of moving slider almost completely to the right.
If you want to hear more, you need higher gain, but higher gain means more electronic noise.
Noise is not good for you.
What you hear is against a so-called noise floor, and lower noise floor is better.
A preamp brings down the noise floor, less gain = lower noise floor in the dongle.
The best preamp out there is called LNA4ALL, made by a Croatian guy called Adam, costs about 30 dollars shipped.
He will send you a bias-T ready preamp when you ask him. Plus, if you have a question, he will answer, no matter what time of the day. I got replies from him at 0400.
RTL-SDR.com's dongle comes with a bias-T option, takes a few minutes to make it work.

Adam's insight:


Due to a comment/suggestion regarding LNAs and use for bias-T, emailed Adam, who replied with the following:

Quoting:

Hello Akos,

regarding the LNA4ALL and Bias-T enabled but powering the LNA4ALL using the side pads at the same time.
In this case, the DC power from the side pads will be routed through the on board LNA4ALL bias-t to the coaxial cable going to the receiver.
If the receiver / dongle input is  AC coupled then nothing will happen to the receiver / dongle but if the receiver / dongle is DC coupled then the damage may occur on the dongle.
Just imagine the DC power coming on the BAV-99 diode at the input of the dongle. It will smoke if the diode is DC coupled to the antenna connector or in parallel to the connector.

Regarding the LNA in front of the dongle. Yes there is a benefit in both cases, LNA close to the dongle and LNA close to the antenna.
LNA close to the dongle will lower the system noise figure. Usually the dongle noise figure is 5dB and higher. The LNA noise figure is let say 0.8dB and the system noise figure will result roughly 1dB. In this case we improve the S/n for about 4dB. This is significant on the UHF and higher. If you connect the antenna strait to the LNA input you will see the benefit of the LNA for sure. Actually this was the idea of using the FlightAware pro stick with the small antenna.

Placing the LNA close to the remote antenna and feeding the signal to the receiver over the coaxial cable will result in same, and will overcome the losses in the coax problem. If there is no LNA close to the antenna the losses in the coax will increase the overall noise figure for the same value. Friis formula is well known and there should be clear enough what is going on in this case. If this coaxial cable is short, then no visible difference in the S/n.

Adam

End of quote.

Enabling bias-T on RTL-SDR.com dongle


Two options: solder, or place a wire across the pads and fix it in place.

Soldering: Heat up iron for 10 minutes, feed solder wire until you get a nice blob, drop it on the two designated pads.

Wire: If you got no soldering station, cut a really small piece of wire from coax center, fix in place with hot or instant glue. Keep wire in position with toothpicks.

Bias-T with LNA4ALL


Please forget the notion you can do it.

Ask Adam to send you a bias-T enabled LNA4ALL, don't even start contemplating that you can do the same job.
Do-at-home: connect a wire between two really tiny pads and add a 10uH inductor, and when I say small, I mean half the size of a rice piece.
No, probably you can't, leave it to the professional.


Where to put an LNA


A preamp should be as close to the antenna as possible.
If you have lots of cable before it, it will amplify all the junk picked up by the coax.

What not to do


Never connect a bias-T enabled dongle to a DC short antenna without a suitable LNA, it will trip the fuse on rtl-sdr.com's dongle, and will probably fry any other dongle.
Examples of a DC short antenna: folded dipoles, a QFH, Adam's ADS-B antenna and any other antenna where a multimeter beeps in continuity mode.
Never connect a bias-t enabled LNA4ALL to a non bias-T enabled dongle, more than likely it will smoke the dongle.

Update: Adam's insight on this is included in the post.

Warranty? 


Officially, doing the bias-T mod, you might lose the warranty on rtl-sdr.com's dongle.

In real life, I don't know. Probably, you write to Adam, or rtl-sdr.com, and they send you a new one.
Been using Adam's pre-amps for years, and they still work without a hiccup.
RTL-SDR.com's dongle is tough, drove over it with a family sedan, been using them 24/7 for over a month and both still work.

Conclusion


For $60 shipped, you won't go wrong with RTL-SDR.com's Silver dongle and a bias-T enabled LNA4ALL.
Both use the same SMA connectors, enabling bias-T is kid's play on the dongle, no wiring with the LNA4ALL.

If you enjoyed this post, please order my RTL-SDR
Guide Book on Amazon Kindle.
Close to 200 pages of knowledge, information and diagrams for all levels of expertise.

Kindle is available for computers (Windows and Mac), and for smartphones and tablets (Android and Iphone, iPad and iPod touch).

If you're interested in the Raspberry Pi, my book details the ins and outs of setting up, using and enjoying the Pi 3 microcomputer.














































3 comments:

  1. Hey Akos,

    thank you for this post!

    I found some paragraphs confusing. Here they are:

    > Never connect a bias-t enabled LNA4ALL to a non bias-T enabled dongle, more than likely it will smoke the dongle.

    I think Non-Bias-T dongle have zero voltage at antenna connector, neither LNA4ALL will send any current back to the dongle... I cannot see why dongle would smoke.



    > A preamp brings down the noise floor.

    I understand where you are coming from, but this phrase might benefit from rewording. Something like this:

    - when installed at the antenna, the LNA increases the voltage from antenna terminals, overcoming huge losses caused at coax cable.

    - when installed close to the dongle (not ideal, but it might be the case), allows you to have readable signal with low gain. Low gain can help reducing the presence of local generated spurii signals.

    Thank you!

    P.S.: I did Amazon's workaround (visit My Books & Devices) and it work just fine, the correct version of the RTL book has been finally delivered.

    ReplyDelete
    Replies
    1. Hey hg,

      Updated the post with manufacturer's comments, hope it clears any questions.

      Let me know what you think about the book, email in foreword, or comment here.
      Again, thanks for feedback.

      Delete
  2. Many thanks for feedback, I will revise the post.
    Can't describe how happy Amazon sent out an email, let me know here, or via email (details at the start of the book) how you like it.
    Without feedback it's impossible to write a better book or a post, so your comments are truly appreciated.

    ReplyDelete