Tuesday, 16 August 2016

Review: Soft66RTL3 All-in-One

August 16th, 2016

$38 dollars shipped: RTL-SDR based receiver with upconverter and band preselector, thermal pad and heatsink. In a metal case. Covers DC to daylight.
Too good to be true, so I ordered one.
Available on maker's webpage (link). Turns up on ebay from time to time, not available at the moment.

First impressions

Delivery took 15 days from Japan to Ireland, unassuming brown postal envelope, receiver carefully covered in bubble wrap and a final plastic layer - unwrapping literally felt like Christmas came early.
Marginally larger than a regular RTL-SDR dongle, connectors protected by translucent yellow caps, tough-looking aluminum shell (not a full enclosure) sided by two rubber/plastic/unidentifiable material at connectors.

Side panel gap on one side was significant enough between cover and metal housing that light shone through.
No antenna, power cable, instruction manual, thank you card, but that seems to be the norm with small-scale manufacturers.

Importance of cable

Needs a USB Mini-A cable for data/power: noise pickup was terrible with a plain and long cable. One more cable and connector type to hunt down - could please manufacturers settle on Micro-USB and offer the option to order one along with the receiver? Thanks.
If buying new, shop for the shortest cable with one or two blobs on each end. Those metal cylinders are called ferrites, and will lower radiated noise pickup from household items.

Snap-on ferrites are an alternative and only cost a few dollars.


Antenna connectors: SMA Female, compatible with handheld transceiver antennas and premium dongles' antenna cable.

Two separate connectors, one for HF (below 30 MHz) and one for higher frequencies - two antennas can be connected at the same time, like a discone for 30 MHz and up, and a dedicated wire for below.
HF connector is right next to selector switch, easy to identify, red = HF.

Preselector and LED: 4 bands selectable with red rotary switch, which is also removable - slot for screwdriver on both, attention to detail.
Small hole: says "RF gain" on the product webpage, does nothing above 30 MHz. Needs small cross-headed screwdriver.

Take it apart

Remove two massive screws (largest and toughest ever seen on any dongle-related product) from the preselector side, board slides out from grooves integrated into the side shell.
Text on board by a tripping post-apocalyptic beauty pageant:

FUKSHIMA Human extinction

Architecture is pure genius: a tiny dongle (marked by red lines in the image) piggybacked onto a printed circuit board (PCB). Dongle's antenna connector is soldered onto the PCB's SMA connector, presumably by hand, fingerprint visible on back of board. Talk about personal touch.
Thermal pad and heatsink stuck onto dongle, blue material is much more stickier than $1 China pads or the SMArt's layers; remained sticky after countless removals.

Top of heatsink and chips on other side leave a minimal gap.

Band preselect and upconverter

Uses 50 MHz local oscillator.

Product webpage shows wrong settings.
Red knob with clicks, numbers on the dial, turn to a number and signals in the corresponding range should be better (called a band-pass filter in radio speak).
LED lights turn red for 30 MHZ and up, and green for HF:
Position 4: 0.4  to 1.2 MHz (LED green)
Position 5: 1.2 to 5 MHz (LED green)
Position 6: 5  to 15 MHz (LED green)
Position 6: 15 to 30 MHz (LED green)
Other positions: upconverter off (LED red).
Turning the knob in VHF and UHF has a small effect, 8 and 9 are the same, zero raises the noise floor.

On HF, wrong setting will completely eleminate received stations.


Worked with SDRUno straight away (beginner start guide here link), latest SDRSharp did not recognise receiver, but previous version functioned well.

Drift, heat, tuner

Weird behavior, three stages:
1. Extremely rapid drift, then a jump,
2. Gradual drift to 26 ppm in 5-6 minutes, then
3. Slow final settle in an additional 8-10 minutes.
31 ppm final once thermal equilibrium reached.
Case temperature by hand is well below other metal enclosure dongles.
R820T designation refers to the tuner part of any RTL-SDR dongle: determines receive performance. Newer and more sensitive version is called R820T2, used in recent dongles from $8 generics to $20+ premium products.

Performance VHF and UHF

Same outdoor pro discone antenna I use for years, and staple testing signals were simply not there. Constant and weak data dowlink in the image below, 420-ish frequency, exact value removed as I suspect it's government comms:

Sensitivity is terrible all across the spectrum, and not just on weak signals: even local airport information system required the most gain to get the lowest Signal-to-Noise ratio, and sounded much less clear and noisy.
From top left: rtl-sdr.com dongle with R820T2, $8 chinese generic, old Terratec with R820T and Soft66RTL3 in bottom right corner, very strong local signal, numbers in dB are Signal-to-Noise ratio, higher is better:

ADS-B performance with an outdoor half-wave antenna on a ground plane metal box from a first-floor windowsill was well below average: 5 messages per airplane, when 30-35 is normal for my location.
After a week of daily use, and with a choice of either the Soft66RTL3 or any other dongle, I don't reach for the tiny black box.
Audio quality is not great, similar to listening to music with a thrift-shop earphone.
Constant buzzing, noise everywhere, like the neighbor shaving on a washing machine whilst simultaneously fighting a hive of bees.

HF performance

50 MHz is added to received signal, so 7 MHz will be heard on 57 MHz.
City center with 20 foot / 6.5 meters of wire, screenshot of 40 meters, signals everywhere:

At least it works reasonably well on HF with a nearly quarter-wave antenna for this particular band.
Have a dongle and thinking of plugging the Soft66RTL3 in with a USB cable? A Ham-It-Up from Nooelec costs $42 plus shipping, doesn't come with a metal case, but soothes with brand-name peace of mind and 6 months' warranty.
And better performance, R820T also used to give the Soft66RTL3 a chance, gain adjusted in software for best audio on strong shortwave broadcast station with a discone.

Ham-It-Up v1.3 with either dongles resulted in understandable audio, Soft66RTL3 was unintelligible.
Strongly suspect that the PCB board / upconverter part is quite noisy, compounded by less than stellar R820T dongle performance.


Disappointing, because this little all-in-one promises tremendous value for money.
Doesn't deliver: performance above 30 MHz is absolutely and comparatively sub-standard, HF works with a long wire.
38 dollars for the Soft66RTL3 is a waste of money; buy a generic chinese for $8 and try direct sampling if funds are short.
Better buys exist: spend 25 bucks and get a full, usable and high-sensitivity premium dongle package from either rtl-sdr.com or Nooelec.
Maybe if small size appeals, and you are desperate for a dongle plus upconverter.

PS. Soft66RTL3 for sale, one lady owner, always garaged, low mileage.

Sunday, 7 August 2016

SDRUno First Steps Beginner Guide - Updated Aug 07

Update Aug 07, 2016: Expanded and revised post with lots of info and images. If you see an error, know a better way or stuck with a problem, comment below, or email me (address in Manifesto).
Really. No man is an island. Share your wisdom, so all of us can learn from each other.
Many thanks to all who made this guide possible.

An RTL-SDR is an affordable ($10 for unbranded, $30 for premium models)
Software Defined Radio receiver, covering the radio spectrum from 25 to 1800 MHz.
The SDRUno is the "software" in SDR, and compatible with RTL-SDR dongles.
Why bother with SDRUno? Because it has the best receive audio I've ever heard.
Previously known as Studio1, used to be expensive software for high-end SDRs, then SDRPlay acquired Studio 1.
Rechristened to SDRUno, now you can download it for free (pdf, download link).
Official manual included in the software; a separate 28-page quick start guide by enthusiasts is also available to download for free (pdf, download link). Understandably, it focuses on SDRPlay users.
I migrate primarily from SDRSharp and to a lesser extent, from SDRConsole; and been using SDRUno for a little over a month, therefore far from completely familiar with it. It's complex with awesome performance, but the initial hurdle was daunting - exactly the reason for this post: to facilitate a less steep learning curve for others.
Background information and testing notes in Manifesto, underlined text are links, bring you to a new page in a new window. Click / tap images for full-screen glory.


Tested with the following dongles:
- Generic chinese R820T2 - you probably get this if buying off eBay (read previous post here how to avoid rip-offs),
- RTL-SDR.com R820T2 TCXO, old (four screws) and new (two screws) versions,
- Nooelec SMArt and other NESDRs, R820T and R820T2 chipsets, with and without TCXO, both old and new versions,
- Terratec R820T,
- Generic chinese R820T dongle.
List above covers most RTL-SDR dongles on the market, so if your RTL-SDR based receiver specifications say RTL 2832U and R820T or R820T2, 99% chance it will work with SDRUno.

Download extIO file

Update: Go here for original download. Thanks for commenting.
Required for operation, must be placed in "Documents" folder.
Here's a link to my Dropbox if you don't have one saved.

Download and install software

Go to SDRPlay website under Platforms = > Windows (webpage link).
Other operating systems were not tested, as 90% of humanity uses Windows, principles are the same under other OSs.

Download SDRUno, straightforward install, step-by-step guide is available from SDRPlay should you run into trouble (pdf, download link).

You still need Zadig

Use if new to RTL-SDR dongles, skip this part if dongle is working fine with SDRSharp or other SDR software.
Zadig tells the computer that an RTL-SDR dongle is not a TV tuner.
First-timers: download and run Zadig in administrator mode (link for download), select all devices, select Bulk 0 from drop-down menu, Install driver.


This may not affect you, but was annoying for me.
SDRuno uses a central menu - which is a fixed size, so on a full HD (1920x1080) screen, text and buttons will be hard to read and operate.
Really small text is not good for you.
See image how much space Menu and Receiver control occupies at different screen resolutions.
Unlike SDR#, changing text size in Control Panel has no effect on this.
Adjusting screen resolution to a lower setting improves readability, so I went down to 1366x768 to learn where's what.
On a 15.6" laptop, this is the native resolution anyway.

Select 0.96 Msps

SDRUno only works at less than 1 Msps sample rate.
Again, ExtIo file must be in "Documents" folder, then connect dongle to an USB port (should be the same USB port each time, or needs Zadig driver reinstallation), then start SDRUno from the Start Menu or via desktop icon.
Central menu comes up, press letter "h" on keyboard for instant access, or with a mouse:
Press on OPT (top left corner),
- then move cursor over "Select Input" (top of list),
- then click on "Generic RTL2832U OEM" (yellow square with dot in it).
Keyboard shortcut again: press letter "h" for instant access to settings screen.
Sample rate (left hand side, third from top) will be 2.4 Msps by default, this needs to be changed.
Click on the little downward-pointing arrow to highlight it, then press the up key on keyboard five times until 0.96 Msps is displayed.
Tuner AGC (bottom right corner) is automatic gain, uncheck it for manual control.

Frequency Correction can be also set here, if value is known - not an issue with TCXO dongles, will be a constant value after warm-up with regular dongles.
PPM setup needs to be done only once. Omitting this makes no difference to reception, but displayed frequency will not be accurate.
1. Tune to a known and always-on frequency, such as local airport ATIS.
2. Wait 10-15 minutes for dongle to warm up, time depends on dongle.
3. Check signal on SP1 window, peak of signal should intersect red line.
If not, adjust with Up and Down arrows between "0" and "ppm".
Recent generic chinese dongles are pretty good with a maximum of plus/minus 20 ppm, older dongles can be 70-80 ppm off - this depends and varies from dongle to dongle.

Finished, but leave this window open, as proper gain setting is crucial to good reception (detailed later on).
Gain settings screen will stay on top of any other open window.

You'll get familiar with the above as SDRUno does not save its settings, so changing the Sample rate to 0.96 Msps is necessary every time.

My sequence is:
- Start software with white EKG curve from desktop,
- main control window pops up, one left-click into blue, press letter "h" on keyboard,
- Click on downward pointing arrow next to Sample rate 2.4 Msps (Second white rectangle above green rectangle),
- Press "Up" button five times,
- Bottom right corner click into third box from bottom to turn off Tuner AGC.

Taskbar buttons

Main menu will still be open, click on SP1, SP2 and RX.
SP1 is Waterfall / Signals, SP2 is signal/waterfall in close detail, RX is the main control panel.
All clicks start as a separate window, so at this stage you have 5 buttons taking up space on your taskbar.
If this is not to your liking, right-click an empty space on taskbar, click on Properties (bottom one on list), then adjust settings.

Closing SDRUno Main with the red X in the top right corner closes all daughter windows.

Change mouse wheel behavior

By default, mouse wheel behavior is the exact opposite from SDRSharp, rolling down changes to a higher number.
This can be changed in Main window, press SETT (top left corner), select misc, then click on "REVERSE MOUSE WHEEL".

All the numbers

Signals have a number, written in either kHz or MHz. This number is the frequency of the signal. SDRUno displays the whole number/frequency.
The small k, short for kilo, means multiply by one hundred, so 198 kHz equals 198,000 Hertz - the Hz abbreviation is used to honor physicist Heinrich Hertz (Wiki link).
The M in MHz means mega: multiply by million, 14 MHz means 14,000,000 Hertz.
7255 kHz is the same as 7.255 MHz, this is just a custom in the radio world.

Adjusting sliders - example: adjusting sound volume level

All sliders can be adjusted the following way:
1. Left-Click into the slider area, adjustment jumps to the point of clicking, or
2. Bring cursor over the slider, then move mouse wheel up and down for fine adjustment.
SDRUno sound level control applies only to SDRUno, if volume is at maximum, but still no sound when you should be hearing something: 
- adjust system volume with speaker icon next to clock (usually bottom right corner to left of clock), or 
- check mute next to slider (should be white text on blue background as in image above).

Resize and move SDRUno windows

SDRuno windows can be resized to your liking; but not all of them, arrow in bottom right corner indicates if a window can be resized:

Start SDRuno

Press SP1 (Main Spectrum window) and RX (Control), then press the Play button in SDRUno Main, starts reception.
Every window has a SETT button (top left corner), controls that particular window's appearance.
SP1 - Main spectrum below, adjust Spectrum Base (orange in image, height of waves from bottom of screen) if you can't see any visible signal.

Eye candy: spectrum and waterfall can be combined into one window, also try various checkboxes to adjust for personal preferences.
With a bit of tinkering, display(s) will make a CIA station chief happy.
Dwelve into SETT options, checkboxes and sliders: SDRUno rewards with a great visual experience.
Don't forget to save your creation - remember Ctrl and two clicks after reading the next paragraph.

Workspace management

One of the coolest feature of SDRUno: set up different environments for different tasks, then save them. For example, have a separate appearance and windows for shortwave and broadcast listening, accessible with just one click.
Nine individual workplaces are possible.
Simple description: workspace name is at the bottom of Main window, change name with Right-click, Enter to accept. Set up workspace. Hold Ctrl, then click on workspace name and click again to select slot.
Example in image and text creates a workspace called "Demonstration".

Detailed description:
1. On the main screen, find the text "Default Workspace" under the green bars.
2. Left-click on it once, jump-up window comes up with available workspaces. In the images, I use Workspace 5.
3. Left-click on Workspace 5. This step selects it as the actual workspace.
4. Right-click on Workspace 5, becomes blue, means you can change the workspace name.
Enter a name, I use Demonstration in the image.
Saving a workspace:
5. Move windows around, set up workspace to your liking.
6. Hold (push down and and keep it pressed) the Ctrl button on the keyboard (bottom left), then left-click name visible under the green bars in the Main window, THEN LEFT-CLICK AGAIN on the workspace slot you want to use. Used windows and their position is saved now.
That's two clicks, once on the workspace name and once on the slot name.
(Took a while to figure this out... . Anyone emailing that description above is excessive will be jinxed to compulsively watch reruns of early Friends episodes.)

Tuning to a frequency - Direct entry

Press RX in Main menu, window called Receiver control pops up.
Long number in the middle is frequency: left-click with mouse, changes to zero.
Default entry in kHz, three zeros after entered number. For example, to get BBC at 198 kHz, enter 198 and press / click Enter.

Onscreen keyboard has the same number layout as a traditional keyboard, numbers in yellow. Bottom right Enter in blue has same effect as Enter on a keyboard.
Dots every three digit make life and frequency identification easier.

Amateur Bands quick access

White numbers on onscreen frequency entry keyboard refer to amateur radio bands - pressing the button No 5 (yellow) / 20 (white) will tune to the bottom of the 20-meter band.

Amateur "meter band" direct click only works if direct frequency entry is not enabled - should be one long number / tuned frequency in receiver control, not just one zero.

Browsing the spectrum - move cursor over number

Change one unit at a time: in receiver control or Main SP display, arrow changes to a hand and yellow line appears above number.
Move mouse wheel up or down to change number.

Received signal is centered under red vertical (up and down) line, how much received each side is called bandwidth, shown as grey area left and right from red line. This is the same in SDRSharp, but it's possible to move bandwidth directly in SDR# - not in SDRUno.
To do the same, open SP2 from Main menu (rectangle where you click Play to start software).
SP2 looks like the child of Spectrum/Waterfall, smaller area, adjust bandwidth there by dragging grey area edges - main Spectrum/Waterfall grey area will also change.
Bandwidth in numbers is also displayed on top of this smaller window.

Tuning a juicy narrowband signal - using Zoom.

HELP! Know a better way? Please let me know!
1 MHz visible is enough to get lots of transmissions and chatter on action bands; problem is tuning to that signal.
Click on the signal, either in Spectrum or Waterfall, then click on Zoom. This will increase the center area of Spectrum/Waterfall, but if the signall falls outside of the enlarged range, move to the number area below, then left-click and hold (grab) to move displayed frequency range. Then zoom again, then grab again, then zoom then...
The procedure above is my greatest gripe with SDRUno, so if you know a setting, checkbox or prayer to make zoom focus on the frequency tuned: please comment, send an email, carrier pigeon or smoke signal.

Automatic Frequency Control (AFC)

Tunes to maximum signal strength nearby - if the signal looks like a mountain, it will move the red line to the top of the mountain.
From Receiver control, left-click on EXV (left of SDRuno sign), then left-click on AFC (bottom left corner, turns white).

It's a great feature for casual broadcast FM listening.
Won't work if there's no steady signal, so for example, if you're monitoring airplane to airport conversations, takes a few minutes to move to the maximum signal.

Using an upconverter (example for shortwave listening)

An upconverter is called an "up-converter" because it adds a number to the received frequency; for instance, a Ham-It-Up adds 125 MHz to the received frequency number.
Frequencies usually in kHz for shortwave broadcasts, signals below 30 MHz and specific amateur radio channels.
Vatican Radio used for the image below on 15595 kHz will be heard on 125+15.595 = 140.595 MHz with an upconverter.

Help / Solution needed:  Under Main => Sett => Offset => Ext Converter Offset parameters can be set for up- and downconverting. Doesn't work for me. Ideas?

Gain control

Accessed from Main window, press letter "h" on the keyboard, or Main top left corner OPT => Select input => Realtek - same setting panel as earlier for 0.96 MSPS setup.
Same values available as in SDRSharp Cogwheel setting, but instead of left and right, slider moves Up (more gain) and Down (less gain).
Less overall gain is required for me with SDRUno, e.g. same station sounds better with 20.7 dB in Uno than with 28.0 dB in SDR#.
Using preamps / LNAs on really strong signals e.g. broadcast FM overloaded with SDRuno even with 0dB gain, whereas in SDR# same station was fine. Turn down preamp gain if you can - this happened with Janilab's preamp which has this feature.

Selecting proper mode according to signal

Pushbuttons below direct frequency entry for a particular mode - which one to use depends on frequency.
Selected mode turns white as in image:

Mode settings above the red line determine mode used: NFM, MFM, WFM, SWFM are only subsets of FM mode, clicking WFM will not change the "boss" mode to FM.

Routing output to external software

Such as for weather satellite images, AIS etc.
Click on SETT (top left) in RX panel, OUT tab, select Cable Input / your solution.

Access official help

Main Menu, top left "OPT" button, second from bottom will be User Manual.
Opens pdf document in separate window, requires Adobe Acrobat reader (download link).

Closing thoughts

The SDRUno has great audio. Even on commercial FM stations, the output sounds better, airplane band is clearer, and it's easier to tell how much coffee the taxi driver had that evening.
Complaints? Software is a highly personal choice, just like choosing your partner, but most males would agree on Scarlett Johansson. Similarly, it's hardly debatable that SDR# has the most plugins, and SDRUno offers only 0.96 Msps visible frequencies and a tedious start-up process for RTL-SDR users, factors which might tip the scale for some. I find SDRConsole great - but this is the realm of personal preferences, so lest I forget:
I truly and fervently hope SDRPlay adds full 2.4 Msps RTL-SDR support as soon as possible. It would make users familiar with the software environment, and might entice potential buyers to upgrade to an RSP.
Give SDRUno a go, you won't regret it.

If you enjoyed this post, or want to get the best performance from RTL-SDR dongles and SDRuno: 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.

Saturday, 6 August 2016

Review: Nooelec ADS-B Discovery 3dBi Antenna Bundle

Two small antennas, one for 1090 MHz and one for 978 MHz, plus two SMA - MCX pigtails for $13.95 + shipping (manufacturer link). Amazon also sells them, check your local store.
What you need to know: these are specialist antennas for mobile applications - perfect for a Raspberry Pi based Stratux receiver, but a size vs performance compromise if you don't own a Cessna, Piper or similar light aircraft. However, two high-quality pigtails sweeten the deal, and both antennas work acceptably with strong local signals.
Background information and testing notes in the Manifesto, underlined text are links, bring you to a new page in a new window. Click / tap images for full-screen glory.

In the bag

Both antennas look exactly like small WiFi antennas found on routers and Internet access points - only visible differences are Nooelec sign and frequency designation, and SMA connector on the ADS-B antennas, versus no sign and Reverse Polarity SMA on the WiFi antenna:

Do not remove the outer plastic cover, as it might damage the antenna; it's surprisingly hard anyway, construction quality is much, much better than a cheap WiFi aerial's.
Receiving elements below - the large bottom one is a high-gain WiFi antenna:

SMA Male connector: works with equipment sharing that standard, such as rtl-sdr.com dongle, Nooelec SMArt, SDRPlay, handheld communications receivers etc.
Two pigtails: washer and nut on SMA end (finally...), angled connector for MCX dongles on the other end, 6" / 15cm 50 Ohm radio coax inbetween, heat-shrink tubing on both ends.
I won't fault you, dear reader, if you don't get ecstatic about pigtails. My mental approach to pigtails and adapters was "Get the cheapest" until I started using quality Nooelec pigtails, and the difference is heaven and earth - I connect and disconnect dongles a lot due to testing.
The same pigtail costs $8 at Noolec (link), so if you need these pigtails, you actually save money and get two antennas for free.


Larger version with 5 dBi gain, costs a bit more (link).
WiFi antennas are obvious candidates, but will require a small wire inserted at the connectors - hence the designator Reverse Polarity in the connector standard.
Doing so might violate local laws, and you'll crawl around a lot if you lose the small wire when unscrewing.
Discretion is the better part of valor - WiFi antennas, or rubber duckies were the weapon of choice for certain listening environments until I got these. Both antennas in this bundle work better than WiFi antennas, and smaller, easier to conceal and less expensive than a rubber ducky from a handheld transceiver.


The telescopic antenna found in the SMArt package (any telescopic for that matter), when extended a little bit was significantly better. 2,756 vs 1,517 total reports, 116 vs 66 total aircraft seen.
For general use, either the signal has to be very strong (e.g. commercial FM broadcasts) or you must be near the signal source. For example, local ATIS got understandable 1.3 miles from the airport, and conversations between Tower and landing aircraft only became clear very close to the runway.
Not recommended as a first antenna due to limitations imposed by small size. For a ready-made do-it-all, search for "SMA Male antenna" on your favourite online store and choose a telescopic similar to this $3 one (link).

Who's it for?

I guess these antennas are primarily for pilots and light aircraft owners. Combine freely available software (Stratux, link) with a Raspberry Pi 3, two SMArts and these antennas for aviation weather and traffic reception at 1090 MHz and 978 MHz. (Note that 978 MHz UAT information is only available in the USA).

For mobile ADS-B reception small size outweighs shortcomings compared to larger antennas. 10 versus 12 position reports per second, or losing a contact at 35 instead of 45 miles makes no difference if you just want to know information about a plane.
Create a WiFi hotspot on your tablet or smartphone, connect with a Raspberry Pi 3 and PiAware, add a battery bank, RTL-SDR dongle and this antenna for a go-and-take anywhere ADS-B monitoring station.

Is it worth it?

Yes, as an antenna pack for general use and ADS-B on the go.
No, it's not designed for maximum ADS-B signal reception performance.
Yes, for $14 you get two antennas and two SMA - MCX pigtails.
The answer is up to your wallet.

Friday, 5 August 2016

Why use a preamp?

When you increase gain in software, like moving the slider in SDRSharp from left to right, you not only increase the desired signal, you also increase unwanted signals: commonly called noise.
Not enough gain, signal will be not strong enough.
Too much gain, signal will be swamped by other signals - like someone shouting into your ear.
You get the best signal - received audio or cleanest weather sat image etc - when Signal-to-Noise ratio is highest.
RTL-SDR dongles are electrical circuits, so they also generate noise when you increase gain, and this lowers your Signal-to-Noise ratio.
One way to combat this is by using a preamplifier, also called preamp or Low Noise Amplifier (LNA).

Gain and Noise Floor

An easy way to demonstrate internal noise generated inside RTL-SDR dongles is by using keenerd's rtl_power (link). This handy and free utility scans the frequency range of RTL-SDR dongles, and returns a value. Plotting these values are possible with Excel, so we can examine returned values versus gain settings.
No antenna involved, only a 75 Ohm resistor, so any visible changes will be the result of changing gain settings in software.
The plotted graph provides a line, which is called noise floor: should be as close to the bottom of the chart as possible.
Let's see what happens to the noise floor at 0dB, 30dB and 50 dB - common settings with RTL-SDR dongles.

The difference between blue (0dB) and orange (30dB) lines are not that significant on screen, but will be audible and visible during use.

Practical example

Consider the following image: airband signal with 0dB gain, signal to noise ratio on right marked with arrow (17 dB). Also note that the blue part of the waterfall does not register any noise.

Let's increase gain to 19.7 dB, the closest value available to a preamp's 19dB. SNR ratio jumps to 31, but the waterfall is also full of noise, represented by white lines and speckles:

Enter a preamp: it adds gain to the received signal, but with lower noise than the dongle on its own.
Turn down dongle gain and add a preamp.

(Signal at 121.85 is a transmission from Ground and unrelated).

SNR ratio will be slightly higher, and blue waterfall area will be cleaner.

The "low noise" in LNA

As seen above, dongles generate noise during amplification, but an LNA's noise figure will be lower than the dongle's.
The equation only works as long as the pre-amplifier will be a low-noise type.
Noise figures are specified for a particular preamp - if there's no noise figure for a product, don't buy it.
Should be as low as possible: for example, Janilab's preamp has a 2.1 dB noise figure at 2 GHz, and the LNA4ALL from Adam has a 0.98 dB NF at the same frequency. Noise figures are just one element of the jigsaw, as other specifications also play a role, but a noise figure is easy to understand.


Amplification is common with radio signals, so more expensive software defined radios will have an LNA on board: examples are the $150 SDRPlay and $300HackRF. Flightaware's ProStick also has a preamp, but that dongle is aimed at ADS-B enthusiasts, not for general radio use.
Cheap preamps are available online, such as various wonders from the Far East, often selling for less than $10. I've always stayed away from them, so can't comment from personal experience.
Satellite or TV preamps might look like a good idea, but despite trying a few I consider them unsuitable for RTL-SDR use: cheap versions have high noise figures, better ones cost just as much as a dedicated RTL-SDR preamp, but offer less signal amplification. What's more, the upper amplification range might stop at 1 GHz, so bye-bye ADS-B airplane position signals.
I've tested and use the following preamps:
Janilab preamp: $25 shipped, 0 to 3000 MHz, 2.1 db Noise figure, easy power connection, manufacturer link.
LNA4ALL: $28 shipped, 28 to 2500 MHz, 1 dB Noise figure, many options for power incl. bias-T, manufacturer link.
LNA4HF: $28 shipped, 150 kHz to 30 MHz, 1-2 dB Noise figure, lots of power options, manufacturer link.

Further reading

An easy and well-performing setup for $60 is an rtl-sdr.com dongle with a bias-T enabled LNA4ALL, post here (link).

The HackRF (review link) features built-in bias-T and the LNA4ALL (review link) can be ordered with a suitable modification direct from the manufacturer.
The SDRPlay (review link) also comes with an LNA onboard for half the price and better performance than a HackRF.