As a user of RTL-SDR devices for tasks such as aero monitoring, ACARS, ADS-B, or perhaps VHF maritime or satellite listening, you may sometimes question the sensitivity of your radio. The answer is more complex than, "how high is the S-meter indication?" To have precise metrics, it is necessary to connect the dongle to a test rig, measure the noise, and inject a weak signal to compare with the noise. But hey, you can make practical assumptions about your receive sensitivity by knowing your configuration and examining the layers of background noise you can receive.
Manufacturer's data indicates that an RTL-SDR has fair sensitivity - enough for local and more distant broadcasters and typical aero / maritime mobile stations. With a quarter wave vertical or one wavelength loop, mounted on a rooftop, high altitude flights should be audible out to 200 miles. Boats or aircraft on an airport should be audible out to about 20 miles. Distances may be increased significantly, even without a preamplifier, with the use of a directional antenna.
If you use the stock antenna (the whip usually included when you buy an RTL-SDR), the problem is that the antenna won't be high up, in the clear, and exposed to better signals. Also, it doesn't have gain or directivity. The noise floor will be set by the RTL-SDR internal RF amplifier.
In the chart above, notice that the level of noise from neighborhood sources is below the receiver's internal noise. Also, there is another noise level, for distant sources (including cosmic noise), which is far below the hiss generated in the RTL-SDR.
Improve Your Antenna First
Simply increasing your gain settings does not help. The problem is that the first RF amplifier (or the mixer) in a radio is what sets the noise figure. A sensitive radio will have a noise figure of about 1 or 2 dB on VHF and UHF. The RTL-SDR has a noise figure of 5 to 8 dB, depending on the frequency and whether you received a good unit in terms of manufacturing tolerances. While it is possible to set your device gain too low and lose the front end noise floor below other noise added by the mixer and IF amplifier, there is no advantage to set the gain above about 25 on the RTL-SDR. Too high a gain setting simply causes more overloading and spurious junk to be created within the dongle.
Use a Quality Preamplifier
After installing a directional antenna as high as possible, and using a low loss feedline to the receiver, station sensitivity can be improved by adding a low noise preamplifier. The term "low noise" means the preamp should have a noise figure below 1 dB, and the lower the better. Install the preamp at the antenna feedpoint, or through a short length of low loss cable. Typically, high gain is not necessary - 15 dB is plenty; more gain than 20 dB is often a waste and probably will cause troubles with overloading. If you do experience problems with overloading, especially from out of band stations, consider to install a filter ahead of the preamplifier. There are inexpensive and effective filters available for weather satellite, maritime, and ADS-B spectrum bands.
The Experience of a Sensitive Receiver
How do you know if your receiver is sensitive enough, after installing a better antenna and preamplifier? On an omnidirectional antenna, such as a ground plane vertical you should be able to hear background noise from your surroundings. Listen for automobile or truck spark plug noise, lightning from nearby thunderstorms, and powerline noise (yes - at VHF and UHF). These comprise the next limit - the next noise floor once you overcome the noise within your receiver.
Reception should be better for the things you seek, and more distant stations should be detectable when enhanced by tropospheric bending on good days or evenings. When you disconnect the antenna from your RTL-SDR, the neighborhood background noises should disappear. When you reconnect the antenna, those noises should return.
Replacing a dipole, vertical, or single loop with a three to five element yagi antenna can bring weak signals up above the neighborhood noise and extend your max range by a solid fifty percent (limited by the radio horizon). What you should observe is stations within range having stronger signals, then a sharp drop for any beyond the visual horizon, then fluttery and weak signals out to approximately thirty percent past said visual horizon. Whenever there is propagation enhancement due to tropospheric bending or scatter from aircraft, you can receive signals at greater distances.
On a receiver with a noise figure much lower than of the RTL-SDR, the beam antenna and preamplifier combination can make a significant improvement in station sensitivity. Antenna directivity is important, as it receives more energy from the aimed direction and less noise from around the local neighborhood. When local noise is no longer marking the floor, the next floor is distant sources. A superbly optimized station can detect cosmic sources, such as the sun, moon, Jupiter, and Saggitarius (our galactic center).
If you consider the above information and make improvements to your receiving station, you can take steps to assure that your rig is "sensitive enough" to pick up whatever is in your area.
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