Search found 136 matches

by hotpaw2
Wed Jun 17, 2020 2:56 pm
Forum: Signal Identification Help
Topic: Is this interference i see at 137MHz?
Replies: 19
Views: 2199

Re: Is this interference i see at 137MHz?

One additional trick that can help is to use ferrite beads or toroids. I prefer toroids, as multiple turns through the donut seem to work better than a string of multiple cores/beads on the cables. Try 1 or 2 meter passive USB cable, but loop several turns of the cable through a ferrite toroid, mayb...
by hotpaw2
Thu May 28, 2020 8:55 pm
Forum: Antennas
Topic: Airspy YouLoop
Replies: 13
Views: 1706

Re: Airspy YouLoop

A YouLoop type antenna requires a receiver with a high-gain low-noise RF front end, such as an AirSpy HF+ Discovery. IIRC, an ordinary RTL-SDR in direct sampling mode doesn't have that much RF gain, and its 8-bit sampling creates a relatively high noise floor, so you might want to try using an LNA b...
by hotpaw2
Thu May 14, 2020 2:43 pm
Forum: RTL-SDR Discussion
Topic: GQRX - unexpected plot when changing frequency range
Replies: 14
Views: 4403

Re: GQRX - unexpected plot when changing frequency range

The bigger the input signal, the more likely the spectrum is to show spurious signals (intermodulation products, etc.). That's because semiconductor devices are non-linear components. They are only approximately linear enough for a range right above the smallest signals detectable. Near the maximum ...
by hotpaw2
Sun May 10, 2020 7:44 pm
Forum: Troubleshooting Help
Topic: Installing RTL-SDR: Cmake error - LibUSB 1.0 required to compile rtl-sdr
Replies: 10
Views: 2460

Re: Installing RTL-SDR: Cmake error - LibUSB 1.0 required to compile rtl-sdr

Make sure you installed libusb itself, and not just the libusb build essentials developer tools.

or download libusb from GitHub and build, and then make install it.

https://github.com/libusb/libusb
by hotpaw2
Wed May 06, 2020 5:56 pm
Forum: RTL-SDR Discussion
Topic: GQRX - unexpected plot when changing frequency range
Replies: 14
Views: 4403

Re: GQRX - unexpected plot when changing frequency range

The first step is being able to detect real signals is to test against real signals and their absence and make sure your equipment is calibrated to tell the difference. You might even need some sort of signal generator and some attenuators, etc. to do this.
by hotpaw2
Tue May 05, 2020 10:31 pm
Forum: RTL-SDR Discussion
Topic: GQRX - unexpected plot when changing frequency range
Replies: 14
Views: 4403

Re: GQRX - unexpected plot when changing frequency range

The signals in your waterfall screenshots look more like spurious intermodulation products or oscillator birdies than real RF signals. Try again after you find a "real" signal, e.g. one you can listen to or decode. Shift up or down a quarter of a frame after you find a signal with a station ID or so...
by hotpaw2
Tue Apr 28, 2020 4:53 pm
Forum: Antennas
Topic: Low Frequency Antenna
Replies: 14
Views: 15326

Re: Low Frequency Antenna

Another possibility for a portable VLF/LF antenna is an old-fashioned loopstick antenna, made by winding a fine wire coil around a ferrite rod. Add a variable capacitor of the appropriate range for a tuned loopstick. That's how those old AM pocket radios worked down to the 100's of kHz without any e...
by hotpaw2
Thu Apr 23, 2020 5:54 pm
Forum: RTL-SDR Discussion
Topic: RTL-SDR Frequnecy Range
Replies: 3
Views: 1296

Re: RTL-SDR Frequnecy Range

Below 13 MHz the RTL2832U can use direct sampling of the RF (to Q channel only). Above 29 MHz, the RTL-SDR acts like a superhet receiver, with the R820T2 heterodyning down to an IF of 3.57MHz, and the RTL2832U quadrature sampling to baseband IQ.
by hotpaw2
Sun Apr 05, 2020 9:55 pm
Forum: RTL-SDR Discussion
Topic: Performance of R820T v3 dongle
Replies: 7
Views: 1421

Re: Performance of R820T v3 dongle

For a pure sinusoidal signal at the exact center frequency, an IQ demodulator produces a constant output. This constant in the FFT result at frequency zero looks almost exactly the same as an unwanted DC bias or IQ imbalance in the IQ demodulator, so is sometimes removed (“compensated” or zero-ed ou...
by hotpaw2
Wed Apr 01, 2020 5:18 pm
Forum: Troubleshooting Help
Topic: MacOS: Stopped Detecting Device
Replies: 3
Views: 816

Re: MacOS: Stopped Detecting Device

Did you upgrade to macOS 10.15 Catalina recently? If so, you may need to rebuild libusb and rtl_tcp (included inside Gqrx), and/or reinstall recent copies and remove all the macOS quarantine flags. The instructions on how to install libusb and rtl_tcp on a Mac here: http://www.transitiontechnologyve...