SDR up the pole on a usb 2.0 extension

Has anyone put the SDR in a small box high at antenna and use a type 2 extension cable with the Pi down in the shade?

I have a similar situation except the antenna, LNA, and SDR dongle are in the attic with a long (5 meter) USB extension down to the Pi in a closet. Seems to work fine with no dropped USB data – tested with rtl_test at 2.4 Msps. However, might be different with a higher sample rate for an Airspy for example.

Whether or not that works depends on the quality and length of the USB extension cable. It’s generally best not to use more than a very short (6 inch/15cm. or so) extension, just if you need one because SDR dongles are so big they overlap the other USB ports on the Pis, then use low-loss coaxial cable between the SDR and the antenna. I tried pitting my SDR dongle near my antenna then connecting it to my Pi downstairs and it worked for a few weeks then stopped working, and I had to move the dongle back to the Pi and go back to using coax.

My pi4 / rtl-sdr / 5v supply are in an outdoor box (roof height) at the base of a pole about 1m below my antenna. Antenna is low so that I can try out my antenna experiments easily. If I raise the pole higher later I have to choose between a longer coax or a second smaller box and a usb extension.

Well, as I mentioned above a 5 meter USB extension cable seems to work reliably with an rtl-sdr dongle. Coincidentally, 5 meters is the longest extension cable specified for USB Type 2. If you need more length, probably you should use coax between the antenna and the rtl-sdr dongle rather than a USB extension cable to the Pi. Realize that any loss in the coax directly increases the noise figure of your receiver dB for dB.

You are right.
Nine years ago (in 2013), I tried to use 45 ft USB extension for the Dongle. I could not find extension cable longer than 15 feet (5 meters), so I purchased three 15 ft cables and used them in series, making total length 45 ft. The dongle failed to work with 45 ft long USB extension. I removed one 15 ft piece to make total length 30 ft, the Dongle failed on that also. However when I used only one 15 ft cable, the dongle worked OK.

Finally I purchased a 50 ft coil of RG6 coax from nearby Dollar Store for $7, and ran it between antenna & dongle, which was plugged into Windows PC. This worked, but gave very poor reception due to long length of coax. I then added a Satellite Amplifier (18 dB, 950~2050 MHz) between antenna & coax. I have purchased the Satellite inline amplifier from nearby Satellite Dish Sore for $4. This arrangement worked very good. I had to make a DIY Bias-Tee to feed the Satellite inline amplifier



Yeah, when the USB extension cable was 30 or 45 feet probably the IR loss lowered the voltage to the rtl-sdr dongle too much for proper operation of the dongle.

Adding an LNA to the setup with a gain of at least 20 dB followed by a long coax to the rtl-sdr dongle is a real win as far as noise figure is concerned. A cascaded noise figure calculator like the following is useful to calculate the resulting noise figure.

I have initially used two Satellite Amplifiers in cascade. Each has 18 dB gain.
The reason to use two amplifier were:
(1) I thought “bigger the better”
(2) I got the Satellite inline amplifiers very cheap ($4 each).

However after a while when I removed one amplifier, I found performance improved instead of decreasing :slightly_smiling_face: Seems dongle has got overloaded with too much gain of two amplifiers cascaded.


When an LNA is used in front of a rtl-sdr dongle, the gain is adjusted down in the dongle. One thing that isn’t mentioned is that as the gain is adjusted down, the noise figure of the dongle will go up. The nameplate noise figure of the rtl-sdr dongle is 3.5 dB, but that’s at maximum gain of about 60 dB. What happens when you reduce the gain by 30 dB ? I haven’t seen any data on-line on that.

In my experience (using Linux) they can be very finicky with USB extension cables and it’s often because they’re not getting enough power. If you can use a powered USB hub with an AC adapter or an active USB extension cable that boosts the signal then it tends to work better.

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