Long USB-cable vs long antenna-cable?

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hotpaw2
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Re: Long USB-cable vs long antenna-cable?

Post by hotpaw2 » Sat Nov 09, 2019 6:55 pm

I think the key is that ethernet cable is both cheaper and works reliably over far longer runs than USB cable. Both types of cables can radiate RF noise, so need to be wound (multiple turns) around ferrite cores at both ends.

So I use a Raspberry Pi 3 with the RTL-SDR plugged in directly into it, no USB cables at all, plus a medium length of coax from the Pi up to the antenna to keep the Pi away from the antenna, and then long ethernet cable run from the Pi to my network router. For higher frequency work, I might add a filter plus LNA near the antenna. Then I run rtl_tcp (or hfp_tcp with my Airspy HF+ Discovery, or rsp_tcp with my SDRPlay RSP2) from the Pi to my laptop or iPad.

So, neither a long USB cable (I use none, or 10cm), nor a long coax antenna cable.

aurgathor
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Re: Long USB-cable vs long antenna-cable?

Post by aurgathor » Sat Nov 09, 2019 9:57 pm

But you also need a Pi or something equivalent which can pretty much negates the cost advantage. I don't have a Pi, but have plenty of USB cables and hubs.

I'm not trying to say that one approach is better than the others (they are all different with different pros and cons) only that it should be possible to reach over 100 ft with USB cables without using any extender that runs over Ethernet, fiber, etc. One just need to chain some hubs, or use active USB cables (which are cables with embedded hubs). It should work as long as enough hubs get external power.

hotpaw2
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Re: Long USB-cable vs long antenna-cable?

Post by hotpaw2 » Sun Nov 10, 2019 6:17 am

Don't forget that each USB hub, and its power supply, is likely an RF noise source. That can add up to a lot of ferrite toroids and beads. Or course ethernet switches and Raspberry Pi's are also RF noisy. So, like you said, it's a trade-off.

aurgathor
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Re: Long USB-cable vs long antenna-cable?

Post by aurgathor » Sun Nov 10, 2019 8:50 pm

The output of the SDR is a digital signal, and that should not be affected by the relatively low level electrical noise you could find in USB hubs. Only the noise that can get to the input of the SDR (presumably mostly through the antenna) is the one that can make a significant difference.

But If you really worry about electrical noise, you could use a USB cable with a ferrite choke or two and/or filter the last USB hub's power.

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