Art, If you are after weak signals on VHF/UHF, the RTL dongle is a poor choice a= t best. The ADC in the dongle is at best 10 bits, so your dynamic range is = limited to about 60dB before you hit the quantizing noise. If you have any = strong signal in the area you live in, like FM broadcast, those might also = affect your receiving signal. I do not think that adding a long USB cable i= s going to help. A much better solution would be to add a bandpass filter t= o eliminate unwanted signals, but you do not seem willing to do that. Try a= lso to EMI shield your RTL dongle. Your idea of using the RTL dongle near the antenna on the mast will need so= me creative engineering, but a long USB cable will not cut it. Your idea of= a fiber link is good, but you could also try a USB to WIFI point to point = link. Just my $0.02, Jean-Paul N1JPL > On Jul 8, 2018, at 4:21 PM, Art wrote: > > Hi Jason, > > I'm after weak signals on the vhf and uhf aircraft bands, so I need all > the shielding and suppression I can get. I've never needed an extension > usb cable, so I don't have anything to test with (other than the 10 foot > cable I just bought). > > I do have a 6 foot cable on order, with double shield and a ferrite on > the braid. But, if I have to spend more money, I'd rather have something > that I know is right, which is why I hoped to build my own cable and be > done with it. > > I know about the RTL-SDR's shortcomings and about the need for front end > bandpass filters. There is no way it will function without them-so many > users are oblivious to the ramification of out of band signals causing > overload. And, many of those that do know about overload assume a > digital signals can't be compromised by strong out of band signals. > > BTW, the RTL-SDR is in a constant state of overload, even without the > AGC and the proper 50 db gain control setting. With only 40 db dynamic > range, the RTL-SDR doesn't even need out of band signals to cause non > linear mixer responses, and the degradation of the desired signals > reception. > > Ultimately, I'd like to just put the thing outside, at the mast > location, but I need to do some USB cable testing before going any > further. A fiber optic link would be neat, although it's overkill for > the 50 to 100 feet transmission distance I'd like to ultimately have::> > > Any idea where I can find specs for the proper usb (raw) cable? > > Regards, > > Art > > > > > On 07/08/2018 03:40 PM, Jason White wrote: >> Have you tried using a different USB cable? Conventional USB cables with >> their single layer of shielding should be more than adequate for your >> application. I suspect that manufacturing your own cable may not the >> easiest, cheapest, or fastest way of getting things to work. >> >> I have a RTL-SDR type dongle and a 125MHz upconverter. In my experience = it >> is noisy and strong signals produce aliasing* every 2.4MHz (sampling >> frequency), every ~27MHz (SDR crystal frequency) and every 125Mhz (from = the >> upconverter). The internal 30-40dB AGC typically will typically boost >> signals past the point where aliasing occurs. As a result AGC needs to b= e >> disabled and the the dongle's RF gain has to be manually set - typically >> never exceeding 20dB. >> >> * aliasing -> ghost signals, lines and spikes in the SDR's decoded spec= trum >> >> -Jason White >> >> >> On Sun, Jul 8, 2018 at 3:09 PM, Art wrote: >> >>> I recently bought an RTL-SDR >>> (https://www.nooelec.com/store/nesdr-smart-sdr.html), a small radio >>> receiver that is powered by the USB connector on the punchbox. It >>> digitizes the received radio signal and sends the digital information t= o >>> the punchbox for processing via USB. >>> >>> It turns out that the computer generates to much noise for the receiver >>> to operate in close proximity to the punchbox. >>> >>> So, I have to move the receiver and/or the antenna further away from th= e >>> computer. I choose to move the receiver as low loss coax for the uhf >>> band is pricey. >>> >>> So, I bought a high end 10 foot USB 2.0 extension cable, with 2 layers >>> of shielding, and emi suppressors (ferrite) on each end of the cable. >>> The cable has 24 gauge power bus conductors, mote than adequate for >>> powering my RTL-SDR receiver. The RTL-SDR receiver uses 370 ma, worst >>> case, so the 24 gauge power conductors in the cable should be adequate. >>> >>> The RTL-SDR unit has an internal (smt type) emi absorber on the power >>> input terminal. The computer is a very dirty place in terms of rf noise= , >>> and having rf noise on the USB lines is like rolling out the welcome >>> wagon for rf interference to the RTL-SDR receiver. >>> >>> The new USB cable works fine for the smaller usb flash drives I have, >>> but it causes the RTL-SDR device and my USB HDD unit to fail when >>> connected through the new cable. Neither of the higher power devices ar= e >>> recognized. The RTL-SDR requires the full USB 2.0 throughput, without >>> it, it can't send data fast enough to enable the software in the >>> punchbox to function. Both the HDD and the RTL-SDR work fine when >>> plugged (directly) into any of the ports on the computer (without the >>> extension cable). >>> >>> I concluded that I need to roll my own cable. That way, I know the cabl= e >>> itself is proper. I can buy the usb ends for the extension cable and >>> solder them to the cable. >>> >>> However, I can't find the technical specs for the cable, so I don't kno= w >>> which type of raw cable to buy....in order to insure my homebrew cable >>> works at it's full capacity (which is 480 MB/sec). There are USB >>> repeater cables that extend the max cable length out to 50 feet, but >>> they're expensive, they draw a lot of power and they don't work at the >>> full data rate for USB 2.0. And, none of the repeater cables have >>> shielding at the housing for the driver electronics. >>> >>> I've thought about using an ethernet cable and putting usb connectors o= n >>> the ethernet cable. But, ethernet uses balanced transmission line >>> (twisted pair)...it might not work well for usb because USB doesn't use >>> a balanced line-so there is little benefit to be had by moving to >>> ethernet cable for usb data transmission. >>> >>> I sure wish the RTL-SDR had an ehernet cable connector instead of a USB >>> connector! >>> >>> My problem is I cannot find the usb 2.0 specification that outlines the >>> critical parameters for the cable itself. Any ideas? >>> >>> Sorry to be so long winded::> >>> >>> Art >>> >>> >>> >>> >>> -- >>> http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive >>> View/change your membership options at >>> http://mailman.mit.edu/mailman/listinfo/piclist >>> >> >> > > > > -- > http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist -- http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .