Connecting SDR to Antenna With An Active USB 3 extension Cable

[Pollys13a]

Hi,
I'm very much a newbie with all this, so I'm hoping somebody can help me out? Thank you :)
I have an RTL-SDR v3 . I also have a MegaLoop MLA-30+ magnetic loop antenna which I intend to put in my loft.
Recently received, The Hobbyists Guide To The RTL-SDR. He talks about using an active USB repeater cable.
"Therefore ideally the RTL-SDR dongle should be situated as close to the antenna as possible, replacing the lossy coaxial cable in favour of non-lossy digital USB cable instead ( in practice you should add about 1M of coax between the antenna and dongle to avoid the antenna picking up any local electrical noise from the dongle )
He then goes on to discuss using an active USB repeater cable. Maximum recommended length for a USB 3 active cable is 18m. 10m would be sufficient for me. I saw this cable on Ebay UK item number 156008012309 " Active Optical USB 3.0 Extension Lead " I'm not sure about the optical part in the description, would this be the correct cable to use, do I plug directly into the dongle? " ( in practice you should add about 1M of coax between the antenna and dongle to avoid the antenna picking up any local electrical noise from the dongle )" That confuses me.
Cheers.

[Adam Nielsen]

> He then goes on to discuss using an active USB repeater cable. Maximum
> recommended length for a USB 3 active cable is 18m. 10m would be sufficient
> for me. I saw this cable on Ebay UK item number 156008012309 " Active
> Optical USB 3.0 Extension Lead " I'm not sure about the optical part in the
> description, would this be the correct cable to use, do I plug directly
> into the dongle?

Yes it plugs directly into the dongle. That cable says in the listing
that it's USB 3.0 only, and it won't work with USB 2.0 devices. I am
not sure what the RTL-SDR v3 is but all the old ones were USB 2.0. So
in that case this extension cable would not work. You'll need to work
out whether the RTL-SDR supports USB 3.0 or not.

The reason is that USB 2.0 and USB 3.0 use different physical wires in
the cable, and so the extension lead you have found apparently only
extends the USB 3.0 wires and not the USB 2.0 ones.

> " ( in practice you should add about 1M of coax between
> the antenna and dongle to avoid the antenna picking up any local electrical
> noise from the dongle )" That confuses me.

The USB dongle produces radio noise, so they recommend your antenna be
around a metre away from the dongle to avoid picking up that noise.
They are suggesting to use a length of coax between the dongle and the
antenna for this.

You may also wish to investigate other options, such as putting an old
computer or Raspberry Pi up in the attic with the dongle plugged in,
and then running a network cable back to your PC instead and accessing
the SDR device remotely over the network. This way you avoid dealing
with USB issues which can be troublesome, and instead use Ethernet
which has been designed from the outset to comfortably work at
distances of up to 100 metres.

If you do decide to get a USB extension cable, make sure you test it
for a while *before* installing it, just in case it doesn't work.

Cheers,
Adam.

[markotime]

I bought  such a  cable years ago, but found that the voltage at the dongle was somewhat lower than 5v,

causing several (at the time) mysterious problems.  There may be better ones available, but the current

capacity of mine seems to be too low.

Virus-free.www.avg.com

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[Pollys13a]

" You'll need to work

out whether the RTL-SDR supports USB 3.0 or not. " I bought a genuine dongle from TechnoFix UK. Have looked on their site but there is no information on whether USB 2 or USB 3. I then did a Google search, trying to find out if a USB 2 or 3, no joy. The telescopic antenna I fixed that to the dongle, then plugged into the laptop that I intend to use. Thinking device manager would show the type of USB port the dongle was plugged into, no luck again. As I can't find any reference to USB 2 or USB 3, then assume it only comes as USB 2, if not any idea how I find out if the dongle is USB 2 or 3?

I only need an active USB cable, don't need a 1m length of coaxial cable?

The MegaLoop MLA-30+ magnetic loop antenna has a long length of cable with an SMA connector on the end,see attached, the dongle also has the dongle attached to it, see attached. I'm afraid I'm getting very confused as to how I connect up the antenna in the loft via some type of cable to the laptop in a room below? Regarding USB 3 active cable, when I was trying to find if the dongle was USB2/3 I read a bit about compatibility between an active USB 2 cable and USB 3 port on the laptop, seems to be a whole can of worms, my laptop has USB 3 ports.

Please clarify things?

Cheers.

[Adam Nielsen]

>> You'll need to work out whether the RTL-SDR supports USB 3.0 or not.
> I bought a genuine dongle from TechnoFix UK. Have looked on their
> site but there is no information on whether USB 2 or USB 3. I then
> did a Google search, trying to find out if a USB 2 or 3, no joy.

It can be difficult to find out. You can look at the USB connector and
compare it to photos on Google. If it's USB 3.0 it will have an extra
set of pins at the back of the connector. If it's missing those extra
pins, it's 100% definitely USB 2.0 only. If it *does* have the pins,
it could be USB 3.0 but it may also be USB 2.0 and it doesn't use the
extra pins so more checks are needed.

I think in your case you will probably find looking at the connector it
is only USB 2.0 though, so that will answer the question for certain.
If it does have the extra USB 3.0 pins then more investigation will be
required.

It's been a while since I used Windows but Device Manager used to have
an option to show by connection type instead of by device type. If
that still there then it will show you whether the dongle is listed
underneath a USB2 or USB3 controller which will answer the question for
certain as well.

> I only need an active USB cable, don't need a 1m length of coaxial
> cable? The MegaLoop MLA-30+ magnetic loop antenna has a long length
> of cable with an SMA connector on the end,see attached, the dongle
> also has the dongle attached to it, see attached. I'm afraid I'm
> getting very confused as to how I connect up the antenna in the loft
> via some type of cable to the laptop in a room below?

I wouldn't worry about the cable at the moment. It's not going to
break anything if you don't have the extra cable, you may just get some
additional noise on certain frequencies. It's something you can deal
with later if you're finding you are having problems receiving a good
signal. More important to get the dongle working first, you can deal
with signal quality issues later, if you even have those issues as you
may find the noise is on bands you aren't trying to listen to anyway.

> Regarding USB 3 active cable, when I was trying to find if the dongle
> was USB2/3 I read a bit about compatibility between an active USB 2
> cable and USB 3 port on the laptop, seems to be a whole can of worms,
> my laptop has USB 3 ports.

USB in general is very difficult to get working reliably at longer
cable lengths. I've used active copper extender cables before and even
at only five metres long they will work fine with some devices but not
others. The worst was a USB 3.0 flash drive that appeared to work fine
but it would write corrupted files - too me a while to figure that one
out.

Personally I avoid USB extension cables wherever I can because I've had
so many issues with them.

Cheers,
Adam.

[Pollys13a]

Ok, thanks for the helpful reply, appreciated :)

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[Pollys13a]

I also read it was a good idea to run the laptop that the RTL-SDR is connected to, via the laptop battery, rather than the laptop PSU because of possible RFI.

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[Adam Nielsen]

> I also read it was a good idea to run the laptop that the RTL-SDR is
> connected to, via the laptop battery, rather than the laptop PSU because of
> possible RFI.

Again I wouldn't worry about it until it becomes a problem. Many
people here run them on normal PCs in noisy environments and have no
trouble receiving signals. The RFI may well cause interference but if
it's on a frequency you aren't interested in listening to then it's not
really a high priority to fix.

If you're trying to receive a low frequency transmission from the other
side of the world then you might have to pay attention to this, after
you've installed your multi-thousand dollar external antenna mast of
course, but for anything located near you or with an indoor antenna,
there are many other things you will be able to improve before you have
to worry about a laptop power supply.

Not to mention that if you're running long USB cables then your laptop
power supply is probably going to be nowhere near the antenna anyway to
begin with.

Cheers,
Adam.

[Pollys13a]

Ok, cheers.

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