Long USB-cable vs long antenna-cable?

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JEL
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Joined: Wed Feb 07, 2018 2:08 am

Long USB-cable vs long antenna-cable?

Post by JEL » Mon Jan 28, 2019 4:44 am

Which option is more advisable?

Running the SDR close to the antenna with a long USB-cable going to the computer in a different room

or

Running the SDR close to the computer with a long antenna-cable going between the antenna in the other room and the SDR near the computer.


I ask because I would like to experiment with moving the antenna around to find any possible sweet-spots that may exist where I live.

But the PC is stationary, so I can either create an antenna/SDR combo (Basically connecting the SDR directly to the antenna and move that combo around, having a long (15-25 meters active USB, or perhaps something crazy like a fiber-optical USB extension if I can find one) USB-extension going to the computer)

Or I can have a long antenna-cable between the antenna and the SDR (the old-school way) an then just move the antenna around.

I obviously want to minimize any unwanted radio-signals, but I'm not sure if more noise will be picked up by a long USB extension or a long antenna-cable.

The long antenna-cable solution is clearly the cheapest one (I'm not sure fiber-optical USB-extenders are worth their, from the examples I've found so far, very expensive price-tag)

Thanks
jacob.

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

Post by Airwind » Mon Jan 28, 2019 5:33 am

I once asked this exact question for a WiFi card and I was told a longer USB cable is preferred.
But in your case I suppose you could also get away by using a longer coax with an LNA instead.

JEL
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Joined: Wed Feb 07, 2018 2:08 am

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

Post by JEL » Mon Jan 28, 2019 5:53 am

Airwind wrote:
Mon Jan 28, 2019 5:33 am
I once asked this exact question for a WiFi card and I was told a longer USB cable is preferred.
But in your case I suppose you could also get away by using a longer coax with an LNA instead.
Thanks.

I would prefer having the SDR close to the antenna, but the price for an optical USB-extender is around 400-500 US$ (Which is way above what I want to even think about spending :) )

I am thinking your suggestion is the only one that makes sense (price-wise at least)
And especially since I already do have some 50 Ohm COAX cables I might be able to use if I get some plug-adapters so I can match them to the SMA-input of the SDR.

My only worry is that they will pick up a lot of noise (I am in a cramped apartment-building, so there are plenty of noise-sources) I might only be able to really minimize by getting the SDR close to the antenna.

snn47
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Re: Long USB-cable vs long antenna-cable?

Post by snn47 » Mon Jan 28, 2019 5:05 pm

I obviously want to minimize any unwanted radio-signals, but I'm not sure if more noise will be picked up by a long USB extension or a long antenna-cable.
While wavelength and therefore antenna get smaller with increasing frequency the attenuation of the cable will increase. The higher the frequency the thicker/heavier the cable will be to have to have the same loss as at lower frequencies.

It depends therefore on
the frequency range you want to receive,
which antenna type you intned to use,
the location and height for the antenna to recommend the best solution for you.

E.g. you can reduce noise/interference significantly if the antenna is optimized for elevations above the horrizon, so it will not receive to much noise/interference originating below/around it.

Since the RTL-SDR Rx is very broadband without a narrowband antenna, bandpath-/notch-filter to limit unwanted radio-signals the Rx is easily overloaded and will generate additional false signals. Therefore it is the the frequency range and antenna you want to use, that defines the best approach.

WHile an active antenna with a preamp can cover the whole range, it will overload the Rx without the use of filter.

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

Post by aurgathor » Tue Jan 29, 2019 2:30 am

Last year I've measured the current draw of several SDR's (RTL-SDR v3, some Nooelec, HF+, etc.) and posted the numbers here.

Both v3 and Nooolec were a bit over 300 mA, and while that's not a terribly high current draw (the USB 2.0 spec is 500 mA), the voltage drop can be significant over 10' or 15' long cables, especially those cables that do not use thicker conductors for GND and PWR. This can be alleviated by utilizing a hub (or a chain of hubs) and ensuring that there is good quality cable between the the last hub and the SDR.

In any case, with USB hubs/repeaters, and 15' long cables in between you can probably get close 100', and I think that should be enough for most people. I've never tried anything longer than about 36 ft.

snn47
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Re: Long USB-cable vs long antenna-cable?

Post by snn47 » Tue Jan 29, 2019 3:02 am

I know of a max. of 40 m (~131 ft) if you cascade active USB 2.0 cables, like Delock Kabel USB 2.0 active 20 m USB cable Art.-No. 82690 price ~30 €. I used one cable with a Haupauge video grabber, but I never checked for RFI.

While a USB 3.0 port is rated 900 mAh vs. 500 mA for a 2.0 port it is the voltage drop across the cable that is the limiting factor in using longer cable. For a GSM stick a passive no name 5 m cable worked, but I never got another one to work.

PS.: When I checked the up to 100 m (305 ft) Lindy USB 2.0 Ethernet active Extender (no. 42694) starts at 80 €, when I checked https://geizhals.de/logilink-usb-3-0-ak ... 64967.html, while two 20 m active cable would be already 60 m. Again I have no idea how much RFI the USB LAN extender will produce.

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

Post by JEL » Tue Jan 29, 2019 6:55 am

I want to try a discone...

indoors...

at the ground-floor of an apartment-complex...

surrounded by noisy neighbors and all their gadgets...
(including those inherent to the building and city's infrastructure)

so pretty much the worst of all worlds :)

I expect my max cable-range to remain below 100 feet (when 'snaked' around corners etc)
Probably below 60 feet.
End-to-end distance (straight-line) will likely be below 30 feet.

I have tried active and non-active USB extension-cables, and could not get anything working when going above 15 feet (which is not enough length for what I want)

I have not tried USB via lan cables though. Those extension-cables are indeed much cheaper than optical, but since lan cables are much thinner I wasn't considering them an option.
But I don't know if they are better or worse in terms of noise, compared to 'normal' USB extension-cables (which where those I tried but couldn't get working when extending longer than about 15')

snn47
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Re: Long USB-cable vs long antenna-cable?

Post by snn47 » Tue Jan 29, 2019 8:16 am

Similar to Coax cable, there are high quality cable and there are cheap low quality cable, though the price of many cable may be high it is not necessarily a way to get a good USB cable. So you might have been just unlucky with the USB cable you bought.

You have two (three) reasons for problems:

1. the voltage loss on the power lines of the USB cable,
which is why HDD often don't work with cable even when they are < 1m. Due to large resistance and the resulting voltage drop, the input voltage to the USB device is below the 5 V minus tolerance.

2. Degration of the USB signals on the data lines of a USB cable is another .

(3. )When The active USB extensions I use a en external DC-power source via a 5 V input at the Hub, located at the end of the extension cable, you can draw the 500 mAh, but any additional power supply or a power bank can also be a source for additional RFI.

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

Post by jevgienij » Tue Jan 29, 2019 12:34 pm

Cons of using an active USB extender are that your device could be more susceptible to external disruptions. Not like a noise on the band itself but for example when I'm running the washing machine in my house, its motor start can crash RTL-SDR. When it does, you have to replug the device. If you don't access that PC physically you can use a tool like Dev Eject to replug the device. But this doesn't work with the extender. At least I didn't find a way.

JEL
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Joined: Wed Feb 07, 2018 2:08 am

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

Post by JEL » Tue Jan 29, 2019 4:37 pm

jevgienij wrote:
Tue Jan 29, 2019 12:34 pm
crash RTL-SDR. When it does, you have to replug the device.
I have had that problem myself with a cheap SDR when run on a combined 10 meter (30 feet) USB cable (one passive 5-meter and 1 active 5-meter cable)

That was one of my first SDR experiments and back then I didn't know about the USB length-limits :)

I got that SDR to run more stable by lowering the sample-rate, but occasionally it just locks up and has to be physically unplugged and replugged to get back to working.

At one point I suspected a wireless phone-charger was disrupting the connection (I have a few placed in various locations, one close to the SDR), but I think it might be something else that causes it (Like a fridge or a neighbor's device or something. Similar to what you found with your washing-machine)

The Airspy flat out refused to run on anything USB longer than 5 meter (15 feet), which is why I started researching alternatives (such as the optical USB extenders)

Currently I'm waiting for the mail to arrive with some plug-adapters so I can test out a long antenna-cable setup, which is the option I'm currently aiming for :)

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