Go below 30 MHz with direct sampling

July 12, 2016


No soldering required.
Some RTL-SDR dongles can receive frequencies below 30 MHz by pushing a small wire through a hole on the printed circuit board (PCB). It's not possible with every RTL-SDR dongle, and requires adjustment in software.
Generic chinese, rtl-sdr.com dongle and SMArt dongles work, the whole process takes a few minutes, and results are surprisingly good with a proper antenna.
Many thanks to the author of rtl-sdr.com for his input; also, for inspiration and stunning possibilities, please visit the website of Martin - G8JNJ.
Please comment below if you have a dongle which is not listed, you know a better solution, or an easy improvement without soldering, or wish to share your experience with direct sampling.



What's direct sampling?




Direct sampling means that the received signal is not processed by electronic circuitry, it goes directly into the brain of the dongle - think of pouring beer into your ear and getting drunk. How much I love explaining complicated signal processing in a few words.
The process has been around for years, but required soldering equipment and skills. More and more dongles feature easy direct sampling, and the list includes $8 generic chinese dongles.
The whole mod requires just a small screwdriver to remove the case and plugging a piece of wire through a hole, then connecting the other end of the wire to an antenna. 
Hole size determines wire size, use your imagination, wires and objects within reach to suit your personal circumstances.
Jumper wires universally work and recommended with Generic / rtl-sdr.com / Nooelec SMArt dongles. If you never came across jumper wires, they can be ordered directly from Nooelec (link) or supplied with Raspberry Pi component kits.
Solid wire coax cable center conductor also works, but might require some filing - paper or metal nail files do the job in less than a minute.
Downsides? The signal enters the dongle's brain directly, just like a small amount of alcohol injected into the bloodstream will get you morbidly drunk, and the equivalent of six shots will kill you instantly. Similarly, the direct sampling mod can fry the dongle - you've been warned.
To ease your worries, know that direct sampling modded dongles are on the market for a while, and people been using them with reportedly good results.


Cheap generic chinese dongles




Recent dongles have a hair-width hole for direct sampling. USB port in the left side of the image below.



Routing the cable to the LED hole on the plastic cover:




Dongle from rtl-sdr.com



Manufacturer website store link, official image for direct sampling mod link.



Routing the wire to pass through screw hole:





Nooelec SMArt


Manufacturer link.
Wire can be inserted from the heatsink side, images below are for illustration purposes only.
Do not remove thermal pads, impossible to glue back. 
The hole closest to the USB connector is the one you want.

The largest hole of the three dongles here, jumper cable had to be folded in half:








Software adjustments




In SDRSharp go to Settings (Cogwheel in top left corner), then click on Sampling Mode / Quadrature sampling, in drop-down list choose "Direct Sampling - Q branch".




HDSDR


Press ExtIO (blue) to bring up the settings screen, the select Direct sampling - Q input.








Antenna



With 20 feet / 6.5 m wire, strung out from a window, I received many stations from 183 kHz to 15 MHz, all without an upconverter.






Closing thoughts



Upconverters cost around 50 dollars or more.
A chinese generic RTL-SDR dongle costs around $8, and by plugging in a suitably long piece of wire enables the reception of radio signals below 30 MHz.

6 comments:

  1. What about the grounding of the antenna?

    ReplyDelete
    Replies
    1. I would not ground... Treat it as an unbalanced antenna. Contacting that antenna wire with any ground could cause a ground loop (especially if there is stray voltage running around). Radios work in microvolts (millionths), and ground loops can be rather nasty (as bad as thousands of volts if there are powerline problems in the neighborhood). Just unplug your external antenna when not in use, especially in thunderstorm conditions.

      Hams with single point grounds will not be asking this question (it's an unbalanced affair), but are usually safe from ground loops too.

      Delete
  2. Thanks for a great article.

    I will definitely be doing this to my Chinese dongle.

    ReplyDelete
  3. Excelente aporte, no cuesta nada probar, les estare comentando.

    ReplyDelete
  4. Since the wire is essentaially connected directly to the RTL2832U chip what do we need to protect from frying it?

    ReplyDelete
    Replies
    1. You can try to protect it by checking for voltage on it vs. the shield ground on the USB. If it shows 0V (AC and DC scales) as it should, you can put a couple of silicon diodes facing cathode to anode, and anode to cathode in parallel between the antenna and the shield ground on the USB. That should take any voltage above .4-.6 V and short it to ground.

      Delete