Update: There is now a newer driver that allows HF tuning without hardware mods via a different mechanism. The new mod seems to work better than this one. Read about it at http://www.rtl-sdr.com/new-experimental-r820t-rtl-sdr-driver-tunes-13-mhz-lower/.
Over on the Reddit RTL-SDR forums user Jengal has posted a modified RTLSDR.dll dll file for SDR# which allows the direct sampling mode to be used on the R820T without the need for any hardware modifications. The modified dll is compiled from keenerds experimental branch of the RTL-SDR driver, which uses code for the no hardware mod written by tejeez. The no hardware direct sampling code was inspired by Anonofish’s discovery where he found that the E4000 based RTL-SDR could tune to AM radio without the need for the direct sampling mod (though this appears to be now patched in the newer rtlsdr drivers).
The direct sampling mod is a hardware modification to the RTL-SDR which allows it to receive HF frequencies between 0-14.4 MHz without the need for an upconverter. It works by connecting an antenna directly to the RTL2832U chip, thus bypassing the tuner. Teejez’s modification tells the RTL-SDR to bypass the tuner in software, allowing antennas to be connected to the normal antenna port. HF reception with the experimental driver is very poor in comparison to the direct sampling hardware mod or an upconverter, but even so Jengal was able to receive AM Radio, an SSB ham radio signal and an HF weather report with a simple longwire antenna.
To use the modified dll, simply download it from this link, rename it to rtlsdr.dll, and replace the original rtlsdr.dll in the SDR# folder. Then connect an HF antenna to the normal antenna port and in SDR# tune to a frequency between 0-14.4 MHz. Next turn ON the RTL AGC option in the configure menu. Jengal replaced the function of the RTL AGC option with the direct sampling mod. He found that best reception occurred when he set the gain to 48 dB.
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