Quite a while ago I tried the HF Mod (aka Direct Sampling Mod) on one of my dongles. The simplest way of doing it is by connecting one of the RTL2832U’s ADC (pin 1 or 2) inputs to an antenna which is exactly how I started. Shortly after that mikikg told me about using a transformer in order to generate a differential signal for the ADCs differential input, as well as boosting the signal and matching impedance. As a quick and dirty solution I desoldered a H16105DF 10/100Mbps Ethernet decoupling transformer from an old ADSL modem and used it. It actually matches the frequencies we care about quite well and in addition serves as a low pass filter. At the moment it is used in a 1:1 configuration, though 1:2 will probably perform better, thus I plan to try that as well. Using it together with an antenna made from a few meters of litz I receive many AM radio stations as well as lots of other signals.
Update:
Since someone over at reddit claimed that mikikg originally came up with using Ethernet transformers for the mod, implying I wouldn’t credit him properly, I feel like clarifying. On 10/3/12 I sent him a mail explaining that I had tried a variation of the HF-Mod using an Ethernet transformer and that the results were promising, I also attached a picture to that mail. Seemingly we then both blogged about it quite a while later, yet he did so before I did.
Update #2:
I have tried using the transformer in a 1:2 configuration and the signal actually got worse. That makes me wonder about the RTL2832U’s input impedance.
I tried this mod last night after wrapping my own balun. Tried to solder it in place and I toasted 2 dongles, lol. I bumped the wires to the I- & I+ and ripped off the traces. I did manage to get some HF receive tho, with one partially salvaged dongle bridging the I+ to the antenna center pole. I strung up some split twin lead 25feet in the house and received some faint 3.6MHz LSB. I used your OSX port of gqrx with device string direct_samp=1, not sure if the device string is needed for your port.
I didn’t feel confident enough to wrap my own one, I think picking the right core is also quite relevant for the resulting frequency response. Sad to hear about the dead dongles, I guess we can be glad those things are that cheap. The direct_samp=1 is only needed if you leave the e4000 tuner in place, as you can see my dongle doesn’t have it anymore, thus it always switches into direct sampling mode.
“The direct_samp=1 is only needed if you leave the e4000 tuner in place”
a’ha! Thank you! – My good Santa is bringing a Aoyue 968A+ rework station, soon I will pull the E4000 and the R820T in the $10 RTL2832U dongles. Air + solder paste should also make for easier connection to the I+- pins directly (on the 2 dongles with 1 lifted trace). I’m thinking of adding a pin header for the Balun on one of the dongles to allow for quick re-wrappings/experimenting. I also have a 3rd un-modded dongle to play with, not that I really need these things, I also have the SoftRock Ensemble II Receiver (1.7MHz to 3MHz), it just fun to see what can be pulled in with a $10 – $20 dongle.
I also picked up some Mica Caps, BNC connectors and a metal box to build this ‘Simple’ Low Pass Filter to remove the FM broadcast intermod. http://www.dxzone.com/cgi-bin/dir/jump2.cgi?ID=12590
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