Some time ago I’ve 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.
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.
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.