A couple of days ago when cracking open an RTL-SDR dongle I noticed that there was a DC blocking capacitor between the antenna connector an the input to the R820T tuner IC.
This got me thinking whether I could modify the dongle to apply a DC voltage to the center conductor of the coax so an external LNA at the antenna can be powered through the coax cable. This way you only need one cable to run between your receiver and your LNA/Antenna combo which might be sitting outside or on a mast.
The mod is very easy and only includes a couple of steps:
- remove the ESD protection diodes (D6, SOT-23) from the input
- solder a 10uH inductor onto the free pad on the transmission line and the other end of the inductor to the nearest 3.3V pad on the power rail
- you may want to add some filter capacitors from directly between where the inductor is connected to the DC rail and ground
In the picture you can see the modified dongle (right) in comparison to an unmodified dongle (left).
Please excuse the crude soldering this was done in a hurry. The damage at the connector was done when replacing the stock MCX with an SMA connector because I didn't have a soldering iron tip with enough thermal mass to heat up the ground plane sufficiently. As a result I used a pair of old wire cutters to get the old connector off and damaged the PCB in the process.
Keep in mind that by removing the protection diodes the input to the receiver is now much more sensitive to ESD, so be careful when connecting an antenna that could build up charge over time (i.e. an antenna that doesn't look like a short at DC).