The VHF/UHF input of the DVB-T dongles is nominally 75 - some may be 50 ohms, depending on if they had the factory make them from scratch using a different design.
The difference between 50 and 75 ohms on VHF input is fairly minor.
I was actually talking about the pins on the chip's impedance.
Its not the same thing.
As you can see here..
https://www.youtube.com/results?search_query=%22direct+sampling%22+RTL2832Direct sampling is not the same at all as using a dongle on VHF and UHF.
For years now, people have been modifying their cheap RTLSDR dongles to use them for HF. Why not, since they cost so ,little and considering the price they work really well.
Pins 4 and 5 of the RTL2832 chip's input are used for HF when
direct sampling their input impedance is specified at something much higher than 75 or 50 ohms, plus its a differential pair.
Normally that input is not something consumers ever deal with because its not used on dongles with an R820T tuner chip.
But, its fairly easy and for another few dollars in parts (if that) you can set up a filter to filter out the various unwanted signals, so you can receive HF.
It seems that a number of companies that sell dongles - for example, RTLSDR blog and CosyCave, a UK company, now sell dongles that make it significantly easier to do that by providing better access to the pads.
Most people shouldn't though unless they know what they are doing because otherwise, say if you just connect up wires with no protection for the inputs, the risk of frying your dongle with a bit of static electricity is substantial.
That said direct sampling isnt hard to do.
You dont need to buy a special dongle either, any of them make a good starting point.
This video shows what can be done direct sampling with a generic Newsky dongle - using a home made filter and RF amp daughter board setup.