Which antennas are best for RTL-SDR?

This question is from a viewer on my YouTube channel. But when I approved the comment for giving a reply, it disappeared! So I thought of posting it here. Actually the question was on SDR in general and not specifically on RTL-SDR. I have been using my regular ham radio antennas for receiving amateur radio signals with RTL-SDR. As you can appreciate from the video clip, I am using an SMA to SO239 patch cable to connect to the PL259 termination of my HF antennas. In a wider sense, my FT-710 radio is also a software defined radio. Yet this discussion will focus only on RTL-SDR Blog V4 which I have been using for various interesting applications.

A simple long wire, like 50 feet long insulated copper wire thrown into a tree, can be very effective for HF frequencies. But I have not tried that option. For a beginner, it is easier to build a dipole antenna of two quarter wavelengths to either side of the feedpoint. That will be about 10m to either side, for the popular 40m amateur radio band. RG58 coax which has an impedance of 50 ohms can be used to connect the antenna to the RTL-SDR in the room, with connectors as mentioned initially. Even though the readily available TV cable has an impedance of 75 ohms, that will also be fine for receive only purposes. In fact, I have used it for transmission also, long back, as I did not have access to 50 ohms cable at that time.

If you want to have dipoles for multiple bands, they can be mounted on the same support in the form of a fan dipole. In that case, third harmonic frequencies have to be avoided, like combining 7MHz (40m) and 21 MHz (15m). 40, 20 and 10m is a good combination for a fan dipole, which will also work on 15m as third harmonic of 40m and even 6m as the 7th harmonic of 40m!