Shortwave on an RTL-SDR: the V4 upconverter, direct sampling and converters

The tuner chip in an RTL-SDR stick starts at about 24 MHz. Everything below, which is most of shortwave plus mediumwave and longwave, needs a different path into the RTL2832U. The app offers three, under Source → HF (0–28.8 MHz).

RTL-SDR Blog V4: the upconverter

The Blog V4 has an upconverter built in. With HF on Auto · V4 upconverter, tune below 28.8 MHz and the app switches it in for you; tune above and it switches it out. There is nothing else to set, and you keep the normal gain controls. The device info lists HF via the upconverter from 0.5 to 28.8 MHz. The V4 offers no direct sampling, so Auto, External converter and Off are its only choices.

Other sticks: direct sampling

Most other sticks, the RTL-SDR Blog V3 among them, can feed the antenna straight into the RTL2832U and skip the tuner. On those sticks Auto uses the Q branch below 24 MHz, which is the one the V3 and most modified sticks wire up. Direct sampling I and Direct sampling Q let you pick a branch by hand if your stick is wired the other way.

Direct sampling works, but it is a compromise. There is no tuner in front, so no tuner gain and no front-end filtering: strong stations from elsewhere in the HF range can show up as ghosts, and the noise floor is higher than on an upconverter. Under Source, RTL digital AGC is worth trying here (leave it off for everything else). A modest antenna often sounds better than a huge one, because there is less to overload.

An external converter

An upconverter box such as the Ham It Up shifts HF up into the tuner’s range. Choose External converter and enter its offset in Converter offset (the app’s hint uses −125 MHz for a Ham It Up), and the frequencies you see and type are the real HF ones. The offset applies to the whole receiver, so switch it back to Off when you remove the converter.

Antennas for HF

The small dipole from a stick kit is far too short for shortwave. What works, roughly in order of cost:

• A random wire of 5 to 20 m outdoors, fed through a 9:1 unun, with a ground or counterpoise on the other side. Cheap, strong, and noisy if it runs past the house wiring.
• A passive loop such as the YouLoop: quiet and directional, so you can turn it to null local noise. Its output is low, which suits the V4’s upconverter better than direct sampling.
• An active loop or active whip: powered by a bias-tee, good in small spaces. The stick’s own bias-T gives 4.5 V; check that your antenna takes that voltage before you rely on it.

Most HF antennas end in a connector other than SMA, so you will need an adapter to SMA male; see the adapters guide. Noise matters more than signal on HF: get the antenna, and the stick on a USB extension, away from chargers, monitors and the computer itself.

Once you hear shortwave, the EiBi schedule in the Stations tab tells you who is on air, and decoders such as FT8, WSPR, CW, RTTY, NAVTEX and weather fax read many HF signals for you (see decoders).