Also known as: upconverter
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An upconverter is an external device that shifts HF signals up into the tuning range of a VHF/UHF SDR such as an RTL-SDR, letting radios that cannot tune HF directly receive shortwave.1 It is essentially a single mixer stage in a box: feed in a low-frequency antenna signal, and it comes out translated to a higher band the dongle can reach.
How it works
An upconverter is a heterodyne frequency translator. It mixes the incoming HF signal with a fixed local oscillator — commonly 100 MHz or 125 MHz — in a mixer, producing sum and difference products.2 The sum product (input + LO) is kept: a 7 MHz signal with a 125 MHz LO appears at 132 MHz, comfortably inside the dongle’s tuning window. The whole HF spectrum, 0–30 MHz, translates as a block to 125–155 MHz. In the SDR software you simply tell the app the LO offset and it subtracts 125 MHz so the display and logs read the true HF frequency.
Because the LO is quartz-referenced, its stability and phase noise add directly to whatever the dongle contributes — a poor upconverter LO smears weak signals. Mixing also creates an image: a signal at LO + 7 MHz and one at LO − 7 MHz both land at the same output if the front end is not filtered, so a good upconverter includes a low-pass filter on its HF input to reject anything above ~30 MHz before mixing. Cheaper units also let the strong FM broadcast band leak through as spurs.
Variants
- Passive vs powered LO — most hobby upconverters (the classic Ham It Up, the SpyVerter) use a crystal oscillator and are USB- or bias-tee-powered.
- Switchable / bypass — better boards let you route VHF/UHF straight through, so you can leave the converter inline and only engage it for HF.
- Different LO choices — 100 MHz keeps the arithmetic round (subtract 100); 125 MHz pushes the FM broadcast image further from the HF output.
In practice
An upconverter is the budget route to HF on an RTL-SDR and works well for casual shortwave, ham bands, and utility listening. Its limits are the ones you would expect from adding a mixer ahead of an already-noisy tuner: modest dynamic range, LO phase noise, and possible images from strong signals. For serious HF work a direct-sampling receiver or a dedicated Airspy HF+ — which digitises HF natively with far better dynamic range — is the higher-performance alternative.
Relevance to SDR
Upconverters are a staple of the RTL-SDR-plus-HF hobby, opening shortwave broadcast, ham SSB/CW, and digital HF modes to a dongle that otherwise stops around 24 MHz. GopherTrunk targets trunked land-mobile systems in VHF/UHF and does not itself do HF work, so an upconverter is not part of a typical GopherTrunk setup; it is included here as core front-end vocabulary for SDR users whose interests reach below the dongle’s native range.
Where to buy
For adding HF to a VHF/UHF RTL-SDR, the Nooelec Ham It Up v1.3 (around $50) is the long-standing default — a 125 MHz upconverter that lifts the entire 0–30 MHz HF spectrum into the dongle’s range. The Ham It Up Nano (around $45) is the compact version of the same board.
Check price on Amazon → Ham It Up Nano →
If HF is your main interest, a native receiver like the Airspy HF+ outperforms an upconverter — see the best HF SDR guide for the full comparison.
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Sources
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Frequency mixer — Wikipedia, on the mixing principle an upconverter uses to shift HF up into a VHF/UHF tuning range. ↩
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Heterodyne — Wikipedia, on producing sum and difference frequencies by mixing with a local oscillator, the basis of frequency translation. ↩
Frequently asked questions
Which upconverter should I buy for HF on an RTL-SDR?
The classic pick is the Nooelec Ham It Up v1.3 (around $50): a 125 MHz LO upconverter that shifts the whole 0–30 MHz HF spectrum up into an RTL-SDR’s tuning range. The smaller Ham It Up Nano (around $45) does the same job in a more compact board. Set the app’s LO offset to 125 MHz and the display reads the true HF frequency.
Do I still need an upconverter with a modern RTL-SDR?
Not always. The RTL-SDR Blog V3 and V4 add direct-sampling HF built in, so they reach the HF bands without an external converter. An upconverter is for older or other VHF/UHF-only dongles, or when you want a cleaner front end than the V3’s direct-sampling branch.
Upconverter or a dedicated HF SDR?
An upconverter is the budget route to HF on a dongle you already own and is fine for casual shortwave and ham listening. For serious HF work a native HF receiver like the Airspy HF+ digitises HF directly with far better dynamic range and no LO phase-noise penalty.
Does GopherTrunk use an upconverter?
No. GopherTrunk decodes VHF/UHF trunked land-mobile systems and does no HF work, so an upconverter is not part of a typical GopherTrunk setup — it is covered here as front-end vocabulary for SDR users whose interests reach below the dongle’s native range.