Choosing an SDR Front-End for the R-390A: A Comparison Matrix

A field guide for pairing the Cold War’s finest analog HF receiver with a modern spectrum display.

The R-390A/URR was the most precise general-coverage HF receiver the US military ever produced, and seventy years on it remains a reference instrument. Its single-conversion-above-1-MHz architecture, crystal-calibrated PTO, and mechanical filters deliver recovered-audio quality and frequency accuracy that most modern SDRs can’t match on their own. But the one thing the R-390A can’t do is show you the spectrum. No waterfall. No band scope. No visual cue that the DX station you’re chasing just QSY’d 3 kHz down the band. That’s where a software-defined radio companion earns its place in the station.

The question isn’t whether to add one — it’s which one, connected how, for what purpose. This article compares six SDR front-ends commonly paired with the R-390A: the Quantum SDR QCom, KiwiSDR / Web-888, Hermes Lite 2, RX-888 MkII, SDRplay RSPdx, and Airspy HF+ Discovery. Pricing reflects approximate 2026 street prices and is subject to currency fluctuation and availability.

Two Architectures, Two Different Problems

Before the comparison itself, it’s worth being explicit about what the SDR is actually doing. The same device can serve the R-390A in two fundamentally different ways, and the right choice depends on which role you want it to play.

Architecture A — IF-tap panadapter

Tap the R-390A’s 455 kHz IF before the mechanical filters, feed a buffered 50 Ω sample into an SDR tuned to 455 kHz, and you get a spectrum window of roughly ±96 kHz (or wider, depending on the tap point’s bandpass) that tracks wherever the R-390A’s PTO is pointing. The R-390A remains the primary receiver; the SDR is purely its eyes.

What you gain: The R-390A’s front-end performance is fully preserved. No antenna-sharing complexity. The panadapter automatically follows the R-390A because both are referenced to the same LO chain — tune the PTO, the waterfall shifts with you.

What you give up: Spectrum bandwidth is limited by whatever passes the tap-point bandpass — typically the 80 to 100 kHz IF envelope before the mechanical filters. A buffered IF-out modification is required (see the V506 tap discussion on r-390a.net). No automatic CAT sync to the SDR, since the R-390A has no frequency output bus — the dial reading must be entered manually as the panadapter’s center frequency.

Architecture B — Parallel direct-sampling receiver

Split the antenna between the R-390A and an SDR, operate them independently. The SDR sees the whole HF spectrum; the R-390A does what it does best on whatever frequency you’ve chosen.

What you gain: No modifications to the R-390A. Full HF coverage on the SDR side, including areas well outside the R-390A’s tuned window. Either receiver can be used alone.

What you give up: Antenna sharing requires a splitter or switch (with attendant loss and isolation concerns). The two receivers don’t know about each other — you’re tuning twice. SDR front-end performance matters considerably more because the SDR is seeing the full band of strong signals simultaneously, not the pre-filtered slice the R-390A already handled.

A useful hybrid: Some stations run both — an IF-tap panadapter for the R-390A plus a separate wideband SDR on a second antenna port for band survey. This is the configuration I’ve settled on here, and it informs the recommendations below.

The Comparison Matrix

Grading is against the R-390A use case specifically, not general SDR merit. A device that’s brilliant in a modern ham shack may be poorly suited to sitting next to a boatanchor, and vice versa.

Criterion	QCom	Web-888 / KiwiSDR	Hermes Lite 2	RX-888 MkII	SDRplay RSPdx	Airspy HF+ Disco
Approx. price (USD)	$349	$300	$325 assembled	$300	$220	$169
ADC resolution	16-bit (IF DSP)	16-bit (Web-888); 14-bit (KiwiSDR)	12-bit	16-bit	14-bit	Dual 18-bit equiv
Tunable range	HF + VHF (RX); 80–10 m (TX)	0–62 MHz (888); 0–30 MHz (Kiwi)	0–38.4 MHz	0–32 MHz direct; VHF undersampled	1 kHz–2 GHz	0.5 kHz–31 MHz, 60–260 MHz
Max. display bandwidth	192 kHz	32 MHz (888); 30 MHz (Kiwi)	384 kHz per slice, multi-slice	Full 32 MHz	10 MHz	768 kHz
Interface	Standalone (5″ touchscreen)	Ethernet / web UI	Ethernet	USB 3.0	USB 2.0	USB 2.0
PC required?	No	Browser only	Yes	Yes (capable host)	Yes	Yes
IF-tap panadapter suitability	Excellent — native IF input	Workable — tune to 455 kHz	Good — 455 kHz within range	Good — overkill but works	Good — SDRuno handles 455 kHz	Excellent — ideal HF DR
Parallel HF receiver suitability	Fair — modest front end	Excellent	Good	Excellent — best DR/$	Good	Good — narrow window
Remote access	No	Native, multi-user	Via PC/server	Via PC/server	Via PC/server	Via PC/server
CAT sync with R-390A	N/A — R-390A has none	N/A	N/A	N/A	N/A	N/A
Physical / aesthetic fit	Matches vintage feel	Small box + display elsewhere	Small box + PC display	USB + PC required	USB + PC required	USB + PC required
Software maturity	Young, evolving	Mature web client	openHPSDR, Thetis, Quisk	SDR Console, HDSDR, ka9q	SDRuno, SDR Console	SDR# native, SDR Console
Transmit capability	Yes, 0.1–20 W	No	Yes, 5 W	No	No	No

Deep Dive — Each Option’s Fit

Quantum SDR QCom

The QCom is the only unit here designed around the premise that the user doesn’t want a computer in the signal path. Its 5″ colour touchscreen, onboard DSP (1027 DMIPS), and dedicated IF input make it the cleanest IF-tap panadapter solution for a station that wants to stay visually and operationally coherent with a vintage receiver. Slip-tune lets you cursor-tune within the 192 kHz window without disturbing the R-390A’s PTO, which is the closest thing to “modern interactivity without desecration” you’ll find.

The trade-off is front-end performance: a QCom on the antenna is outclassed by an RX-888 MkII or Airspy HF+ Discovery in dynamic range. But that’s not its job here. Fed the clean, AGC-controlled signal from a pre-mechanical-filter tap on the R-390A, it has margin to spare. The 20 W transmit section is a bonus rather than a primary feature for this role.

KiwiSDR / Web-888

These are not IF-tap devices in any serious sense — they’re standalone wideband direct-sampling receivers with a browser-based multi-user interface. Their real value for the R-390A station is remote monitoring and band awareness. Point one at the antenna farm, leave it running, and you (or anyone) can see the full HF spectrum from anywhere. The R-390A stays untouched; the Web-888 is a parallel eyes-and-ears resource.

The Web-888 succeeds the original KiwiSDR with a 16-bit ADC and 62 MHz of coverage, meaningfully better than the Kiwi’s 14-bit/30 MHz for strong-signal environments. If a Kiwi is already in the station, it’s still perfectly useful; if you’re buying fresh, the Web-888 is the clear choice.

Hermes Lite 2

The Hermes Lite 2 occupies an unusual niche: it’s a capable direct-sampling receiver, a 5 W QRP transceiver, open-source, and Ethernet-native. For a station that wants both a parallel wideband receiver and a way to actually get on the air with modern modes, the HL2 is the single most versatile unit in this comparison.

As an IF-tap panadapter it works fine — tune the VFO to 455 kHz and take a 384 kHz slice — but it’s not particularly optimised for that mode. Its strength is as a dual-purpose companion receiver / QRP transmitter sitting alongside the R-390A, sharing an antenna via a suitable T/R or receive-only splitter.

RX-888 MkII

Receive-only, USB 3.0, 16-bit ADC, 130 MHz sample rate. On pure performance per dollar, nothing in this comparison beats it as a parallel wideband receiver. With ka9q-radio or SDR Console on a decent host it will record entire HF bands simultaneously, which opens up use cases the R-390A alone can’t touch — band survey, WSPR spotting on every band at once, propagation monitoring.

As an IF-tap panadapter it’s overkill but serviceable. The limitation is the USB 3.0 tether to a capable PC, which is aesthetically at odds with a clean boatanchor station. For serious work rather than casual listening, though, it’s hard to argue against.

SDRplay RSPdx

The RSPdx is the generalist of the group. 14-bit ADC, 10 MHz display bandwidth, coverage from 1 kHz to 2 GHz, and the mature SDRuno / SDR Console software ecosystem. As an IF-tap panadapter it’s entirely workable — SDRuno handles the 455 kHz tuning cleanly and its visual presentation is excellent.

Where the RSPdx stands out is versatility. Today it’s your R-390A panadapter; next week it’s monitoring aircraft on the airband; the week after that it’s decoding AIS on 162 MHz. For a multi-interest operator it offers the best utility-to-price ratio on the list.

Airspy HF+ Discovery

The HF+ Discovery is purpose-built for HF dynamic range. Its dual-channel 18-bit-equivalent architecture delivers the best strong-signal handling in this group on HF, at the lowest price point. The penalty is narrow instantaneous bandwidth (768 kHz max) and limited tuning range.

For IF-tap panadapter duty that 768 kHz is already four times more than the R-390A IF envelope will deliver — you will never run out of bandwidth on the tap. This is arguably the most technically correct IF-tap SDR choice when PC-based operation is acceptable. Pair it with SDR# or SDR Console and the spectrum quality is genuinely superb.

Recommendations by Use Case

Best standalone IF-tap panadapter (no PC in the station)

Quantum SDR QCom. It’s the only unit here that lets you keep the station’s visual and operational character intact. If “no laptop on the operating desk” is a hard requirement, the QCom is in a category of one.

Best IF-tap panadapter (PC acceptable)

Airspy HF+ Discovery. Best HF dynamic range per dollar, entirely adequate bandwidth for the IF envelope, and the most polished HF-focused software. If a mini-PC or NUC driving a second monitor is acceptable, this is the technically cleanest choice.

Best parallel wideband HF receiver (local operation)

RX-888 MkII. Nothing else in the price class delivers full-HF-at-once with 16-bit resolution. For band survey and multi-band digital-mode monitoring alongside the R-390A, it’s in a class of its own.

Best remote access / multi-user monitoring

Web-888. Modernised successor to the KiwiSDR with better ADC and broader coverage. Set and forget; access from anywhere.

Best transmit-capable companion

Hermes Lite 2. Open-source, Ethernet-native, 5 W out, capable receiver. If you want the R-390A for listening and a QRP path for working what you hear, the HL2 is the natural pairing.

Best generalist / multi-band utility

SDRplay RSPdx. Not the best at any single thing, but no weak areas and unmatched coverage range. If you want one SDR that does R-390A panadapter duty and everything else up to 2 GHz, this is it.

Closing Thoughts

The R-390A doesn’t need an SDR to be a great receiver. After seventy years it remains one of the finest HF receivers ever built, and adding a modern companion shouldn’t be framed as “fixing” something that isn’t broken. But the R-390A is blind to the spectrum around its tuned frequency, and that blindness is a real limitation in the modern band environment where activity is increasingly clustered in narrow pockets.

A properly chosen SDR companion — connected via a clean IF tap or sharing the antenna through a good splitter — turns the R-390A into a genuinely modern station without compromising what makes it special. The right choice depends less on which SDR is “best” than on which role you need it to play: eyes for the R-390A, or a parallel receiver that happens to sit beside it.

My current station runs a buffered V506 IF tap feeding an Airspy HF+ Discovery for the panadapter, with a Web-888 on a separate antenna port for band awareness and remote listening. Your priorities may point to a different mix. The comparison matrix above is a starting point, not a verdict.