Choosing an SDR for the Collins KWM-2A Transceiver

The fifth in the Boatanchor SDR series — and the receiver Collins already built with external-receiver integration in mind.

Companion articles: R-390A, Hammarlund SP-600, Collins 51S-1, and Collins S-Line 75S-1 / 32S-1 Station. Read the S-Line article for the four-SDR-roles framework (panadapter, TX monitor, off-air monitor, digital frequency reconstruction); this article applies the same framework to the KWM-2A and focuses on what’s genuinely different about a one-box transceiver.

The KWM-2A was Collins Radio’s MARS/military-extended version of the KWM-2, the first practical mobile SSB transceiver and the most-produced Collins tube amateur equipment — over 24,000 units built between 1959 and the mid-1970s. Where the 75S-1 / 32S-1 station put receiver and transmitter in separate chassis that shared an IF frequency, the KWM-2/2A put them in the same chassis and went further: the same oscillators, the same mechanical filter, and the same RF amplifier stages do double duty for receive and transmit. That architectural integration — and the KWM-2A’s factory rear-panel provisions for an external receiver — changes the SDR question meaningfully from what the S-Line article covered.

The rear-panel feature that matters most

The KWM-2/2A rear panel carries a jack labelled “TO EXTERNAL RECEIVER IF USED” and another labelled “MUTING CONTROL FOR EXTERNAL EQUIPMENT”. Collins provided these in 1959 so that operators could connect an auxiliary receiver — typically a 75A-series boatanchor for monitoring while the KWM-2 did the talking — with automatic muting during transmit to protect the external receiver’s input.

In 2026, that “auxiliary receiver” is an SDR. The infrastructure for cleanly integrating a modern software-defined radio alongside a KWM-2A was built into the transceiver from day one. No drilling. No modification. Plug in, configure the muting polarity, done.

KWM-2 vs KWM-2A — and the Rest of the Family

The distinction most often misunderstood is between the KWM-2 and the KWM-2A. They are the same transceiver electrically except for crystal board configuration:

KWM-2 — ham bands only. Single crystal board with factory crystals for 80/40/20/15/10 m amateur segments plus WWV at 14.8–15.0 MHz. 200 kHz tunable segments.
KWM-2A — MARS and military variant. Adds a second crystal board with 14 additional sockets, covering any 200 kHz segment in the 3.4–5.0 MHz and 6.5–30.0 MHz ranges (the 5.0–6.5 MHz gap is hardware-enforced). Front-panel switch and indicator toggle between the two boards.

Everything else — IF architecture, mechanical filter, VFO, rear-panel interconnection, power requirements — is identical. For SDR integration purposes, “KWM-2A” can be read as “KWM-2, with additional frequency coverage that extends the panadapter’s potential usefulness into MARS, international broadcast, and maritime bands”.

Variant	Years	Distinguishing Features	SDR Integration Notes
KWM-1	1957–60	Predecessor. Different architecture, narrower coverage, not considered part of the S-Line family in the strict sense.	Different IF scheme; most of this article does not apply directly. Worth a separate write-up.
KWM-2 (Winged Emblem)	1959–early 1960s	Original production run. Ham bands only. 455 kHz second IF, 2.955–3.155 MHz first IF.	All recommendations apply.
KWM-2A (Winged Emblem)	Early–mid 1960s	MARS crystal board added. Same signal path.	All recommendations apply. Extended frequency coverage increases the value of panadapter work on MARS bands.
KWM-2/2A (Round Emblem)	Late 1960s–mid 1970s	Later production with round CCA-era emblem and accumulated service bulletin improvements.	Identical integration points. The more common variant on the secondhand market today.
URC-32 / R-1051 family	1960s	Navy/military transceivers derived from KWM-2 architecture. Substantially different packaging and interfaces.	The IF-tap philosophy applies but physical access differs significantly. Specific to the unit.
HF-380 / HF-2050 / KWM-380	Late 1970s–80s	Solid-state successors. Entirely different architecture.	Out of scope for this article.

What the KWM-2A’s Shared Signal Path Means for SDR Integration

In the S-Line, the 75S-1 had its own RF front end, mixers, and IF chain; the 32S-1 had its own (matching) chain. A tap on one didn’t touch the other. In the KWM-2A, the same RF amplifier, the same first and second mixers, the same oscillators, and the same mechanical filter serve both functions, with relay switching determining signal flow direction.

Three consequences flow from this:

A single IF tap serves both directions. Tap the 455 kHz IF (or the 2.955–3.155 MHz first IF) and you see received signals in RX mode, and you see the transmitted signal forming up (pre-PA) in TX mode. One SDR, both roles, automatically.
You cannot run the SDR unattended through a T/R cycle on the IF tap. Signal levels at the IF points are much higher during transmit than receive. Most SDRs will survive the excursion, but AFC and AGC behaviour will be disturbed. Muting coordination is important.
The 2.955–3.155 MHz first IF is a uniquely good panadapter tap point. It’s a fixed 200 kHz passband into which the variable first conversion places whatever band segment the crystal selects. A broadband SDR tuned to ~3.05 MHz with 200 kHz capture sees the entire active band segment simultaneously — without needing to re-centre as the VFO tunes, because the VFO tunes within the passband, not above or below it.

Tap Point Comparison

Three potential internal tap points, with practical implications:

Tap Point	Frequency	Bandwidth Available	Suitability	Modification Required
455 kHz second IF, pre-mechanical-filter	455 kHz	~200 kHz (full segment)	Same approach as S-Line; works for all SDRs with 455 kHz IF-mode support.	Small coupling capacitor to second-mixer plate, route to spare chassis point.
2.955–3.155 MHz first IF	~3.05 MHz centre	200 kHz, fixed	Preferred for panadapter work. Fixed centre frequency; VFO tunes inside the window; full band segment visible at all times.	Capacitive pickoff at first-IF transformer, routed to rear panel.
455 kHz IF, post-filter	455 kHz	2.1 kHz (mechanical filter bandwidth)	Narrow; useful for demodulation-chain monitoring but not panadapter work.	Minimal; can often be picked off product detector input.

Why the first-IF tap is the preferred choice: On the 75S-1 / 32S-1 station, tapping the 455 kHz pre-filter IF is the only practical way to get a full-segment view, because each band-segment crystal produces a different 455 kHz-referenced first-IF frequency that must be mixed down. On the KWM-2A, the first IF is the 200 kHz passband — always the same 2.955–3.155 MHz window, regardless of which band you’re on. One tap, one SDR centre frequency setting, every band segment. It’s the cleaner architecture.

Historical precedent: the DFD digital display for KWM-2

Tony I0JX and others have documented digital frequency display installations on the KWM-2/2A using essentially the same approach as the DFD2-S on the S-Line: probe the VFO, crystal HFO, and BFO signals, compute the display frequency in real time. For the KWM-2 the probe points are internal tube shields rather than rear-panel RCA jacks, because the KWM-2 doesn’t share the S-Line’s external-cable interconnect scheme for those signals (the corresponding signals are all internal to the single chassis).

The same three oscillator probe points can feed an SDR-based frequency reconstruction system if digital readout is the goal rather than spectrum display. For most operators the panadapter view is more useful than a numeric readout, and the 2.955–3.155 MHz first-IF tap delivers that directly.

Four SDR Roles, KWM-2A Edition

The four roles defined in the S-Line article — IF-tap panadapter, TX spectrum monitor, off-air station monitor, and digital frequency reconstruction — apply directly here, with KWM-2A-specific nuances:

Role A (panadapter): The 2.955–3.155 MHz first-IF tap changes the calculus vs. the S-Line — see the dedicated matrix row below.
Role B (TX monitor): Applies identically. The KWM-2A puts out ~100 W; any RF sampling must include proper attenuation.
Role C (off-air monitor): Applies identically, with the bonus that the KWM-2A’s factory “TO EXTERNAL RECEIVER” + MUTING CONTROL connections give you a clean way to use the KWM-2A’s own antenna with automatic mute during transmit. This is genuinely unique among the receivers and stations in this series.
Role D (digital frequency reconstruction): Done via internal probes rather than rear-panel jacks. Same idea, messier installation.

RF sampling safety, again: 100 W at the KWM-2A antenna jack. Any TX-side sampling requires a proper directional coupler, sampling tap, or attenuator rated for the power. Do not connect an SDR directly to any point that sees full PA output power.

The Comparison Matrix — KWM-2A Context

Criterion	QCom	Web-888 / KiwiSDR	Hermes Lite 2	RX-888 MkII	SDRplay RSPdx	Airspy HF+ Disco
Role A: 455 kHz IF panadapter	Excellent	Workable	Good	Overkill	Good	Excellent
Role A’: 2.955–3.155 MHz first-IF panadapter	Good — if IF mode tunes to 3 MHz	Excellent — natural range	Excellent	Excellent	Excellent	Excellent — best HF DR
Role B: TX spectrum monitor	Good	Workable	Good	Good	Excellent — SDRuno tools	Excellent
Role C: Off-air monitor via “TO EXTERNAL RECEIVER” jack	Good	Not the use case — designed for separate antenna	Good	Excellent	Good	Excellent
Role C’: Off-air monitor on separate antenna	Fair	Excellent	Good	Excellent	Good	Good
Muting input compatibility	None	Via software	Via PTT input	Via software only	Via software	Via software
MARS band value (KWM-2A specific)	Good	Excellent — full range coverage	Good	Excellent	Excellent — up to 2 GHz	Good
Physical / aesthetic fit	Matches vintage look	Small box + display	Small box + PC display	USB + PC required	USB + PC required	USB + PC required
Approx. price (USD)	$349	$300	$325 assembled	$300	$220	$169

Deep Dive — KWM-2A-Specific Considerations

Quantum SDR QCom

The QCom is purpose-built for IF input at 455 kHz, and its 192 kHz display bandwidth fits the KWM-2A’s 200 kHz band segment almost exactly — same happy coincidence as with the S-Line. Physical pairing with a KWM-2A sitting in its PM-2 dust cover, or mounted next to a 312B-4 console, is aesthetically coherent. The limitation is that the QCom’s IF mode is optimised for ~455 kHz inputs; tuning it to the 2.955–3.155 MHz first-IF tap is possible but not where the unit shines.

For a KWM-2A owner who wants a single self-contained panadapter and doesn’t want to deal with a PC, the QCom on a 455 kHz tap remains the right answer.

Web-888 / KiwiSDR

The Web-888 is arguably more valuable on a KWM-2A than on any of the prior receivers in this series, for one reason: a transceiver station actively transmits, which creates the need for remote monitoring of your own signal. A Web-888 on a separate receive antenna, positioned a few wavelengths away from the transmit antenna, gives you objective evidence of what your KWM-2A sounds like on the band — from anywhere, with a browser. For mobile operation (which the KWM-2 was designed for), a home-based Web-888 watching the bands while you’re on the road is particularly useful.

Hermes Lite 2

The HL2’s value proposition on a KWM-2A is split-band / cross-mode operation. The KWM-2A transmits SSB and CW beautifully; it does not transmit FT8, FT4, JS8, PSK31, or other modern digital modes natively (though audio-in hacks exist). An HL2 on a second antenna handles the digital modes while the KWM-2A handles voice and CW. No KWM-2A modification required; no compromise to the transceiver’s vintage integrity.

RX-888 MkII

The KWM-2A is the one case in this series where the RX-888’s 32 MHz instantaneous capture maps almost perfectly to the transceiver’s coverage: 3.4–30 MHz fits neatly inside the RX-888’s range. A parallel RX-888 on a separate antenna, or fed from the “TO EXTERNAL RECEIVER” jack with proper muting coordination, captures everything the KWM-2A could possibly tune to, simultaneously, at 16-bit resolution. For a contester or DX chaser, this is a genuinely powerful addition that doesn’t require any transceiver modification.

SDRplay RSPdx

The RSPdx’s appeal on the KWM-2A, and especially the KWM-2A specifically rather than the KWM-2, is its extended coverage all the way to 2 GHz. If you’re using the MARS crystal board to operate in high-HF MARS frequencies, the RSPdx is already comfortably within its range. If you’re curious what’s happening on VHF/UHF while operating the KWM-2A on HF, the RSPdx is the only SDR in this group that can show you. Add SDRuno’s measurement suite, and it’s also the best TX-quality diagnostic tool for maintaining a well-tuned KWM-2A.

Airspy HF+ Discovery

The HF+ Discovery remains the technical-performance leader for pure HF work. On a KWM-2A, connected to either the 455 kHz or 2.955–3.155 MHz IF tap (both within its tuning range), it delivers the cleanest panadapter view in this comparison. Its compact USB form factor tucks away neatly beside or behind the transceiver; a small Mac mini or Raspberry Pi can drive the display without dominating the operating position.

Recommendations by Operating Style

Fixed station, classic vintage look

QCom on a 455 kHz IF tap, plus optionally a Web-888 on a separate antenna for remote monitoring. The QCom sits next to the KWM-2A like a miniature modernised SB-620; the Web-888 is invisible in the background. Matches the aesthetic the KWM-2 was always meant to have.

Technical restoration / TX quality focus

SDRplay RSPdx on the “TO EXTERNAL RECEIVER” jack with proper muting, running SDRuno. The combination of factory-supported antenna sharing, automatic muting, and SDRuno’s measurement suite makes this the strongest diagnostic setup for verifying KWM-2A transmit cleanliness without any modification to the transceiver itself.

Mobile / portable operation (the KWM-2’s original intent)

Airspy HF+ Discovery fed from an internal 2.955–3.155 MHz first-IF tap, displayed on a small laptop or tablet. The entire active band segment visible at all times regardless of VFO tuning; minimal PC footprint suitable for mobile or portable installation. A capable single-SDR solution for an operator who’s actually using the KWM-2A the way Collins designed it to be used.

Contest / DX operation

RX-888 MkII connected via the “TO EXTERNAL RECEIVER” jack with appropriate muting, recording the entire HF spectrum throughout the contest while the KWM-2A handles QSOs. Post-contest review of the recording often reveals stations you missed, or confirms partial callsigns you weren’t sure of.

Bridging to modern digital modes

Hermes Lite 2 on a separate antenna port. The KWM-2A stays on SSB and CW; the HL2 handles FT8, FT4, and other modern digital modes. No transceiver modification; the vintage station remains pure.

Extended frequency coverage (KWM-2A MARS use)

SDRplay RSPdx as parallel receiver. The 1 kHz to 2 GHz coverage comfortably encompasses any frequency the KWM-2A’s second crystal board can reach, plus lets you listen above 30 MHz when the MARS schedule doesn’t need you.

Closing Thoughts

The KWM-2/2A is the one radio in this series that Collins explicitly designed with external-receiver coexistence in mind. The rear-panel “TO EXTERNAL RECEIVER” and “MUTING CONTROL” jacks anticipated the 75A-4 or R-390A sitting beside the transceiver; they serve the SDR use case equally well, and no operator in 1960 would have been surprised by the concept. Add the particularly clean panadapter tap at the 2.955–3.155 MHz first IF — a happy consequence of the KWM-2’s double-conversion architecture — and the KWM-2A emerges as possibly the easiest vintage transceiver in existence to pair with modern software-defined radio.

For most operators, the practical recommendation collapses to two setups. Either a QCom on a 455 kHz IF tap plus an optional Web-888 for remote monitoring (the “vintage look” answer), or an HF+ Discovery on the first-IF tap with a small PC driving the display (the “technical best” answer). Both cost less than a single replacement 6146A tube pair from a premium vendor; both add capability the original KWM-2 designers would have appreciated but couldn’t have built with 1960 technology.

Ed Andrade and the KWM design team built a transceiver that expected to be used alongside other equipment, and that expectation ages well. Sixty-seven years later, “other equipment” happens to mean something very different from what they imagined, but the rear panel is unchanged, and the jacks still do what they always did.