N9EWO Guest Review :
HF Transceiver
(as a MW / SWL Broadcast Receiver)

FLEX-6400M DSP HF Transceiver. (manufacture photo)

N9EWO : Dean Bianco provides a guest review (but NOT as a transceiver). Our thanks to Dean. Excellent information on this 10.5 lb SDR transceiver. However being the internal fan operates in just "receive" mode is deal breaker for me no matter how well it performs ! Even if being a very quiet fan, it's not just the noise that is a factor. Dust and other room air nasties are inhaled inside of it more (will require it to be cleaned internally more frequently just like with a desktop computer). Yes, for many this always operating fan will not be an issue. The Icom IC-7610 also suffers from a "receive only" operation fan trait but here it cycles on and off (which is still unacceptable in our view). The discontinued Yaesu FT-450D transceiver also has a cooling fan that operates full time (see our review here).

Approx. Serial Number of Test Sample  : (not noted, a mid 2020 manufactured sample)
Firmware Version(s) Tested : (not noted)

Dean Bianco guest review of the FLEX-6400M (used as a SWL Broadcast listening receiver).

A User Review of the Flex Radio 6400M---An SWL's Perspective

The Flex Radio Systems 6400M is a currently-produced amateur radio transceiver that is built on state-of-the-art SDR technology.  It's main appeal, in addition to high-performance, is its stand-alone form-factor, thus eliminating the need for a PC to operate the software, displays, and deliver audio.  It uses a combination of touch-screen and old-school knobs, thus replicating many of the old analog heterodyne interfaces, yet with the features of an intuitive touch-screen interface. My review will be solely based on its use as an SWLing receiver.  Covering 30 kHz to 54 MHz, it is a complete general coverage receiver that features:

- Direct Sampling SDR Technology Utilizing DDC (Direct Down Conversion), Vertex-6 FPGA, 16 bit ADC, 122.88 Msps, TXCO Stability 0.5 ppm
Physical Dimensions:  (H) 6.47" (W) 14" (D) 13.25"-----(H) 17.1 cm  (W) 35.6 cm  (D) 33.7 cm,  Steel and Aluminum Case, Black Matte Crinkled Finish, Front Bezel Hard Plastic, Weight 10 lbs---4.53 kg, Optional Rack Mount Adapters

- 8 inch, 1920 x 1200 IPS Resolution WXUGA Touch-Screen Display, Panadapter Spectrum Display, width adjustable from 5 kHz to 7 MHz, and Waterfall Display, Adjustable (Waterfall defeatable via a work-around)
- Large Main Tuning Knob, Tuning Steps Adjustable down to 1Hz, parameters are mode dependent
- Internal Speaker (defeatable), External Speakers (Powered Hi-Fi recommended), Headphones
- Class D Audio Amplifier, In Push-Pull 
- AM/Sync AM w/continuously variable sideband selection/USB/LSB/CW/FMW/FMN/RTTY
- PBT in all modes except FM
- 2 Automatic, Adjustable Notch Filters, all modes except FM, AM and SAM
- Continuously Variable Adjustable Bandwidths in all modes : 50 Hz to 24 kHz, mode dependent---In AM and SAM out to 24 kHz, In SSB and CW, out to 12 kHz
- Filter Hi and Lo Audio Contouring, Adjustable in all modes except FM
- Sharp/Soft CW DSP Filtering in narrowest bandwidths
- RX Filter Sharpness/Latency, Adjustable, all modes
- Wide and Narrow Noise Blankers, Adjustable, all modes
- DSP White Noise Reduction Filter, Adjustable, all modes except FM
- Automatic Threshold Control, Adjustable, all modes except FM
- Three AGC Decay Speeds and AGC Off
- Two Separate, Fully Independent, Adjustable Receivers w/ their own controls
- Eight-Band Audio Equalizer, Adjustable, all modes
- Squelch, FM only (untested)
- Various Antenna Inputs, SO-239 and BNC for under 30 MHz, dedicated RX-ONLY BNC input disables accidental tx keying, LINE IN and LINE OUT use a 15 pin HD-15 connector, optional external GPS-disciplined frequency control
- Flexible Memory Storage, Theoretically Unlimited Memories (untested)
- RF Pre-Amplifiers, 8,16,24, and 32 db gain, defeatable to "off"
- RF Attenuator, -8 db
- Frequency Offset Accuracy "Synchronizable" to WWV or via external GPS
- Separate Speaker and Headphone Controls, internal speaker defeatable
- Bandpass Preselection, amateur bands only
- S-meter in s-units only
- Dedicated line-level output for external digital mode demodulators
- Powered by a user-supplied, regulated external 13.8 volt DC supply, minimum of 25 amps recommended, never exceeding 16 volts

The 6400M has the front panel interface built-in, allowing complete operation without the need for any PC, keyboard, mouse, and external display monitor.  This to me is the sole reason I chose the 6400M over the 6400 (non-M) which offers the exact same functionality, but requires a computer set-up for use.  Using a PC and peripherals is not my idea of radio, and if you are of like-mind, you might consider the M over the non-M model.

The radio is rather light in weight, but is well-constructed and feels solid.  It sits on your desk courtesy of large, non-skid rubber feet.  The knobs are less-robust feeling (more on that later).  My 6400M uses the optional rack mount kit as I prefer my station receivers in a console desk-rack, tilted at a 19-degree slant, which I find optimal for hour-after-hour finger/hand interface to the radio. On the rear are a myriad of inputs for external speaker, headphone, line-in and line-out, three antenna RF inputs (SO-239 and BNC), USB for ethernet connections to upgrade firmware, transverter input, and an external display jack are the major connections. 

The display is comparatively large, bright and sharp, with many of the display parameters fully adjustable, including flexible dimming and color schemes to suit most tastes.  It has a wide viewing angle.  One slight negative is the main frequency display is rather small. The S-Meter is well-implemented, although I wish it had the option of including signal measurement in dBm, such as is found on the ICOM IC-R8600. The panadapter is easy on the eyes and is well-implemented.  Very similar to the IC-R8600 in function and look, but with a larger display and better resolution.  The touch screen interface needs very little break-out force, which might bode well for reliability.  There is a main menu to access the rich feature set and a "dash-board" that allows quick and easy access to the direct frequency-input keyboard, pre-set bandwidths, AGC, pre-amp, noise blankers, notch, and noise reduction settings and adjustment.  I feel it is well-implemented and simple to use.

In addition to touch screen access for feature choices and parameter adjustments, there are large, old-school knobs that allow volume, main frequency tuning, bandwidth settings, PBT, filter edge adjustments, automatic threshold settings, and six user-definable settings such as AGC decay rates, NB on/off, NR on/off, mode selection, notch on/off as just a few examples.  This is a great feature!  My taste in operating prefers the least amount of touch screen interaction for constantly changing common parameters and settings in hands-on radio use.  With the knobs and touch screen working in concert, the ergonomic balance is much less fatiguing.  Kudos to Flex!  The knob's encoder quality is my main concern.  They feel WAY too loose to my liking.  They feel cheap.  While the main tuning knob has a smooth-as-silk, flywheel motion, it needs more weight and friction to slow its free-wheeling behavior.  It is easily bumped and it will detune from where it is resting all too easily.  This is a minor, but still annoying gripe.  Room for improvement.

AUDIO:  How is the audio quality?  In a word, spectacular. Clean, crisp and with a wide dynamic range:  bright highs to deep bass are what can be realized when you use high-quality, external powered speakers, such as from Bose, JBL and others.  I use a pair of Bose Companion 2 Series III's and am pleased with room blasting volume and sound quality.  The 6400M uses a class D push-pull audio amp that kicks major butt.  Now, mind you, if you prefer a more limited, less-dynamic sound, all you need do is adjust the audio characteristics via a menu-accessible 8 band equalizer, with sliding controls to tailor the sound to your preference.  I have found that best weak signal recovery works with a near maximum of high end and a moderate setting of the bass range.  Combine the fine audio reproduction with the SAM mode and you have a real aural treat.

Speaking of the 6400M's SAM, how effective is it?  I have been a radio hobbyist since I was a boy in 1970, so I have had a fair variety of receivers over the ensuing years.  Some of these radios have included an R-390A and a JRC NRD-515 (still have the '515!) hooked up to the legendary Sherwood SE-3 outboard synchronous detectors.  After spending many hours using the 6400M with its sync detector on all kinds of signals, under varying ionospheric conditions, I have concluded that this Flex has equivalent performance of the second-to-none SE-3. This is the SINGLE BEST REASON to justify the expense of the 6400M, and what made me sell my otherwise amazing high-performer, the ICOM IC-R8600 ! I have had and used these receivers with native sync detection:  the aforementioned ICOM IC-R8600, the JRC NRD-535D, the Eton/Grundig Satellit 800, the Eton E-1, the Tecsun PL-660, the Sony ICF-2010 (ICF-2001D) and my beloved WJ-8711A.  These receivers had sync detectors that range from barely acceptable to very good.  The best of the lot was the ICF-2010 for portable use and for desktop use, the WJ-8711A. 

However, the Flex 6400M, with its continuously variable passband shift, tone contouring, audio equalization, robust and well-performing AGC, and tenacious signal lock holding even during rough fading and nearby adjacent channel slop, is simply the best I have ever used. It has renewed my appreciation and enjoyment of MW and SW AM broadcast listening.  I can even make out about 75 per cent of the excessively bassy audio from the Voice of Korea!  With in-the-clear, strong signals, one can use up to 24 kHz bandwidth for near-FM quality fidelity.  Another aspect of the SAM detector is you can tune away from a signal and onto a new one with your tuning dial and you can leave the sync engaged, since there is no squealing when the detector has to lock onto a new signal.  Has it ever lost lock?  Just once!  This was under very weak reception of WWV.  The steady 600 Hz audio tone they transmit very briefly warbled.  Amazing.  I am blown away.

(editor note : Dean has told us that the internal speaker sound quality is just plain unacceptable BAD. So a decent "powered" external speaker(s) is a requirement.)

SENSITIVITY:  Not having any test equipment, I can only refer to my JRC NRD-515, having sold my otherwise excellent ICOM IC-R8600, for comparison.  The 515 is very sensitive and the 6400M matches it, though never exceeds it.  I have noticed that the noise floor on the Flex improves as you increase pre-ampflication, a characteristic also shared by the WJ-8711A.

SELECTIVITY:  Excellent.  Better than any superhet I have used.  Huge range of choices, up to a sublime 24 kHz in SAM and AM.  ECSSB excels with bandwidths that stretch out to 12 kHz!   When the going gets rough with interference, the narrow bandwidths can sound quite good, even down to 2 or 3 kHz bandwidth in AM/SAM.  Passband tuning truly aids with fidelity here.

AUTOMATIC NOTCH FILTER:  This is not very well-implemented in my experience.  It is fussy to adjust, and it just plain doesn't seem to work very well.  Of all the things I would say are not so good with the 6400M, and its major drawback, would be the notch filter is not very affective.  For a point of interest, the 8711A's notch filter performance was simply the best ever conceived.  The ICOM IC-R8600 was no slouch either!  Adding to its negatives is the fact that it disabled in AM and SAM!!

DYNAMIC NOISE REDUCTION:  This feature helps reduce listener fatigue and improve signal recovery from the distraction of hiss and white noise.  This works very well, and it does it without much if any water-gurgling digital artifacts as found on other DNR's.  However, in order to get best results, it must be adjusted to a very conservative setting, as the 6400M has a very powerful DNR here.  The slightest setting works extremely well and noticeably aids in audio enhancement.

NOISE BLANKING:  Here we have two NB's---Narrow and Wide.  They work in concert as opposed to being one or the other for the most effective reduction or elimination of such man-made irritants as line noise, dimmers, fluorescent bulbs, timers, ignition, electric fence pulses and other modern atrocities to radio reception.  Overall, these work quite well.  Experimentation and patience is the key here for successful noise mitigation.  They work more often than not, but be aware there are times when I have had line noise that could NOT be reduced by very much, so there are nuances here.  Again, overall, leaps and bounds improvement on impulse noises of various types.

AGC:  The only user-adjustable parameter is the decay rate.  The Flex offers Fast, Medium, Slow, and Off.  The attack and hang are factory-set, at least with the latest firmware iteration.  Not really a drawback in my experience with it. The attack is quite adequate judged on what my ears tell me; there is no syllable-onset pop or click and overshoot in SSB or CW, the two most noticeable modes for the shortcomings of a too-slow attack time. Also, there is no static-crash induced swamping of the receiver, which plagued my IC-R8600.  It really is a pleasure to be able to make out marginal signals under heavy summer static crashes.  The DNR reduces the harshness of the static crashes as wee. In contrast, my beloved WJ-8711A had a sub-optimal AGC attack speed that had to be mitigated with judicious use of manual gain reduction and use of their AGC threshold control, which tamed but not completely resolved static crash harshness, the clipping distortion of pops on SSB, and overshoot distortion.  No such worries here.  Smooth SSB reception. 

AGC-T:  The "T" meaning threshold. This variable control sets the point at which the AGC kicks in to keep varying signal strengths as even as possible without the fatiguing pumping of the noise level with a signal's amplitude changes. It is set by the prevailing noise floor where no signal exists, and then starting from full gain, reducing it to the point when the noise just starts to decrease. This works well, in making the signals pop out above the noise, often resulting in quieter signal to noise ratios.  Works in CW/SSB/AM and SAM.

MEMORIES:  I have not used them yet.

DYNAMIC RANGE/OVERLOAD: I think that the dynamic range is excellent, as the only time I had experience overloading was when I received WBCQ's 500 kW flame-thrower one night (9330 kHz) when that signal was 60db over s-9!! Yes, the 6400M has a preselector, but it is only to be found on the ham bands!!  The non-ham frequencies are wideband. (I don't believe under normal circumstances this will be a problem as my WJ-8711A was also wideband., having no preselector until I finally bought an aftermarket one). In order to resolve that incredibly strong WBCQ signal, the judicious use of the 8 db attenuator and backing down of the AGC-T completely resolved it.  A 60 db over s-9 signal is a rarity for my rural reception location.  I use a122 foot long T2FD antenna, so I can assuredly state that this receiver ca handle high-signal strengths with no intermod issues. Images are a thing of the past found in superhet designs, so not an issue here. I found only three weak birdies throughout out the entire LF-MF-HF range!! 



- Outstandingly smooth and distortion-free audio recovery, whether on very strong signals or very weak signals.  AGC well-designed with no fading weirdness. Punchy and high audio dynamic range add to the aural pleasure, especially when paired with excellent power speakers.

- Large, easy-to-use display with its Panadapter and Waterfall (See CONS)

- Runs cool with very little heat detectable. It does use a fan on receive, but it is very quiet. (See CONS)  You can barely hear it.  I was pleasantly shocked as fans are notoriously noisy and intrusive to good listening sessions. 

- Smooth, effortless main tuning knob has a silky-smooth action (See CONS)

- Selectivity, sensitivity, narrow-spaced dynamic range, all top of the line

- Extremely effective dynamic noise reduction and noise blanking

- Touch screen has low-break-out force

- Menu system better than most other receivers that use them.  Very short learning curve.

- Dedicated knobs for most frequently adjusted signal parameters, making the user interface fast and less convoluted

- Waterfall is defeatable if you find it distracting and of little value to your needs, as I find it.

- Light weight, but durable case.  Although the receiver has a large foot print, and is tall, it is easily moved when needed.  I find its larger size very attractive, as I like radios that are big vs the tininess of say the ICOM R-8600, while a wonderful receiver, always irked me.


- Automatic Notch Filter very difficult to adjust.  Sub-optimal performance.  Does not work in AM or SAM.  Probably the biggest CON for an otherwise sterling radio!

- Receiver uses a continuously running fan.  The fan is VERY quiet, incredibly so.  That's the pro.  The con is that if the fan should die, the heat might damage the internal circuitry.  I hope the fan Flex designed in is robust, but only time will tell.  Like I said, it is very quiet, so that's OK.

- The manual is 178 pages.  YIKES!  There is a lot of redundancy of explanations.  It is overly long and lacks a cohesive, concise, step-by-step approach.  Contrast this with the concise, well-written manual for the ICOM R-8600. There is no included printed manual, it is only available as a download to your computer or smart phone.  Also, some functions are not explained clearly and requires a bit of filling in the missing info on your own.  URGH.

- Flex's on-line presence is quirky.  There seems to be a lack of cohesion in coordinating customer service responses to customer inquiries.  They appear to rely too much on customer on-line community interaction.  I prefer a 24/7/365 official employee-based, comprehensive customer relations dept myself.

- Major firmware revisions are few and far between from what I have discovered.  Also, a major revision (not minor bug fix revisions) costs almost $200.

In conclusion, the Flex-6400M, despite some quirks, is a truly strong performer for high-quality LW, MW, and SW reception.  High dynamic range, clear, pleasant audio, highly effective noise mitigation, smooth AGC, and outstanding synchronous detector combined with ease-of-use, very good ergonomics and large spectrum display make this state-of-the-art radio a true joy and pleasure for hour upon hour listening sessions.    

Dean Bianco
ver 1.2

Rear View of the Flex-6400 and Flex-6400M Ham Transceivers (manufacture photo)

Links for additional Information (all subject to change without notice) :

eham reviews on the FLEX-6400M

Manufactures Web Site : FLEX-6400M

Why I Chose The Icom 7610 Over The FLEX-6400M

QST Printed Review February 2019 (8 pages) : Flex-6400M

You Tube Video : FLEX-6400M QST February 2018

You Tube Video : FLEX-6600M Start up time by KN4JX

You Tube Video : FLEX-6600M MENU OVERVIEW by KN4JX

You Tube Video : FLEX-6400M Quick Look

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