All content Copyright 2017 Mark Abraham and may not be reproduced or redistributed without consent.

Prolouge (if you have read my other reviews, please skip the Prologue and the Disclaimer)

A lot of my fun in Ham Radio is the tinkering with these software defined radios.  I have reviewed and owned several at this point, the Flex 5000 Dual RX, Anan 100D Dual RX, the MB1, The DUO, Afredi, Ettus, Funcube, RTL, QS1R, KX3, Winradio, (if you want to count it as one) and also reviewed others and perhaps plan a few more for the future.   As you can see by the shack and other articles on the site, I like to see what all I can do with them and experiment a bit.  I hope you'll enjoy the this review as it takes the experience from all the others, my enthusiasm and passion and explores this newer SDR and shares my thoughts on operating with it.  I have been a licensed Ham since 2011, wow time fly’s, I dug right in to HF and SDRs not too long after that and never looked back. I hope you enjoy the review!



I have no affiliation with with the vendor nor am I compensated in any way for these reviews.  I do not have an alliance with any Ham Equipment Manufacture and try to retain a very independent and unbiased view.  I simply don’t care about brand loyalty, I care about great products so I try to tell it like it is and focus on what’s good and what seems bad.  I also try to write these reviews so all level of Hams can read and enjoy!  If you know of a great product and would like to see it reviewed here, let the manufacturer know about SDRZone so they can contact me and arrange a loan the review equipment.  Much of the equipment reviewed here was purchased by me directly and then sometimes later resold. Far too much of it has remained with me though as I am an enthusiast and in general share the excitement of SDR Radio.  


Review Setup - feel free to skip if you know what radio servers and slices are already

I think a little background might be in order before we dive in to the details.  Some will be reading this without a lot of SDR knowledge and a common question that arises is: “what is a slice receiver?”

When we think of a traditional superheterodyne receiver transceiver, it has one or two receivers in it with 1 to 2 sub receivers.  Sometimes the second receiver is the sub receiver and in more modern incarnations there can be two dedicated receivers and as many as 1 sub receiver attached to each.  I am taking great liberty here in speaking to this at the very high laymen level since its not my intent to present a course on radio architecture.

In the digital realm, slices could be thought of as virtual sub receivers created from a larger digital receiver.  The larger receiver captures a larger swath of spectrum and presents it in a way that it can be digitally carved up in to multiple smaller virtual slices, or AKA slice receivers.  Since the slice receivers are only virtual they can be created and destroyed as needed.  The # slices for all practical purposes are limited by larger receiver’s total bandwidth capture capability and the slice processors ability to simultaneously carve out and serve them up to an end user.  Hence why you see Flex and or other SDR makers speak in terms of 2,4 and 8 slices.

In general, in SDRs the ADC is the capture unit and the FPGA/CPU and DSP/CPU are the slice processor where a computer or device can represent the CPU and or the DSP or they could be circuitry onboard the radio with a radio operating system to do the processing.

The more ADC's and Processing power the more elaborate the slices can be and or the larger quantity of slices can be had.  A more precise overview in Flex terms can be found on the flex website at the link above in the user’s guide.  This was meant to be a more general synopsis related to SDRs overall.

I mention this also because the concept of a slice receiver is a foundational element on how Flex Radio Systems thinks in terms of SDRs.   Functionality must be available to each slice so that the slice may cover whatever need the user assigns.  This might be working QSO’s or skimming bands for local spots or gathering and or displaying other information that could be fed and or used in another program. 

What we end up with in essence is more than a software transceiver.  In the bigger picture we get a radio server that sits on a network and feeds a client or possibly clients as the future looks to unfold.  Clients might be anything from a device such as an iPad or computer end user or eventually users to software consuming and processing data for enhanced operations.

In turn this created a paradigm for a thin client.  A fat client is a full blown software package that sits on a computer or device and does all the data processing and manipulation to serve a client.  Its generally bulky but also usually fully featured.   Data is sent to the fat client to be processed to its end result.

A thin client might be thought of as processing the data to the fullest extent that makes practical sense and sending a much lighter data feed to a much smaller less capable client.  In the olds days we had main frames and terminals attached to the mainframe.  The main frame did all the work and the terminal was an interface to that work product or a means of requesting more work product.  The terminal was a client and the mainframe was the server.

We then moved into a world where the desktop started doing the work in a distributed manner and servers were used to store the resulting work product and make it available to others.

Today we have a mixture of paradigms out there and ironically Flex Radio Systems has examples of both in its older architecture with good ole fat PowerSDR and its newer thinner offering of SmartSDR.

The PC was used to do the heavy lifting in the old model and the radio server now does the heavy lifting internally and sends out lean work product for the end user to consume.

The arguments between each camps fan’s goes something like this:

  • The fat client can do more for my individual needs.
  • The thin client is more nimble and portable and thus more powerful.

Your cheerful SDRZone site owner will tell you that they both have great argument and that he has seen these models come and go and come and go again over and over.  Today we call the cloud the server and everything else clients.

So in summary, we get a radio transceiver that is a server, does the heavy lifting and serves its clients with slice receivers.

Review Notes

The FLEX 6500 was purchased used and will be traded up for a 6600.  By unfortunate chance, this creates a 20 day window for me to review the radio and will mean that it will be more limited than my usual work. Given this situation I would encourage you to also read the Flex 6700 review (in 3 parts) located in the archive.  Mike Alexander by far wrote on of the more professional reviews for this site with his Flex 6700 review and he was so kind as to have shared his time and thoughts.  I should also note that the upshot of this situation is that a few weeks after the new Flex models are released I should receive a 6600 to review.  I promise a deep dive and more comparisons when it arrives. It will also be at that time I will review SmartSDR V 2.0 which should be released with the new models.

Additional notes may appear here until the review is marked final which usually happens 6 months after the review is first published.

The Specs –  From the Flexradio Systems Website

FLEX-6500 Feature Highlights

  • Maximum Slice Receivers:Four (4)
  • Maximum Panadapter Bandwidth: 14 MHz
  • Antenna Connectors:SO-239x2; BNCx1; XVTR-BNCx1
  • Wideband Frequency Coverage:30 kHz - 72 MHz
  • Transmit Frequency Coverage:160-6m amateur bands, 100W nominal output
  • Transverter IF Frequency Coverage*:100 kHz - 72 MHz
  • Digital Audio Exchange (DAX) Channels:Four (4)
  • DAX IQ Channels/Bandwidth per Channel (DAXIQ):Four (4) @ 24 kHz - 192 kHz
  • Preselectors:160 - 2m (except 60m)
  • Microphone Connectors: Unbalanced 8-pin Foster, Balanced XLR/TRS
  • Antenna Tuner Unit (ATU):Integrated
  • Full Continuous Coverage Transmit:MARS/CAP Capable

 Receiver Specifications

  • Receiver Architecture:Direct Digital Sampling
  • Spectral Capture Units: One (1)
  • Maximum Slice Receivers: Four (4)
  • Maximum High Resolution Spectrum Displays:Four (4)
  • Maximum Panadapter Width: 14 MHz
  • ADC Resolution:16-bits
  • ADC Sampling Rate:245.76 Msps
  • Wideband Frequency Coverage:30 kHz - 72 MHz
  • DAX IQ Channels / Bandwidth Per Channel:Four (4) @ 24 kHz - 192 kHz
  • DAX Audio Channels: Four (4)
  • Amateur Band Preselector Coverage:160m - 6m (except 60m)
  • Preamplifiers / Attenuators:-10 to +20dB
  • Spurious and Image Rejection Ratio:100 dB or better
  • External Powered Speaker Output Impedance Level:600 Ohm Stereo Unbalanced

Transmitter Specifications

  • Transmitter Architecture:Direct Digital Up-conversion
  • TX DAC Resolution:16-bits
  • TX DAC Sampling Rate:491.52 Msps
  • RF Output Power:1-100W nominal SSB, CW, FM, RTTY, Digital; 1-25W nominal AM
  • Amateur Band Coverage at Rated Power Output:160m – 6m
  • Low Pass PA Filter Bands:1.8 MHz - 30 MHz continuous, 50 MHz - 54 MHz
  • Transverter IF Output Power:+0 dBm Typical; +15 dBm max
  • Transverter IF Frequency Coverage:100 kHz - 72 MHz
  • Modulation System:Digital Low Power at Carrier Frequency
  • Maximum FM Deviation:±5 kHz
  • DAX Transmit Channel: Yes
  • Carrier / Unwanted Sideband Suppression: <-80 dBc typ / <-80 dBc typ
  • Harmonic Radiation 1.8 - 50 MHz Amateur Bands: <-60 dBc; -70 dBc 6m
  • Transmit Bandwidth:Default 100-2900 Hz (Variable 50-10000 Hz)
  • Microphone Connectors:Unbalanced 8-pin Foster, Balanced XLR/TRS
  • Microphone Impedance:600 Ohms Nominal (200-10kΩ)

Antenna Tuner Specifications

  • Matching Range 80m - 10m: 8.3 Ohms - 300 Ohms
  • Matching Range 160 and 6m:16.7 Ohms - 150 Ohms

General Specifications

  • USB 2.0 Ports (peripheral control): Two (2)
  • Master Clock Frequency:983.04 MHz
  • Master Clock Phase Noise:-147 dBc@ 10 kHz; -152 dBc@ 100 kHz
  • 10 MHz Reference Clock Stability:0.5ppm TCXO
  • GPSDO Frequency Stability (GPS locked):15 x 10 (-12) over 24 hours
  • Emission Modes:USB, LSB, CW, RTTY2, AM, Synchronous AM, FM, NFM, DFM
  • Frequency Resolution:1 Hz min.
  • Antenna Connectors:SO-239x2, BNCx1, XVTRx1
  • Antenna Impedance (w/o tuner):50 Ohm Unbalanced


  • Power Supply Requirements: +13.8V DC nominal ±15%
  • Current Drain (Receive/Transmit Max):1.7A / 23A @ 13.8V


  • Height:4" (10.2 cm) with feet; 3.5" (8.9 cm) without feet attached
  • Width:13" (33cm)
  • Depth: 12" (30.5cm) or 13" (33.04cm) with front handles
  • Weight:Approximately 13 lbs. / 5.9 kgs
  • Operating Temperature:0 to +50 degrees C; +32 to +122 degrees F


  • 2U, 19inch (48.26 cm) Rack Mounting Kit
  • Front Handle Kit (adds 1 inch/2.5 cm to total depth
  • GPSDO with Rear Panel 10 MHz Reference Output

Specifications subject to change without notice.
1Requires optional GPSDO module
2Requires 3rd party software.

Purchase Rationale

Why buy a Flex 6500?  Well, right now, they are cheap as relative to there original asking price and they are a lot of radio for your money.  The receiver is top notch and the software seems to have all the basics there all be it different user experience.  As far as the 6500, you can also turn around and trade it up for a 6600 series when the time comes and get dual ADC’s.  I like the fact that the 6500 has 4 slice receivers and I can use all of them towards benefit while working the radio. The idea of a radio server is also intriguing, if you wish to sit on your deck and ham, all you need is a tablet and maybe a headset.  We will talk about all this more as we move forward.

Ordering and Shipping

This will not be applicable for this review since the radio was purchased used.  I will show a used unboxing.  I did also speak to flex about the 6500 trade-up to a 6600, they were polite, answered all my questions and helped me right over the phone.


 Unpack 170249



Double Boxed - thats good!


Unpack 170249

Unpack 170249


Warehouse supervisor, shes a real tuff customer! 


Unpack 170249








Unpack 170249

Unpack 170249

Front face of the radio, clean and modern looking

Unpack Impressions

The Flex 6500 is a very professionally made radio server.  It very much reminds me of a small PC in its case design, however, it certainly packs a very nice transceiver inside.  Like the MB1, its simple, a unit, cables, and small pamphlet and a CD.  It does make me wonder a bit about the Flex 6600, not having a front LED, I’d rather have paid more and have it like the panel 6500 does.  I 

will however learn to live without it given its more affordable price and Dual ADCs.


Unpack 170249


Rear Radio Panel


The rear of the radio has the connectors one needs, is clean and professional.  Of course you'll need an Ethernet connection, ground, antenna and power as a minimum to get on the air as well as the handset or microphone for SSB. The radio is actually very easy to setup and install and works well with both Windows software and the Ipad app I purchased.  I was a little surprised at first for the $50 price tag on the App, although I guess if one is paying $200 for the software then $50 is not bad for the app.  I will admit, I don’t like it that the software costs extra, however, if it guarantees continued development and enhancements then I guess I can and will live with it.


    I had no real issues setting up the radio, my previous SDR experiences in setting up radios directly translated over to the 6500.  In fact, this radio again was one of the easier rigs to setup.  Plug it in to a wall power source, plug in the Mic, hook up the antennas, hookup network, ground it and power on.  Because I bought the iPad app, I used it first for a few days before even loading windows with SmartSDR.

    SmartSDR setup is straight forward, a nice feature is that your virtual audio cables and soft com ports for CAT are also installed with the base software.  This makes the setup of 3rd party programs very straightforward.

    The manual is excellent and very professionally written, you can easily find them on the Flex Website.

    If you are overall new to SDR's and interested in these radios, I suggest signing up for the Flex Community, there you will find the answers to many common questions, and some of the Flex Radio Systems staff frequents the website answering questions.   Fully reading the manual a few times will help you prepare for owning your new radio.  

    On SDR Zone you can find additional guides as well so that if you are a new to PC’s running SDRs and digital operating modes then the Digital Modes Guide will help you understand what the setup for those modes looks like.  Both guides offer a little theory here and there as well, explaining some of the basic whys.  I highly suggest reading everything you can prior to your SDR radio arriving, if you do you will have a much greater chance of having a very positive experience like I ended up having.

    Setup can be summarized as follows:

    1. Unpack
    2. Check Contents for completeness and or damage
    3. Place Radio in shack testing position
    4. Connect Mic
    5. Connect Ethernet CAT 5/6 Cable to radio and PC or Hub (No Crossover cable needed) to your network
    6. Connect Dummy Load and or Antenna depending on whether you wish to transmit
    7. Connect Power
    8. Load app or software
    9. Power Up Radio

    You’re ready to get on the air!

     SDR Manuals and Improtant References


About the Flex 6500


Let’s talk about the physical aspects of the radio first. I think the case is highly professional. It’s solid, rugged, and very modern, polished and sleek as well. Amazingly small when you think about the power that resides inside the case! The connectors all seem standard (259 & BNC) as well as solid and, everything seems very professional nice and tight!  

This rig would seem to be professionally manufactured.  I have never been a fan of Flex's rear panels, however, it is functional and since you only see it when you are connecting things, so long as its solid and works, I am good.  I think everything one needs is pretty much there, so I don’t have any critical commentary about the radio. The display is professional looking and is nice to be able to see status.  Again, rather sad that the newer models only allow for a Maestro now for the display.  I like that the radio doesn’t require top ventilation, makes it easier to install with other radio equipment.


I have actually been waiting a few years now to try SmartSDR.  I really wanted a 6500 when they were first announced.  SmartSDR is a very nice SDR package, but its not quite the user experience I once imagined it would be for the end user.  I think long term I would definitely want a tuning knob and do have a Flex control now.  I think someday I might get a Maestro for my 6600.  To be honest, the Flex control knob feels cheap, they should work on this more with a really nice heavy perhaps metal tuning knob and buttons. This is subjective and I do realize some might like the current model.




Everything is pretty easy to find when you get right down to it.  As you can see there are tabs in the upper right corner from which you can make several setting changes.  These range from EQ's to power settings, Mic settings and filter settings. On the left you have DAX, Display Bands, Antennas, RX's, ect.

I actually am not going to do a deep dive on Smart SDR because I will do that with their version 2 when I get the 6600.   I have been wanting a firsthand answer as to whether SmartSDR is prime now, and I would say that it all seems to be there, this being said after operating both Digital and SSB and simulated CW via DM780 without a Key.

Setting up Digital modes is made very easy now with SmartCAT and DAX.  See the screenshot below as to how easy it is to setup HRD and DM780.  ComCat works well off HRD as you would expect.






SmartCat is Flex's virtual serial port emulator and is nice as well, you can assign a port to each slice.  This is really nice because it means you can have a lot of different third party ham software packages hanging off slices.  Like Skimmer for example, where you could be skimming and then using another slice for digital and yet another hooked to a logger working SSB.

DAX is Flex's version of VAC (Virtual Audio Cables) and its built in and I should say, its very nice and cleanly implements.  You have already established DAX cables ready to use after install.  You just need to set listen to on the recording device so you can hear your receivers if you’re not using a hard wire to speakers and just want to use your PC's soundcard and speakers.





All this makes integration very simple out of the box.  I have to give Flex an A+ for this as it should make it a lot easier for newbie SDR users.  I do recommend if you have never used an SDR before that you get at least a Flex Control and better yet a Maestro so you have knobs to help you until you get more comfortable using software defined radio controls.  I am not overly thrilled about tuning, especially to off frequencies and while the Flex control makes it easier, I do prefer other packages that allow one click tuning and mouse wheel on the panadpter fine tunning.  

Other Comments

A big deal regarding SDRs for me personally is the ability to support an external set of knobs such as the Hercules DJ Controller or other midi based controllers.  SmartSDR relies on Flex Control ( A simple knob) and Maestro ( a very elaborate display and controls external portable device) if you want physical controls.  I am thinking its only a matter of time before someone eventually tackles the DJ panel.  That is one potential issue with a fully commercial SDR, the vendor wants you to buy and use their branded accessories.

The fact is, that Flex with these radio servers are hands down more advanced in this further along than the other manufacturers. We will talk about this more later though as a Radio Server may not be the ticket for everyone.  There are real advantages and some disadvantages to this approach as far as looking at overall transceivers, SDRs and non SDRs.

Ironically the use of SmartSDR dispels the illusion that newer is better.  It sure is prettier, however, in reality, its more of a new user experience paradigm than it is as far as innovation goes.  SmartSDR could benefit from additional advanced tuning controls.  If you look at HPSDR's work, they have far more facilities to fine tune your operating experience and now we will have Professional TX and RX controls adding even more possibilities for fine tuning.  The same holds true for ExpertSDR as well.

Operating Comments

I have started to run some PSK and RTTY via Ham Radio Deluxe, DM780 and Comcat for Logging.  It’s all working great! There have been no radio specific issues integrating digital modes.  It all works with ease.  Fine tuning frequencies takes getting used to, again I highly recommend a tuning knob or maestro if your adverse to using a mouse.  Click Tuning is a bit weird, you have to double click.  There is both good and bad in my mind to this.  Accidental clicks are greatly reduced; however, single clicks would be faster and sometimes if you double click too fast while moving, you'll miss your target as it will move you to another.

The Flex receiver seems to fair comparably to other new radios.  Had I wrote this review when it first came out, it would have been deemed as quieter, however, the world of SDR has progressed significantly and there are new features and technologies that are beginning to move the Flex 6K series to its first signs of aging unless Flex works faster to add more details around the overall operating experience.  If you want less and believe less is better then a Flex would surely appeal to you as far as SmartSDR goes.  If you want full control, then it may not be for you.



  • Clean Radio Server Package
  • The 6K series and SmartSDR do not seem like they are experimental offerings. 
  • Well featured, the software seems to be at a point of adding newer features verses the basics and essentials.
  • Well engineered and I don’t see issues with extraneous RFI
  • Very very Flexible
  • Easy integration with 3rd party Software
  • Excellent thin client remote access
  • The iOS App is a pleassure to work with.


  • Need to buy a $4000 6600 for diversity reception
  • There is a lack of detailed controls for those that expect to see these in the level and price range of the 6500.  SmartSDR seemingly ignores those that would want them and I don’t think we will ever get them.
  • Add on knobs and dials are expensive
  • App costs extra for tablets
  • Did expereince network hiccups with the App and also in a virtual machine.

There are not a lot of real cons to this transceiver, it seems very solid right now at this point in the Flex 6K evolution.


User Manual

The user manual comes in pieces verses one large document, hardware and software guides are seperate.  I spent a good amount of time combing through these to make sure I wasn't missing anything about the radio and I have to say, these are very well done and only assume a intermediate level of knowledge.  The SmartSDR manual for example explains each Noise Reduction setting, type of noise they combat and when and how to set them.  As a side not, if you plan on printing the SmartSDR manual in color, you seriously might consider taking it to Office Depot or similar because it is very graphically heavy and ultimately a color laser or copier type printing will work better and likely cost less.  I suggest 3 hole punch so you can add revisions.  At this point in time, you may just choose to hold off until SmartSDR 2 is released and the new guide comes out.


Testing (Any changes will be highlighted in RED, otherwise if you read the MB1 review you may skip this section.)

About the Testing

Disclaimer, these tests I do are very informal and those that perform professional testing would likely poke holes in both the methods and results.  To that end, I use what we jokingly have referred to as the Joe Ham method, which is based on seeing and hearing how radios perform in a real shack with real world signals.

The basic premise of the test is to pit a given radio against the KX3 I keep in my shack.  This is done by using a mini circuits splitter and a manual 100DB attenuator.  A live signal is fed into the splitter and then equal cables are run from the splitter to each receiver.  Each radios audio output via the headphones jacks are fed into a Steinburg UR44 DAW, one radio on the left channel and the other on the right. 

Before I describe this in more detail, I want to convey the concept behind these tests and the bottom line premise that ultimately the product of any transceiver we buy and use is its ability to render audio of weak signals and provide clear reception and separation from the noise floor.  Therefore I feel that these tests may be of interest to hams seeking to purchase new SDR transceivers.  Of course, the other products of a radio purchase are the transmit side (being able to successfully DX), and the overall user experience with the desire for it to be easy and productive if we are contesting.

Before running the tests, all EQ and audio enhancements are turned off in the radios.  The volume for each radio is set high and then a VST Plugin meter in the DAW is used to equal the levels that will go into a chain of other VST plugins.  The audio signal is reviewed in the following ways.

The two signals are reviewed on an oscilloscope super imposed over each other.  This looks at the audio components amplitude, envelope, and delays of the two associated signals.  Again the volume levels are tweaked to ensure parity.

The signals are then fed into a frequency analysis plug-in and filters on the radios are adjusted to look at the overall impact on the audio signal.  Typically, the KX3 physical filters create steeper cutoff slopes verses digital ones. Again the volume levels are verified.

Next the signals are reviewed on two different spectrum analyzers;

The first looks at a 3D spectral heat map of the received audio and helps visualize the quality of the audio.  It is easy to see if the audio is balanced or unbalanced in this view.

The second is more of a pure traditional audio spectrum analyzer basically letting you see the performance at a typical range of octaves.  This can help visualize whether a radio tends to differ at various octaves from another.

All of these variables are reviewed as the signals are manually attenuated.  The attenuator effects the pre-split signal so that both receivers are guaranteed to be impacted equally. 

I use a live signal because we want a real-world view of receiver performance.  I also typically run this test with about 10 different signals, about 2-3 per each band that my antennas are able to cover, 40M, 20M, 17M, 15M and 6M.  Sometimes this is not possible due to band conditions.  Both SSB and Digital signals are reviewed while attenuating the master signal.

At a certain level of attenuation the signal becomes week enough that one or the other receivers loses track of it while the other still holds it, this usually occurs somewhere between negative 34-45DB.

SSB we look for legible audio through a set of equal speakers.  Volume is raised near the end of the cycle to see if it helps, often it just raises the noise and the signal is still not legible.  One might say that at a certain point the signal to noise ratio flips and the signals ability to produce audio is gone and all that remains is the noise. In the case of digital modes we look for which radio stops decoding first and begins to garble with its audio adjusted as best as it can to still copy a signal.

As the informal tests are performed I take notes and share anything remarkable in the readout in the review.  I should note that I have not had a receiver at SDRzone that has beaten the KX3.  It usually tends to win by about 2-5DB depending on the other SDR radios being tested.

If you would like to learn more about testing ham radio audio, please find the Ham Audio Analysis article located in the article archive at SDRzone.

The actual Testing and Comparisons.

 * Note that real world live signals are used, they are messy as they contain noise and other real world artifacts, this will in turn impact the visual elements reflected in these tests as they are not pretty signals created by laboratory signal generators. 

  • Some of the tests can be viewed on my YouTube Channel.

Flex 6500 vs KX3 - tested on 20M and 40M

The Flex 6500 tends to seem significantly quieter, however the noise floor and S Meters tend to perform equally on an unaltered signal.  The Flex beat the KX3 by a DB or two. Consistently in CW, PSK31 and SSB.  The same was true with the Pre-amps engaged with the exception that with the flex set to +20 it beat the KX3 by roughly 8DB in CW & PSK31 and SSB.

Flex 6500 vs MB1 - tested on 20M

These two radios ran Neck and Neck with preamps set equal.  In the end the Flex has a 20DB preamp that allows it to go past other radios like the MB1 that has a +10 Max.  I feel like the audio on the MB1 is much nicer, it has a richness to it that I have not seen the other SDRs I have, it has that Kenwood sound to it like a TS-990.  The 6500 and Anan seem to have what I would call lighter crisper audio as the higher notes stand out more while on the MB1 the lower notes tend to give it a very pleasant balanced sound. The noise floor was lower on the MB1, but I don’t think we can really pay much attention to this as I am not sure there is anything scientific behind the difference. The MB1 edged the flex without Pre-amp by about half DB with Preamps equal whereas the Flex started to pull away with them both set to 10DB and clearly had 10 more DB worth of signal grabbing set at 20DB..

Flex 6500 vs Anan 8000DLE - tested on 20M

This was a bit of a surprise, I expected the Flex to beat the Anan, minus diversity and to lose to it with Diversity on by a smidge.  It really didn’t play out that way, the Anan just flat out beat the 6500 until the Flex Preamp was turned on, then at +10DB the Flex edged it out by a hair and at +20 beat it handedly.  The Anan has a pre-am at the detector which is not controllable, this likely accounts for the performance boost seen in this model.  The Anan only has a contollable preamp function on 6M.  Too bad they don’t add a full featured one!  Diversity was not actually tested, I just ran out of time because I didn’t have an attenuator for the second antenna so obviously in that case its not a valid test.  I will get to this better in the 6600 review when the time comes along.  In digital the Anan 8000 DLE won by several DB without the pre-amp engaged on the Flex.


The flex played nicely with my Amp and shack switch matrix, no RFI issues whatsoever.  The volume level on the flex headphone output seemed a tad bit lower overall, however it seemed cleaner when pushed to the max than the other radios with less noticeable distortion.  Overall the audio is crisp and clean and a pleasure to operate with although perhaps subject to early listening fatigue.

All said the Flex Signature series appears to still be leading class offerings.  Also, its really evident that when we talk about Joe Ham test results, a solid pre-amp can rock the results.  Truly without the Pre-amp the 6500 is starting to fall behind the others.  That said, it has one, you can use it and its like a shot of nitrous oxide when a boost is needed.


Some Thoughts

I think a few thoughts are in order before I go further and need to be shared.  Look, lets be honest, people read reviews because they want things to go a certain way and validate their purchase decision while others look to them to help make purchase decisions. I have already had people writing me asking me which is better given that I am living a Hams SDR dream right now with the stack I have amassed in the rack.  

I have bad news for those that want one answer and that is, there is no magic SDR that will serve all.  For each ham, its all going to be about what you want and value and need verses an all in one solution.  

If you want a radio server, the Flex 6700 has to still be the best ticket in town.  If you want some of the latest innovations at the cost of fancy looking software, then an Anan 8000DLE is a hard ticket to beat.  And if you want knobs and a rig that is both a traditional rig and an SDR, then the MB1 sure is cool and a worthy ticket to consider as well.  

Dual ADCs are getting to be a must have now, especially after you experience them, so a radio like the 6500, or and MB1 is going to lose points for some people. There will be others that totally don’t care and want a kick ass transceiver with all the knobs and buttons of a classical rig, well, that is indeed your MB1 and just like a radio server, there is no other quite like it.  

The team working HPSDR... wow, so much radio for your buck and PureSignal for ultra clean transmit and Diversity for enhanced reception via two antennas and receivers rocks and it are game changers.  See where this is heading now?

While the Anan seems to be the better receiver now as far as real world usage goes, I feel like the MB1 is the better all around single receiver SDR.  While the MB1 can’t quite serve up slices like the 6500 can, it still manages to cover the ground I want with its 4 slices.  The MB1 seemingly stumbles into hidden functionality with the ExpertSDR software in its ease and native integrating with Skimmer or serving up IQ output for input into other software packages.

This was a difficult review due to the dynamics of the current market offerings.   So let’s drive to the finish line now in saying that the Flex 6500 is still a relevant offering for those looking for a $3000 high performance radio server capable of 4 slices.

 Wrapping Up and Comments

I certainly had a blast with the Flex 6500, near the end of my time with it, I was almost sorry I traded it in for a 6600, however, the 6600 will have the second ADC, be able to work diversity and still give me 4 slices which in my case seems plenty adequate since I wont be using the Flex to skim.

There is definitely a paradigm switch to the user interface and it takes a little bit to fully understand and appreciate how it really works.  I was a little bit rough on the 6500 in some eHam posts about Smart SDR.  I am guessing that if I made a detailed matrix on essentials verses extras that the Flex would have the essential bases covered.  I think if one wants detailed controls then perhaps an Anan might satisfy that need at the cost of a modern interface and fewer actual available useable slices.  The more I used the 6500 the more I was comfortable.

I enjoyed the app, despite its price tag and initial learning curve.  The more I learned about the Windows version of SmartSDR the more I learned about the app as they work very similar and more or less you get the full featured radio control via the app, plus some extra goodies like the log book and DX cluster integration.  Below is a screen shot from the App.




Note, just above the UTC time in the bottom left corner you see a black bar with tick lines on it.  This actually simulates a VFO knob and allows you to easily fine tune by either touching a click mark or swiping the wheel to make it spin further up and down the dial or fast and and quicly if needed, especially if you set the step rate to 100.  While this screen looks cluttered, I had the various windows open to provide a better feel for the overall app, you wouldn’t need all these while operating.  Other than the issues I had initially getting my headphone mic to work with the app, it ends up being straight forward after you get familiar.

There is no substitute for testing and comparing radios in your own shack.  A ham can advance past all the bias if they so choose and find objective answers on performance and gauge that based on the way they actually use their transceivers.  This opportunity for me allowed me to see and hear with my own eyes the differences.  I do understand that most hams can’t have all these rigs in their shack and am truly fortunate to have had the chance to do this.  I came away so far having great respect for all 3 rigs.   I also believe it’s time for me to sell the KX3.I have too many radios and since all these are beating it now, it makes since to move up to a better reference radio.  



Scoring:  All categories are weighted equal and averaged then rounded up or down to the nearest 10th.



*Score 1-10 is high




Based on previous experience and current t experience, Excellent Communication and Packaging

Build Quality


Very solid Professional Build

Design Quality


Fan Noise & Power Output and Power Connectors - definitely excels here, however, you need a device to operate this radio.

Ease of Setup


How hard is it to setup and Operate - the 6500 was very easy! :)



Easy to locate and follow, mature to the point that user supplements don't seem to be needed.



There is lots of room to add on and experiment. GPSDO, Flex Control, Maestro, and lots of possibilities via add-owns, apps, etc.  Trade up to more powerful transciever offerings are available.

Operating Experience


Software is very mature and robust and complete, I did not find it to have bugs or issues.  I think that the intermediate ham would find most of what they need.  An expert Ham might kick this score back to a 7.5 and say that SmartSDR needs more detailed controls.  The App definitely increases the score because its very nice for remote operation.



Very clean receive audio, good on weak sigs.  Chart topping receiver top 5%, performs very well in Joe Ham tests, low latency. Pre-amp allows it to win the receiver battle against other radios tested.



Fast Support!



You’re getting a well featured radio server transceiver, upper end performance, you must add on to get knobs, buttons, display, etc.  This is subjective, if you want a radio server, it's tops, if you want a complete SDR, it's less compared to an MB1.

Overall Score (Average)


Excellent Overall

*8-10 = best in class, 5-7 Above Average, 3-4 Below Average, 2 Poor, 0-1 Unacceptable

I will have to change this matrix a bit now due to the dual receiver options some rigs have and others dont.  That has to be worth something in the end.



In summary I really ended up enjoying the Flex 6500.  It certainly is a different user paradigm and I particularly enjoyed the app, so much so that I might have to see a Maestro before actually buying one as the app is so nice.  I think app wise the Flex takes the gold ribbon, even using the full ExpertSDR interface via NoMachine didn’t compare as its almost too much for remote usage while the Flex App seems just right.  Apps and addon screens in general, still have a long ways to go in reality to reach a full rich and complete experience.  Your not going to do PSK31 ect via the App and there a Maestro or the full blown NoMachine experience might rock.  Rotator control, seeing your amps power output, all things that still need to be addressed and are better managed remoting into a dedicated Ham computer to view and manage.  All that said, the 6500 is a fabulous radio and a hell of a deal to buy used right now, you can have one under $2800 if you shop it just right.  So all in all, I am a little sad to see it go and excited at the same time for the 6600 to come.  I think it will be fun to have and keep.

You may particularly like this SDR if:

  • You want to operate remotely from the radio
  • Tired of all the cables running from your SDR to your PC.  
  • Would like to just be able to turn on the radio and go like you can with a traditional transceiver.  
  • If you do not need Diversity
  • If you don’t care about PureSignal.


The Bottom Line

The Flex 6500 is an outstanding transceiver.  This is in part due to the point and time of this review given that SmartSDR has had several years now to develop.  I continued to run the Flex just for pure hamming for about a week after the testing, more time would have been nicer.   The radio was a little frustrating at first learning the new paradigm and it all grew on me the more I played with it.  Post review notes follow at the bottom of the review to reflect my expereince fo rthe remainder of the review period.  One last not, I highly suggest you join the Flex Community prior to purchasing your Flex to get a finger on the user base pulse.  There are real issues users are facing every day and I suggest you tune into those prior to pulling the trigger on a purchase to make sure they are not deal breakers.


About The Reviewer

Reviewer NI0Z, Mark Abraham - Licensed as Extra 2011. You can read more about the reviewer’s background on QRZ.com



I will share my week of operating the radio purely for hamming here and these notes will close on Jul4th, 2017.

June 21

Today  I had great and Ugly within 10 minutes.  iPad on the portch SSB, awesome QSO and unsolicited awesome audio report.. was using a set of beats Bluetooth headphones (They have a tiny Mic built in on the cord about mouth level).. was suprised because I hadnt adjusted anything between the sould headphones and the Beat Bluetooth headphones.  I never even tuned the soul headphones either, guess I was lucky.  Then the next QSO was an R3 DX, as as I asked for the call again the app froze in transmit.  Much to my horror then 6500 was froze in the shack in transmit as well.  NOT GOOD and scary!   

Regular SSB ops for the remainder of the day, again unsolicited great audio witht he hand Mic.  I am rewiring my studio mics and so while I did test them early on with the 6500, they wont be available again likely for the remainder of my review, the hand Mic will be used.

Resetting it I returned to the deck and operated a while longer with a few more QSO's and there were no further issues.


June 22



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