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Can someone please explain the difference between 6400M vs 6600M


Looking to buy a flex and can't decide which model will suit my needs best. Could someone please explain to me what are the major differences? What does 3rd order vs 7th order mean? Water fall 7Mhz vs 14Mhz what does mean?

Thanks, Angel

Best Answers

  • John KB4DU
    John KB4DU Member ✭✭✭✭
    Answer ✓


    The main difference is the receive capabilities. The 6400 has only one “scu” which basically means one receiver using one receive antenna and can display two bands at one time. The 6600 has two scu’s and each can display 2 bands for a total of 4. The dual scu’s are also higher performance, capable of displaying 14 MHz spectrum at one time, vs 7mhz at a time for the 6400.

    I have a 6400 and 2 bands at once is all I can handle.

  • KD0RC
    KD0RC Member, Super Elmer Moderator
    Answer ✓

    Hi Angel, the filters keep out of band signals out of the receiver. For casual operation, the 3rd order filters of the 6400 work great. For SO2R operation, or any contesting where there will be multiple transmitters on different bands (20 and 40, for example), the 7th order filters will provide over 50 dB of isolation on the contest bands

    The 6600 and 6700 can view a 14 MHz wide portion of the spectrum at one time, where the 6400 can see 7 MHz at once. You will not normally have your panadapter set this wide.

    The more important difference to me, is that the 6600 and 6700 have two spectral capture units (SCU) as compared to the 6400 which only has one.

    With two SCUs, you can operate SO2R, you can operate satellites with transverters, you can do diversity reception on two antennas, etc, etc.

    The radio that is right for you depends on the kinds of things that you want to do.

    If you like contesting, the 6600 can do things that the 6400 can't. If you are strictly a casual operator and don't need the second SCU, the 6400 may be fine.

    Best of luck in your search!

  • John KB4DU
    John KB4DU Member ✭✭✭✭
    edited September 2021 Answer ✓

    But be aware the 6400/6600 do not have the loop feature.

  • David Decoons, wo2x
    David Decoons, wo2x Member, Super Elmer Moderator
    Answer ✓

    Two things I didn’t see mentioned in this thread

    ATU - optional on 6400, included on 6600.

    Number of transverter ports - 1 on 6400, 2 on 6600.

    Bonus - same for number of RX antenna ports - 1 on 6400 and 2 on 6600.

    With either radio the RX antenna can be independent of the TX antenna and as others pointed out already the 6600 can receive two frequencies on two different antennas at the same time.

    Last, to clarify - the 6400 has two slice receivers (think VFOs) and maximum of two panadapters while the 6600 has four slice receivers and maximum of 4 panadapters.

    73 Dave wo2x


  • Ted  VE3TRQ
    Ted VE3TRQ Member ✭✭✭

    The major difference between the 6400 and the 6600 is the number of SCUs (spectral capture units, analogue to digital converters). Each SCU can handle a separate antenna, thus allowing two antennas to be used simultaneously with the 6600, but only one with the 6400. That also means being able to listen to two separate bands on their own antennas at the same time with the 6600 - great for satellite work, or anything full duplex on different antennas. You also get two XVRTR inputs/outputs on a 6600, and only one on a 6400. Unfortunately for those who want to use receive loops to add filters, preamps, etc. there is no ability to output the receive signal and re-inject it - for that you need a 6700.

    The 6400 can have 2 panadapters with a receiver (slice) on each (2 total panadapters, 2 total slices) - the 6600 can have 4 panadapters with a slice on each (4 total panadapters, 4 total slices).

    As to the order of the filters, the 6600 has better (more effective) filters to screen out large signals from other bands when running on the contest bands - good when you work close to someone else on a different band. The 6400 is good, the 6600 is better. Both radios are very good at handling strong signals within band.

    The SCU on the 6600 is “wider”, in that the SCU handles 14 MHz of bandwidth, meaning you can put two receivers on the same SCU 14 MHz apart. The 6400 is limited to 7 MHz. I really think that difference will not be meaningful to many hams.

  • Neil D Friedman N3DF
    Neil D Friedman N3DF Member ✭✭✭
    edited September 2021

    The 6600M has both a balanced microphone input and a pseudo-balanced microphone input. The 6400M has only a pseudo-balanced input. The 6600M has two receive-only antenna inputs while the 6400M has only one.

    The year the 6400M/6600M were introduced, a Flexradio speaker at the Dayton Hamvention’s Flex dinner described the 6600M’s additional filtering as a one thousand dollar value.

  • angelo
    angelo Member ✭✭
    Hi Ted VE3TRQ, can you please elaborate on the receive loop comment? This is a configuration I was looking to implement, in order to improve reception or noise, because my current antenna is a vertical omni-direction, so in order to reduce noise, I am planning on using a receive loop.
  • John KB4DU
    John KB4DU Member ✭✭✭✭

    The 6400 normally uses the same antenna for tx and rx. However, there is also an rx in connector to allow the use of a receive only antenna, separate from the transmit antenna. I use it for a low noise loop on ground, but any low noise receive only antenna can utilize it. The 6600 has the same facility, plus the 2 antenna inputs, 1 for each scu.

  • Ted  VE3TRQ
    Ted VE3TRQ Member ✭✭✭

    Hello Angelo.

    The “receive loop” I mentioned is the ability to have the receive antenna of the radio directed to an output connector, which can then have processing applied before being redirected to a receive (only) input connector. The 6700 has such capability (as does my K3 :-)

  • oldtimer
    oldtimer Member
    A HAM newbie here -- but an oldtimer in years and especially eyesight. My eyesight problems have directed my attention towards a 6400. (not a 6400M).
    I have a question which is a bit "off subject" to this stream - but
    John and Ted - In simple language please, can a 6400 be configured to operate HF SSB , with a Receive Only Antenna (loop for low noise) on Antenna-1's connector, and a Transmit Only Antenna (Vertical) on Antenna-2's connector. These of course would not require simultaneous operation.
    Any help appreciated.
    Many thanks
  • Ted  VE3TRQ
    Ted VE3TRQ Member ✭✭✭
    edited December 2022

    I'm sure there are better qualified folks here to answer your question, but here goes :-)

    The answer is "Yes" you can do what you ask above, although I would vary that a bit by putting the receive-only antenna on the "Rx In" input jack (BNC) and allow for two Tx antennas of your choice on "Ant-1" and "Ant-2". They will all be connected to the same SCU (Analog-to-digital converter / receiver) on a 6400, but the radio will take care its input circuits while transmitting. And since there is only one transmitter, the 6400 and 6600 are no different while transmitting.

    I use a two-SCU radio (6600M), and do exactly what you do, with one antenna output dedicated to one tx antenna, and the other one to another tx antenna. I just happen to also be able to listen while I tx if I want to (which is never :-) My only advantage (that I use) is being able to listen to multiple bands at a time - that's harder with a 6400, and impossible using two different antennas. [Edited a spelling error]

  • John KB4DU
    John KB4DU Member ✭✭✭✭

    Yes. There is some loss of capability that way.

    The 6400 actually has 3 antenna connections. There are the two that are mentioned in the question that can be used as rx and tx in the conventional way. There is an additional rx only antenna connection. If the low noise antenna is connected to the rx only input, then two transmit antennas can be connected at the same time. For example a tri-bander to ant 1 and a vertical to ant 2 AND the low noise antenna remains connected to be used with either of the tx antennas. This allows the monitoring of 20 and 40 (for example) at the same time. The rx only is selected as the receive antenna on both bands. The Tribander on ant 1 is used on 20 and the vertical on ant 2 is used on 40. I use this to chase Parks on the Air stations. I monitor the spotting web site and select the band needed to call the park station.

    Also, I agree with the choice of the non M radio. The box for the 6400/6600 is pretty big, so the M versions take up a lot of desktop space. I have the 6400 on a shelf under the desk along with the power supply, network switch, and the computer. On the desk is monitor, keyboard, mouse and key.

    I hope this helps. If more is needed, just let us know.

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