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Monitoring 1 Flex With Another

Bill Roberts
Bill Roberts Member ✭✭
I have a Flex 6300 and a 2 RX/ATU 5000 as the only HF rigs in the shack.  Both radios share a single antenna through a 2 position coaxial switch.  I'd like to occasionally monitor the transmitter of one rig with the receiver of the other.  So basically, one receiver would be connected to the "disconnected" port while the other would be transmitting.  Is this advisable?  Would I be in danger of burning up either front end?  Thanks in advance.


  • Dave KD5FX
    Dave KD5FX Member ✭✭
    edited March 2017
    I do this all the time! I have my 6300 and a Kenwood TS850 on a 2 position coax switch. I usually kick in the attenuators on the 850 and plug in a headset to monitor my SSB or PSK signals.
    73, Dave
  • Mark-NA6M
    Mark-NA6M Inactive Employee
    edited May 2016
    Howdy Bill,

    The critical component here is your 2 position coaxial switch. Who makes it and if unknown, has it been measured for isolation between ports? How much power are you running into it?

    The port isolation for this popular switch:


    Is rated at more than 50 dB between ports.

    If this is typical of your arrangement you can expect to hear yourself *extremely* loud and clear in your other receiver which will probably require you to pay attention to its RF gain control in order to avoid swamping the front end.

    Your mileage may vary. Proceed with caution.

    73 de na6m
  • Bill Roberts
    Bill Roberts Member ✭✭
    edited July 2018
    I'm using an MFJ 1702 2 port switch.  I would only run 100 watts or less when monitoring one with the other.
  • Ken - NM9P
    Ken - NM9P Member ✭✭
    edited June 2020
    I do it all the time with my alpha delta-4. It shorts the unused terminal. Depending on the band, I hear my barefoot 6500 at about S9+30/40 on my 1500. If you use DDUtil, you can **** one rig to the other so it follows you around the bands. It has been helpful to record my 6500 on the 1500 to test different audio configurations. If I want to hear a weaker signal, I just reduce power. I also have both rigs connected to my Bose companion 2 speakers and they sound very similar to one another.
  • Tim - W4TME
    Tim - W4TME Administrator, FlexRadio Employee admin
    edited March 2017
    I highly recommend connecting the receiving radio to a dummy load for receiving.  This is how I do all of my over the air testing and there is no way to make a mistake and accidentally transmit 100W into one of the radio if there is some sort of switch failure.  Putting 100W into the radio will damage it immediately.
  • Bill Roberts
    Bill Roberts Member ✭✭
    edited July 2018
    Good idea. What is it they say about an ounce of prevention?
  • Tim - W4TME
    Tim - W4TME Administrator, FlexRadio Employee admin
    edited December 2016
    It can save you a bunch of $$$ on repair costs ;-)
  • Reg
    Reg Member ✭✭
    edited March 2017

    1500 watts reduced by 50 dB yields 15 mW.  About 1 year ago Graham KE9H posted an Official Response that the maximum power that a 6500 or 6700 receive input could accept was "about +8dBm."  +8dBm is approximately 0.0063096 watts.  This seemed illogical because +8dBm is approximately S9.  Therefore I am using front end protectors on all RCVR receive only inputs.  Some are DX Engineering DXE-RG-5000's and the others are Array Solutions AS-RXFEP's.  All RCVR inputs are connected to receive antennas and so far none of the front ends have been damaged so I don't have a basis to recommend the DX Engineering vs Array Solutions front end protectors (receiver guards.)  They both appear to work.


  • Tim - W4TME
    Tim - W4TME Administrator, FlexRadio Employee admin
    edited December 2016

    Your conversions are off.  +8 dBm is not approximately S9

    S9 = 50.2 uV @ 50 ohms = -73 dBm = 0.00000000005012 W

    +8 dBm = 0.00630.01W

    And the FLEX-6000s have a clamping relay to disconnect the SCU if the input power exceeds the point where the SCU can get damaged.

    But it is never a bad idea to add a little extra insurance by using RF limiters.

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