Welcome to the new FlexRadio Community! Please review the new Community Rules and other important new Community information on the Message Board.
If you are having a problem, please check the Help Center for known solutions.
Need technical support from FlexRadio? It's as simple as Creating a HelpDesk ticket.

Clean TX at Low Power?

K1ESEK1ESE Member ✭✭
I am thinking of removing the attenuator form my Expert 1.3K.  My Flex 6500 would drive the amp with about 5 watts.

Is the Flex 6500 transmit output just as clean at 5 watts as it is at 20 watts?   I am aware that some radios lose transmit quality when power is very low.  If that's the case with the Flex, I will leave the attenuator in the amp.

73 de K1ESE


  • Martin AA6EMartin AA6E Member ✭✭
    edited January 20
    Interesting question.  Somebody should measure this.  You can think of reasons why low power settings might have higher percentage distortion, but it's not clear if it would be significant in the real world.  Operating your amp with higher gain could also (theoretically) lead to worse RF feedback or other amp linearity issues.

    Personally, I wouldn't make a mod like this if there's no big benefit.  You only save a few watt-hours.

    73 Martin AA6E
  • k3Timk3Tim Member ✭✭
    edited May 23
    Any 6x00 can make the measurement by using the Full-Duplex option and an audio two tone test generator such as:


    On the (windows) sound menu set the playback default device to Dax Audio TX, enable DAX and VOX on the 6x00. Set volume levels etc.

    In Full Duplex be ultra-careful!  Connect a dummy load and transmitter port to the load. I set the receiver antenna to an unused port (xcvr). There is plenty of isolation but best to test with 1 watt at first.

    results 3rd order IMD test for 2 tones(550 and 1800):
    5 W :   -49dB

    100 W : -60dB   !!!!!!

    Hmmm.  Can't be..  60dB down on 10 meters at 100 watts. I've measured this before and found -43dB at 100 watts. Below are the graphs. Perhaps someone else can try the measurements.



    100 Watts:

  • K1ESEK1ESE Member ✭✭
    edited June 23
    If those readings are accurate, it would appear to be much cleaner at higher power.  Not sure if that difference also shows up at 5 watts vs 20 watts.
  • Ken - NM9PKen - NM9P Member ✭✭
    edited February 2017
    Be careful about believing what you see on the receiver panadapter when transmitting with FDX.  

    While what you see within the TX Slice's filter flag is fairly accurate, I have found that the rest of the display can exhibit some spurious artifacts due to overloading and leakage within the rig when transmitting.  This often shows up during digital transmissions, causing people to freak out when they see phantom spurs on their rtty or psk31 signals.

    The 6000 receiver is almost as good as an expensive spectrum analyzer, but there are a few caveats...this is one of them.

    Perhaps recent software updates have changed this.  If so, I will be glad for someone from FRS to correct me on this.
  • Tim - W4TMETim - W4TME Administrator, FlexRadio Employee admin
    edited February 2017
    Perhaps recent software updates have changed this.  If so, I will be glad for someone from FRS to correct me on this. 

    No deltas
  • John - K3MAJohn - K3MA Member ✭✭
    edited April 2018
    I have been running my 6500 and SPE 1.3K with 4-6 watts of drive for over a year and never a issue reported with 3rd order products not being quite acceptable.  I think the larger problem will come from trying being conservative in your expectation of the max amp output.  I keep mine down around 1K watts and do not try to drive it to 1.3K or higher.  If you do the same you should expect a clean output.
  • edited April 2018
    My SPE 2K has no issues running as low as 2 watts with this new release.

  • Gerald-K5SDRGerald-K5SDR FlexRadio Employee ✭✭
    edited April 2019
    I am not sure I can add much without doing measurements myself at low power.  Monitoring the internal RF leakage on the panadapter is not a good method for the reasons Ken stated above.  You would need to put the transmitter through a power attenuator (e.g. 40 dB/100W) to get the signal level down to receive levels and then do the measurements.  In that case, you could "carefully" make measurements with a FLEX-6000 or a commercial spectrum analyzer.  With a power attenuator in line, you can't get a much better spectrum analyzer in terms of dynamic range than a 6000.
  • Tim - W4TMETim - W4TME Administrator, FlexRadio Employee admin
    edited February 2017
    Thanks for sharing the plots, Doug.

Leave a Comment

Rich Text Editor. To edit a paragraph's style, hit tab to get to the paragraph menu. From there you will be able to pick one style. Nothing defaults to paragraph. An inline formatting menu will show up when you select text. Hit tab to get into that menu. Some elements, such as rich link embeds, images, loading indicators, and error messages may get inserted into the editor. You may navigate to these using the arrow keys inside of the editor and delete them with the delete or backspace key.