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Figuring out Full Duplex with transverters

arodland Member ✭✭

I recently picked up a pair of DEMI transverters (L144-28 HP and L432-28 HP) and I'm trying to use them together with my Flex 6400 for satellite work using the instructions in W5RKN's January 2016 AMSAT article (available at https://www.flexradio.com/wp-content/uploads/assets/AMSAT-Jan2016-FullDuplex-VHF-UHF-Satellite-System-Using-FlexRadio-SDR.pdf), with one going to RX A and the other going to XVTR A (since the 6400 doesn't have a B set I have to switch externally).

It works but I have a pretty big "desense" problem while doing full duplex. When I transmit on UHF, the signal I have tuned on the VHF side drops about 3 S-units. A very strong local signal can still punch through weakly, but an actual satellite signal is swamped. Changing the RF gain on the Flex or the RX gain on the DEMI don't seem to make any improvement (but currently I'm using 0dB gain on the flex and max gain on the DEMI). It also doesn't matter how high or low I set my RF POWER (even 0!) and it doesn't even matter if I unplug the UHF transverter, so I guess the issue is internal to the Flex.

I do understand that both IFs are in the same band, but this doesn't seem to cause a problem for W5RKN — he says everything is fine unless the IF frequencies end up right within a few kHz of each other, which isn't true in my case — e.g. for the center of the CAS-4B linear my TX IF is at 31.280 MHz and my RX is at 29.925 MHz. I thought maybe "wideband mode" was the culprit, but choosing a different TX freq that puts the IF below 30MHz doesn't change the symptoms at all.

Is there something I'm missing, or is this just not going to work with my rig?


  • Tim - W4TME
    Tim - W4TME Administrator, FlexRadio Employee admin

    The 6600 has 2 SCUs so he is using a separate digitizer for each IF. The 6400 only has one and the IFs are in the same band on the same SCU.

  • arodland
    arodland Member ✭✭

    The article says he settled on the 6500, not the 6600. That's a single-SCU rig as far as I know.

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