What TX amplifier do you plan to use?

  • i know...

    but antenna and "FEED CONCEPTS" from 2001 can be used also on 2018/2019...

    i cant remember that the mainstream physics have changed since 2001...

    so the antennas will work now also...


  • Sure, but this topic was about "which TX amplifier to use" and with AO40 a TX amplifier for 13cm was not required (at least in the operational state that it has been in).

    So probably it belongs in a topic about antennas.

  • remco@Es'hail-2 [/home/remco]> rm ao40

    rm: remove regular empty file `ao40'? y

    remco@Es'hail-2 [/home/remco]> ls ao40

    ls: cannot access ao40: No such file or directory

    remco@Es'hail-2 [/home/remco]>

  • AO40 was 13cm downlink, not uplink.

    Not exactly correct as I remember it.

    AO-40 also had a 2.4GHz receiver that was used a few times. I believe it was connected to the 24GHz downlink for the tests. I am using the same helix now as part of my dual band feed. At that time, I used a HB PA with FLL120 GaAs FET for the uplink, which is still working with about 5W output.


    Charlie G3WDG

  • Hello Charlie,

    maybe you have some pictures of the PA?

    I read somewhere that with this FET it is very important to apply the right bias voltage prior to applying the drain VCC. Or am i wrong?

    vy73 DB8TF

  • Here's a photo of the PA.

    Yes, it is always best to apply gate voltage before drain on any GaAs FET PA. Procedure is to start with about -5V on the gate before applying drain voltage, then carefully take the gate less negative until the correct value of quiescent drain current is applied. This only has to be done once when setting up. Normal operation would have that gate voltage present all the time (ie on receive as well as transmit) and then just switch the drain voltage on for TX. Good designs of power supplies include an interlock so that should gate voltage fail drain voltage cannot be applied.



  • Not exactly correct as I remember it.

    AO-40 also had a 2.4GHz receiver that was used a few times.

    Of course it had receivers and transmitters on every VHF and higher band, but after the major failure it was mainly used in L/S mode (23 up 13 down) so most people had 13cm downlink only. I had a G0MRF converter that was only for receive.

    (and no uplink equipment on 23 so I never worked over it)

  • Hi Charlie,

    can you elaborate on this a bit more? I believe your procedure is correct for GaAs Fet PAs but we have nowadays sometimes MOSFET or LDMOS and here the gate bias is switched while the drain stays alive as the source-drain channel is non conductive at zero gate bias.

    73s Achim DH2VA

    PS: great to have your expertise in this forum!

  • Hi Achim

    Yes, the comments applied to depletion mode devices like power GaAs FETs. With enhancement mode devices like LDMOS it is different, in that drain current is zero with gate at 0V, and increases as the gate goes more positive. Very commonly people switch the gate voltage from zero to its operating value to perform T/R switching.

    Glad to be here and lots of interesting discussions going on!



  • In the first weeks I'll wait and see how it works for others with QRP in CW.

    My first try on 2.4 GHz TX NB will be done with the following equipment:

    1) WiFi WLAN Signal Booster 5W (ebay No. 112945159735, Fake! not 5 W ---> 1.8 W cold, 1.5 W after 15 minutes quite hot


    2) WiFi WLAN Signal Booster 5W (ebay No. 263850483595, Fake! not 8 W ---> 1.8 W cold, 1.6 W after 15 minutes quite hot


    3) Long-Range WLAN Grid antenna 24 dBi, 2.4 GHz ---> Polarisation not RHCP but only linear H or V (= 3 dB less)

  • DG4BX: do you mean an "Andrew T-2400" WLAN antenna?

    I have one of those here and will compare it to my 80cm dish with 4 turn helical feed as soon as we are allowed to transmit to EH2.

    Those WLAN boosters are almost every time a lot weaker than advertised, and they contain a RX/TX switching circuit which maybe needs to be deactivated for HAM use.

    vy73 DB8TF