Increasing noise on LNB during transmission

  • Hello, I am using a 1,2m offset dish and a POTY feed. Never had that problem before with different amplifers from low power up to 70W. But now I have built an new amplifier using a Doherty transistor pallet module. I am getting increasing noise on the RX (MiniTioune Scan & Tune), if I increase the transmitting power. I don't have this with the diver stage alone, which is also good for 20W DATV. I do have a very sharp band filter (-70 dB) just before the final amplifier. And I cannot see any spurious signal up to 2,9 GHz on the HP spectrum analyser. Shoulders at 75 W output are at 30 dBc. The signal on the WebRX is very clean and I can produce a 16APSK signal at 15 dB without shoulders. I don't have an increase of noise on my nearby 85 cm dish, while I am transmitting with the bigger dish.
    So, what can it be? I don't have oscillations, I don't have a spurious emission on 2,6 GHz. The only difference to the other amplifiers is, that this one is a Doherty type. The operating point of the transistors are correct according to the data sheet and changed only a little bit to increased linearity, but without any change to this noise problem.

    Has anyone had this problem before or has some idea what I could check?

    73 de Gerhard OE3GBB

  • Gerhard,

    Can you check your spectrum up to 12-13 GHz somehow? May be there's something spurious you wouldn't expect.

    Or harmonics (did you measure?):

    2405MHz x 4 = 9620MHz; will probably not fit into the waveguide. Not sure.

    2405MHz x 5 = 12025MHz; will pass the waveguide. Could overdrive the LNB stages.

    The noise increase is not independent from output level, when does it start (level)?

    How long is the waveguide of your POTY? Did you shorten it?

    73 Mike

  • Hi Mike,
    that is a very good point. I do not have a SPA for this frequencies, but I will try to tune my LNB to 12025 and see what is there.
    If the 5th harmonic is the problem: This is the first PA I am using without a circulator. So, I also can try ma 200W circulator for 2,4 GHz and maybe this device is acting also as a low pass filter ? How to build a LPF for 200W on 2,4 GHz with minimum losses? I also will try to change the amplifier from Doherty to push-pull A-amp as a last try as maybe the problems arise when the C-part of the amp is activated.
    Will keep you informed.

    73 Gerhard

  • Yes, interesting indeed. Please let us know.

    A 2.4 GHz low loss lowpass could be something mechanical (big). May be Achim DH2VA could have an idea. Or someone else.

    An 2.4 GHz circulator will have insertion loss. I think out of band it has much more attenuation which is helpful here.

    73 Mike

  • Hi all,

    Many thanks for all the contributions.

    Here are the results from my investigations:

    Yes, it is the 5th harmonic problem. I could tune my LNB to that frequency on 12.038 MHz using SDR-Console and could see some TV transponder signals at -115 dBm. But when I transmit at lower power I get the 5th harmonic of my transmission at -80 dBm. When I increase the power I get -45 dBm. So it seems that the activation of the doherty path of the amplifier is making this problem.
    I inserted now a 200W circulator and the situation improved a little bit. I can transmit now 333 kS at beacon level without increasing the noise level.
    I am using bandpass filters for 10W between driver and final amplifier, built for Wifi on 2410 MHz, 27 MHz bandwith, but 10W CW only.
    High power low loss low pass filters could be built according this:…mpedance_Lowpass_Coax.php. But I don't have the ability for that kind of high precession work.
    Alternativly those could be built from low loss PCBs with increased copper layer thickness.

    I also do have a WLAN duplexer, but only rated for 20W and I don't want to risc this eather.
    So next will work on optimizing the doherty amplifier to reduce the 5th harmonic.

    73 de Gerhard OE3GBB