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Flex 1500 XVRX port and active receive antenna voltage

KD7YDW Member
edited February 2020 in FLEX Series (Legacy) Radios
I'm thinking about using Active Antenna Amplifier model AAA-1C by LZ1AQ to make an active receive antenna on the XVRX port of my Flex 1500. Below is information about the voltage to the radio. Which voltage is safe to use: 6V pp or 4.2 V pp?
"This amplifier can give up to 19.5 dBm of undistorted power at 50 ohms load which is equal to 5.6 v pp amplitude (at equivalent input field intensity 125 dBuV/m or 1.8 V/m). Be aware that this voltage can occur only if there is a powerful transmitter in the vicinity of the antenna. For higher intensities the amplifier enters saturation state and the maximal output voltage is limited to 6 V pp. Generally this voltage can not damage the input of the analogue RX and TRX. But now on the market there are some direct sampling SDR where the input is a fast ADC (connected to the antenna usually through some kind of amplifier/driver). Usually these are 3 V devices and this 6V pp voltage can be dangerous for the chips. Unfortunately the producers of these radios do not specify the maximal allowable input voltages. For this reason there is an additional limiter in the Control board – see Mounting Instructions. The limited voltage is 4.2 V pp – this is a compromise between safety and reduction of the amplifier dynamic range. This voltage is 0.6 V higher than 3.6V supply of the 3V chips and usually is assumed safe. Check carefully the documentation of the radio for maximal accepted input voltages (if there is such item)."


  • John
    John Member
    edited June 2018
    If you are talking about a preamplifier between the aerial and the flex radio, then it should have a dc block capacitor on the preamp output anyway, The gain of the preamp could overload the flex front end and you might have to use an attenuator in line. Is this what you are talking about ?
  • KD7YDW
    KD7YDW Member
    edited February 2016
    Here is the link to the Active Antenna Amplifier I'm wanting to use:

    This would be similar to the Pixel active antenna. 
  • EA4GLI
    EA4GLI Salvador Member ✭✭✭
    edited November 2016
    I currently use that amp and I think the easiest is to take the 12v source that is also powering your 1500. The remote connector takes the 12v and is where you connect the rj45 that comes from the antenna and the bnc to the XVRX port on the 1500.

    Check the mounting instructions


    Page 2.2

    All you have to do is to bring power from your power supply into the J7 connector.

  • John
    John Member
    edited June 2018
    yep, its just a preamp which needs +12v and its already got the bypass relays so that on transmit, you dont put rf into the preamp, its goes around it via the relays. Probably, you will need to put in a small deay on the flex transverter setup to ensure the relays are either fully engaged or not, as they case may be, before you apply rf. Normally, the relays are not energized for transmit, so that if the power supply to the preamp goes off, then you wont destroy it.
  • Tim - W4TME
    Tim - W4TME Administrator, FlexRadio Employee admin
    edited March 2017
    First off, the 1500 is not a direct sampling SDR.  It is a QSD SDR with a mixer to down convert to an audio baseband frequency.  

    Second, the saturation point for the 1500 is fairly high, 0 dBm using the -10 dB preamp setting (attenuator).  And -10 dBm with the preamp off.  Coverting dBm to dBuV, 0 dBm = 107 dBuV, so you have to be mindful that this RX preamp can expose the ADC in the 1500 to signal levels beyond the overload point. I would enable the limiter in the Control board for an added measure of safety.
  • KD7YDW
    KD7YDW Member
    edited February 2016
    Thank you all for your input.  I'd buy all of you an adult beverage of your choice if I could.  The support of the Flex community and Flexradio is the best I have ever encountered.  Great products and support!

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