Minimum safe transmitting antenna distance

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  • November 27, 2020 at 12:28 pm #1465
    DavidR
    Participant

      Can someone confirm these numbers?

      I wanted to get an idea of how far a transmitting antenna should be from a connected SDRPlay. Under the table are the resources I used to calculate these distances, which are approximations. Assumptions are rigs in general use, 100W HF sets, 10W QRP HF sets, 50W mobiles for 2M/70CM, and common walkie talkies at 5W.

      Assuming this is correct, it would seem impractical to have the SDR connected while operating an HF without some device disconnecting the SDR antenna, such as the MFJ device I have seen. It would appear that operating a walkie talkie is practical outside the distances below, which isn’t difficult to achieve.

      Band Power Minimum safe distance (meters)
      160M 100W 1500
      160M 10W 500
      80M 100W 800
      80M 10W 300
      40M 100W 400
      40M 10W 150
      30M 100W 300
      30M 10W 100
      20M 100W 200
      20M 10W 75
      17M 100W 150
      17M 10W 50
      15M 100W 150
      15M 10W 50
      12M 100W 150
      12M 10W 50
      10M 100W 100
      10M 10W 30
      6M 100W 60
      6M 10W 20
      2M 50W 15
      1.25M 50W 10
      70CM 50W 5
      MURS (151MHz) 5W 5
      FRS/GMRS (462MHz) 5W 2

      https://www.random-science-tools.com/electronics/friis.htm

      http://www.sdrplay.com/community/viewtopic.php?t=1355&p=4389

      https://www.rapidtables.com/convert/power/dBm_to_Watt.html

      using flat dipole antenna gain as 1.64

      http://www.arrl.org/files/file/Regulatory/Band%20Chart/Band%20Chart%20-%2011X17%20Color.pdf

      December 26, 2020 at 9:12 am #1566
      Andy2
      Participant

        I use an HP RF limiter type 5086-7248. It will limit the RF arriving at your RSP’s antenna connector to a few mW. They are quite old now and obsolete, but can sometimes be found on the net. I’ve had mine since the 1980’s!

        February 25, 2021 at 2:44 am #1771
        N7BYD
        Participant

          Andy, when I check the 5086-7248 it comes back as a mixer. Would the HP / Agilent 5086-7261 be a good choice? The question I have is it is rated at 0 to 2.5 GHz, 10 W, I am assuming that 10W would be max power it was rated to handle as input can’t seem to find what their output would be.

          Thanks

          Tom

          February 24, 2022 at 9:55 pm #2003
          fresnoboy
          Participant

            Hi. I think he meant the 5086-7284, not 48.

            I can’t tell the difference between the 7248 and the 7261. Does anyone know where a datasheet for these parts can be found?

            The goal is to limit input power to 0db right? I am doing planning to put a set of antennas on my roof, and I need to protect a couple RSP2s from a nearby Diamond 2M/70cm antenna with a transceiver that can do 50W out.

            January 8, 2024 at 10:34 pm #2269
            kq4avz
            Participant

              Can this really be true?? With these distances, what’s to keep someone in your vicinity from keying up on their ham radio and killing all the SDRPlays around them?? Nothing malicious — there’s just no way they could know. And the above is calculated only at 100 watt max. Legal limit, at least in the US is 1,500 watt.

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