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Sub Antennas

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  • #16
    Originally posted by Bob Gato View Post
    In the old days of CB radio-an SWR meter was commonly used-since the MHZ range is the same (27), could one be dusted off and rigged to answer that question?
    NO. SWR (Standing Wave Ratio) is (almost) never an indication of whether or not an antenna will work or not. There are antennas on satellites with SWRs in the 20 to1 range (or higher) that still work because that RF energy "gotta go somewhere". The transmitters in satellites are designed to handle such large SWRs.

    As I said, I have my doubts that the antennas currently be used are resonant at their intended frequency. They might be but only of they use a lot, and I mean a lot, of inductance (a coil) at their base. More likely ( and I'll admit I really don't know what kind of antenna is used) it's a active antenna which is a chosen piece of wire that is then "tuned" to sorts/kinda work efficiently. You could do all kinds of measurements on land, in the dry, but when you submerge that antenna your work is pretty much useless.

    Hell, I surprised the effing things work at all.

    Dan
    Born in Detroit - where the weak are killed and eaten.

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    • #17
      You would know better then I, considering your line of work -I do know if you call a RC radio manufacturer (especially about a MHZ radio ) they will tell you that the antenna length is critical? Why so if it doesn't really matter..

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      • #18
        Originally posted by Bob Gato View Post
        You would know better then I, considering your line of work -I do know if you call a RC radio manufacturer (especially about a MHZ radio ) they will tell you that the antenna length is critical? Why so if it doesn't really matter..
        They don't want to be culpable if you plant your model airplane into someone's living-room.

        David
        "... well, that takes care of Jorgenson's theory!"

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        • #19
          Good point..

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          • #20
            Originally posted by Bob Gato View Post
            You would know better then I, considering your line of work -I do know if you call a RC radio manufacturer (especially about a MHZ radio ) they will tell you that the antenna length is critical? Why so if it doesn't really matter..
            I never said it doesn't matter if the antenna is in the air. The manufacturer has gone to some lengths to get the length correct for whatever RFamp or Mixer circuit the antenna feeds - in air. What I said was once you surround that antenna with something other than air - like water - the carefully calculated lengths go out the window especially if it's a resonant antenna. If it's non-resonant like an active antenna I still feel the water is going to mess up their calculations.

            I should have added that all the manufacturer's data was based on an antenna that was straight. Any deviation from straight - like curled up in a ball and stuffed in a WTC - also throws all the calculations out

            Full disclosure - antenna design was never my line of work. It is a passion I have had for over 60 years (I am an old timer) as a ham radio operator. I have designed antennas for several friends - they seem to like my work.

            Dan
            Last edited by roedj; 03-12-2019, 09:12 PM.
            Born in Detroit - where the weak are killed and eaten.

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            • #21
              It would be interesting to see what the consensus is among the model submariners as to what antenna length works best for , say a 75mhz radio....I am sure that over the years people have tried different lengths besides factory spec in search of that sweet spot under water.

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              • #22
                What wire would you use to replace an already cut antenna wire?

                As far as lengths go, I have used David's cut and use external. I have used an antenna twice the length of a regular one. I try to keep it close to original antenna length normally or never cut it and run it inside the tube.
                If you can cut, drill, saw, hit things and swear a lot, you're well on the way to building a working model sub.

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                • #23
                  To replace an already cut wire-In checking other RC treads and forums- the answer seems to be buy a replacement from the MFG. As to cut and external, I would just keep total length, including thru WTC fitting the same and the same type of wire-strand/solid, copper//plated, insulation type as original. Personally I go for a full length out stretched antenna in plastic tube (clear for water inspection) run thru a nipple fitting out of the WTC with a plug in the end. Not much to brag about but I am happy with control in the deep end of my chlorinated pool using that technique.
                  Last edited by Bob Gato; 03-13-2019, 06:35 AM.

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