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Radio Transmitter Options for Submersibles

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  • Radio Transmitter Options for Submersibles

    2.4ghz wavelengths have difficulty penetrating water, which is why 72mhz is used, and 75mhz is even better. Please post your radio gear, experiences, what you've found to work, and what you've found doesn't work. Hopefully we can pool our experiences and save each other time, money, and headaches!
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  • #2
    What I have been able to find out is that surface use 75Mhz radios are difficult to find in the US that have enough channels to operate the typical static dive sub. The old surface use radios prior to 2.4Ghz were mostly just 3 channels. Yes, the lower frequencies are needed to penetrate the water. 27Mhz, 72Mhz, 75Mhz etc. as opposed to 2.4Ghz. It is unfortunate but the lower frequencies are not a big market in the US for radio manufacturers as most RC cars or boats have transitioned to 2.4Ghz as well which is fine for them. In addition, it is sometimes desirable to have proportional control along with analog switches for some of the dive systems out there. Radios that would fit the bill are more likely to be 72Mhz which are still acceptable for aircraft use but risk interfering with someones plane or being interfered with if on the same channel. I have also heard that most regattas require 75Mhz channel sharing via clothes pins so if you desire participating in such you need 75Mhz equipment. In Europe 40Mhz equipment is readily available but is not legal in the US for RC. All is not lost however as it is possible to "re-tune" some 72Mhz radios to 75Mhz operation. One such radio is the Futaba T9-CAP. I recently snagged one off of eBay in excellent condition for a little over $100.

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    It has enough channels and the right type, is programmable, will hold enough models to satisfy me and it can be "re-tuned" to 75Mhz for about $50 by sending the Tx module and Rx to a place called Radio South. This is what I am going to do. There may be other radios out there that could be "re-tuned" but it has to do with the availability of the necessary crystals. I originally inquired about a JR radio I found for free but was informed that the crystals are not available for them. At least that is what I was told. The problem is that while not impossible, radios like this are becoming harder to find because they are considered "old tech" which makes it harder for the true sub captains out there who want a sub that dives.

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    • #3
      Bob Martin of the Nautilus Drydocks has a stock of VEX 75Mhz radios which are 6 channel and work fine for most sub applications in the US using normally aspirated water tight cylinders. If I am not mistaken they were at one time used for robotics. They don't have the proportional control functionality needed for some applications but can be set up with the servo stepper he also provides to allow additional control capability beyond what the radio itself can do. That is another option at least.

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      • #4
        Something that I am running into is that the BLM or Battery Link Monitor and AD2 pitch controller I got from the Nautilus Dry Docks don't seem to play well with the PCM receiver I am trying to use in my Redoutable. I have ordered compatible PPM receivers to try and all indications are that they will work fine. I will have to send them off to Radio South for re-tuning to get the best range but the crystals in my PCM receivers should work in them as well. All part of the learning process. In time, unless a solution is found we may be confined to just surface runners but I hope a solution can be found.

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        • #5
          Alpha, RCjetdude , How many channels are typically needed for these submarines? Best, LB
          Captain: Got any ideas?
          F/O: Actually not.
          — Captain Chesley B 'Sully' Sullenberger III and F/O Jeff Skiles—

          You'll never be good at something unless you're willing to suck at it first.
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          • #6
            Originally posted by Elbee View Post
            Alpha, RCjetdude , How many channels are typically needed for these submarines? Best, LB
            Good question. I depends on the sub. The very basic controls of a surface runner would be rudder and throttle just like a boat but to control something that dynamically submerges you would need at a minimum rudder, throttle and dive plane. To make something that is able to statically dive it would take rudder, throttle, most likely a dive plane and a ballast control. It goes up from there as you add features. My Redoutable has rudder, throttle, stern dive plane, ballast control and the forward dive plane (or in my case fairwater plane) is on a separate channel than the stern dive plane. The reason for this is that the stern plane is on a slider and ties in through a pitch controller called and AD2 which acts like a gyro to try and keep the sub level. The fairwater planes are on the control stick like the elevator on a plane and are used to dynamically control depth once the ballast tank is flooded. Obviously the sub has to be moving to accomplish this. On my Redoutable the ballast servo also controls a backup ballast system or emergency blow what is compressed gas which is Badger airbrush propel. The Battery Link Monitor or BLM senses either a low voltage condition from the 3S LiPo or a signal loss from the Transmitter and acts like a failsafe to blow the emergency ballast and hopefully surface the sub or return it to within radio range. It is pretty slick actually. Of course if one wanted to add additional functions they would need extra channels for them. Some very sophisticated subs use ballast tanks that can be very precisely controlled with a combination of analog and proportional channels in addition to the basics. It is fascinating to me.

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            • #7
              Update - My current Ohio class project will be 2.4 GHz. Basically you extend at least one of the antennas using RG-178 cable and secure the end up one of the masts on the sail. The sub is below the surface but the antenna is sticking up above the water. You can't dive deeper than that but it is really not that necessary to go much deeper unless you are in really clear water. It is a bit nerve-wracking to have your sub completely disappear below the surface and not be able to see it. Cruising at periscope depth is the ticket.

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              • #8
                RCjetdude, Seems like a reasonable compromise. Looking forward to seeing your newest project. All the best, LB
                Captain: Got any ideas?
                F/O: Actually not.
                — Captain Chesley B 'Sully' Sullenberger III and F/O Jeff Skiles—

                You'll never be good at something unless you're willing to suck at it first.
                ~Anonymous~

                AMA#116446

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                • #9
                  Radio does a really bad job penetrating water. The lower the frequency the better the penetration but even then it's limited.

                  In the US I would say 27 MHz would be the lowest available frequencies legally I believe.

                  The question here is what kind of penetration are you getting from 27 or 75 MHz? Or should I say how deep and how far from the transmitter creating an angle which signals reflect off the water instead of penetrating.

                  The other thing to consider is freshwater or saltwater.

                  Saltwater having very little penetration depending on salt content. I've lived in the Northeast and the southeast. Very salty in the NorthEast and not so salty in the southeast United states.

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                  • #10
                    In very clean mineral free water, 75MHz can penetrate up to 30 ft. 27MHz even better. The trouble is finding the equipment. I have managed to secure what I need for my purposes. We run at Cohutta, GA for Subfest every September in clear water to depths over 8 ft no problem. Of course 2.4GHz remains periscope depth.

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                    • #11
                      Thinking and Texting out loud.

                      Underwater range


                      I assume 30 ft is distance from receiver and transmitter. Preferably in pool water or a freshwater pond or lake. Some pools may have chlorine or salt based chlorination.


                      I imagine some people have stuck their antennas in the water. Actual contact with the water may or may not be good but if the antenna was enclosed enough to minimize actual physical contact with the water. I imagine people have already done this. Maybe use a coaxial feed from the transmitter to the water with antenna enclosed in the water but not in contact with. You do not know if there is a way for tuning underwater in antenna. Not that the transmitter antenna is tuned on something like 75 MHz or even worse 27 megahertz.

                      I know 2.4ghz is too high but the signal is boostable using Wi-Fi booster 4 watts.



                      If the signal is only injected into the water and not radiated into the air then I would think that the frequency is less of an issue interfering with another source receiver. Reception is often dependent on height of Tx antenna above ground or surface eliminating chance of interference. The problem with having equipment with enough channels could be eliminated.

                      I have a 8 channel 72 MHz dual conversion system which I use on aircraft. Still works fine in 2022 and the fact that very few people use 72 now makes it even more reliable flying in the Miami fort Lauderdale area. Also the option of retuning both TX and receive to 75. Don't care for that idea.


                      I do have a three channel 27 MHz radio. More like toy grade but works. Maybe I'll have to set up some containers with pressure relief watertight with LEDs for indicators and do some experimenting. Signal also can be boosted.


                      Even another option is go much lower in frequency if you could inject the signal only into the water not radiating it in the air. Take a radio with the PPM control signal and feed it into transmitter and reconvert it at PPM converter preferably with failsafe.

                      Some RC receivers actually have controllers in them with separate receiver module that can be removed and just connect the control signal to wire or another device.

                      Or just get a separate controller (no receiver) and connect your signal to it any way you like.


                      Audible sound also passes well through water. If you can convert the PPM into an audible sound in the water and reconvert it faithfully. Just an idea. Taking into consideration the wildlife in the area. I have done some underwater recording of sound using a simple homemade underwater mic. Plenty of sounds underwater in harbor and intercoastal and canals but not so much at the beach at open ocean.


                      Also easily done just with a direct wire with PPM signal which may need a little amplification with longer wire. Just a controller or PPM converter to pwm.



                      If you want a video Link then it would have to be something on the surface or a direct cable which would solve most problems so long as you don't run out of cable. Totally different beast compared to all other RC craft.


                      Many years ago I used what was called a rabbit on TV video and audio to extend to different locations in a large house using a simple thin 2 conductor wire. Not underwater but I would be curious to see how something like that would work. Transmit both video and audio. Then maybe another wire for control if not using the one wired for full or half duplex.

                      I would like to add a fish finder to a underwater vehicle. Not that I fish anymore. It's just cool.


                      I imagine you could buy this stuff now. Underwater remotely operated vehicle with video. I think I did see one somewhere.
                      ​​​​​​






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