In need of assistance on antenna design and controller/receiver selection
 

Hello, I have found a trove of information on this site and would like to say that I appreciate all the posts! I am in process of building a vessel to carry a sinker, and tow the bait (small fish) out to sea with the objective to be able to operate to 500m with a threshold of 300m. I've already checked the local game laws where I will be using it and I'm gtg. I am confident in my design with the exception of selection of a controller, receiver, and antenna. At the moment I'm considering using the RCmall Flysky FS-T6 High Precision 2.4G 6 Channel 6ch Radio Controller Transmitter and Receiver Kit, an SMA connector through the hull, and make an 38 inch antenna (close to hull length) from coax cable, attached to a 3/16 fiberglass rod for rigidity, with 31mm stripped at the end for 1/4 wave-all insulated with shrink tubing. I would like any suggestions on this antenna setup; specifically on the overall design, controller selection, coax cable type, or additional modules I should consider incorporating. If you know of an available antenna that I could buy I would appreciate that as well. Thanks!

Your best bet would be to contact a licensed R/C equipment repair shop and have them fabricate the antenna for you. I would recommend contacting Radio South and see what they can do.
https://radiosouthrc.com/custom-work/

2G4 coax loses a scary amount of signal over any length much longer than the actual wavelength. The systems as bought for model use are tested and certified to FCC or CE standards, any alterations that do not appear in the makers sales list are not recommended. At the receiver end, performance is likely to be degraded and range reduced, at the transmitter end, you run the risk of having an illegal transmitter.
If siting the receiver aerial is the problem, the best answer is to mount the receiver and its supplied aerial where it needs to be and extend the leads from where the boat works are to where the receiver now is.
38" boat at 500 metres? No doubt it will be sort of visible, but control involves not only telling the boat what to do, but knowing when it is doing it. I have problems steering a yacht about that size at 400 yards. There is a certain amount of guesswork involved but it helps that I can see the sails on their 4 foot 6 mast. Seeing what a bait boat with a stick is doing at that kind of distance could be a problem in anything but perfect flat calm conditions.

Hence my suggestion to talk to Radio South. Since all they do is R/C radio gear, if it's possible, they would know how to make it work. I won't say it's possible to meet the given requirements but I personally wouldn't want to take a small boat that far out, especially if doing so involves salt water

I appreciate the replies thus far! I should have included a bit more information, assuming I can control the boat to the minimum required distance I will upgrade to include a GPS unit with return to home function, but at a later time. I also will have a ball of sorts mounted to a non-transmission whip antenna for viability purposes. Lastly until I build quite a bit of confidence in the system it will have a separate line attached to the bow which will be used to retrieve it once the mission is accomplished. I will also have a spotter with binos, and have considered using my monocular head mount if needed. Also the design includes a mount for a gopro if wanted. Back tot he original question I will likely reach out to radio south in short order, thank you for the recommendation
In all reality if it can reach a bit over 200m it will do what I need it to do, which is carry bait about 20 yards past the first breakers in NC and VA. The reasons for the longer distance requirements is: a. I may go to other beaches in the future that require longer tows and b. why not? Tim Taylor would say more power-more distance.

I have a gut feeling that Radio South are excellent at customising the control panel to fit customers needs, but modifying the RF bits might be outside their preferred working area. In these times, repairing an RF section might involve replacing the dead unit with a working one, rather than doing any modification which would involve recertifying, which is not a cheap process.
Receivers tend to be effectively a sealed unit in that the important bits are not easily accessible. Increasing sensitivity is generally a matter of providing an aerial amplfier, but this usually compromses selectivity and in practical terms the unwanted noise becomes a bigger part of the received signal and might well not offer any benefit. Open the window further for more fresh air, but that lets more flies in.
Gain something from being higher, lose something from having a much longer coax, get some of it back from an amplifier, lose something because of increased noise pick-up. Choices.
Looking for a more traditional set using crystals on 75MHz (is 75 for surface in the US?) might work better.

First off, 75MHz is the surface band here in the US, good call(guess?)
Mfr02, I can't disagree with much of what you said in your previous post. That said, I referred him to Radio South since they can modify and tune transmitters as well as receivers to meet FCC regulations. I'll be sending in a radio and several receivers for retuning in the near future due to that capability. IF what he wants is doable without violating FCC regs, great. The other issue that you didn't touch on is RF transmitted power. IIRC, the FCC regulations allow no more than .5 watts of transmitted power out of a non-licensed transmitter. This applies to R/C gear and CB radios, off the top of my head anyway. The biggest issue in the case of getting out between 300 and 500 meters is the amount of power available. The general rule of thumb is that an R/C system can reach approximately 402 meters over water and 800 meters in the air. With the plan being to get out passed the breakers, with an antenna just a shy meter above the water, I see major issues. When using a 2.4GHz system, the water will reflect the signal, as you know, so the signal won't get to the boat IF the breakers are over a meter high. If he goes with a 75MHz system, the signal won't be as heavily affected but it will still be attenuated due to the electrical properties of salt water. I saw several boats crash when the Spectrum 2.4 wheel radios first came out, due to reflected signals, making most avoid them. It wasn't until they had done a major redesign of the receivers that the system was finally usable for boats.