Hi again, I'm continue to work on developing a wireless flash trigger for under water strobes (see previous threads linked below on ultrasonic and VLF options). I've come up with a third possibility, which is to run a receiver antenna wire just below (or even touching) the transmitter antenna.

I've attached a crude diagram, but here is the basic idea: My underwater camera housing has plenty of space in it to mount a trigger to the hot shoe which activates the transmitter to send a brief pulse (my plan is to use a roughly ~3-6V, 27 or 40 mHz system from RC cars). The antenna wire can wrap around the inside of the housing, terminating below the camera. Each flash housing would have an external aluminum attachment point for an antenna wire (the attachment would be built into the housing so that on the inside, it could connect to the receiver board to activate the flash). Those antenna wires could then run down the flexible arms of the camera rig, terminating just below the housing, under the transmitter wire. Thus, even though they are VHF, the signal only has to go through a few mm of plastic and water.

An additional possibility is to actually have a direct connection. The camera housing has a metal plate mounted in the bottom, and I could easily contact the transmitter antenna to it on the inside, and the receiver antennae on the outside.

White this is not a truly wireless option, it does solve most of the issues I had with big, bulky, waterproof wires that needed waterproof connection points to each flash and camera housing.

My questions are:
1. What happens if I make contact between transmitter and receiver antennae? Does that guarantee that the signal transmits, or does it make the whole thing non-functional?
2. Assuming they don't touch, how important is antenna length in a scenario like this where the distance between them is minimal? I know you are meant to calculate ideal antenna length based on frequency, but it is not clear to me what happens if you don't follow that in a scenario like this where all I need is to transmit a quick pulse over a short distance.
3. What happens if the two flashes are on a single long antenna (wire) stretched between them?
4. Are there other issues I haven't thought of that make this a non-viable option?

Thanks again

https://forum.allaboutcircuits.com/...ter-flash-strobe-trigger.199138/#post-1889190

https://forum.allaboutcircuits.com/...ver-for-underwater-flash.199232/#post-1890173

 

You can couple them with tuned coils for near field RF energy transfer. It wouldn't really act like an antenna, it would be more like a transformer with coupled magnetic fields
The camera housing could have a tuned coil on a RF transparent plate sealed to the Camera housing and receiver pickup tuned coils mounted outside connected to rf coax for each flash unit.

A open coil Resonant transformers.
https://arxiv.org/pdf/1810.09436.pdf

 

If ANYTHING stays nearer than lambda half to transmitter antenna, then transmitter effectivity is heavily destroyed.
The same about receiver antenna, but less severe.
However there may be some exclusions
1) the resonant frequencies of both antennas are different, far standing, and may not fall in harmonics one of each other. Then one field sue paralyses another, BUT at least the receiver input amplifier will not burn out as it will sure happen at first case.
2) Both antennas yet may be connected through antenna relay - factually that is pin-diode based electronic switch. That is frequent problem for yachtsmen UHF radio and AIS transmitter. Then both antennas may be substituted with single and switcher just commutates two outputs between single antenna. But it price is drastic, nearer to 1 k$ than to half of k$.
3) both antennas may be orientated orthogonal, what is next method to make sailship both antennas not conflicting. One antenna is mast-head, where lightning arrestor may be mechanically pushable in and out, but AIS antenna hangs in 45 degree angle between shrouds. Yet some cross-job is imminent between aluminium mast pipe and AIS antenna, as well the steel shrouds and antenna, but 45 degrees makes the antenna SWR falls from 1.1 to 1.6 and it is satisfactory.

 

If ANYTHING stays nearer than lambda half to transmitter antenna, then transmitter effectivity is heavily destroyed.
The same about receiver antenna, but less severe.
However there may be some exclusions
1) the resonant frequencies of both antennas are different, far standing, and may not fall in harmonics one of each other. Then one field sue paralyses another, BUT at least the receiver input amplifier will not burn out as it will sure happen at first case.
2) Both antennas yet may be connected through antenna relay - factually that is pin-diode based electronic switch. That is frequent problem for yachtsmen UHF radio and AIS transmitter. Then both antennas may be substituted with single and switcher just commutates two outputs between single antenna. But it price is drastic, nearer to 1 k$ than to half of k$.
3) both antennas may be orientated orthogonal, what is next method to make sailship both antennas not conflicting. One antenna is mast-head, where lightning arrestor may be mechanically pushable in and out, but AIS antenna hangs in 45 degree angle between shrouds. Yet some cross-job is imminent between aluminium mast pipe and AIS antenna, as well the steel shrouds and antenna, but 45 degrees makes the antenna SWR falls from 1.1 to 1.6 and it is satisfactory.

That's great but the OP is talking about underwater at pretty low power.

 

RE: NsaSpook: Have no an idea why at underwater the parallel to antenna other resonating rod be less harmful as at air. Know only that 5 mW of 50 kHz at bovine stomach may have about 100-120 dB attenuation in one feet thick bovine meat. Simply had a hands on such measuring.