Ideas for a passive Beacon / Tag Detector

[snoozy]

Huoy!
First off all, I'm new here, I'm an electronics engineer and I love Dave since years
This is a very competent forum, so I hope I get some new ideas to get progress in my project.
Please correct me, when I'm wrong and forgive me that I'm not a RF expert. Any comments and suggestions are welcome.

I want to build a detector to locate lost surfboards. The range could be ~100 m
I'm looking for the most adequate technology to achieve this.

My favorite solution right now is a passive beacon consisting of a diode and antenna.
Similar to the Recco system. The transceiver need to send a VHF (or UHF) pulse, the beacon will reflect it with the doubled frequency.
The transceiver also contains a receiver with two antennas and is possible to find the direction, since of signal strength.

Pros:
- Beacons will be very cheap
- No battery needed

Cons:
- Water is a problem for RF
- Multiple beacons (no identifier)
- Building the transceiver/receiver could be very hard (actually I could not found any help on this)


GPS
Pro:
- This could be easy because there a a lot of this devices on the market
- No need of a detector

Cons:
- It will need a SIM Card and Network connection. This will be a problem for travelers
- Water, when the board is upside down, I'm pretty sure no connection will be possible

Bluetooth (low energy)
Pro:
- A lot devices (chips) on the market
- No detector needed (smartphone)
- Direction is not given

Cons:
- The range might be very low
- Battery

Active RF transmitter:
Pro:
- Good range (at least I think so)

Cons:
- Without an activation mechanism, the battery will die very fast
- Receiver and transmitter is hard to build

Again: Thank you in advance 

[dmills]

Nothing RF with an aerial you can fit on a board is going to work for toffee immersed in salt water, just not going to happen, the path loss at UHF is **stupid** in sea water, and only a bit better in fresh.
Signal strength as a DF method is pathetically poor in a real environment, as it ignores all multi-path effects, forget that, if you must get a bearing use doppler or just a directional aerial on the recever.

The usual approach to locating lost things in the sea is an acoustic pinger, but even they are easier to locate in deep water rather then shallow with a sea running, if you make it a transponder then you can get range, and sometimes even bearing.

Regards, Dan.

[DaJMasta]

RF of some sort could be an option if you know the device is going to be above water, though, like if it was embedded in a surfboard.  You would still get loss, sure, but even intermittent contact would probably be sufficient for finding it, though the triangulation of the signal isn't really a trivial task - not insurmountable, but not necessarily easy.

100m range and surfboards (big, floating) simplifies your task somewhat.  An RF-remote triggered audible alarm would not have to be hugely loud to be found.  A similar device with a reasonably bright LED could make location after dark easy too.  And for during the day.... why not send up a drone with a camera?  An off the shelf drone with a GPS and an HD camera should give you plenty of resolution to ID a big floating surfboard and you could even automate the flight path to just make a sweep - only issue would be finding them when the weather isn't suitable to fly.  Maybe you could just get a small balloon and hang a wifi camera off it and leave it up there!

[snoozy]

Thank you a lot for your answers.

As I assumed the water is a problem and floating on the surface partly under water could be also a problem for pinging it acoustic.

But I like your idea of triggering a rescue alarm to easily find the board again. That could be a practical way to get it back.
What technologies could be the best for the trigger signal? Since the the board could be still under water and maybe out of range soon.?
Maybe a mechanical mechanism could be a better option...

I will think a bit more about that.

Hang loose!  🤙

[SeanB]

Surfboards invariably float, and thus will be on the surface, unless they have been in the Mavericks grinder and are a collection of small bits spread out on the shore.  In most cases locating is more a matter of making them a bright contrasting colour that is different from the sea, so you find all those flourescent yellow and orange boards, and very few blue and brown/green ones. An acoustic pinger would probably be your best bet, combined with a flashing LED, though that would only work night time. Triggering would probably have to be based on a pressure transducer, that is set off by being under 5m of water for more than 10 seconds ( being dumped will get you down that deep easily enough I have found, but you will float up eventually), but I do think that the cost of this will be kind of more than the value of a new board, especially if you want to have it be reliable enough to survive in the surf for a few years.

It has to be thin enough to fit in the board thickness, and cannot be right at the spine, so you have under 4cm to play with thickness wise in the foam, and it will not survive in the active standing area ( or survive the wax either) and in the rear it will weaken the board, and in the front it will be easy to damage in the sand.

I do not see a market for this, custom boards are too specialised, and are more likely to be made in pairs because they break, and also for conditions, while the common mass produced boards are mostly cost driven, as the typical surfer here is either a student or hardcore, and will have limited budget.

[snoozy]

Thank you for your thoughts!

Maybe I should have been a little bit more accurate in the description.
I was not only taking about surfboards, also specific about kite boards.

They will float, but sometimes up site down. You won't know.
There are thinks like GOJOE which turn the board, but that's only for beginner I guess.

I don't see a automatic trigger for this kind of sport, because there are a lot of cases were you will dump in the water for example when you put on the board.
Therefore I was thinking about a RF trigger.

So, still some thinks to think about