I am working on a device that floats around in the water and transmits data to a fixed receiver.
The device must be battery powered, so low power. The radio is point to point because there are no other transceivers available for a mesh. This must work on a globally available, unlicensed frequency band. There is not much data, so the bit rate can be very low. For the most part, its line of sight, and I would like to have a range of at least 6 miles but 15 or more is much better. Ideally, it would use a PCB printed antenna so the device can be easily enclosed in a water tight container with no holes or connectors. Finally, I would like the radios PCB to be as physically small as possible, ideally only an inch wide by a few inches tall.
I don’t ask for much, hua?
I looked at wifi, too low range and too much power consumption.
Bluetooth is of course much too short range.
Zigbee and/or XBee seem like the leading contenders.
What else should I consider looking at?
You got a lot of contention there; if you're floating on the surface, then line-of-sight is very limited, and six miles may as well be the surface of another planet. Fifteen is in the next solar system.
I think you can forget any high frequency stuff, so that tends to rule out short, printed antennas.
Yeah its a big challenge. Especially since I think the 480MHz might be the only global band available. And as you said, that means big antennas. I could live with 2 to 3 miles if I have to, but even that may not be doable.
Although, I believe that 480 MHz might be compatible with antennas used for near field communication... I realize the bigger the antenna the better (dipole vs 1/4 wave, 1/4 wave vs 1/8 wave and so on).
Would it be practical to use a tethered balloon to support a much longer aerial? Otherwise, I would have thought that a tall whip aerial was essential.
You need three feet of height above "surface" at both the tx and rx ends to reach 4 miles @ line of sight, says here:
I'm not a sailor, but aren't wave troughs & peaks typically >3ft when you get some distance away from the shoreline?
Or is that only when stormy weather is approaching?
I’m no sailor either, but I would have thought that offshore waves < 3ft would count as a dead calm.
Waves are a problem. The receiving antenna will typically be 45 to 50 feet above the surface. Radio at ~480 MHz will have a better chance of making it through than 2.4GHz, but needs more power. But also, as the device floats, it will find itself at the top of the wave as often as at the bottom.
Is it a problem then with line of sight being lost with both transmitter and receiver being at the bottom of waves for indeterminate periods of time?
Or isn't that the situation you are expecting to use this in? A PCB printed antenna woudn't seem to have much chance at all.
Hmm, you said fixed receiver tho - like on shore somewhere, atop a high tower? Still have the loss of line of sight problem.
Really need an autonomous aerial platform above the transmitter that can relay data back. Than can transmit from water on one frequency, and tranmit to fixed receiver on another. Water transmitter can be inexpensive use & lose device with small battery, aerial platform something more costly that is re-used over & over.
Well if the transmitter is just beaconing or repeatedly sending the same telemetry at fixed intervals, maybe every several seconds, and 1/3 of the packets get through, this is very acceptable. The idea is to send small packets of data and although an acknowledgement is very nice it may not be feasible under all conditions.
Fixed receiver does imply on shore, so I worded that badly. The receiver could be on a boat or on shore but typically will be within a few miles of the transmitter.
The key to this thing is small and watertight. An external antenna both potentially compromises water tightness and gets in the way. A 440MHz antenna is about 4 inches long, although there might be a way to design it so it is inside the enclosure.
To the poster who suggested deploying a balloon, that’s a very cool idea, thanks!