VHF Power Supply - switchboard or independent?

[[164147]]

Obviously - but that's not what you said here and it's what stats007 is disagreeing with:

The height of antenna i probably less significant than the fact the fixed VHF is 25w and the handheld is only 5 or 6w.

 

[JumbleDuck]

What evidence is there that all this faffing about solves a real problem?

It doesn't solve a problem. It might, however, enable one to avoid having to deal with a problem in the future.

So what lots of things. How many yacht distribution panels are bursting into flames and how is that problem helped by adding another one with the associate risk of it bursting into flames.

"It all sounds like a rather complicated to a solution to a problem which if it ever existed has been long since displaced by the existence of satphones, EPIRBS and handheld VHFs"

But it's a very simple modification - inexpensive and easily implemented. And if we're not interested in making a vessel safer - and reliable communication is a safety issue - then why not expand that philosophy and not bother carrying lifejackets, liferafts, EPIRBS and so forth? You and your crew will hopefully never need those items - but they're there in case you do.

It's a matter of balancing perceived risks. Your preferred wiring system solves some problems and introduces others. I don't carry a liferaft.

 

[lw395]

Sorry but you're wrong, it's basic mathematics that dictates transmission distance. A handheld is more than capable of transmitting beyond the horizon if at the right height.

If it's 'at the right height' the horizon is a lot further away.
Height is often more use than power. Height will be helpful when the boat is not on the open sea, but up a river or there's a headland in the way etc.
Power always helps, particularly when there are others on the same channel....

 

[JumbleDuck]

Only if the main battery bank isn't flat!

If it is, you turn the main switch to "emergency link" as well - if the engine battery is flat too then it really isn't my day and, for all the rats' nest of wiring he hankers after, it wouldn't be the OP's day either.

I have toyed with fitting an emergency battery for the radio and GPS, so that the first step in the Idiot Guide to Calling for Help would be "Turn the big red switch by the radio to Emergency", but it's not that simple.

First of all, you can't just connect a gel cell across the boat supply and expect it to survive the range of voltages it would see. Gel cells have to be carefully charged, ideally with a current supply, and would not take at all kindly to having 14.4V shoved into them. Emergency batteries which burst, either physically or into flame, are not a good idea. I have looked around and nobody, as far as I can see, makes a nominal-12V input charger for gel cells. The RC flying world would love to buy them, but they just don't exist. There are, of course, 12V chargers for NiMH and Li-ion batteries (I have one myself) but since they put batteries through discharge cycles and measure the voltage they see and you can't use them for a UPS setup. Just to add to the fun, the domestic ones all do 1.5V and 9V batteries only.

Then there's the changeover issue. Whatever solution you use, changeover has to be make (the emergency supply) before break (the main supply) because a time when you have turned the Big Red Switch is unlikely to be a good time to wait for the system to reboot and reacquire. Whatever system is used needs to do a smooth changeover and/or be able to handle having both supplies paralleled.

None of this is impossible to solve (every laptop computer in the world does it) , but it's nowhere near as easy as sticking a gel gel and a switch in.

 

[prv]

Oddly enough, in familiar coastal waters, a depth sounder is the one instrument that I almost never use.

Depends where you sail. In places with rocks and mostly deep water, you avoid obstacles by navigation (or more likely pilotage). In places with gently shelving mud, we avoid it by sounding.

On the other hand, if you want to seriously p**s off a grumpy port captain, try entering a busy commercial harbour without advertising your presence or intentions via the appropriate VHF channel!

I sail out of one of the busiest ports in the country. Being under 20m, VTS have no interest in hearing from me.

Pete

 

[Bru]

OK, I haen't done radio propogation since I was at college and that was a long time ago! And it's not my field of expertise either so I wouldn't stake my reputation on getting it right

Never the less, my interest was sparked (aha ha ha) and I've dredged up what I could remember and looked up the rest ...

Assuming a hand held VHF with 5W output power situated 8' above the water
Assuming a fixed VHF with 25W output power, a 3dB gain masthead antenna 40' above the water and 50' of RG-58U cable
Assuming a base station receiver with a sensitivity of 0.5μV and a 3dB gain antenna 40' above the water and 20' of RG58-U cable

Transmitting on Ch.16 (158.6MHz) and assuming perfect VSWR

Combined radio horizon between transmitter and receiver

Hand held 17.2NM
Fixed 23.8NM

Clearly that is not the whole story since, whilst we all can attest to fixed VHF setups achieving 20 plus miles I doubt many people have managed 15 plus miles with a 5W hand held in normal conditions

So we have to take into account transmitter power, line losses (primarily the cable between the transcievers and the antennas) and path loss (the atmospheric loss of signal power over distance). We also have to decide on the fade margin, the difference between the theoretical sensitivity of the receiver and the minimum power received to achieve reliable communication

The formulas are all readily available on the tinterweb (Google is your friend here) and I can't be doing with repeating them, I'm just going to give the results ...

Selecting a fade margin of 18dB for 99% reliability of voice comms

Hand held, antenna 8' above water line - max range to base station 4.26NM
Fixed VHF, antenna 40' above water line - max range to base station 12.89NM

Reducing the fade margin to 9dB, less reliable but still viable for voice with some interference

Hand held 7.16NM
Fixed VHF 21.63NM

The fixed VHF is now close to achieving max line of sight range. The hand held is nowhere near

If we plug the hand held into the masthead antenna and stick with a 9dB fade margin we get a range of 14.47NM, still nowhere near the radio horizon. Double the range of using it at deck level though

Raising the base station antenna to 80' increases the hand held range at deck level to 10.12NM and the fixed VHF to 30.59NM

The input figures can be played around with to one's hearts content but the conclusions are clear. Both antenna height and transmitter power play a crucial role in determining max range and, contrary to suggestions that have been made, a 5W hand held cannot (in normal conditions) get anywhere near the radio horizon even when it is transmitting through a masthead antenna

Antenna height has a greater effect on range than transmitter power but not by as much as you might think (with both antennas at 8', the difference in range between a 5W and a 25W transmitter is 2.52NM, with both at 40' the difference is 5.63NM)

And do remember that radio propogation is not an exact science. There will be times when the theoretical ranges are far exceeded due to atmospheric conditions etc.

I'd add more but it's blowing a hoolie up the river, the tide is on the ebb and it's getting damned uncomfortable on board so I'm off to do some stuff ashore!

 

[lw395]

Taking the power as 5W and the sensitivity as 0.25microvolts, that's 37dBm to -119dBm
A path loss of 156dB.
In free space at 160MHz with no antenna gain or cable loss, that would be a range of about 6000 miles.

So, generally we are nothing like free space!
That's usually because the ground is in the way.


There are other things going on too, handheld antennas are compromised by having a watery lump next to them and poor/variable grounding.