Join Date: Apr 2006
Thanked 162 Times in 159 Posts
Rep Power: 12
Re: VHF Output
"obviously you can get any amount of power from any voltage or convert one voltage to another if needed." No, you can't. There are separate limits on voltage and current, depending on source and other physical limits (like wire size and length). While you can convert voltage to current and vice versa, you need special equipment to do that with DC circuits, or transformers to do it on AC. Since we're talking about DC here, the answer is that you can't convert voltage to current, or vice versa, without considerable extra expense. Think of what an MPPT solar controller costs--that's active conversion.
"Nothing is stopping them at the least from throwing a DC-DC on the front end."
Wanna add even 1/4 of that MPPT cost to your average VHF radio?
"Also it's unlikely for circuits to run off external voltages directly." Unlikely only if you don't know about electronics. While some circuits are regulated, there is no reason to internally regulate devices that are powered from a specific and limited DC power source. Got a flashlight that uses 2 D cells? No need to regulate it, you KNOW it is running off 3VDC maximum and the amperage doesn't matter, the bulb will self-limit that.
Same thing for a DC car stereo or a VHF radio. Regulators cost money AND consume power AND take up space in the product AND add to warranty costs because anything can and will fail in some percent. Vendors know--for a fact--that a "12VDC" system is going to be regulated to a maximum of 14.4 volts, by industry convention. And that a dead battery will limit the lower end to about 12 volts at any effective power draw. So there's no need to regulate the equipment, because your alternator already is regulating it. Yes, that regulator can fail too and when it does, it is expected that everything running off it will have failures.
So the way these things are done? Unregulated, and that's plenty good enough. Your problem has got nothing to do with regulation or power conversion, you've probably got a simple problem of old cheap wiring, or wiring that was botched. And those problems can kill any electronics, no matter how many redundant layers of "protection" you add onto them.
"This ...doesn't suggest by itself that output power would go down at lower voltages. " You're reading a CONSUMER manual designed to help you operate the radio, not an engineering course. Odds are there's a section up front from the lawyers, telling you not to operate it in a bathtub or the rain, but very little about what lightning and alternator failures and other issues can do to the radio. And they've got no reason at all to confuse you with technical graphs showing how the circuits degrade once you go outside the operating parameters. Moisture? Temperature? Dew point? They'll all affect operations, just like high or low voltage will. See the graph anyplace that tells you how much they'll affect it? Or even operating limits?
And do you think anyone wants to scare the customer by confusing them with numbers, like, "This product will suffer an additional 10% total harmonic distortion when operated on 11.9 volts, and 20% harmonic distortion plus blah blah intermodulation and frequency splatter at 11.5 volts..."
Oh yeah, that would help everyone.
If you want technical information, look to technical sources, not a consumer manual. Look to technical radio reviews. The ARRL has them for members, for ham radios, and they're very explicit about these issues. For marine radios...damfino who does lab reviews of them.