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Go Back   SailNet Community > On Board > Gear & Maintenance > Electronics
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  #1  
Old 10-22-2012
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VHF Output

I just installed a new antenna on my sailboat. My old one was blasted off by a lightning strike. I used a shakespeare tester to make sure the antenna had a good connection and the tester said it did. My question is I tested the radio output and it showed 15w. I know the radios are supposed to put out 25w why would it show just 15w? The radio is new.
Thanks, Rick
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Old 10-22-2012
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Re: VHF Output

Radio could be out of tune, they are factory tuned for a maximum of 25W but can put out less.

Radio might not be getting a full 14.4 volts, engine power. At battery power it will have a lower output. Check again with the engine running, and see what voltage is making it to the radio itself.

Antenna cable could be bad.

And of course the tester could be off.

If the radio came from a local store, one option is to stop by and do a show-and-tell, see what's wrong with which part. Or send it back for warranty replacement, if time is not an issue and you just want to be sure. (Still need to confirm the voltage available to the radio, etc.)
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Old 10-22-2012
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Re: VHF Output

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Originally Posted by hellosailor View Post
Radio might not be getting a full 14.4 volts, engine power. At battery power it will have a lower output. Check again with the engine running, and see what voltage is making it to the radio itself.
and check the voltage while transmitting, in case you have an issue with a voltage drop in the wiring when current is being drawn.
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Old 10-24-2012
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Re: VHF Output

Thanks all, I guess I should test the radio with the engine running although I keep the boat plugged into the dock so I should have full power. I also tested another spare radio and it showed 15w. I'm just hoping that something else is not damaged that I overlooked. If my antenna cable was damaged would the tester show a good antenna? Rick
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Old 10-24-2012
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Re: VHF Output

If two different radios each showed as 15W when connected to the same power line, that would indicate you are not getting enough power to the radio. Could be undersize wire too long, could be crud at connections, could be other issues but wiring sounds like #1.

Try getting a 20-30W light bulb, a cheap incandescent bulb, and hooking it up where the radio goes. Use a dmm to check voltage and amperage, because "filament" bulbs are self-limiting and if it says 25W on the bulb, it will damned well draw 25W from the lines if that is available. If you don't see it drawing that much power, you have a supply problem.
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Old 10-24-2012
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Re: VHF Output

Quote:
Originally Posted by hellosailor View Post
Radio might not be getting a full 14.4 volts, engine power. At battery power it will have a lower output. Check again with the engine running, and see what voltage is making it to the radio itself.
How come you're assuming this? Is it something common with VHF's? I would have assumed that any decent piece of marine electronics can handle the full range of inputs (roughly 9-15V) without degraded performance.

Edit: Nevermind, I assume you were using this?
http://shakespeare-marine.com/access...menupick=art-3

Last edited by asdf38; 10-24-2012 at 06:10 PM.
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Old 10-24-2012
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Re: VHF Output

"How come you're assuming this?"
From comments and instructions from various radio manufacturers, and observing the operations (including power output) of various transmitters operating at reduced voltage or restricted amperage.
In order to get power out, you need power in. The amount of amplification is fixed, so if there's not "enough" power in, you'll get less power out. Often stated in transmitter specs, where they'll say the rig is designed for 13.8 volts (typical "12" volt radios) or in the case of very good equipment, it may actually be rated for full power at 12 volts or less.
And fwiw, at 9 volts, or even 11 volts, you can expect significant distortion and other problems, not just a weak signal.

We had an old VHF that was refusing to transmit. I figured, old transmit button, something's worn out, fix the button. Autopsied the button, couldn't find any problem. DID find a punked out fuse connection--and that's all it took to stop the radio from transmitting completely.
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Re: VHF Output

Ok. Just piques my curiosity because obviously you can get any amount of power from any voltage or convert one voltage to another if needed. Nothing is stopping them at the least from throwing a DC-DC on the front end. Also it's unlikely for circuits to run off external voltages directly. Except for special cases most chips run from 5V or lower (usually 3.3V, 1.8V, 1.2V or less) so all electronics in a 12V application have some form of regulation that usually leaves them immune from reasonable changes on the input. That said I know little about VHF amplifiers specifically and their voltage needs.

I just skimmed the manual for one random radio (UM380) and it does repeatedly cite 13.8V, but it's done in the context of specifying current. I.E. 6A @ 13.8V when transmitting at 25W. This implies current might vary based on voltage but doesn't suggest by itself that output power would go down at lower voltages. Anyway, fairly minor detail.
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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.
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Re: VHF Output

Hello,

Buck converters (step-down), boost converters (step-up) and buck/boost converters (anything to anything) aren't special. They're in almost every electronic device over a few watts. Not just because of efficiency but because efficiency means less heat and thus smaller size (dissipating heat takes size). I've designed them and specified them and a 5W fully integrated (mosfet/inductor included) buck starts at a couple bucks in low quantities. A 25-50W converter, non-isolated, fully integrated is on the oder of $15 in lowish quantities.

As a neat aside there is a simple device called a Hydraulic ram that can literally move water upwards and is exactly analogous to a step-up DC-DC boost converter.

A cheap incandescent flashlight won't have regulation but a good LED one will. Sure if there is no reason for regulation there will be no regulation but being able to put out 25W on your VHF at 11.5V in a boat is a pretty damn good reason for regulation.

An MPPT controller is a buck/boost or perhaps a more exotic topology or variant, marinized and controlled by a microcontroller with a special charge algorithm. The expense results from the standard combination of ruggedization and low quantity production and perhaps an emphasis on efficiency adds a bit more cost. But the power conversion involved, variable dc-dc by itself isn't especially expensive. This is why it's so common.

If the lawyers really looked over the manual AND the performance does significantly degrade while still within the stated operating range of 9.5-15.8V THEN there would be a problem. Hence why I still suspect it's unlikely.

Note, all this stuff I'm talking about has come down in price significantly over the years along with everything else computer related. Power supply design used to mean discrete op-amps and lots of other chips. Now IC's 2x2mm include all the control circuitry for 50W+ supplies and their higher frequencies cut down the size of the inductors, capacitors or transformers needed. So perhaps in years past nothing I'm saying now applied.

Back to the thread I do agree that checking the voltage is a good idea. Certainly outside the stated operating range all bets are off. But if I saw 10 or 11V while transmitting at full power I'd look elsewhere for the problem. Hence why I asked why you thought otherwise.
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