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Old 22-11-2021, 15:36   #1
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VHF Transmissions setting off alarms

This is a weird problem, and I'd appreciate any advice on how to fix it.
Whenever I transmit on VHF at high power, my oil pressure alarms go off for both engines. This affect is caused by all three radios on board, 2 fixed-mount and one handheld. Interestingly, when I move the handheld away from the control panel, the problem ceases.
Some troubleshooting revealed 2 circuit cards under the control panel, one for each engine, and each card has an audible alert siren. They both go off when transmitting in high power. Low power is not a problem.
So somewhere, RF energy is getting into those circuit cards. Each circuit card has inputs from engine oil pressure, temperature, and transmission oil pressure. Since the problem only occurs when the transmissions are engaged, I can assume the RF energy is getting into those circuits.
What to do about it? Maybe I'll try putting some aluminum foil around the cards as a sort of shield.
Your advice is greatly appreciated. DT. MV Escapade, Ocean Alexander 456.
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Old 22-11-2021, 15:47   #2
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Re: VHF Transmissions setting off alarms

Your coaxial cable is probably damaged or not connected properly (corrosion?).

Or the coax shield is not soldered. Check also the weld on center connector of your coax on both ends.
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Old 22-11-2021, 17:51   #3
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Re: VHF Transmissions setting off alarms

I would agree with the above. You are getting stray RF into the shack, or boat. You have a seriously bad connection somewhere.
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Old 23-11-2021, 11:36   #4
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Re: VHF Transmissions setting off alarms

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Originally Posted by MARC D View Post
Your coaxial cable is probably damaged or not connected properly (corrosion?).

Or the coax shield is not soldered. Check also the weld on center connector of your coax on both ends.
Handhelds don't have coax. Now what?
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Old 23-11-2021, 12:22   #5
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Re: VHF Transmissions setting off alarms

You said when you move the handheld away from the control panel then no problem. So, the posters comments on a coax/connection problem with your fixed radios may be correct.

Sounds like you have one, or two problems, depending on if you consider the second a problem. Addressing the second issue first - your electronics for your oil pressure alarm are sensitive to RF in the VHF frequency range. This is why your alarms go off if you transmit with a handheld in their vicinity. You can probably provide some shielding, a ferrite bead or coil on a power lead, or some other modification that will reduce the sensitivity to RF.

However, your description is also suggesting that your fixed mount radios are leaking RF. Assuming your antenna is at mast top you should not have enough of your transmitted RF energy hitting your control panel to cause a problem. If you can find and fix the RF leak your fixed radios won't bother your oil pressure warning and you will have better radio performance. You can then decide if the issue with the handhelds is worth addressing.
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Old 23-11-2021, 15:45   #6
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Re: VHF Transmissions setting off alarms

When I key the portable handheld VHF radio , the LED light over my chart table turns off.

It lights back up when I release the transmit key.
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Old 23-11-2021, 16:02   #7
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Re: VHF Transmissions setting off alarms

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Originally Posted by sinnerman View Post
When I key the portable handheld VHF radio , the LED light over my chart table turns off.

It lights back up when I release the transmit key.
Just following this thread to see what it turns up.
Please let us know what you find.

Al, S/V Finlandia
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Old 23-11-2021, 21:16   #8
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Re: VHF Transmissions setting off alarms

The cards are receiving the radio transmissions via the wires coming into the board. Not sure how many wires come to the board, but a good start will be the oil pressure wire.

Adding a toroidal ferrite choke to the wire should help, or eliminate the problem.

You will need about 4 to 6 turns of wire wrapped around the toroid.

In effect this allows the low frequency oil pressure or any other signal to arrive at the alarm board, but will block any RF reaching the board. You can also use the little clip on ferite found on laptop chargers and some USB cables. You do need at least 3 or 4 turns of wire wrapped through them for it to work, which may mean you have to wrap some wire onto a ferite, and add it between the wire and the board....and you may need to do all the wires coming onto the board. They don't all need their own toroid, one or two toroids will suffice, with all the wires going through before going to the board.
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Old 24-11-2021, 11:50   #9
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Re: VHF Transmissions setting off alarms

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Handhelds don't have coax. Now what?
Your tinfoil cage isn't the worst idea with if grounded. I guess my wisecrack about some of us can read, re: hand held was edited out?
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Old 24-11-2021, 17:28   #10
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Re: VHF Transmissions setting off alarms

Are your radios bonded,obviously not talking about the handheld.
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Old 24-11-2021, 21:34   #11
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Re: VHF Transmissions setting off alarms

Yes, handhelds do not have coax, but they do have antennas and if the antenna on the radio has a faulty (re: corroded) connection inside, it will cause issues with surrounding electronics. Again, possible common mode, or stray currents being picked up by nearby wiring.

Next, it is possible you have a faulty switch in your AC or DC panel. If the switch has a bad connection, but good enough for low level DC current, it could be prone to RFI and therefore causing stray currents in the wiring.

Sometimes wiring can form a loop antenna if there are multiple ground points. There really should only be one point in the boat where ALL grounds connect. If you possibly have different ground points then you might be creating a loop.

An example of different grounds would be if you have separate batteries. Where one battery located somewhere is providing power for something, and is using maybe a different ground return than they house battery. If those two batteries are not grounded together, then you will form a ground loop.

JMHO
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Old 25-11-2021, 11:54   #12
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Re: VHF Transmissions setting off alarms

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Your tinfoil cage isn't the worst idea with if grounded. I guess my wisecrack about some of us can read, re: hand held was edited out?
Why don't you try it. Probably a total investment of $.50. Send me the rest of the roll of tinfoil i need a new hat.
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Old 25-11-2021, 15:51   #13
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Re: VHF Transmissions setting off alarms

Thank you, IOM! Much appreciated - will order toroids shortly.
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