Supressing VHF Noise from Inverter/Charger

[CaptTom]

I have a VHF radio that produces an awful hum (heard by the receiving station) when transmitting. I've identified the source as the Heart 2KW inverter/charger that's just a few feet below and behind the bulkhead where the radio is installed. Turn it off and the transmission is clear as a bell. The unshielded, untwisted power cables from the radio run down and through the same compartment as the inverter. The hum sounds like it's in the 60Hz range.

Anyone have an suggestions as to the best solution?

A ferrite choke on the power cables?
Shielded twisted pair power cables?

Obviously I'm assuming that the noise is being introduced through the power cables. Or could there be something else? The antenna cable goes straight up and away from the inverter.

I'm hoping the ferrite will be the solution, because that would be cheaper and easier than running all new cable. But will they help with that low a frequency?

[a64pilot]

I'd certainly try a choke, my experience with them is that it will help, may not completely eliminate the noise though, but it's too cheap and easy to not at least try.
I think you want the choke closest to the VHF radio as possible

[SoonerSailor]

Do you get this when charging on shore power, or is it when inverting off the batteries for 120V appliances?


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[CaptTom]

Originally Posted by SoonerSailor

Do you get this when charging on shore power, or is it when inverting off the batteries for 120V appliances?

Both.

I've ordered some ferrites, due in later this week. I'll try that first unless someone has other ideas I can try.

[transmitterdan]

Ferries won't help I fear. Hum. as in 60Hz, requires huge inductors and capacitors to filter.

Look at where the inverter gets its DC power. Is it direct to the battery or is some of the wire shared with the DC power circuit to the radio? If so, consider running the DC radio wires straight to the battery. Same for the inverter. Always use proper size fuse or circuit breaker in each circuit.

Also, is this a newer modern VHF radio? Modern radios should be better in this regard.

[SoonerSailor]

I had very poor luck trying to filter interference from the Freedom 20 charger/inverter installed on my boat. The problem stemmed from the hash put on the DC bus whenever the charger was active. I went so far as to parallel a large capacitor with low ESR (equivalent series resistance) across the power leads to the radios. This knocked the noise down a little bit, but really not enough to matter. Somewhere I have a oscilloscope screen capture of the hash but not sure where it is right now.

As transmitterdan has informed, ferrites are unlikely to cure the problem.

Wish I had an answer for you short of replacing the inverter/charger, or just not running it when radio comms are needed.

[CaptTom]

Thanks for the heads-up on ferrites. I was afraid that I'd find that 60Hz is too low for them to filter out. Sounds like that's the consensus here, too.

The brand-new radio sounds almost as bad as the circa 2002 radio it replaced in the same location. The radio on the flybridge, on a different circuit and farther from the inverter, but powered from the same breaker panel, sounds fine. The wire to the problem radio takes a rather circuitous route; the PO just tapped an existing lighting circuit where it passed through the compartment with the inverter. It's a good hefty wire, about 14 Gauge, with good connections, and there's no voltage drop when transmitting. Just the noise.

So, it sounds like I'm going to be running a dedicated power cable sooner, rather than later. Something I've wanted to do anyway. I'll route it from the other direction, away from the inverter.

Any value in buying shielded cable? Should it be a twisted pair or is flat OK?

[transmitterdan]

The radio might work better if the power wires are run together directly from the battery plus and minus. Shielding will not help in my experience. Twisting the wires together at about 3-4 twists per foot can make a difference. A lot of times a breaker or fuse panel has a lot of distance between the plus and minus wires back to the battery. This can be a significant source of noise.

A lot of times when this happens the problem is not in the plus side but rather the minus side of the system. If the inverter is connected to the same battery return (minus) bus as the radio that can be a huge source of hum. Inverters are usually much higher current than any other device on board and they unavoidably make a lot of AC noise. The inverter really wants to be connected directly to the battery plus and minus or as close as possible with oversize wires back to the battery. Consider wiring the inverter directly to the battery and not through the breaker panel if that is how it is now. The rest of your electronics will thank you (or at least not hate you).

[transmitterdan]

I just thought of something else. Tell us about the antenna for the noisy radio. Where is it mounted and how does the coax get from the radio to the antenna?

Sometimes radios don't have DC blocking between the SO-239 connector and the battery minus. This can be a source of noise pickup.

[leftbrainstuff]

You often need to reach into the bag of noise suppression tricks. You've identified the source so you're halfway there. You might get lucky with a ferrite choke but it sounds like you have a more fundamental issue.

I'd try seperating power and signal wires. It's never rewarding lumping them together. Check both runs of your power feeds to ensure you dont have any loose connections. It sounds like you're inducing stray rf into your aerial. Seperation might help here.

Then I'd try a filter. If this doesnt work then I'd start looking at potential faults inside the heart. You might also try relocating the vhf and / or aerial as a trial. Sometimes this might do the trick.

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[Weyalan]

Turn off the charger/inverter when broadcasting on the VHF?

[Reefmagnet]

I had a cheap inverter that caused the same issues. Was nothing to do with power supply as the hand-held suffered the same transmit interference. I think turning off the inverter may be the only answer shortage replacing it with a less vhf xmit noisy one.

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[sy_gilana]

Is the inverter case grounded?

[SoonerSailor]

The observation that the hum is heard by others listening to his transmission indicates to me that dirty power is modulating the vhf signal. Not anything to do with his antenna.


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[transmitterdan]

If the inverter is sharing a ground with the radio then current could be flowing in places it doesn't belong. The antenna may be grounded in a place that allows inverter return current to flow back to the radio along the coax outer conductor.