VHF noise ONLY on CH 73 - Only with Instruments ON

Ocean View

Hi All

Long time since I've posted and looking for a bit of knowledge base help re Ch 73 noise. (Did a forum search no luck)

About to do an offshore and regatta later this month and Ch73 is in the Si's - need to fix a Ch73 noise problem.

New boat I'm on has lots of noise on CH 73 since a new GME VHF DSC radio,  B&G Vulcan 9 and depth transducer was fitted a few months ago - Apparently the problem began after the old old radio was removed, and new items added - new radio - few months later B&G vulcan 9 and depth transducer we installed - There is still a mix of NMEA 0183 gear onboard too - ie GPS to DSC Radio and original outdoor instruments etc.

I'm about to start disconnecting bits to work out the cause - but any tips would be much appreciated as the race is later this month.

The Problem

Noise on CH73 (and only CH73)
VHF GME DSC radio is clear as a bell on ALL channels with NO noise or interference WHEN the instruments power switch is off.
VHF is also connected to a NSPL-500 Splitter and B&G NAIS-500 AIS unit - which powers up when the VHF is main switch is on

Instruments ON
As soon as the instruments power switch is turned ON Ch 73 makes lots of noise - as though no squelch was on.
The Instrument switch turns on the B&G mast and cockpit instruments, Digital yacht Wi-fi NMEA module (I might rip that out as it's not needed now I think)
B&G Depth Transducer, original paddle wheel  (Vulcan 9 is powered on the same switch - but it makes no difference to the noise if the Vulcan is on or off).

Ch 73 shows busy signal all the time with lots of background noise on the speaker
The noise is continuous most of the time but chops in and out a bit and I thought it was due to bad coax terminations on the splitter leads as it changed when i touched them - but it seems to be a coincidence and not a cause.

I moved all the cables away for the VHS ant leads and power leads the other day to see if it was cable related with no change.

Now thinking it's depth transducer induced and will disconnect it on the weekend

But any tips on other items to check and suggestions to implement to rectify it are much appreciated.



El Borracho

Verified User
Pacific Rim
Tough problem to solve.

1 - How does it affect actual race course communications on 73? Might work okay because the auto gain control of the receiver boosts quiet channel noise more than actual signal.

2 - The noise source is more likely some processing device rather than one of the sensors. Grounding or fault in the Vulcan 9 would be a prime suspect because it likely has a processor clock capable of producing noise in the VHF band. But you say it is off...is it really?

3 - What does the handheld do? Hunt it with that. Even to the masthead.

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duncan (the other one)

Super Anarchist
Turn things off one by one and find the culprit.

Also cheap and easy to try a few simple noise suppression solutions such as ferrites on the DC power to the instrument CPU &/or VHF.



Super Anarchist
Noise in the VHF frequency is usually from a DCDC converter buried in the power supply of some piece of equipment. The best way to locate the source is to disconnect equipment until you identify the culprit. LED-based running lights, especially those on the mast have increasingly been a at the heart of these problems. If you have an LED running light that has a wide range of input voltage (12-24), then there's almost certainly a DCDC converter in that light and you might want to start there. Tricolors, anchor lights and steaming lights are the worst offenders because they are close to the VHF antenna cable.

There are two kinds of RFI interference - radiated and conducted.

  • Conducted interference is conducted out of the offending equipment on its wires - power and/or data. This kind of interference can be addressed with chokes, but I would recommend replacing the offending equipment
  • Radiated interference is radiated out of the equipment and picked up by your radio's antenna or possibly its power/data cables and conducted into the VHF.

Note that once the RF is out of the source, either by conduction or radiation, it can get into your VHF via its antenna or via its other connecting wires - data and/or power. I would disconnect the data connection and power the radio from an independent (shore power 12v wall wart $10) supply to better understand. Hopefully it's getting into the VHF by the antenna.

Using a hand-held VHF radio as a sniffer is a good aid as was suggested.

Another possibility is that your VHF antenna has broken down and is overly susceptible to noise that would otherwise not be an issue. In-mast antenna cables don't last forever.

I would start with the easiest, disconnect the splitter and power down the AIS and try it, next different ant and whip at the radio to verify cable run and mast head whip.  I have a vague memory of 73 being brought up before for the same issue related to LED units like MD said.

The good part is the troubleshooting from that point is not to bad, you just start disconnecting stuff till it goes away.

Guessing you have already checked but 1st thing is to verify its not you, check another boat or handheld with everything off.  Sounds unlikely but we had a AIS virtual signal from Manzanilla that was on the fritz and would scramble 12 or 17 in a ancorage several miles away.  Lots of people were scratching there heads messing with there radios till someone sorted it out. Never know


Ocean View

Thanks for the tips above.

I'll try all of those options on the weekend and see what we find.

The VHF handheld if a great suggestion or starters
Was going to try the emergency antenna on wednesday but ran out of time and didn't want to to have the panel in bits before we did a windy 27+kn twilight race in a bit of swell.

Location: It occurs on long offshores, inshores, and in dock - same problem in all locations - Pretty sure it's an on boat problem.

Was going to take a 10 Ahr SLA to power up the VHF separately to isolate PWR as a cause.
Have also heard that some DC/DC switch mode convertors replaced with a regulator can fix issues like this.
It's bit of rats nest behind the panel so will do a bit of tidy up and draw diagrams of the cabling and re-run the cables neater while I'm at it.

LED's: Aware of the LED issue - pretty sure LED's are not being switched with the instruments - but will do an isolation check and see what we find.
It doesn't occur or change when only nav lights or internal lights are turned on  - but will do more checks in this area -
Also have a bunch of Ferrite cores in the workshop I can implement.

Vulcan - I pulled it's power cord mid-week - so pretty sure that it is not the cause as it was totally without a power source

Thanks for the info and advice

Will spend a few hours digging around (instead of minutes) 
Disconnecting and sniffing with the handheld VHF and the multi-meter and hopefully we can find (and rectify) the cause.

Cheers for now.


Ocean View

Spent a few hours working on this one and it looks like it's the B&G Hydra processor that's creating the noise.

Disconnected the depth sensor and powered up the processor as a standalone device using a live feed from another power source (nothing else was on) and it instantly makes the constant background noise on Ch73 on my handheld - and is louder when I hold the handheld antenna near one corner of the B&G processor. It's an older CPU with the Hydra 2000 displays and associated mast displays

The only frequency it is creating noise on is 156.675 MHz (Ch 73) and the radios I know it affects is a raymarine VHF handheld and a pretty new GME DSC VHF radio.
(have not tried any others)

check connections, poor earths, loose wires etc, NF-5 DC noise filter I had in my box of tricks, no filter, the installed DC filter and made a makeshift RF shield with a bit aluminium foil - - None of them made any difference

Next step will be to disconnect all the instrument displays and sensors and just power up the B&G - but didn't have time to do that.

Any ideas on how to get rid of the noise if it's coming from the B&G
and has anyone seen this problem before from a B&G on one VHF channel ?



Super Anarchist
No, there are 1000s of those system sailing around the world without RFI-related issues.

To figure out what's going on, disconnect the power wire on the Fastnet bus from your Hydra process so that all your Fastnet devices (mostly displays) loose power.

If the problem persists, the problem is with your processor.

If the problem goes away, then the problem is a Fastnet-connected device. To figure out which one, you can start to disconnect the power at the junction boxes and the middle and work from there towards the offending end.



If they share the same bus bar for a ground, you might want to ground one of them elsewhere.  This is a shot in the dark, but satisfying when it suddenly cures the problem.


Ocean View

Quick update

Used an external power source to power up the processor only and still noisy on CH73 
Disconnected the devices connected to the fastnet bus junction box and it stops - So time to start disconnecting the power for each display as suggested by Moonduster to determine the culprit.

One step closer - but not fixed yet

Thanks for the tips

You've helped a lot as this is my first foray in B&G fastnet system so starting to understand it a bit better after your tips - and reading these sites.




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