Isolating a fridge from the vhf and chartplotter?

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OrwellTiggerToo

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Following on from my problem of noise (buzzing) on the vhf, whenever the fridge compressor runs. I have isolated the problem to the -ve, common between the fridge compressor and the -ve return on the nmea link (radio to GPS (Garmin)).

Does anyone have any experience of this problem?
Is there a way of totally isolating (filter wise) the compressor, that would prevent any noise from entering the -ve?
Would a change of compressor get rid of the fault (it is an old unit)?

I'm at my wits end, hair is falling out, sleepless nights!!

Who'd have a boat?
 
AS frdiges and VHF radios do not have to be on 24/7, perhaps you can switch one or the other off occasionally, to allow sanity to return. I know many people always sail with VHF on channel 16, but you dont have to. Same with fridge they still keep cold if only connected 60% of the time.
Hopefully you will get aradio buff who can advise a choke or similar to provide a filter.
My only thoughts on the matter were the capacitors fitted to the LT side of ignition coils on cars, which made the difference between hearing the radio or not. Might work??
 
I've tried capacitance suppression and ferrites. They had absolutely no effect whatever.

I'm beginning to think that the "Switch the fridge off when you use the radio" is the best I can hope for!
 
My tu'pneys worth - I suspect that there is a grounding issue, ie one of the connections on the -ve system isn't as conductive as it should be. A way to check would be to disconnect the -ve of the fridge from the existing cabling and then run a single test cable directly from the -ve of the battery to the -ve of the fridge.
 
OrwellTiggerToo said:
. . . . I have isolated the problem to the -ve, common between the fridge compressor and the -ve return on the nmea link (radio to GPS (Garmin)).

Does anyone have any experience of this problem? . . . . . I'm at my wits end, hair is falling out, sleepless nights!!
Click to expand...

I am an RF Engineer and have specialised in radio frequency interference. Looking at this problem from a different angle, do you have your Garmin GPS mushroom antenna inside the cabin or is it mounted outside on the stern guard rail? If it is inside the chart-room/cabin, it is probably nearer the fridge and also would find receiving satellite signals more difficult through the cabin deck-head and would therefore increase the RF Gain and this would then be susceptible to other interfering signals, like the fridge.

Another useful test,other than those suggested by the previous forum members is to completely wrap the GPS antenna in tinfoil to see if it lessens or cures the RFI? There is a possibility that the Antenna RF Gain circuitry is working very hard, as mentioned above and fridge is 'radiating' RFI as compared to 'conducting' the interference to the VHF radio even though the -ve cable eliminated the problem. Another test is to remove the VHF receiver antenna from the set to see if the interference is eliminated or decreases?

There are other things which can be done to stop this problem but it is difficult to suggest anything without using a spectrum analyser and oscilloscope, both of which I have, to work out the frequency of the interference and then possibly making an 'L' - 'C' filter at the interference frequency? Ferrite filters, beads or rings, are OK but you have to know the RFI frequency as there are many, many differing types and sizes and they all suit different problems even though they might look the same.

Are you coming down to the Solent for Cowes week, if so, give me a shout or phone me.
 
VHF and Autopilot motor

sorry for slight thread hijack. Every time my autopilot hydraulic motor starts it seems to broadcast a signal on the ship VHF ch16. Drives me nuts; autopilot unusable. Would the suggestions above be equally applicable to my issue?
 
OrwellTiggerToo;3612591 . . . problem of noise (buzzing) on the vhf said:
I have little knowledge of electrickery, but when this happened to me, the fridge operator's manual said to check that the wires to the compressor were twisted (i.e. plaited together). Mine were not, and when I did plait them it cured the problem. But don't ask me why!
Click to expand...
 
ITH said:
I have little knowledge of electrickery, but when this happened to me, the fridge operator's manual said to check that the wires to the compressor were twisted (i.e. plaited together). Mine were not, and when I did plait them it cured the problem. But don't ask me why!
Click to expand...

It greatly reduces magnetic coupling to another circuit.

The NMEA wires should be twisted with a ground too.
I have avoided NMEA for a while, is the '-' isolated, i.e. not earth?

Most boat fridges seem to contain an inverter which generate a high voltage at a fairly high frequency to drive the fridge motor via a commutating circuit.

I would try twisting the cables and running them straight from the battery.
As well as the normal suppresor caps, a substantial electrolytic may help.

You could try running the VHF and GPS from a separate battery, this may tell you if the problem is conducted or radiated.
 
Robih said:
sorry for slight thread hijack. Every time my autopilot hydraulic motor starts it seems to broadcast a signal on the ship VHF ch16. Drives me nuts; autopilot unusable. Would the suggestions above be equally applicable to my issue?
Click to expand...

I presume you are referring to my thread above?

What ever interference you are suffering from, the method of interference (RFI) has to be traced and eliminated.

From the interfering source, it is either conducted which means it travels along the wires into the victim equipment or ir is radiated which means that it effects the victim equipment by getting into the front end RF circuits via the antenna lead.

Disconnecting the antenna or in the case of the Garmin GPS mushroom, you completely and fully enshroud it in tin foil. These are standard test methods used in discovering which mechanism is causing the RFI.

Also, if you can, operate the victim equipment from a spare battery. On a yacht you can use the engine battery as long as it is not connected to any house battery.

If the RFI is eliminated or dramatically lessened by using a spare battery, then the RFI is conducted. If it is only lessened, you might possibly/probably find that you have sensitive victim equipment which is suffering from both conducted and radiated interference. :confused:
 
David, yes I was referring to your post, thank you for the response. If the engine battery is connected to the domestics via a common negative circuit I guess that breaches the independence of the engine start battery?

Rob
 
Robih said:
. . . . . If the engine battery is connected to the domestics via a common negative circuit I guess that breaches the independence of the engine start battery?
Click to expand...

Assuming the Fridge is connected to the house bank of batteries, the principle of disconnecting the VHF radio from the house battery supply is to confirm whether the RFI is induced by conduction down the DC supply from the fridge to the VHF radio.

You can run a red/black supply from the back of the VHF radio to the Engine start battery and just disconnect the engine battery negative and positive terminals to isolate from the house bank.

If having done this, you are still experiencing RFI, disconnect the antenna input. as you are possibly getting RFI via the antenna lead.

With an independent battery supply to the VHF radio and a disconnected antenna lead, you should not get RFI into the radio (discounting reactive field RFI or electro-static discharge directly into the VHF radio!) :confused:
 
Thank you!

Just want to thank you all for your suggestions and expertise.

I'd worked out that the noise was condutive from the fridge and into the -ve nmea. Ran an 8.5mm negative from the fridge directly to the batteries and it's lessened the noise considerably. I don't really like having flying leads off the batteries but I think in this case it'll be ok.
Once again, many thanks

Steve
 
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