VHF interference

[SteveGorst]

Further to my post last week I have investigated the source of interference on channel 80 and it is definitely the echosounder. I have checked the coax cable to the radio and the screening appears to have gone black and brittle which would I guess render it pretty useless. As I am a beginner in the field of VHF could any of you guys point me in the right direction regarding grades of coax to buy and where to buy from.

All help appreciated.

Steve

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[mainshiptom]

I would check that all your earth connections are in good order !

Tom

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[john_morris_uk]

You need a marine grade of 50 ohm impedence co-ax. Marine grade means that the braid is tinned copper. Remember copper is expensive and that is one of the reasons the better the co-ax the more it costs - its got more copper in the braid. Be very careful with the conncetions. Water is death to co-ax (and to the efficiency of your VHF).

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[SteveGorst]

do you mean the earthing of the coax shielding? I have tried to earth it as best I can but it is very brittle and in generally poor shape which is why I think I may need new cable.

I think the power leads are connnected fine they are all bright and shiny and soldered.

Are there any other earth connections I should be looking at?



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[SteveGorst]

Thanks for the advice John, I suspect the cable the previous owner put in was not marine grade. Do you think I am right in assuming that defective shielding will increase the susceptability to interference?



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[misterg]

Hi, Steve

(Me again!) Crude test to distinguish antenna bourne interference from supply bourne is to unplug aerial lead from radio. If still present with aerial lead unplugged, then it is from the DC supply.

To re-wire aerial, you need RG58 cable (50 ohm, low loss or RG8X if you're dead keen). Available Maplins or RS (#388-338 £27+vat for 100m), your local CB/ham radio shop (Rhyl??) or your friendly computer networker (cheapernet/thin ethernet cable)- Need to make sure that braid & core are tinned, and there is a solid polythene insulator (looks like you'd expect it to). Some cables (particularly computer ones) have an aluminium sheath over braid which I don't think is any good for a salt water environment. The term RG58 should be widely recognised. Strip a little off the end of any prospective candidates - steer clear if the braid is whispy copper. Proper stuff has a tinned braid knitted as tight as chainmail! Make sure all connectors are fitted to cable properly - soldered where necessary. Try to avoid joints.

I will be away from the computer until next Sun. PM me if you are still scratching your head then.

Regards,

Andy

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[William_H]

Further to Mistergs comments. If the interference is not coming down the antenna and possibly if it is you should try test running the sounder on a separate battery near the sounder. If this stops the interference then the radiation is coming out of the power lead of the sounder and either going along the power lead into the vhf or radiating out of the power lead. Try a separate earth lead to the case of the sounder The next step is to make an inductor from the ferrite rod (aerial rod ) from an old portable radio. It looks like hard carbon. Take the coils off and wrap your power lead or an additional heavy insulated wire fitted in the power lead around the ferrite about 10 turns as close to the sounder as possible. You may also need a capacitor of from 1000 picofarad to 100000 picofarad also known as 10 to 100 nanofarad to connect from the power lead into the sounder to ground of the sounder. The capacitor may come from a TV repair man or Maplins for a very few pence or an old radio if you can identify one and get it out intact. It looks like a small often green or brown disc of ceramic with two wires Good luck will.

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[SteveGorst]

Wow I think this is starting to look like a job for an electronic engineer. Luckily I had the foresite to encourage my son to take up electronic engineering and he has just completed his first year at Edinburgh Uni. He will be home next week so you can guess his first assignment if I don't manage it!

I have removed the aerial and found that the interference stops as soon as I remove it. There is a joint in the coax at the foot of the mast and I think the corroded section is from there downwards, only 2m at most so I will replace that for starters and see if I still have a problem. Hopefully better shielding will filter out any weak interference that the echosounder is giving off.

I've also purchased a handheld now as a back up, an Entel HT640 with a lithium battery as I hate Nicads! I had been meaning to but this has forced my hand.

Thanks for all your encouragement

Steve


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[bruce]

you should take apart all the connections and clean each wire/connector and blocks and remove and clean your fuses and their holders, you could even lub connectors with sil grease before tightening. if your coax is cracked, that could be your problem. would recoment not runnint transducer and vhf in same area of the boat. local ham operator should be able to help you.

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