VHF trouble

[Rob_Webb]

I recently bought a new ICOM 401 VHF and I'm having trouble with weak transmission. I've had an electrical guy check/replace the obvious areas of antenna/cable/connectors and he has declared that part of it sound. But I am told by other stations that my signal is "clear but quiet".

The dealer who sold me the radio is happy to ship back to ICOM for a check on the transmission power etc. But before that he reckons it could be a problem with the power supply with the cables not allowing enough current to the unit on transmit (new 85 AH battery by the way). I'm new to the boat but the whole switch panel looks only a couple of years old with very clean, neat wiring, so I'd be surpised if that's the problem. And the set itself is wired directly into the switch panel with no long extension to the power supply cables.

Does anyone have any other ideas before I send it back and lose another 10 days before I can start my holiday?

Thanks
Rob.

 

[jimi]

Have you checked the masthead connection between the antenna & the cable? I had a similar problem & it was a loose connection there.

Jim

 

[Rob_Webb]

Yup, my guy has been up the mast this morning and replaced the antenna 'just in case'. He's also replaced/re-soldered all the connectors and the deck plug fitting.....

Bugger!

R.

 

[JeremyF]

Rob,

Obvious point, but have you tried your hand-held attached to the masthead antenna? It should narrow down where the problem lies. I can lend you an emergency antenna so you could do the reverse, and check out the VHF that way.

Jeremy Flynn

 

[tome]

Are you sure you are transmitting at full power? VHF sets have a high (25W) and low (1W) setting. Just a thought...

 

[brianrunyard]

Can you try the radio on someone else's boat, or try another radio on yours, this would either eliminate the antenna or the set.

Brian
http://homepage.ntlworld.com/brunyard

 

[ZEBEDEE]

Has your man done a VSWR on your Ae as well as a visual inspection if not then this should be the next step a power output check of the set could also be done at the same time with the set in situe Good luck

 

[Bergman]

If your signal really is "strong but quiet" the probably not a antenna/connection problem.

More likely to be the set that is under modulating.

This could be microphone related or could be a setting within the radio that needs adjustment or repair.

Probably best get it back to Icom soonest. I think you are wasting money having man run up and down mast.

 

[Chris_Stannard]

I would agree if the signal is really strong but quiet that the set is under modulating and needs to go back for a check. this could of course be a microphone problem so why not ask the dealer, or a friend to lend you one to make sure it is not that.
At 25 watts output most radios will draw between 6 and 10 amps on transmit, the manual will tell you how much your draws. If the cable to your radio is undersized, and an electrics handbook will tell you what size it should be, then you will get an excessive voltage drop on transmit. It is not only a question of the thickness of the cable but the distance it has to run, or rather has been run. A thin wire may do for a short distance but you need a thicker one for a longer run. Under normal circumstances a voltage drop would not cause to much trouble but the radio may be programmed to reduce the output of the transmitter if the drop is high enough to get towards the bottom end of its tolerance range.
My programme with you set would be
1. Check the VSWR which requires a specialist meter but which is easy to use. If you cannot borrow one of those try the radio using an emergency antenna to see what the performance is like then.
2. Check the wiring to make sure it at least meets the standard required for the current drawn and the length of the run to your radio.
3. If it is okay on both of these tests try to borrow a micreophone to eliminate that from the equation.
4. If all else fails take it back.

Chris Stannard

 

[Rob_Webb]

Thanks for all your advice, chaps. Basically, I have now just taken it back for ICOM to check it out.

We tried attaching an emergency antenna to the set and when holding this at deck level got exactly the same response from SCG i.e. "clear but quiet".

Also, the cabling from battery to switchboard to VHF is a good gauge and definitely up to the job. During standby, the set has 12.8V going to it and during transmit this only drops to 12.3V, which is a very reasonable drop I understand. So I don't think it is a problem with the supply to the set.

So iI've now reluctantly sent it back for hopefully a quick turnaround!

BTW, all this hassle prompted me to buy a handheld as a backup, which is probably no bad thing, other than another £200 out of pocket!

Cheers
Rob

 

[VMALLOWS]

Why would a 25w set draw between 72w and 120w ?. I can accept 50w (4amp) as an absolute maximum??

 

[Mike_Martin]

Hi Rob,

The simplest thing (and probably the cheapest) to do would be to contact ICOM UK direct and ask them if they will check the radio unit before you get someone running up and down your mast.

Contact Jon Brookes on 01227 741 741 and tell him Mike Martin suggested you speak to him. They may test the radio for free and give you a quote for repairs if needed.

Mike.


Manager,
Aeronautical & Maritime Section,
Radiocommunications Agency

 

[oldharry]

25 watts refers to the RF power being applied to the aerial, not the power consumption of the set. 12 volt transmitters require quite a high supply current to generate 25 watts of RF power, and a supply requirement of 120 watts is not exceptional.

Most modern MB VHF transmitters are arranged so they will operate on a reduced supply, but the RF ouput will fall away very rapidly as the supply reduces. This is why the transmitter ouput of a portable is so much lower - 5 watts requires significantly less power, so the batteries can still give an adequate supply without being prohibitively large and costly.

 

[Rob_Webb]

Good news!

Thanks for all the suggestions - the problem has now been resolved. As the set was so new (3 weeks old and not yet been proven to work), ICOM agreed to a simple swap for a replacement new set. Marine Superstore (Port Solent) who sold me the original provided me with a new replacement under ICOM's instruction - and once fitted in my boat it worked perfectly. So it was a problem with the transmission on my new ICOM all along..... and it was the last placed I looked (as always)!

Thanks for all your help. At least I now know I have a perfect system!

Rob

 

[ccscott49]

Re: Good news!

Moral of the story, check with an ASW meter first, before assuming something else is wrong, it would have told you straight away where the problem lay. Still you now know all your connections are fine!