VHF Problem - Help!

[mobeydick]

I asked last summer, but could not resolve the problem, and it is still there. So I'll try again with some more information. Any radio experts out there?

I have an Icom fixed VHF (2 years old). Last summer it started recieving Solent VTS on channel 16, so I get all the ship movements when I am trying to listen to the coastguard. This lasts most of the way down the Solent.

I have investigated and found:
- Main set (CH16 selected) picks up CH12 when keyed on my handheld VHF
- Main set (CH16 selected) does NOT pick up CH12 when connected to emergency aerial, even when H/H is right next to the aerial.
- Main set (connected to mast top aerial) picks up Ch12 from the H/H when channels 07,08,10,12,14,16,66,67 or 76 selected on the main set
- The aerial has been checked with a SWR meter and seems OK (2 years old)
- Icom say not a known fault mode
- TX/RX with coastguard, marinas seems OK
- Dual watch NOT selected...!

Any ideas anybody? Next step would be to get Icom to check the set out, but I am reluctant as it seems Ok when connected to the emergency aerial.....

MD

 

[matelot]

what is happening is difficult to explain but easy to cure. If the set works perfectly OK using the emergency aerial but not with the masthead aerial, then its the aerial that is faulty. People on here glibly talk about SWR meters as if they are the ultimate check but they arent. Doesnt matter what the swr meter says, there is clearly something wrong with the masthead aerial.

Do you do anything more than simply switch over the aerial connection when you go to the emergency aerial? For example, move the set itself? And is the emergency aerial properly connected and mounted as its intended to be?

 

[misterg]

To add to matelot's post: Be really suspicious of the cable from the set to the masthead aerial, the connector at the back of the VHF, and any join that there is in the cable at the base of the mast. (My gut feeling is that there's a poor connection to the mast-head cable screen at the back of the set).

Late thought:

Are you testing the masthead aerial wiht the HH next to the set? If so, are you testing the emergency aerial with the HH in the same place, or do you put that next to the emergency aerial? It's possible that the HH signal is punching into some intermediate circuit within the fixed set when the HH is close, and it's nothing to do with the aerial (and not really a fault, just compromised design).

1st step: check the coax and any connections from the masthead to the set. If the cable is damp or blackened anywhere, it's junk.

Andy

 

[mobeydick]

Thanks to both replies.

-I just unscrew the main aerial and screw in the emergency one, which is in the cabin, near to the HH
- The HH is the same distance from the main set in both cases

Can anybody suggest why those frequencies and not, e.g. CH80, are affected?

MD

 

[Cliveshep]

I don't know anything whatsoever about your setup, but when we used to use PMR & CB in REACT up in East Anglia years ago the problem you describe was bugger-all to do with antenna but a function or rather malfunction of the channel selector mechanism/electronics. Standing Wave Ratio checks do indicate that the transmission is getting out of the antenna and that the antenna is tuned properly (or not). Your SWR reading would show a duff antenna connection straight away as it would show a previously-tuned antenna now out-of-tune with the rig INMO.

 

[mobeydick]

Further investigation shows:
- The H/H test may be a red herring. Many (most?) of the channels being picked up were due to local coupling through the set rather than the aerial. Proved by moving the H/H a few meters further away. CH12 still strong on 16 though.

I believe the problem is in the screen somewhere, as was suggested, so I wil focus on that (re-make the connectors etc)

Thanks

MD

 

[Refueler]

Really you cannot (IMHO) test such by having them in same vicinity. Get someone to walk up the marina a ways - so that your not getting interaction direct between the sets without antena. Signals do not just radiate or get picked up from the actual antena at end of a cable - there are also signals emanating from set and cable itself, albeit low enough to not normally be a problem.
We used to have Storno VHF deck Walkie Talkies in one co. I worked. Awful things ... if you got close to another set even on different channel - the howl and cr*p that crossed to other set was atrocious that they became unuseable.

You may just have poor connections that are allowing this 'cross over'. As others have suggested - remake or check connections ... make sure a good distance between HH ( note they give out often 5W - a reasonable power) and main set - try again.

 

[Channel Sailor]

As an ex wireless engineer, I have a strong preference to test Antenna and transmitter connections with a Bird Through line meter, or similar. Not an SWR meter. But not many people can get their hands on a Through Line. If the antenna and cable feeding it show good with a Through Line, then I would borrow an old VHF (non GDMSS) an try it for a weekend. There must be loads of them about these days. I would agree that testing between handheld to mast head when next/close to each other could be misleading.