VHF reception problem

jonathankent

jonathankent

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Now I know there has been many a thread on this in the past, so I apologise to some of you who seem the same thing time and time again, but I have been left confused from some other threads, so.......

I haven't been out sailing very much at all this season, and haven't needed the VHF at all, but today I decided to make sure that all was OK - it is a new VHF & antenna installation this year. I knew the time of the weather forecast so this was an ideal time to make sure all was receiving OK, however all is not OK /forums/images/graemlins/frown.gif.

I had my handheld to make double sure. At the time, the VHF received a very muffled and intermittent message to change to channel 23 for the forecast. However, upon changing I didn't get the forecast, but the display on the VHF was indicating it was receiving but nothing could be heard.... volume & squelch all OK /forums/images/graemlins/smile.gif. I switched on the handheld, and on the same channel received perfect reception with no interference whatsoever.... so there is a problem with the main VHF.

I have read other contradicting posts regarding testing the coax with a multimeter and resistances, voltages, etc but don't know what I should be doing to get sensible results. I will obviously be checking all the connections very thoroughly, but can someone indicate the correct method to test the connectivity within the coax??

If it makes a difference I have got one of those deck plugs aswell. I just hope it doesn't point towards the antenna connection itself - didn't trust those solderless push-in connections anyway, but I don't really want to have to go up there to try and fix it.....

All guidance is much appreciated.
 
Try to borrow or otherwise obtain a portable VHF aerial, with its own coax plug etc. Connect to your VHF in lieu of your own aerial and see if the result is any different.

Pound to the proverbial pinch of what-not the snag will be the deck plug. If the VHF itself proves OK, then dismantle, clean, re-cut and carefully re-connect the coax to the plugs. Try again.

If the VHF was OK and the deck plug re-connection doesn't help then the possibilities include a break or other damage within the mast length of the aerial cable, or the masthead antenna connection itself. These latter two options are the least likely, so don't go aloft until you have exhausted the preceding suggestions.

Good luck!

/forums/images/graemlins/cool.gif
 
I think you should disconnect the antenna at the set. Fit an emergency antenna or a 1m or so long piece of coathanger type wire and see if you get any sort of signal. If you get better reception than with the main antenna you know it is the antenna/connections. Don't transmit with the wire antenna.
If it is the antenna system, it may well be the connections. With the antenna disconnected at both ends there should be no continuity between the core and the braid of the coax. If you disconnect at the deck plug you can at least check the system is OK from radio to deck plug. With the antenna connected a multimeter will give no useful measurement so at that point you need to call in the experts or bite the bullet and fit a new antenna.
 
Just in case you haven't checked make sure you have 12 plus volts at the power input to the set.
PM me (with emaila ddress) if you want details of how to make up an emergency VHF antenna. It is essentially a piece of coax cable with a suitable plug to go into the set of a length to get it outside and up a bit. hoisted on a halyard would be good.
The top 1/4 wavelength 17.7 inches of the cable is bared. The braid is folded back over the cable and the centre conductor is left out and becomes the antenna itself. the braid should be 17.7 inches long folded back and the braid especially should be covered in tape or heat shrink. Do not earth it.
You will find it has excellent VSWR for transmitting and can be rolled up for emergency use. or in yourcase comparison with the mast top antenna.
good luck........ olewill
 
Now I had a go at fixing this earlier today, but with mild success. I looked at the connector into the VHF and the connection was a little loose, so I corrected that. Thinking that was probably the cause of the problem I put it all back together to try again. Used the handheld.. no problem, a clear message. Went further up the river and tried again... yet again a clear message received and transmitted. So, I thought I had fixed the reception problem and waited for the weather forecast to make sure. When the forecast came over, it was not really any much better than yesterday... still interupted and very muffled. Why would it be that within a local distance receiving & transmitting is fine, but receiving from further afield a problem? The handheld still receives it OK. If the main VHF antenna connections had a problem, surely I could still receive from the wire that is OK, its not as if I am on the edge of the transmission area. I have stripped the deck plug and redone both the connections... they weren't loose, but renewed nonetheless. I await to see whether this has made any difference, but if not I don't know what to do. I suppose try connecting an emergency antenna into the deck plug to try and rule out that as a problem. but at least I know the VHF can receive and transmit OK.
 
The common type of connectorused is the BNC type a bayonet push in and turn 1/4 turn about 12 mm diameter. These are pretty reliable but one little trap is that the centre conductor shrinks and pull s the centre pin back. You should be just able to touch the sharp tip of the pin with a fingernail. ie just level with the surrounding metal body.

To test the cable you need a multimeter or a lamp and battery. As discussed the antenna may be open circuit to DC (as in the one I described) or it may have a coil across the coax (in the antenna base) which will read low resistance. Test the resistance of the antenna at the deck connection ie looking up. If you get low resistance ie an atenna with a coil, then you should be able to check at the set end of the cable and get the same resistance. If you get low resistance again it means the connections through the deck joint are OK. Although there could be a short circuit outer to inner which would confuse things. Obviously with the deck connection disconnected you should get high resistance from outer to inner. If you can then put a piece of wire between outer and inner at the conection looking back to the set then you should be able to see that at the set end.
Failing all this then it is proabbly the antenna up top that is bad. good luck ............. olewill
 
A defective antenna or antenna conection will result in either no reception or reception with a higher than expected level of background noise. It will not cause muffled reception. This could only be caused by a receiver fault.
Regards Hubert.
 
Will, have you considered thatb there might be a fault with the set?

Some FM detectors will 'capture' a very strong signal, but with a weaker signal that is slightly off frequency for the detector will give very similar results to those described.

If only one could measure the accuracy that the set is tuned, it would eliminate another possible fault. Mind you, if I had it at home, I could also measure its sensitivity so it would prove/disprove the antenna/feeder suggestions
 
I don't think it is the set at fault. As I have mentioned earlier, when transmitting/receiving locally it is clear as a bell, it is only when trying to receive from a further range that there is a problem.

I will test all the connections with my multimeter.... at least from the help on here I know what readings I should be getting.
 
see if you can borrow another fixed set and connect it to your aerial. that way you will isolate whether it is the aerial or the set that is the problem. tbh it is unlikely to be the set if it works at all ie sets usually either work or dont, not in between.

if its not the set after trying the above, then beg borrow or steal an emergency aerial and try that with your own set. I wouldnt be inclined to try transmitting on a home made from coax type aerial, but you might be braver.

If it is your aerial then you have the ballsachingh job of tracing all possible connection faults. there comes a point when its simply easier to replace the aerial. 2 common problems with aerials. the first is the entry of water along the braid usually from a poor masthead connection. the second is the situation where just a whisker of the braid is touching the core at a connection.
 
[ QUOTE ]
there comes a point when its simply easier to replace the aerial

[/ QUOTE ]
It was brand new this winter, so unless faulty from the start it should be OK. I am deffinitely more inclined to point towards my connections.... /forums/images/graemlins/smirk.gif
 
I believe I have identified the problem... the masthead antenna connection /forums/images/graemlins/mad.gif. I have checked all the other connections, including deck plug, and the continuity and all is OK. I can get a very good and clear reception by using just the pin from the deck plug... connecting the antenna doesn't make any difference. Now, when I connect the deck plug through the deck, it loses the signal... therefore the antenna isn't doing what it should be. The plug is obviously OK when out on the deck, but it is probably just receiving the signal from the small pin not the antenna. What a pain in the backside /forums/images/graemlins/mad.gif.

Thanks to all who have assisted... at least I think I have found the root cause of the problem.
 
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