AIS Aeriel Question?

[fireball]

I have the definite back up of a proper VHF transmitting aerial should the need arise to use it, even if it is of limited range?

As I (and at least one other) said earlier in the thread - VHF range is largely dependant on the height of antenna - the higher they are the greater the horizon - so the thought is that if your mast does unexpectedly reduce in height then you've got the backup antenna - and if it's not getting through then you can either dismount it and raise it (on a boathook?) or whip up a new temporary one - this can be done with a bit of coax cable ...

Around the UK, the coastguard antennas are normally raised so high that their range is massive - hence we can regularly hear Solent CG in Cherbourg but only hear other vessels in the (eastern) Solent when we get back within sight of the IOW.

 

[xeitosaphil]

As I (and at least one other) said earlier in the thread - VHF range is largely dependant on the height of antenna - the higher they are the greater the horizon - so the thought is that if your mast does unexpectedly reduce in height then you've got the backup antenna - and if it's not getting through then you can either dismount it and raise it (on a boathook?) or whip up a new temporary one - this can be done with a bit of coax cable ...

Around the UK, the coastguard antennas are normally raised so high that their range is massive - hence we can regularly hear Solent CG in Cherbourg but only hear other vessels in the (eastern) Solent when we get back within sight of the IOW.

Thanks fireball, good point, I will make it easily demountable, and carry a suitable length of coax as an extension lead, I already intend to make the chart table end easily switchable, and I can use my Telescopic Whisker pole which is extendable to 5.5mtrs if needs must as a aerial mounting pole.That should cover all the bases?

 

[fireball]

and carry a suitable length of coax as an extension lead,

you really want as few joins as possible - I believe adding extra joins reduces effectiveness - (and bad connections can render it useless - done that myself! ) ...
our AIS antenna lead is in one run from the chart table to the antenna.
I do have onboard (but not looked in depth at it) the means to make up an entirely new cable - so if I were to need to use the whisker pole as a mast then I'd run that up, not try and extend the existing antenna cable. This cable was a by-product of antenna trials.

I'll re-state that I'm not an RF expert by any means - but I do IT systems building and diagnostics - so have a grasp of the concepts involved.

Before you go away and build an entirely new backup backup cable - just think of the probability of requiring it and the alternatives ...

Consider your existing AIS antenna range - it's receiving ClassA at around 15Nm (well - probably will - you need to check it) - if you were unlucky enough to loose your mast, whats the chances of being > 15Nm from a ship? That depends on your sailing area and the frequency of shipping.
What are your alternative methods of alarm ... Flares, EPRIB?
Bare in mind that DSC alarms will resend themselves until they receive an acknowledgement - and that can only be done by the CG or big ships - so even if at the time of alarm you were outside range the alarm will continue to send - leaving you free to tend to the rest of the vessel/crew.
All that said - a few quid for 10meters of coax plus the appropriate connectors is probably worth it

 

[xeitosaphil]

you really want as few joins as possible - I believe adding extra joins reduces effectiveness - (and bad connections can render it useless - done that myself! ) ...
our AIS antenna lead is in one run from the chart table to the antenna.
I do have onboard (but not looked in depth at it) the means to make up an entirely new cable - so if I were to need to use the whisker pole as a mast then I'd run that up, not try and extend the existing antenna cable. This cable was a by-product of antenna trials.

I'll re-state that I'm not an RF expert by any means - but I do IT systems building and diagnostics - so have a grasp of the concepts involved.

Before you go away and build an entirely new backup backup cable - just think of the probability of requiring it and the alternatives ...

Consider your existing AIS antenna range - it's receiving ClassA at around 15Nm (well - probably will - you need to check it) - if you were unlucky enough to loose your mast, whats the chances of being > 15Nm from a ship? That depends on your sailing area and the frequency of shipping.
What are your alternative methods of alarm ... Flares, EPRIB?
Bare in mind that DSC alarms will resend themselves until they receive an acknowledgement - and that can only be done by the CG or big ships - so even if at the time of alarm you were outside range the alarm will continue to send - leaving you free to tend to the rest of the vessel/crew.
All that said - a few quid for 10meters of coax plus the appropriate connectors is probably worth it

Must say I did think about the number connections, and the possibility of future connection failure, but thought it would ok in an emergency, but it would still necessitate the removal of the stern rail /Aerial connection and the changeover of the AIS end. I suppose one straight BNC connector and one PL259 aerial connector on the aerial feed already does away with one possible connection problem i.e. the BNC/ PL259 adaptor.
As you pointed out, most of my sailing is usually South West, Devon, Cornwall, Scilly Isles, and some Cross Channel, so mostly within 15miles range of CG or shipping.

Will I ever need it? Who knows, hopefully not? But for the sake of a few quid, I think I will get a suitable length made up with a couple of connectors on so it would be ready if needed.

I think once I have it fitted, I'll disconnect the AIS and see how the VHF performs from the pushpit rail mounted aerial, just out off interest for future reference.

Thanks for all your input, any ideas where I can get a really good marine quality plain BNC connector for the AIS? Salty John only does BNC adaptors on his web site?

 

[Salty John]

The insertion loss of a connector at VHF frequencies is very small. As an example, a BNC connector tested at 3GHz is less than 0.2dB loss.

However, it's true that each connection is a potential failure point, so the connector needs to be well fitted and maintained. And, it's much more likely to be a connector to cable joint that goes wrong - very rarely would it be the connector to connector join, so if your AIS cable can reach the radio directly so much the better.

If you put a BNC connector on the AIS feed to go directly to your AIS engine you'll need either a suitable adapter, or an adapter cable, to connect this to your radio in an emergency - adapter better than adapter cable because no extra cable join. The other way is a PL259 on the AIS feed and an SO239/BNC adapter to connect to the AIS engine, discarded when you go to the radio. Both ways are OK.

 

[ithet]

Have followed this with interest. Despite the argument I think all the posters are agreed on the main OP issue (even lustyd and Salty John) -
if you want to mount a second aerial on the puspit for AIS reception and emergency VHF transmission use a STANDARD VHF one. Preferably a 1/2 wave or 1.1m one, not an AIS tuned one and definitely not a stubby.

Have I got it?