AIS 700 - VSWR fault

[MagicalArmchair]

I popped to the boat for the afternoon today to try and get to the bottom of this fault. I took down the headlining at the base of the mast, and there was the union where the coax from the aerial, met the coax inside the boat, that eventually ends up on our AIS 700. It appears that the deck gland that these cables enter through has been leaking for quite some time!!

The union, judging by the corrosion, had been standing in water. Additionally, the PL259 on the Aerial (mast) end, had never been soldered, and it just fell apart in my hands once I had unscrewed the union. This connection would never have been great from when the boat was first made, its frankly a wonder it was working at all.

Where the water was getting in:

I tried cleaning up the union, and reattaching the PL259 to the aerial coax, however, the coax just disintergrated in my hand. I cut back a few inches, and found some metal that didn't just turn to dust, however, it was still quite corroded. On reassembly, my VSWR was better, hovering around 4:1 to 5:1, however, as soon as it went to say 5.1:1 (so anything over 5), the alarm would sound.

My battle plan is this:

• Cut back another couple of inches of coax to try and find some uncorroded core (note: I am running of of cable length!!)
• Get a new crimp on PL259 (working up there its hard to solder the connector on).
• Buy a new PL259 union. The old one, whilst I cleaned it up, was still pretty bad.

If all that fails, I'll be up the mast with a new bit of Coax to pull through the mast!

 

[Daverw]

Personally I would change the coax, water / moisture can get itself a long way along and it effects the impedance severely. Any thing else is not really sorting it out. With swr at 4.1 you are throwing away so much transmit power

 

[MagicalArmchair]

I had a nasty feeling that would be the answer. At least the materials for the job are cheap - the going aloft is just a bit expensive in the courage department!

I've changed other things at the top of my old boats mast so as for kits and method for getting up there I'm fine, however, I've never swapped a VHF cable.

On this Selden mast, what can I expect at the top? I presume there is a PL259 attached to the bottom of the VHF aerial. Where will that then pass into the mast? The Selden documents are anything but clear. Should I be able to chop the cable, attach the new cable to it, cover it in cable lube, and pull that through the bottom of the mast? Or will there be a cable gland at the top of the mast I'll need to take apart before I can pull the cable through?

I presume that cable is RG8 looking at the connector?

 

[Daverw]

I would use binoculars to have a look at the mast head first, the connection could be pl259 or hard wired, if It’s pl259 put a new plug on length of coax first and use the old to pull down through when you are up, take self amalgamating tape up to cover connections. Coax normally easy to pull through as smooth.and stiff.

 

[duncan99210]

It’s likely that it’s the original antenna at the masthead, which is probably fitted with a permanent connection between lead and antenna, no plug and socket. Given you’re heading up the mast, I’d replace the entire thing: antenna and lead. The antenna bracket will probably be a simple bit of bent alloy pop riveted in place to the side of the mast: you might find it easier to remove that and replace with a new bracket matched to whatever antenna you choose to replace it with. Drill out pop rivets and fit new bracket, attach a mousing line to the old antenna and use that to pull the new coax through. Enjoy.

 

[MagicalArmchair]

Thank you all. New VHF Glomax aerial purchased and nerve steeled for multiple trips up the new mast. It's only three more metres up than my old boat and I could get up and down her mast in a lunch break.