One for Rick or Dave I would guess...
My Silva S10 DSC Marine VHF Radio on my Mac is not receiving a signal. This is not surprising because the antenna cable is heavily corroded and has detached itself from the antenna at the mast head; in any case it looks like regular TV cable with inadequate connectors. It also attaches to a connector through the cabin roof, which when detached is uncovered to the elements and allowed rain water in.
Do you have any information on what type of cable and connectors (radio, antenna, and through-deck fitting) I should get? And the name of a suitable supplier would be useful as well. I have a suspicion that I may need another antenna too, as using the old one could have internal corrosion from the look of the cable that fell out?
As always, your expertise is most appreciated.
John,
Generally you will require 50 ohm coax for the connection from the radio to the mast head antenna unit. The connections either end will depend on the equipment at each end of the cable.
any through deck fittings are normally radio/antenna specific. Try looking at:
http://www.force4.co.uk/department/electrical/plugs-sockets-deck-glands/...
2 Pin 3 Amp Waterproof Plug & Socket - Made from chrome plated brass, these traditionaol plugs and sockets are waterproof and come complete with plastic cover. Available in 3 amp (small) and 5 amp (large).
i have and have just replaced the originals with the two pin type.
The type of antenna I am not sure about but try asking a chandler or google the radio type for an installation manual.
Sun downers and calm moorings
Woody retiring JJ
Woody DB
Jack Jeavons, M26C Fair winds to all
The standard 50 ohm coax is type RG58, widely available.
ANY connector in a cable run will degrade the signal, no matter how good it is. The best solution I've seen is to run the cable down the backstay and then directly into the boat and to the radio, no connectors needed, even when the mast is lowered and secured for towing.
A nice antenna for the mast head is the Hawk, it incorporates a windex so you don't need a separate one.
Rick Jones (Treasurer), former 26X & 19 owner, Isle of Wight
Many thanks for the replies.
Rick, I am intrigued by your suggestion of running the cable down the backstay; on my 26X the backstay attaches to a chain plate starboard of the transom, and the radio is mounted above the port aft berth just inside the hatchway. How would you run the cable from the backstay? Presumably down to the cockpit floor and along to the hatchway, but would this not get in the way all the time unless it was clipped down? And how would it be best to run the cable through the hatchway without compromising the weatherproofing (such as it is)?
I'm beginning to think the answer maybe to have a hand-held VHF radio; any recommendations? I'm hoping there's a good one out there for about £150, but maybe this is unrealistic.
I think on the boat I saw that a cable entry had been drilled somewhere near the backstay chainplate, then the cable routed inside. This is a lot better than taking it round the cockpit and the hatchway. You can get gland fittings for sealing a cable through a deck.
Hand helds are convenient, and modern ones very good, but they have more limited range. A combination of the smaller antenna and much lower TX power (5w on high as opposed to 25w on a fixed set). That said that's all I currently have, I use a Standard Horizon HX851 which I'm very pleased with.
Rick Jones (Treasurer), former 26X & 19 owner, Isle of Wight
OK, I can see that routing the cable through a hole in the hull by the chain plate would be the best arrangement, but presumably this means detaching the antenna with the backstay when removing the mast from the boat for river cruising? It's either that or fitting a connector, which is what you suggested trying to avoid.
Hope the Poole trip is going well, hope the weather is better down there than up here!
RG58 is ok but RG8X has much lower loss and is more rugged. You can mix and have one in the mast and the other below decks if you're only changing part of the cable run. You cannot use TV coax as it's 75ohm not 50ohm.
All connectors in the antenna cable MUST be coaxial connectors. The losses and standing wave from a normal two or three pin connector will ruin any signal strength on receive and may actually damage the transmitter with too much reflected power bouncing back into the transmitter.
The most common connectors are PL259 and SO239 plugs and sockets often referred to as UHF connectors.
Better electrically but mechanically smaller and weaker are BNC or TNC (bayonet or Threaded versions).
My personal preference is to use N-type connectors, very low losses and strong.
Each connector has to be the right version for the cable type used.
I have a spare N type plug and socket filled with silicon to use as a blank to cover the deck socket and mast cable when not in use.
Once any water gets into a coax cable it wicks down between the dielectric and the copper and ruins the cable performance even before is corrodes it.
Use adhesive lined heat shrink over the back of every connector.
Unless you are adept at soldering use crimp connectors or better get the cables made up to length. A poor joint or overheating the dielectric will ruin the signal.
Remember you are trying to catch a few microvolts of signal and get it into the receiver. Every signal loss matters.
I have two antennas: mast and aftrail that have the same PL259 connectors at the radio so I can quickly swap them, and a deck mounted N-type socket for the mast step. I like the idea of the back stay connector free route but only if you don't intend to remove the mast from the boat. That said the extra length of RG58 cable is probably similar to the loss from an N-type.
Do not kink, trap, fold or crush the coax. Minimum bend radius is about 100mm.
Don't be tempted to use high gain antennas they do not work on a rolling boat they simply point most of your transmitted power into the sea or out into space as the boat rolls. Rubber flexi or 3' 3dB whips are fine.
Dave. G8YQT (lapsed)
Dave Newton Sailbadthesinner
Great, thanks Dave for such a detailed explanation. I'm sure this specification for a reliable antenna/cable setup will be a very useful edition to the MOA website for future reference. Your point about water ingress makes me wonder how long the radio has been useless, because when I picked up the boat in September 2014 the deck connector was open to the elements; I realised that it should be covered and taped it over, but long after much damage to the cable had probably been done.
I will start shopping for the necessary cable and equipment as you recommend, but I have a feeling replacing the cable down the mast and between the mouldings inside the cabin will be quite a nightmare. I guess the trick is to attach the new cable to the old (taped together?) and pull it through, but I understand the mast is foam-filled? Also, I'm sure it's on the web site somewhere, but do you know the length of the mast? My guess is about 9 metres.
Thanks again Dave, appreciate your help.