Navtex antenna positioning?

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prv

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Sooner or later I'm going to want to mount an antenna for the Navtex (currently just using it as nav data repeater). The boat is largely free of extraneous things strapped to the outside, and I'd like to keep it that way. So I'm thinking of mounting the antenna high up inside the hull, such as inside a cockpit coaming. I understand that as it's a lower frequency the need for height is much reduced compared to VHF etc.

This would probably involve having the antenna horizontal rather than vertical; does this matter?

Alternatively, I'm sure I read about someone who connected his receiver to a chainplate bolt, to use the rig as a large but chaotic antenna. Is this worth considering, or a completely daft idea? Note that the unit in question has a powered tuner box but a "dumb" antenna, so I wouldn't be bypassing any necessary amplifiers or applying a voltage to the rig.

Any general tips on antenna positioning, or how to easily tell the difference between a good setup and a bad one?

Cheers,

Pete
 
I installed my Nav6 in a rush before a cruise last year. Like you, I was reluctant to add external clutter so I put the passive aerial in a locker on a long cable. Reception was pretty poor and I had to lash it on deck to get an improvement.
I have just connected the box to a shroud chainplate via the Hi-Z antenna connection. It seems to be working well. Time will tell if there are problems.
 
Chi34 said:
I installed my Nav6 in a rush before a cruise last year. Like you, I was reluctant to add external clutter so I put the passive aerial in a locker on a long cable.
Click to expand...

That's interesting - when you say "passive aerial", do you mean the white rod that comes with a separate box, which is then connected to the display itself? I ask because there seem to have been several different setups for the ICS Nav6 over time. I don't have any tuner/antenna yet, but the separate box and rod is what I plan to fit.

Pete
 
prv said:
Any general tips on antenna positioning, or how to easily tell the difference between a good setup and a bad one?
Click to expand...

You can usually get away with mounting the Navtex antenna below decks - in a locker, say - but you won't get as good a reception as with it on the stern rail or similar.
Wherever you decide to mount it, the antenna needs to be vertical. You will lose a huge amount of reception with it horizontal. D
 
dk said:
You can usually get away with mounting the Navtex antenna below decks - in a locker, say - but you won't get as good a reception as with it on the stern rail or similar.
Click to expand...

OK - so four feet or so of vertical height makes that much difference? I imagine being inside a fibreglass box vs outside doesn't really have any effect.

Pete
 
Alternatively, I'm sure I read about someone who connected his receiver to a chainplate bolt, to use the rig as a large but chaotic antenna. Is this worth considering, or a completely daft idea? Note that the unit in question has a powered tuner box but a "dumb" antenna, so I wouldn't be bypassing any necessary amplifiers or applying a voltage to the rig.






I first fitted the ICS Navtex 6 antenna inside the boat, polyester/foam composite construction. This worked well in the mid channel area but struggled to pick up anything in the west. A move to the pushpit improved the performance but only marginally. On the advice of the ICS technician, yes there was only one in those days, I connected the chainplate to the screw on the top of the antenna, it was there to fit an optional aerial that was later discontinued. Once the rig became the aerial reception was excellent and I've never sailed out of Navtex range. I seem to recall that the rig must have no direct connecting with the sea for this system to work, so no knockdowns:-)

Peter.
 
Adonnante said:
Once the rig became the aerial reception was excellent and I've never sailed out of Navtex range. I seem to recall that the rig must have no direct connecting with the sea for this system to work, so no knockdowns:-)
Click to expand...

Thanks, that's very encouraging. Hard to have a less obtrusive antenna than nothing at all :). I don't think my rig has any connection to the sea, though it does connect to the pulpit through the base of the forestay. I guess that's not a problem, since the pulpit and guardrails aren't connected to the sea either. I could perhaps insulate the pulpit from the guardrails to reduce the amount of metalwork that mustn't be inadvertently connected.

If I'm getting knocked down, then I think reliable Navtex reception is probably fairly low down my list of priorities :)

Pete
 
I can't advise on using tthe rig. but if using a normal antenna keep it vertical and clear of anything that may block it - stanchions, mast etc. Also, obviously, somewhere where a rope won't snag on it and drop it in the sea! A good idea is to put a long bit of cable on first and try the antenna in different positions, before fitting finally.
Tim
 
We went for the chainplate option and it works very well as long as MCA remember to broadcast.

Get a good earth and you'll have enough signal to keep you getting all the stations you do not need too

Just take the centre lead of the coax to the chain plate and if necessary earth the outer sheathing.
 
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Gwylan said:
Just take the centre lead of the coax to the chain plate and if necessary earth the outer sheathing.
Click to expand...

Thanks. Just need to try to dismantle the woodwork around the chainplate now. Bloody modern yachts with everything boxed in :).

When you say "earth", do you mean a radio style earth like keelbolts (or a metal plate if I had one) or just the 12v negative?

Pete
 
I fitted a Navtext a few weeks ago, stuffed the antenna behind some panelling in the. Aft cabin. Works well on the west coast of Scotland- not sure which way up its pointing.

(The antenna, not the west coast of Scotland...)
 
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prv said:
I don't think my rig has any connection to the sea, though it does connect to the pulpit through the base of the forestay.
Click to expand...

Just a thought, but if there is lightning bonding in place between mast and keel, as there may be, then the rig would be connected to the sea, save only for a coat of antifoul.
 
Simondjuk said:
Just a thought, but if there is lightning bonding in place between mast and keel, as there may be, then the rig would be connected to the sea, save only for a coat of antifoul.
Click to expand...

There isn't. We've had the mast down and I've rebuilt part of the mast step. Definitely nothing there :)

Pete
 
We have a boat in Greece and another in Milford Haven. In Greece we have a Nasa Navtex with the short white tubular antenna on a pole above the solar panels, where it works well. In the past we sited the antenna below decks, where its reception was not too bad but not as good as on the pole. We also have a Nasa Weatherman with a similar antenna that did not perform well at all. I took a wire from the antenna connection, via a small capacitor, to the backstay, which transformed reception. Instructions for doing this, size of capacitor, were in the manual.

In Milford Haven we have a very old Nasa Navtex with the wire antenna. This lives in the wheelhouse, lying at a small angle to the horizontal. Even in the marina it receives very well.
 
vyv_cox said:
I took a wire from the antenna connection, via a small capacitor, to the backstay, which transformed reception.
Click to expand...

More votes for connecting it to the rig, then!

Is your backstay insulated as for SSB transmission, or is that just how you made the connection to the whole rig?

Pete
 
vyv_cox said:
No, there is no insulation.
Click to expand...

OK, sounds good. Thanks.

One thing I do need to check is that the backstay isn't connected to the rudder stock. They meet the deck very near each other and I can't quite remember the arrangement; it's possible that the backstay chainplate also supports the rudder top bearing. This is likely to give the dreaded rig-to-water connection. Not the end of the world, though, as it's a fractional rig with a very lightweight backstay just shackled on, so I could slip a bit of hose over the shackle to insulate it. I would then have to connect to a shroud chainplate instead, but that's ok.

Pete
 
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