Aerials;- where?

[Graham_Wright]

Having read tales of pushpit mounted electronics swept away by following seas and snapped off by those who thought they were grab handles, I look for alternative sites.

GPS and Navtext for example, could their aerials live on the spreaders?

Which way up and are the losses by being shadowed by the mast compensated for by the increased elevation?

Learned voices and those with experience please respond.

 

[Fascadale]

I heard a suggestion at an RYA course that it was best to mount your GPS aerial on a part of the boat with the least movement.

A mast mounted aerial would be swinging about more which, so the story goes, causes the GPS problems with ascertaining your position.

Don't know how true this is but most GPS aerials you see are pushpit mounted and I suspect every one else who has one, has one with an internal aerial.

No idea about Navtex

 

[boatmike]

Agree. Both will work OK at spreaders but if you are rolling around a lot they are better down low. Neither need to be at any height and are waterproof so unlikely to suffer on pushpit.

 

[alan]

You can get away with mounting both GPS and Navtex antennas internally thus saving on wiring runs and they are better protedted from hand-holds e.t.c.

Check first to see if they work in the position selected before mounting permanently.

Alan.

 

[Jonny_H]

Navtex certainly works ok inside the boat. My Raymarine GPS is mounted on the gantry at the back, however I have a USB GPS which works fine from below decks.

Jonny

 

[cmedsailor]

Can't say about Navtex but GPS works absolutely fine with the antenna inside below decks.

 

[Relax]

I mounted my Garmin GPS aerial on the pushpit but when taking the hard I have loosened the screw and inverted it to avoid damage - it still works fin upside down!

Think I'll leave it that way - just to irritate the anoraks /forums/images/graemlins/grin.gif

 

[vyv_cox]

Whilst I agree that a Navtex will work OK with its antenna below, it certainly won't work at its best down there. If you are cruising in an area where Navtex signals are strong and reliable then I would follow that advice. If you are cruising widely or in a weak signal area then you will want to have it working as well as possible.

GPS antenna below seems to be dependent upon the set. However, my old Garmin 76, which worked OK with its internal antenna at the chart table, used to lose signal occasionally, whereas other boats with external antennae suffered no such problems.

I have both of these, plus an emergency VHF aerial, mounted on top of the radar scanner on a stern post. This seems to be an excellent location and I suffer from none of the listed problems.

 

[Salty John]

VHF - line of sight, therefore as high as possible, with 1m horizontal separation from metal spars and rigging. Sitting atop spars is fine. AIS VHF antenna can be pushpit mounted because it receives signals from transmitters mounted high on the superstructure of a ship so distance loss is not great; means you have a spare antenna if you lose the rig. Mizzen or radar arch are good locations.
GPS and Navtex active aerials - not line of sight, so low down, even below on a GRP boat, works. GPS should not be masthead because the viloent acceleration and deceleration can confuse the speed functions.
SSB - backstay.

 

[st599]

GPS should be mounted low due to the effects of multipath, and the poor performance of speed indicators on a mast that's swinging.

 

[Jonny_H]

We have everything bar NAVTEX mounted on the gantry (GPS, Radar, Second VHF etc):

The only thing to beware of is that somethings don't like being near each other. For instance, GPS doesn't (according to Raymarine) like being next to Radar at the same height (we therefore have a short pole to raise it 1.5 feet above the radar dome and its scan field). You can just see it on the above picture in between the radar dome and the wind generator.

I have also heard about SSB / Radar interferance. However, we have yet to mount our Whip (which will be about 4 feet from the radar) so cannot report on that.

Jonny

 

[Oldhand]

[ QUOTE ]
GPS and Navtex active aerials - not line of sight, so low down, even below on a GRP boat, works. GPS should not be masthead because the viloent acceleration and deceleration can confuse the speed functions.
SSB - backstay.

[/ QUOTE ]

Since when was GPS "not line of sight"? I think you will find 1.2 odd GHz propagation is line of sight but it can pass through some dielectric materails, with attenuation.

 

[Salty John]

I think that most people will realise that in this context the term line of sight means one antenna seeing the other antenna. GPS signals come from a satellite so the antenna's height above sea level is unimportant from the point of view of signal reception.

 

[nigelhudson]

We bought our boat with the GPS aerial already mounted on the pushpit. Like you I always felt that it was a but vulnerable but we have not had any problems. When we bought a plotter with its own external aerial we looked around for a different mounting point a) to reduce the vulnerability of putting both aerials close together and b) to simplify installation. We finally chose a position under the sprayhood on top of the instrument housing above the companionway. We get excellent reception in this location and would recommend it to others.