On Tue, 22 Jan 2008 00:24:09 -0500, "Bill Kearney"
wrote in
:

it's best to keep the cable run as short as possible.

Which is usless advice if it's a network-connected GPS antenna. ...

And sound advice if it's a passive antenna, as is often used on smaller
boats.

--
Best regards,
John Navas http:/navasgroup.com

Rob wrote in news:a999739f-3fe0-4e5a-b019-
:

I have been getting conflicting advice about relative position in
regard to my radar dome. Both will be mounted on my mast about 12 to
24 inches apart. I have a choice of having the GPS right below the
bottom of the dome or I could put an extension and have it extend a
few inches above the dome. Any advice?
Thanks
Bob


Wrong attitude. The GPS antenna has no need for ALTITUDE. Aboard
Lionheart, I got tired of them bumping into both the Raystar little dome
and the Garmin active antenna, so I mounted them INSIDE the overhead
cabinet in the galley, behind the helm (just forward of the bulkhead-
mounted wheel on an Amel 41 Sharki ketch). Fiberglass and plastic is
transparent to RF energy (especially RF tearing up the damned HF/SSB). The
signals work perfectly.

On a big center console fishing boat, they were looking for a good place to
mount the GPS so the fishing could tear it off and were looking way up
high, which is crazy. I said, "Let's just lay it INSIDE the overhead
cabinet behind the GPS and radios under the plastic bimini top and see how
it does across the harbor." During my free boat ride..(c;...we noted the
GPS worked perfectly with strong signals in all directions. So, when we
got back to the dock, having run completely out of beer, we mounted the GPS
antenna in the back corner of the INSIDE of that electronics cabinet, out
of the way of the other gear and wires. It's worked perfect there ever
since. If he had had a sheet metal top, of course, that wouldn't have been
possible. Noone fishing can get to the GPS antenna to rip it off, now.
They'll have to pull the overhead electronics console apart to even find
it!

larry wrote:
Rob wrote in news:a999739f-3fe0-4e5a-b019-
:

I have been getting conflicting advice about relative position in
regard to my radar dome. Both will be mounted on my mast about 12 to
24 inches apart. I have a choice of having the GPS right below the
bottom of the dome or I could put an extension and have it extend a
few inches above the dome. Any advice?
Thanks
Bob


Wrong attitude. The GPS antenna has no need for ALTITUDE. Aboard
Lionheart, I got tired of them bumping into both the Raystar little dome
and the Garmin active antenna, so I mounted them INSIDE the overhead
cabinet in the galley

Belowdecks is the way to go in a fiberglass boat.

My friend held the antenna alternately in the clear abovedecks, and
belowdecks hard up against the coachroof in various spots. I watched the
signal strength indicator on the chartplotter as she moved the antenna
between the abovedecks and belowdecks positions. In many spots
belowdecks there was no difference in signal strength from the
satellites with the antenna abovedecks or belowdecks.

My GPS antenna is now mounted in a good spot belowdecks, out of the
weather, away from running rigging and flying feet.

Moonshadow wrote:
larry wrote:
Rob wrote in news:a999739f-3fe0-4e5a-b019-
:

I have been getting conflicting advice about relative position in
regard to my radar dome. Both will be mounted on my mast about 12 to
24 inches apart. I have a choice of having the GPS right below the
bottom of the dome or I could put an extension and have it extend a
few inches above the dome. Any advice?

Belowdecks is the way to go in a fiberglass boat.

My friend held the antenna alternately in the clear abovedecks, and
belowdecks hard up against the coachroof in various spots. I watched the
signal strength indicator on the chartplotter as she moved the antenna
between the abovedecks and belowdecks positions. In many spots
belowdecks there was no difference in signal strength from the
satellites with the antenna abovedecks or belowdecks.

My GPS antenna is now mounted in a good spot belowdecks, out of the
weather, away from running rigging and flying feet.

Better yet, get an internal antenna version of the unit, it
has optimal connectivity with no cabling. My Garmin GPSMAP
172C is mounted inside a pilothouse and always hits on any
satellite shown above the horizon, without problem.

Always best to get the GPS antenna some distance from the radar and out of
the direct beam of the radar.

I agree that low down is generally ok. Mine is inside the fibreglass
flybridge with no apparent ill effects!

Alec


"Rob" wrote in message
...
I have been getting conflicting advice about relative position in
regard to my radar dome. Both will be mounted on my mast about 12 to
24 inches apart. I have a choice of having the GPS right below the
bottom of the dome or I could put an extension and have it extend a
few inches above the dome. Any advice?
Thanks
Bob

On Tue, 22 Jan 2008 19:40:30 +1100, Moonshadow
wrote in :

larry wrote:

Wrong attitude. The GPS antenna has no need for ALTITUDE. Aboard
Lionheart, I got tired of them bumping into both the Raystar little dome
and the Garmin active antenna, so I mounted them INSIDE the overhead
cabinet in the galley

Belowdecks is the way to go in a fiberglass boat.

My friend held the antenna alternately in the clear abovedecks, and
belowdecks hard up against the coachroof in various spots. I watched the
signal strength indicator on the chartplotter as she moved the antenna
between the abovedecks and belowdecks positions. In many spots
belowdecks there was no difference in signal strength from the
satellites with the antenna abovedecks or belowdecks.

My GPS antenna is now mounted in a good spot belowdecks, out of the
weather, away from running rigging and flying feet.

My own tests have been quite different -- I've seen considerable
difference between belowdecks and abovedecks, and agree with
manufacturers recommendations to mount the antenna on something like the
stern pulpit. It makes no sense to take _any_ risk with something this
critical to safety.

--
Best regards,
John Navas http:/navasgroup.com

Why do people persist in putting their GPS antenna on the stern rail. Is it
not one of your most important instruments? Do you want it to be yanked off
by some clumsy git climbing aboard from a dinghy or clipped off by a
shoreline? I have always mounted mine on the stern but directly on the deck
where it out of everyone's way. It also gets a perfect view of the sky
without the pendulum movement of a mast mounting. This is on my third boat
and I have never had one damaged. How many people keep a spare GPS aerial
for these eventualities?


"Alec" wrote in message
...
Always best to get the GPS antenna some distance from the radar and out of
the direct beam of the radar.

I agree that low down is generally ok. Mine is inside the fibreglass
flybridge with no apparent ill effects!

Alec


"Rob" wrote in message
...
I have been getting conflicting advice about relative position in
regard to my radar dome. Both will be mounted on my mast about 12 to
24 inches apart. I have a choice of having the GPS right below the
bottom of the dome or I could put an extension and have it extend a
few inches above the dome. Any advice?
Thanks
Bob

Nicholas Walsh wrote:
Why do people persist in putting their GPS antenna on the stern rail. Is
it not one of your most important instruments? Do you want it to be
yanked off by some clumsy git climbing aboard from a dinghy or clipped
off by a shoreline? I have always mounted mine on the stern but directly
on the deck where it out of everyone's way. It also gets a perfect view
of the sky without the pendulum movement of a mast mounting. This is on
my third boat and I have never had one damaged. How many people keep a
spare GPS aerial for these eventualities?


Mine's coachroof mounted 'limpet' style on our Contessa 26 with excess
cable shortened and a new BNC plug put on. It's predecessor used to be
mounted on a stancheon supporting the mainsheet horse and that was a
nightmare. If it didn't get knocked by the mainsheet, someone would
lean back over it and block the signal, or a warp would abrade its cable
etc. I give the new one a gentle polish a couple of times a season to
keep the water beading up and running off and I get a *far* better
signal. I've replaced the cable into the old one and tested it for a
spare just in case though.


--
Ian Malcolm. London, ENGLAND. (NEWSGROUP REPLY PREFERRED)
ianm[at]the[dash]malcolms[dot]freeserve[dot]co[dot]uk
[at]=@, [dash]=- & [dot]=. *Warning* HTML & 32K emails -- NUL:
'Stingo' Albacore #1554 - 15' Early 60's, Uffa Fox designed,
All varnished hot moulded wooden racing dinghy.

On Mon, 21 Jan 2008 19:52:29 -0000, JohnW
wrote:

Mounting it high up on a sailing boat also makes it subject to
position reporting error due to heel, not to mention confusing
it when rolling.

I was under the impression that GPS results are referenced to the
location of the antenna. So what does heel have to do with it?

Casady

Richard Casady, in article 47b6d18e.1524399671
@news.east.earthlink.net, says...

I was under the impression that GPS results are referenced to the
location of the antenna. So what does heel have to do with it?

When you are heeled over, the antenna, if up the mast, will be
over to the side somewhere, some distance from the boat
centerline where it will be giving an incorrect position
report for the boat. Since heel isn't constant, the error
introduced by heel would be variable.

Not that you should be using the position information reported
by GPS to that level of accuracy anyway :-) However, there
have been several GPS assisted collisions with fixed landscape
features, so perhaps that isn't true anymore?

If you are pitching and rolling, the antenna will be moving
relative to the boat so the GPS will include that motion in
with the boat's forward velocity in its speed calculation.
---
One problem with mounting the antenna at deck level, under the
pushpit, is that from a dinghy, it looks too much like a
handle to help getting on deck. If on the pushpit, it can get
knocked. I have mine under the GPS structure which has no
reported signal strength implications. It also isn't a
visible "I've got a GPS available for stealing" signal...

--
JohnW.
Replace the obvious with co.uk in 2 places to mail me.