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Old 21-07-2011, 14:15   #16
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Re: GPS Antenna Flush on Reverse Transom?

The human body stops GPS waves: if somebody seats on a GPS antenna mounted flush on deck, the receiver operation is gravely disturbed. The same can happen if the antenna is installed below deck and somebody seats on the deck above. Don't ask how I found it...

For this reason, I would recommend installing the antenna where nobody can sit. Mine is on the pushpit.

Alain
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Old 21-07-2011, 14:30   #17
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Re: GPS Antenna Flush on Reverse Transom?

Quote:
When it blows my wind generator (we sleep in the aft cabin) tells me to get up to check things out. I don't think I could hear the beep beep of an alarm when the wind starts honking.
Two plus one things...

First, strong current can make you drag as quickly as a nice wind blow. And even with moderate winds, the constant push along with some current addition can easily make a poor/moderate set fail.

Second, the advantage of an anchor alarm based around a mobile phone (or iPad, etc) is that it'll be right next to your berth watching your anchor all night long. When it goes off right next to your head, you'll hear the beep-beep. I've been doing that for 8 years now.

One additional thing. For existing "dumb" anchor alarms, the equation to set the anchor alarm is:

Given:
R = amount of rode out in feet
G = distance in feet the GPS is located from the bow
A = amount of GPS accuracy you expect in feet (I use 10)
S = amount of slop/error/etc in feet you accept (I generally use 20)

Alarm setting = R + (2 * G) + A + S

It's the 2 * G that most people leave out and it can be significant. This ignores sea floor depth which is usually insignificant when using normal scope ratios.
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Old 21-07-2011, 14:36   #18
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Re: GPS Antenna Flush on Reverse Transom?

Quote:
Originally Posted by Hydra View Post
The human body stops GPS waves: if somebody seats on a GPS antenna mounted flush on deck, the receiver operation is gravely disturbed. The same can happen if the antenna is installed below deck and somebody seats on the deck above. Don't ask how I found it...

For this reason, I would recommend installing the antenna where nobody can sit. Mine is on the pushpit.

Alain
Mine is on the coaming in the port stern quarter. (see earlier pic) While I have yet to sit on it I have sat close to it and sometimes the antenna is covered by an arm or body leaning over. I have yet to lose a signal because of it being blocked. I was worried at first because the rails of my bimini and an arch are over it but have never had a problem. It was installed by the dealer and apparently they install them all like that.
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Old 21-07-2011, 14:40   #19
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Re: GPS Antenna Flush on Reverse Transom?

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The human body stops GPS waves: if somebody seats on a GPS antenna mounted flush on deck, the receiver operation is gravely disturbed.
Yeah. You're right. But I'm a cruiser. The GPS fix can wait until they move.
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Old 21-07-2011, 16:02   #20
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Re: GPS Antenna Flush on Reverse Transom?

Wow! Thanks for all the replies!

OK, firstly - some more background. I've attached a a few pics from how the boat WAS a while ago (don't have any more recent pics unfortunately). Since then, the entire boat was stripped and rebuilt (i.e. even all bulkheads inside), so no electronics/antennas or anything from these pics are left. The rails however are the same, as is the general layout. Cockpit has been shrunk athwartships to approx 2ft wide and two seating positions with minimal backrests created on either side of the companionway. Oh, and we've also added a deckbox on the hatch turtle to squeeze in a pair of propane/lpg tanks.. so finding mounting space is REALLY hard. Oh, and I have no coaming (!).

On the transom - there is now a boarding ladder on the port side of the backstay - so can't mount the antenna on deck all the way aft (like Vasco's option) as it will be stepped on regularly, and mounting on the rail where the GPS on the pic is is also a no-go, since it will be grabbed accidentally each time somebody boards.

The backstay is now a Navtec hydraulic unit so it extends significantly higher than in the pic - so not sure how that will affect the backstay mounting idea? Also - if I mount on the backstay - how does this affect backstay tensioning with the hydraulic ram in the way + pump handle?

Now moving over to the starboard side of the transom, there will be an outboard on the rail, which rules out rail mounting there as well.

On the anchor alarm thread - thanks for the heads up, but I'll have to put that one in the "worry about it later" basket as I don't even have a GPS mounting position yet to get the GPS coords to alarm on - it's an old racing boat and space to mount anything is a real hassle(!)

On GPS mounting orientation - if I were to use one of the suggestions of mounting the antennas on adjustable bases to make them horizontal or if I just welded up a fixed angled bracket etc in stainless - what are your thoughts there about damage from seas? I understand that everybody is comfortable with mounting on the rails etc - but that's another 3ft above the waterline and presumably safer.

I haven't had much experience offshore, but over the next few months will be transitioning to cruising fulltime and will slowly be doing a westabout circumnavigation. But to date, I've never experienced a boarding wave, or been pooped etc - so what are your thoughts about the strength (and corrosion/water resistance) of GPS antennas in general with the bracket idea?

Thanks again!
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Old 21-07-2011, 17:07   #21
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Your over thinking this. Just put it on the stern rail like everyone else.

Dave
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Old 21-07-2011, 17:44   #22
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Re: GPS Antenna Flush on Reverse Transom?

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Originally Posted by goboatingnow View Post
Your over thinking this. Just put it on the stern rail like everyone else.

Dave
Mine's not on the stern rail.
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Old 21-07-2011, 17:48   #23
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Re: GPS Antenna Flush on Reverse Transom?

My choice is on the deck, in the cockpit, behind the helm, at the transom. It won't have any problems acquiring a signal through the fiberglass. JMO but nearly all radomes are made of fiberglass because it passes the rf signal.

If you are using a computer then use a usb GPS. It looks like a flash drive and costs about $19 (US). No mounting required. You can even port this out to other equipment with a usb to serial adapter. In my case I have a dedicated chart plotter and I port the nmea data into the computer (also another chart plotter).
There is no need to have 2 GPS antennas. One will suffice. Just route the nmea into the AIS. Keep the second antenna as a spare. That is what I do, a totally connected system so that ALL data is available on every instrument and the computer. It's what the nmea bus was designed to do.
Just to get you thinking about it.
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Old 21-07-2011, 17:55   #24
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Re: GPS Antenna Flush on Reverse Transom?

Actually, everything on the boat is NMEA2000, so the Maretron GPS will feed output onto the bus, and I even have somewhat of a backup GPS at the masthead as I believe the Airmar PB200 ultrasonic wind instrument has a receiver as well.... however from what I can read of the NAIS-300 manual, while it also gets connected to the N2K network, it looks like it wants its own GPS antenna as well - hence mounting two GPS antennas.
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Old 21-07-2011, 18:12   #25
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Re: GPS Antenna Flush on Reverse Transom?

The Maretron gps is not an antenna, it is a complete gps receiver. So is the one that is part of the PB200. The NAIS300 (simrad AIS) needs a GPS ANTENNA, as it has it's own receiver built into the box. It should have come with one when you bought it. The antenna connects with a SMA connector, which is not the same as a NMEA 2000 connector.

So mount the antenna that came with the NAIS300 on your stern rail, or maybe even inside (Garmin makes an interior bracket to mount on the overhead INSIDE a boat) if you get good reception there.

You will find that you don't need the Maretron GPS at all, but perhaps keep it as a backup, disconnected, to save power.

Chris


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Actually, everything on the boat is NMEA2000, so the Maretron GPS will feed output onto the bus, and I even have somewhat of a backup GPS at the masthead as I believe the Airmar PB200 ultrasonic wind instrument has a receiver as well.... however from what I can read of the NAIS-300 manual, while it also gets connected to the N2K network, it looks like it wants its own GPS antenna as well - hence mounting two GPS antennas.
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Old 21-07-2011, 18:17   #26
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Re: GPS Antenna Flush on Reverse Transom?

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The Maretron gps is not an antenna, it is a complete gps receiver. So is the one that is part of the PB200. The NAIS300 (simrad AIS) needs a GPS ANTENNA, as it has it's own receiver built into the box. It should have come with one when you bought it. The antenna connects with a SMA connector, which is not the same as a NMEA 2000 connector.

So mount the antenna that came with the NAIS300 on your stern rail, or maybe even inside (Garmin makes an interior bracket to mount on the overhead INSIDE a boat) if you get good reception there.

You will find that you don't need the Maretron GPS at all, but perhaps keep it as a backup, disconnected, to save power.

Chris
Oops, yes, the maretron is a complete unit, however due to it's higher sensitivity and accuracy (based reviews and marketing material), my goal is to use that as the primary and have the AIS switchable.

Any thoughts about a horizontal bracket on the transom idea for these and subsequent damage from wave action?
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Old 21-07-2011, 19:35   #27
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Re: GPS Antenna Flush on Reverse Transom?

At one time I had a backup GPS pressed against a vertical window in the cabin. This meant it was only getting half the sky at best. It worked fine. A full sky is ideal but not absolutely necessary.
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Old 21-07-2011, 21:04   #28
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Re: GPS Antenna Flush on Reverse Transom?

I would be really surprised if the Maretron was significantly more accurate than the one in the PB200. If they have the same SIRF chipset (I'll look at this tomorrow) you would be hard pressed to notice a difference.

Chris


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Oops, yes, the maretron is a complete unit, however due to it's higher sensitivity and accuracy (based reviews and marketing material), my goal is to use that as the primary and have the AIS switchable.

Any thoughts about a horizontal bracket on the transom idea for these and subsequent damage from wave action?
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Old 22-07-2011, 14:32   #29
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Re: GPS Antenna Flush on Reverse Transom?

The GPS is such an important instrument that I believe you should do everything possible to ensure the primary GPS receives the best signal. Too many boats accept poor GPS performance, I suspect because it still gives position and speed even with a poor signal.
Get a better signal and the accuracy improves significantly.
Modern GPS chips are much better than the older designs, but they still do better with a clear view of the sky, with the antennae oriented appropriately. Unfortunately the chips fitted to many expensive marine GPS units are not state of the art, and suffer even more from poor installation.
Donít compromise the GPS and make sure its set up for WAAS or EGNOS if appropriate
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Old 22-07-2011, 20:08   #30
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Re: GPS Antenna Flush on Reverse Transom ?

VERY slight clarification...the NAIS300 gps "antenna" is also a complete receiver. It gets its power through the sma plug, and also sends the NMEA data back to the unit. At the antenna end a coil and capacitor is used to split the dc off, same at the simrad box end. Doesn't affect anything written here.
I don't know anyone who makes an actual antenna (without receiver). Older GPS units also did it this way (carrying the signal and dc on the sma cable) but rarer to find today.
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