Aerials on a pole, another question.

[franksingleton]

As someone else has said, if you have a GRP boat, put the GPS aerial in a cupboard under the deck. Our Digitalyachts small mushroom works very well. I once tried a GPS dongle on the chart table. Again it worked well. The late Colin Jones once wrote an article showing his NAVTEX aerial also below deck.

 

[typhoonNige]

I replaced my ICS Navtex antenna with the new type (electronics in a separate box). I mounted the antenna below decks near the autopilot computer in the stern space against the hull. Performance was very poor so I mounted it on our bimini frame. I can now receive Niton from the Canaries!

 

[dslittle]

No need to put GPS aerial there. Why bother.

They work perfectly well behind GRP. Put it inside near the GPS itself. Shorter cable run. No unsightly serial, out of the westher, less prone to damage etc.

My NavTex active antenna works perfectly well in the aft lazarette out of the elements as well...

 

[lw395]

I can't envisage the mechanism by which purely receiving antennae might interfere with one another.

LO and IF leakage spring to mind, as well as shadowing and pattern modification.

I think it's unlikely to be a problem in this case.

 

[doug748]

The thread that would not die

.........

I think it's unlikely to be a problem in this case.

Indeed, I did put the Plotter mushroom and Nasa Navtex aerial below decks - roughly in the area of the cockpit combing. It worked very well. The spare VHF/AIS aerial is a whip, which I re-sited on the pushpit.
All very neat and a good tip from Exsolentboy and others.

 

[lw395]

The thread that would not die




Indeed, I did put the Plotter mushroom and Nasa Navtex aerial below decks - roughly in the area of the cockpit combing. It worked very well. The spare VHF/AIS aerial is a whip, which I re-sited on the pushpit.
All very neat and a good tip from Exsolentboy and others.

GPS may work a lot less well below decks in heavy weather.
If you're happy to run that unquantified risk, that's up to you.

GPS is like digital TV, it tends to work perfectly down to a certain signal level, then a further small drop destroys it completely.
It's not so much the layer of GRP that matters, as the layer of water on the deck when sailing in foul weather.

That's when I most want my GPS to work properly.

 

[johnalison]

GPS may work a lot less well below decks in heavy weather.
If you're happy to run that unquantified risk, that's up to you.

GPS is like digital TV, it tends to work perfectly down to a certain signal level, then a further small drop destroys it completely.
It's not so much the layer of GRP that matters, as the layer of water on the deck when sailing in foul weather.

That's when I most want my GPS to work properly.

I think the answer for many people will be "try it and see". My e7 plotter has its own aerial and is installed on the bulkhead by the chart table, perhaps 3 feet above the waterline, and has not given me any concern, and my HR is not especially flimsy.

 

[lw395]

I think the answer for many people will be "try it and see". My e7 plotter has its own aerial and is installed on the bulkhead by the chart table, perhaps 3 feet above the waterline, and has not given me any concern, and my HR is not especially flimsy.

What weather have you tried it in?
What allowance for lower signal levels from the satellites?
What allowance for the gps in the unit aging?

It's all very well doing 'try and see' in the marina on a sunny day when everything is new.
So long as that's all the performance you want from it.

 

[jdc]

I can't envisage the mechanism by which purely receiving antennae might interfere with one another.

What you may have missed is that all receivers have active circuitry inside them, which 'mixes', aka heterodynes, the incoming RF with a local oscillator to generate an intermediate frequency, or as is increasingly the case in modern designs, a direct conversion to a quadrature signal around DC. This local oscillator signal leaks out of the receiver antenna, and there are limits placed on the maximum amount that is allowed to leak out by the national licencing authorities. The limit is usually around -57dBm, which may seem quite low but has to be compared with the sensitivity of a reciever, which is around -90dBm for Bluetooth or WiFi, ~-110dBm for mobiles and may be -120dBm for low data rate comms standards. And is ~-155dBm for GPS, although the more relevant figure for GPS is the noise power which is nearer -112dBm (GPS wanted signal is buried in noise and only extracted by long correlation with the known wanted sequences).

So the signal emitted by a receiver antenna can be 60dB - a factor of a mere one million - higher power than the sensitivity limit of another near-by receiver - quite enough to cause desensitization unless the frequency plan is managed such that LO leakage from one does not fall in-band or on an image frequency. Most high quality designs have front-end band-pass filters which help to mitigate this, but many old GPS sets do not have any selectivity before the mixer.

That said, the three systems mentioned by the OP are so far away from each other that one would be unlucky were there to be mutual interference.

 

[Gwylan]

I intend to put GPS, Navtex and AIS receiving aerials on a short pole at the pushpit.

Would these three interfere with each other in any way? I would like to keep things as tight as possible so I am thinking of a separation as little as 6 - 8in between each one.

Thanks.

Have a similar project in mind but want to use a telescopic pole to extend to 3m to put a ďeck flood and a wifi aerial.

Any ideas on suppliers?

 

[pvb]

What you may have missed is that all receivers have active circuitry inside them,

Whilst receivers have active circuitry, receiving antennae don't necessarily, and the OP was specifically asking about "GPS, Navtex and AIS receiving aerials". Most GPS antennae are now active, certainly, but as typhoonNige mentioned decent Navtex antennae are now passive. And AIS receiving antennae are almost invariably passive.

 

[johnalison]

What weather have you tried it in?
What allowance for lower signal levels from the satellites?
What allowance for the gps in the unit aging?

It's all very well doing 'try and see' in the marina on a sunny day when everything is new.
So long as that's all the performance you want from it.

Three seasons in the Channel and Baltic in summer weather, mostly fine but open water up to f6. In the event of problems, I have a spare GPS connected to a pushpit aerial, and a hand-held, but the e7 hasn't reported a problem.

 

[jdc]

Whilst receivers have active circuitry, receiving antennae don't necessarily, and the OP was specifically asking about "GPS, Navtex and AIS receiving aerials". Most GPS antennae are now active, certainly, but as typhoonNige mentioned decent Navtex antennae are now passive. And AIS receiving antennae are almost invariably passive.

What difference does active or passive make? There is still some radiation from an antenna connected to a real receiver input, and a few metres of low-loss coax cable makes no difference; it comes out of the antenna port of the receiver.

 

[pvb]

What difference does active or passive make? There is still some radiation from an antenna connected to a real receiver input, and a few metres of low-loss coax cable makes no difference; it comes out of the antenna port of the receiver.

I obviously misunderstood your post. However, despite your concerns, you seem to agree with the general view that the OP's plan wouldn't be a problem.

 

[jdc]

... you seem to agree with the general view that the OP's plan wouldn't be a problem.

Yes - we're absolutely in agreement about that!

 

[Heckler]

From memory, GPS is microwave-ish frequencies, Navtex is a few hundred khz (relatively lowish frequency, though not actually LF), and AIS is of course VHF. So they should be well separated and unlikely to interfere, but RF is funny stuff...

Pete

Ive got my cockpit vhf on a pole on the pushpit, the navtex next to it and I had a garden solar led next to it. The gps plotter mushroom is below and to one side.
When I transmit for a few secs the plotter loses the position and the led lights go out if they are lit!
Go figure!
S

 

[lw395]

Ive got my cockpit vhf on a pole on the pushpit, the navtex next to it and I had a garden solar led next to it. The gps plotter mushroom is below and to one side.
When I transmit for a few secs the plotter loses the position and the led lights go out if they are lit!
Go figure!
S

Seems reasonable to me.
Your vhf is putting out a field far in excess of what the other kit would be spec'd to tolerate.
You are running a risk of damaging stuff, not just deafening it while transmitting.

 

[Daydream believer]

My set up does have more than the proposed 6-8 inches difference but would solve the problem, if you can do this sort of setup
Mine works perfectly . Although not shown the plotter fits just behind the tiller
the poles are from a broken extending sailboard booms & can be extended another 2 feet if I wish. They cost me nothing

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