Where is your primary VHF antenna?

[waterman46]

Originally Posted by SV Bacchus

Good info hello, ours is a transmit AIS and both are presently out of commission due to a lightening strike. I will ask the electronics repair folks about installing the antenna a little lower, that sound like a good idea!

I'm wondering, since both antenna mounts are at the same height, just opposite sides of the mast, could the AIS antenna use its same mount but mount the antenna upside down? Only the base of both antennas would be at the same level that way.

Antenna mounted upside down? Wouldn't that put the antenna parallel to the mast a few inches away from it? That would be a very bad location. On a spreader about 2/3 distance out from mast, is better.

Probably for those mast-deficient boats like sloops and cutters, a single VHF antenna at masthead, with a duplexer below decks to allow both AIS and VHF to use the same antenna, is a pricier but elegant solution.

[markpierce]

Atop the mast.

[SV_Harbinger]

For the primary vhf antena I would only ever use the masthead to get the antenna as high up as possible and use the largest wire I could cram up the mast. I have a second antenna on transom if something happens to the main (just unplug one and plug in the other) For some reason or another I seem to collect hand helds, I know where 3 are and there are 2 more kicking around here somewhere....No AIS though

[hellosailor]

Waterman-
Yes, adjacent to the mast would be no good, that would be transmitting directly into ground. Better if at least 1/4 wavelength away from it, which might mean at the end of a masthead truck, or out on the spreader...position depends on what's in the way up there.

[GTom]

VHF on the masthead of course, but I'll add an AIS antenna soon, planning to get one of these for my radar pole:



Anything speaking against it? As I understand, if vertically separated, the two lobes shouldn't interfere...

[Group9]

For the main, cabin located VHF, 3' ss, at the top of the mast. But, for the secondary, helm located VHF, an 8' fiberglass antenna mounted onto the rack for the solar panels, at the stern quarter (the theory being, we would probably still have one working VHF if we got dismasted).

I learned the dismasting and no radio lesson years ago, one night, when we lost a mast with the only VHF radio antenna at the top of it. (Range was found to be greatly reduced when the antenna was forty feet underwater. )

[GILow]

Originally Posted by SV Bacchus

Good info all on my VHF AIS situation. I'm going to have a sit down with the electronics installer and discuss options. Hopefully I am now armed with enough info to not be led down "the rosey path"!

Don't forget the top spreader as an option for the AIS antenna.

I admit, I am a bit prejudiced against splitters, I feel they are a potential point of failure that could fail silently and I would not know my AIS was not transmitting or receiving as it should.

Having said this, I have not had a chance to run the extra antenna cable for the AIS up the mast yet so mine sits up with the solar panels, about 10 feet above the water.

[GILow]

Originally Posted by Group9

For the main, cabin located VHF, 3' ss, at the top of the mast. But, for the secondary, helm located VHF, an 8' fiberglass antenna mounted onto the rack for the solar panels, at the stern quarter (the theory being, we would probably still have one working VHF if we got dismasted).

I learned the dismasting and no radio lesson years ago, one night, when we lost a mast with the only VHF radio antenna at the top of it. (Range was found to be greatly reduced when the antenna was forty feet underwater. )

Same setup for my VHF radios (but swap helm and cabin antennas) and I chose to learn about the dismasting issues through the experience of others rather than trying it for myself.

[GTom]

Originally Posted by GILow

Having said this, I have not had a chance to run the extra antenna cable for the AIS up the mast yet so mine sits up with the solar panels, about 10 feet above the water.

What range do you get normally for smaller fishing boats (class B?) and big ships? I don't plan to put the AIS one much higher.

[GILow]

Originally Posted by GTom

What range do you get normally for smaller fishing boats (class B?) and big ships? I don't plan to put the AIS one much higher.

Sorry, the installation is quite new, hence I have not had time to run the antenna up the mast, so I don't have any meaningful data on range yet.

But watching friends travelling in company with another boat round the coast of Australia last year was an education in AIS range.

Both boats had the same make of AIS, one had the antenna on a splitter to the top of the 45 foot mast and the other had the antenna on the aft rail of a big cat, so maybe 6 feet above the water.

The boat with the mast-top AIS regularly stayed in range of shore based AIS stations for ten or more miles further than the cat. It was enough to convince me to put my antenna up on the spreader. (But not enough to convince me to use a splitter.)

[GTom]

Thanks, sounds interesting! However, I am fine with 12-15miles total range for ships/coastals and 5-6 miles for fishing boats if I can achieve that with a radar pole mounted AIS antenna. Not fun of splitters either and I am almost certain that I can't squeeze a second RG213 inside my mast.

[skipmac]

Originally Posted by Telemark43

Consider just one VHF antenna, use by both AIS and VHF with Vesper AIS/VHF/FM Antenna Splitter. Pricey, but works AOK. Mine has been in continuous used for two years, uses minimal power and fail safe is VHF only. Solves problem of two (or three) units using VHF antenna. AIS only broadcasts for a very short time so doesn't really interfere with VHF use.

Initially I had reservations with splitters but the Vesper unit was highly recommended by several radio pros. Found a deal on a new Vesper AIS with free splitter so had nothing to lose to try it. So far I love it and as a side benefit, the splitter also has a feed to your FM stereo so you can get great stereo reception as well.


FYI the Vesper is an active splitter so you have to run power to it. If I recall it even has a signal amplifier built it to boost the received signals but I am not certain about this.

[David M]

With the GPS antenna at least, you want it as low as possible but so that it still has a decent view of the sky. The reason is that as the boat rolls, you want a minimal change in what your COG is indicating. More pitching from being up higher would also provide a less consistent SOG.

Same goes for an AIS GPS sensor placed up high, except that others would see a less consistent COG and SOG.

With a VHF antenna, of course, it is good to place it as high as what is practical so as to get the distance to the horizon as far as possible