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Old 29-02-2012, 21:37   #1
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AIS Antenna

Do I need to spend more money to get an "AIS" antenna, or is a standard 3' VHF whip good enough? It will be mounted on our 50' high main mast.
Thanks,

John
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Old 29-02-2012, 21:45   #2
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Re: AIS Antenna

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Originally Posted by jrogers View Post
Do I need to spend more money to get an "AIS" antenna, or is a standard 3' VHF whip good enough? It will be mounted on our 50' high main mast.
Thanks,

John
Here is the one I'm using.

Shakespeare Antenna Specifications: Shakespeare Squatty Body® 5215-AIS VHF Marine Band

It's a 3ft that they say has extra bandwidth for AIS. What we don't know is what the bandwidth of your antenna is. It might be good enough, or it might not. If "not" it's likely that will show up as not receiving AIS targets as far away as you might.

If you are going to install an antenna, why not choose one that's guaranteed to work?
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Old 29-02-2012, 21:57   #3
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Re: AIS Antenna

G'Day John,

Don't get your knickers in a knot over this!! Our AIS antenna is our normal VHF whip, a not very top end item, further hampered by both using a splitter (built into our ICOM AIS rx) and being mounted on our radar/solar arch, about 12 feet above the WL.

Despite these handicaps, we routinely get AIS data from 25 mile distances. Frankly, info from greater range than that isn't of much interest to me.

Save your money for something important!

Cheers,

Jim

PS: Extra bandwidth??? The AIS signal is on one of the normal VHF frequencies, and "extra bandwidth" isn't required. Sounds like a sales gimmick to me!
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Old 29-02-2012, 22:00   #4
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Re: AIS Antenna

Thanks for the advice! I like the idea of saving money for something else and it seemed like a year ago there weren't antennas specifically for AIS. Is this just marketing?
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Old 29-02-2012, 22:15   #5
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Re: AIS Antenna

[QUOTE=jrogers;898785]Thanks for the advice! I like the idea of saving money for something else and it seemed like a year ago there weren't antennas specifically for AIS. Is this just marketing?[/QUO

Looking at the website for Shakespeare, the difference between the 5215 and the 5215 AIS is that they imply (but don't state) that the centre of the bandwidth is higher on the AIS version This makes sense as AIS uses frequencies at the top of the band.

So Shakespeare don't seem to recommend using either the 5215 OR 5215 AIS for both VHF and AIS. This is a worry as I've been using the 5215 AIS for VHF and AIS for a while. Reception of both seems to be very good. I think I might check my SWR on channel 16 though...
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Old 29-02-2012, 22:48   #6
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Re: AIS Antenna

At first I thought that the extra bandwidth claim was bogus until I thought about it a bit more....

AIS uses channels 87 and 88 at the upper end of the Marine VHF assignments. One could presume that a standard Marine VHF antenna has its center either at channel 16 or near the middle of the Marine VHF band. And thus best operation at those channels with increasingly poorer operation as you go to the ends of the band.

With a standard antenna there may be some or significant loss (increasing VSWR) as you move toward the ends of the antennas pass-band and thus a greater loss on the AIS channels.

Thus is is not the bandwidth of the AIS signal but rather that the AIS channels are at the ends of the Marine VHF band and so a wider bandwidth antenna is an advantage.

The AIS antennas list a VSWR of 2 to 1 or less for a 7 MHZ pass-band and most likely do a little better at AIS frequencies. We need to measure the VSWR of both a standard and an AIS marine VHF antenna on the AIS channels to see how much real advantage there is.

This leads to 2 other thoughts.

First, that you may get an advantage by using the AIS Marine VHF antenna for your "normal" Marine VHF use. Better VSWR at channel 68 with perhaps a little less at channel 16.

Second, If you have a dedicated AIS antenna it may be better to take standard Marine VHF antenna and shorten it little by little to tune for the minimum VSWR on the AIS channels. Optimized for AIS.

Regards, Ethan




-----------------------------------------
87 157.375 161.975 Automatic Identification System B 88 157.425 162.025 Automatic Identification System
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Old 01-03-2012, 01:57   #7
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Re: AIS Antenna

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Originally Posted by Jim Cate View Post
G'Day John,

Don't get your knickers in a knot over this!! Our AIS antenna is our normal VHF whip, a not very top end item, further hampered by both using a splitter (built into our ICOM AIS rx) and being mounted on our radar/solar arch, about 12 feet above the WL.

Despite these handicaps, we routinely get AIS data from 25 mile distances. Frankly, info from greater range than that isn't of much interest to me.

Save your money for something important!
Same here.
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Old 01-03-2012, 02:23   #8
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Re: AIS Antenna

Quote:
Originally Posted by Jim Cate View Post
G'Day John,

Don't get your knickers in a knot over this!! Our AIS antenna is our normal VHF whip, a not very top end item, further hampered by both using a splitter (built into our ICOM AIS rx) and being mounted on our radar/solar arch, about 12 feet above the WL.

Despite these handicaps, we routinely get AIS data from 25 mile distances. Frankly, info from greater range than that isn't of much interest to me.

Save your money for something important!

Cheers,

Jim

PS: Extra bandwidth??? The AIS signal is on one of the normal VHF frequencies, and "extra bandwidth" isn't required. Sounds like a sales gimmick to me!
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Originally Posted by mikefossl View Post
Same here.
What these guys said.

The very slight difference that maybe possible SWR wise by shifting the centre frequency of the antenna wil be far overshadowed by far more important aspects of the installation like feeder loss (coax loss), quality of connectors, height of antenna, shadowing from nearby objects (mast / rigging etc). Don't sweat the small stuff and unless you want the very best and are prepared to pay for it, put the monry towards better coax or connectors or 3M sealing tape etc.
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Old 01-03-2012, 04:10   #9
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Re: AIS Antenna

I replaced my AIS antenana last summer with the cheapest 1.1m stainless whip from JGTech in UK. I also replaced the coax with tinned aerial grade (available with the antenna) The aerial is mounted on the stern gantry so is relatively low.

I was expecting 20nm ranges, and achieved some contacts out to 39nm

Who needs more?

Why pay extra for a dedicated aerial?

As a bonus, I now have an emergency aerial that is rigged and tested. I have an extra length of Coax to connect the AIS aerial to the VHF should I be so careless as to lose a mast.
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Old 01-03-2012, 19:28   #10
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Re: AIS Antenna

Always educational to ask a question here! I'm going to use an inexpensive, standard whip and better coax.

Thanks again, all.
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Old 01-03-2012, 19:51   #11
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Re: AIS Antenna

Depends on how you want to be. I stuck the shakespeare ais vhf on a stern rail and left the masthead alone for vhf. Thats pretty cheap and for distance on Ais it works fine. Not sure on miles but its plenty. In this scenario I have a second antenae that is wired for vhf if needed. That has some value. I would look at it in terms of range needed complication and redundant capability.This is really pretty cheap and has some advantages.
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Old 20-04-2012, 17:13   #12
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Re: AIS Antenna

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Originally Posted by Jim Cate View Post
G'Day John,

Don't get your knickers in a knot over this!! Our AIS antenna is our normal VHF whip, a not very top end item, further hampered by both using a splitter (built into our ICOM AIS rx) and being mounted on our radar/solar arch, about 12 feet above the WL.

Despite these handicaps, we routinely get AIS data from 25 mile distances. Frankly, info from greater range than that isn't of much interest to me.

Save your money for something important!

Cheers,

Jim

PS: Extra bandwidth??? The AIS signal is on one of the normal VHF frequencies, and "extra bandwidth" isn't required. Sounds like a sales gimmick to me!
Agreed. AIS uses only two channels, and they are adjacent to each other: 161.975 MHz ("Channel 87B") and 162.025 MHz ("Channel 88B"). Someone in the Marketing department went overboard with the hype on that one! You need practically no "bandwidth" at all to support AIS.

If you really want a "resonate" antenna at 162 MHz:

1) Remove the radiating element with a hex key.
2) Cut 3/4 of an inch off the bottom.
3) Reinsert the radiating element.
4) Tighten the hex screw.

(Or just cut 3/4 of an inch off the top, and save yourself 3 steps.)

It's amazing they charge EXTRA for an antenna with nothing more than a slightly shorter radiator. What a gimmick!

Your ordinary VHF antenna will do fine. No modifications are really needed.
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Old 24-04-2012, 17:04   #13
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Re: AIS Antenna

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This leads to 2 other thoughts.

First, that you may get an advantage by using the AIS Marine VHF antenna for your "normal" Marine VHF use. Better VSWR at channel 68 with perhaps a little less at channel 16.
Actually, it would be worse, since the channel 68 frequency is actually lower than channel 16.

Eric
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Old 24-04-2012, 17:18   #14
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Re: AIS Antenna

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Actually, it would be worse, since the channel 68 frequency is actually lower than channel 16.

Eric
Drats! that's what assuming increasing ch number leads to increasing freq will get you. (Ah, nice to see that someone is reading and understands what I was saying)

I wonder what the history if ch assignments is.

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Old 24-04-2012, 18:31   #15
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Re: AIS Antenna

Originally, there were 28 marine channels spaced 50Khz apart. Eventually, demand for more channels increased and 28 more channels were added between the original ones so the spacing became 25Khz. Channels 29 thru 59 were designated for land mobile so the new marine channels began at channel 60. Channels in the 60 to 88 range are 60 channels higher than the channel just below it in frequency, i.e. channel 68 (156.425Khz) is just above channel 8 (156.400Khz). Two of the channels, 75 and 76, are never used because they are adjacent to channel 16 in frequency. This is to help prevent interference to channel 16.

Eric
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