Best practice for VHF & AIS antenna placement

[Auspicious]

Originally Posted by Jammer

The received wisdom seems to be that the best VHF antenna is a 1/2 wave, center-fed dipole placed at the top of the mast.

Not that I've seen. Most are end fed either quarter wave, five-eighths wave, or stacked arrays (which are usually center-fed dipoles that trade lots of gain from the array for the issues of poor transmission line routing).

Originally Posted by Jammer

What is the best type and placement for a separate AIS antenna on boats with but a single mast?

Vertical separation is more important than horizontal separation. Good places for a second antenna are on a spreader, on the backstay, on a radar pole, on a second mast, on an arch or bimini, or on the pushpit.

Originally Posted by hellosailor

If the VHF (regardless of type) antenna is at the masthead, the best place for a second VHF antenna (which is what the AIS really is) would be below the VHF antenna, so there is very little interaction between the two.

Correct.

Originally Posted by a64pilot

If your going to use a separate AIS antenna, consider Vesper’s dual antenna that is supposed to work for both. That way you would have a back up VHF if needed, although I believe a regular AIS antenna would work well enough as a VHF antenna.

Marine VHF and AIS antennas are not really different. Consider that the marine VHF band runs from 156.050 to 162.000 MHz (channel center frequencies). AIS runs on 161.975 MHz and 162.025 MHz on what were once channels 87B and 88B. No one thought they needed a different antenna for 87B and 88B - why a special antenna for AIS?

A quarter wave antenna ranges from 17.994" to 17.333" (half an inch difference) across the marine VHF band. A decent antenna design, centered on channel 16 at 156.800 MHz will exhibit good performance (measure SWR) across the entire marine VHF band.

Originally Posted by svmobert

Is there a specific reason you want a separate antenna? We have a splitter under the nav station for VHF, AIS, and broadcast Radio. It works extremely well and we have very long range AIS and VHF operation in practice. There have been no interoperation issues in the 4 years we have had the boat with this setup.

I don't like splitters. Commercially available splitters add insertion loss and an impedance bump (which means higher SWR). The addition of a low noise amplifier (LNA) does avoid some of the loss although you should still look at the increase in noise floor and signal-to-noise ratio (SNR). If you are going to use an LNA it should be as close as possible to the antenna (increasing SNR at the input) which is not the case in almost all marine installations.

In addition to insertion loss and impedance changes there are failure modes added with a splitter. The splitter will give preference to one transmitter or the other (usually voice over AIS). That means either switching the secondary transmitter to a dummy load or depending on the foldback self-protection circuitry of the secondary transmitter from damage when transmitting into an open load. If the switching circuitry fails the transmission of either radio may be routed directly into the receiver of the other radio which will certainly result in irreparable damage.

Commercial requirements for Class A AIS are an independent antenna and associated transmission cable. Recreational boaters would be well served to take the same approach.

[continuouswave]

Originally Posted by Auspicious

...Marine VHF and AIS antennas are not really different. Consider that the marine VHF band runs from 156.050 to 162.000 MHz (channel center frequencies). AIS runs on 161.975 MHz and 162.025 MHz on what were once channels 87B and 88B. No one thought they needed a different antenna for 87B and 88B - why a special antenna for AIS?...

Marine Band VHF ship radios never used 87 or 88 as simplex channels. The ship transmitter transmitted at 157.375 or 157.425, and received the shore station at 161.975 and 162.025-MHz

The AIS transmitter will be transmitting at 162-MHz. That requires an antenna with good VSWR. That is the difference that now has resulted in antennas specifically tuned for good VSWR at the AIS frequencies.

[Dockhead]

Originally Posted by Auspicious

Originally Posted by Jammer
The received wisdom seems to be that the best VHF antenna is a 1/2 wave, center-fed dipole placed at the top of the mast.



Not that I've seen. Most are end fed either quarter wave, five-eighths wave, or stacked arrays (which are usually center-fed dipoles that trade lots of gain from the array for the issues of poor transmission line routing). . . .

The Shakespeare Galaxy XT Little Giant is a center fed dipole, and is pretty widely used by sailors. It was recommended to me by another ham, and I have had great results from it.

I once got a "weak but readable" signal report across the English Channel -- 60 miles! -- using ONE WATT transmission power, using that setup.

I don't know what's wrong with the transmission line routing. AFAIK, the radiators are tubular, and the feed line just goes up the middle of the lower one. Quite elegant. Only problem with that is that the antenna then has a pigtail rather than a socket connector, so you have to whack it off and solder on a proper connector, then telecomm-wrap the connection so it's safe to have up there. Small price to pay for having such a good antenna with no ground plane issues. I'm very pleased with it, and I've never had to unwrap the Type N connectors I put on up there -- knocking on wood.

[TreblePlink]

The little giant and many other similar types are technically known among purists as (end fed) sleeve dipoles. The only difficulty with this type is that the feedline exits the bottom tubular element, so is tightly coupled, sometimes resulting in (non-transverse) coax currents, distorting the vertical pattern. Two methods are often used to minimize this; adding another quarter-wave de-coupling sleeve below, and / or using ferrite "chokes" on the line just below the bottom sleeve.

They are popular for marine use because they are among the best at not needing a large metal mounting surface.

The other common sailboat antenna is a 1/2 or 5/8 wave stainless vertical whip, with a matching coil inside the base. These require some form of metal below, if only the coax cable. These work fine on the masthead, although many would be surprised at the true vertical pattern distortion caused by the metallic stays. One advantage with this type is that the center frequency can be adjusted with the length of the whip.

Best practice with any antenna installation is to use an SWR meter or something more elaborate to reveal the frequency where the impedance match is best, and where 2:1 SWR limits are. Otherwise, you don't really know what you've got.

[Dockhead]

Originally Posted by TreblePlink

The little giant and many other similar types are technically known among purists as (end fed) sleeve dipoles. The only difficulty with this type is that the feedline exits the bottom tubular element, so is tightly coupled, sometimes resulting in (non-transverse) coax currents, distorting the vertical pattern. Two methods are often used to minimize this; adding another quarter-wave de-coupling sleeve below, and / or using ferrite "chokes" on the line just below the bottom sleeve.

They are popular for marine use because they are among the best at not needing a large metal mounting surface.

The other common sailboat antenna is a 1/2 or 5/8 wave stainless vertical whip, with a matching coil inside the base. These require some form of metal below, if only the coax cable. These work fine on the masthead, although many would be surprised at the true vertical pattern distortion caused by the metallic stays. One advantage with this type is that the center frequency can be adjusted with the length of the whip.

Best practice with any antenna installation is to use an SWR meter or something more elaborate to reveal the frequency where the impedance match is best, and where 2:1 SWR limits are. Otherwise, you don't really know what you've got.

"To purists . . " -- you are being polite! What he really meant was "to those who actually understand how they work." Thanks for the excellent information!

[Emouchet]

if you go with a dedicated antenna for the AIS, you should then tweak it for the exact AIS xmit/rec frequency.
It should be center tuned to 162 MHz, so a 33.5” whip.

a bit shorter than a regular VHF antenna.

[TreblePlink]

Originally Posted by Emouchet

if you go with a dedicated antenna for the AIS, you should then tweak it for the exact AIS xmit/rec frequency.
It should be center tuned to 162 MHz, so a 33.5” whip.
a bit shorter than a regular VHF antenna.

Trouble is, depending upon mounting and other nearby metal objects, as well as the whip matching network design, the actual required length can depart significantly from the theoretical length...

Normally, before cutting the whip, measure to find the current center frequency, and adjust from there - if it is too low, trim an eighth and re-measure, etc. If it is too high, you need a longer whip.

If you measure 2:1 or better SWR at all your operational frequencies, you're OK. 1.5:1 is a bit better but generally not required for good results.

[redsky49]

Originally Posted by Paul Elliott

This of course depends on what range you want from your AIS system. But typically you can do with the shorter range you will get from an antenna mounted lower than the masthead. My AIS antenna is on the upper spreader now, but I got adequate performance from an antenna on the stern pushpit rail....

First, how did you mount this? Any pics?
And second, aren't you you impeding the performance due to blocking of the signal by the mast and rigging?
Have you noticed any issues?
Thanks

[Paul Elliott]

Originally Posted by redsky49

First, how did you mount this? Any pics?
And second, aren't you you impeding the performance due to blocking of the signal by the mast and rigging?
Have you noticed any issues?
Thanks

Sorry, I can't find any pics at the moment, but I had a rigger mount the antenna to the upper spreader, using a bracket he either found or made. He put a cover over the coax and ran the cable inside the mast (without a conduit, but that hasn't caused any problems).

The tip of the antenna gets close to the shroud as the shroud angles in towards its mast attachment, but neither the shroud or the mast appear to create any serious detuning or directionality. The mast won't create an actual dead zone, since it's got a small diameter relative to the VHF wavelength, and it's not of a length or distance from the antenna to act as a "reflector" or "director" element. For the same reasons neither will the shroud, and also because it's at an angle relative to the antenna. These pieces of metal are probably causing some antenna detuning because of proximity and capacitance / inductance effects, but not enough to be an issue in my case. VALIS is a fairly big boat, this might be more of a problem with the geometry of a smaller boat.

One of these days I should tie an insulating line between the tip of the antenna and the shroud above, since the tip of the antenna has poked a few holes in my spinnakers. Or, I should learn to not fly the kite when the wind is light and flukey, but sometimes even a quarter knot is better than zero.

[Jim Cate]

Wonderful theoretical discussions...

But guys, as I reported above, using a standard VHF masthead antenna fed with good coax and using the Vesper splitter (or what ever you want to call it) we get excellent performance from both VHF and AIS: AIS class A reception routinely > 35 miles, class B 8-12 miles. No specific data on how far ships receive us, but other yachts report similar 8-12 mile range.

I have a backup antenna on the radar arch (~ 12 feet above the water). With it direct (no splitter) I get ships at ~20 miles and class B at 5-10. Not as much data with this rig, but it is clear that it is inferior to the masthead. I have no useful data for VHF voice range, but it seems normal to me (don't use it very much).

So, while I admit that there is an additional failure mode using the splitter, its performance seems damn good. Should it fail, it is simple to remove it from the system and carry on. Could a different system using two antennas work even better? Quite possibly, but it is questionable that this is a significant gain in mission accomplishment.

Jim

[Paul Elliott]

FWIW, on a smaller powerboat I have a Garmin AIS-600 transponder that shares an 8-ft "6 dB" whip antenna with the VHF transceiver, using the built-in Garmin splitter. It works just fine. I'm sure I could improve the performance, but there's no need and I've got more interesting things to do.

[jkindredpdx]

My VHS is on the Masthead, AIS is on the aft Bimini Arch... both working great so far.

[DotDun]

Originally Posted by Jim Cate

Wonderful theoretical discussions...

But guys, as I reported above, using a standard VHF masthead antenna fed with good coax and using the Vesper splitter (or what ever you want to call it) we get excellent performance from both VHF and AIS: AIS class A reception routinely > 35 miles, class B 8-12 miles. No specific data on how far ships receive us, but other yachts report similar 8-12 mile range.

I have a backup antenna on the radar arch (~ 12 feet above the water). With it direct (no splitter) I get ships at ~20 miles and class B at 5-10. Not as much data with this rig, but it is clear that it is inferior to the masthead. I have no useful data for VHF voice range, but it seems normal to me (don't use it very much).

So, while I admit that there is an additional failure mode using the splitter, its performance seems damn good. Should it fail, it is simple to remove it from the system and carry on. Could a different system using two antennas work even better? Quite possibly, but it is questionable that this is a significant gain in mission accomplishment.

Jim

This begs the question that has been discussed previously. Why do you need to know about a ship 35 miles away? IMO, there is zero navigational value in knowing such.

[Emouchet]

Personal experience:
Do NOT try to switch input connection on any splitter between the VHF input and the AIS input.
At some point, I has some trouble with a non working splitter, and trying to troubleshoot, I swapped the VHF with the AIS. As soon as I tried to xmit with the VHF on 25w power via the AIS input of the splitter troubleshooting was done: it fried the entire splitter input stage... Pretty expensive experimentation