VHF vs AIS antenna

[GTom]

Originally Posted by Riddle

Hi,
Regular VHF antenna's are tuned tot the middle of the band. AIS frequencies are with 162 MHz at the edge of the band.
Some AIS transponders do not like high SWR values (> 2,0) This can occur as the SRW for a particular antenna at the edge of the band is not optimal.
Fiberglass antenna's have - because of their construction - a slightly "broader" frequency spectrum and are less likely to show this effect as stainless steel whip antennas. Thin whip antenna's have a slightly "narrower" optimum frequency range.
There is a simple solution: In case of a stainless steel whip antenna, you can cut of approx 1/2 to 1 inch from the top of the rod. This will move the centre frequency (the resonance frequency) slightly up to the desired 162 MHz.
The disadvantage of this operation is that you cannot reverse it, and the antenna is not optimized for some channels on the lower edge of the VHF band i.e. ch 01.
But since this shortened antenna is only used for AIS there is no problem.
One of the best VHF antenna's on the market is the Celmar 0-1 (made in Denmark). Reasonable priced too. Cutting the top is easy.

Interestingly the Glomex RA109 is a stainless whip but it should work fine on both ch16 and on the AIS frequency:


Doing my homework, found some longer, 6' (1.5m) and even 9' (!!!, 2.8m) antennas with 3dB amplification. How can such length result in a 3dB radiation pattern?

[continuouswave]

Originally Posted by GTom

How can such length result in a 3dB radiation pattern?

All gain in an antenna occurs by eliminating radiation in some directions and concentrating radiation into one main lobe. There is no overall gain in radiated power, just a gain in one main lobe relative to an antenna that radiates equally in all directions.

The change in radiation pattern occurs due to constructive and destructive combining of radiation from different parts of the antenna assembly. In the case of vertical monopole antennas, as the radiator length increases there are more high-current points for radiation than on a short antenna.

Manufacturer claims of gain often include speculative gain based on the presence of the antenna above a perfectly conductive and infinite ground that provides a reflected reinforcing radiation.

In essense, a marine antenna advertised as having 3 dB gain really has 0 dB gain compared to a half wave antenna in the same environment.

[CarlF]

I believe a "tuned" AIS antenna would be 0.6" shorter than a standard 3ft VHF whip. Metz used to sell an antenna that was designed to be trimmed to length.

But, as mentioned, the tiny difference in performance is overwhelmed by the low height of a rail mounted antenna. Using a high quality splitter (and good coax) at the masthead would be far better.

That said, any AIS is a great improvement over no AIS. After it is installed, do hail other boats (including ships) to see how far they can see you. For some reason, sailboats have wildly different transmit ranges. Ships see me at 10+ miles but on a friend's boat it's less than 3 miles. This really isn't enough warning for a 900 ft container ship doing 22 knots.

[GTom]

Originally Posted by continuouswave

All gain in an antenna occurs by eliminating radiation in some directions and concentrating radiation into one main lobe. There is no overall gain in radiated power, just a gain in one main lobe relative to an antenna that radiates equally in all directions.

The change in radiation pattern occurs due to constructive and destructive combining of radiation from different parts of the antenna assembly. In the case of vertical monopole antennas, as the radiator length increases there are more high-current points for radiation than on a short antenna.

Manufacturer claims of gain often include speculative gain based on the presence of the antenna above a perfectly conductive and infinite ground that provides a reflected reinforcing radiation.

In essense, a marine antenna advertised as having 3 dB gain really has 0 dB gain compared to a half wave antenna in the same environment.

Thank you! The typical half-wave antennas targeting yachties are always advertised as 3dB and as length defines the gain, I was surprised to see some much longer (5/4ʎ?) motorboat or ship antennas also listed as 3dB.

Originally Posted by CarlF

But, as mentioned, the tiny difference in performance is overwhelmed by the low height of a rail mounted antenna. Using a high quality splitter (and good coax) at the masthead would be far better.

That said, any AIS is a great improvement over no AIS. After it is installed, do hail other boats (including ships) to see how far they can see you. For some reason, sailboats have wildly different transmit ranges. Ships see me at 10+ miles but on a friend's boat it's less than 3 miles. This really isn't enough warning for a 900 ft container ship doing 22 knots.

Not on the rail for sure. I plan a stern mount but above my pole mounted radome, about 12' above sea level. Actually a reason why I am eyeing longer antennas that this 12' could be made more - obviously don't want a narrow lobe on a sailboat though... Mast+backstay mounts I give a miss for the moment as I'd like something that stays on when the mast decides to quit.

A splitter, two additional connectors and 30' more RG213 will easily halve your signal strength compared to a dedicated stern mounted solution.

The ship vs friend's boat riddle: ship has pro setup including 6dB antenna 100+feet above sea level, LMR400 or at least RG213, pro fitted connectors. Friend's boat: RG58...

[Dulls]

Same as us and no problem. Identical set up to us.

[continuouswave]

The VHF Marine Band allocation is 156 to 162-MHz. Antennas for shipboard use are NOT tuned the center of the band because all ships transmit between 156 to 157.400-MHz, the low end of the band. Ship station antennas are usually tuned to be best around 156.8-MHz.

The automatic identification system (AIS) operates on two channels, just above and just below 162.000 MHz, the extreme high end of the band.

In some countries, notably the USA, channels in the middle of the VHF Marine Band are allocated to other services, such as public services or railroads.

All antennas have a bandwidth for their gain and their input impedance. The antenna is usually designed to have the best match to 50-Ohm transmission line. The degree of mismatch is indicated by the antenna transmission line VSWR. The range of frequency for a VSWR of 2:1 or less is generally considered the antenna’s bandwidth.

The radiation pattern and gain of the antenna also varies with frequency, but generally these do not change as rapidly with frequency as VSWR.

Antenna VSWR bandwidth is generally proportional to the diameter-to-length ratio of the radiating element, so fatter radiators tend to have wider VSWR bandwidth.

In the context of the discussion about use of one antenna for both ship station transmitters and AIS transmitters, the antenna tuning is probably moved to be more toward the middle of the band so that the VSWR at either end is not greater than 2:1, and the diameter of the radiating element may be increased compared to ship transmitter-only antennas.

[Sportswagoneer]

[QUOTE=GTom;3188869]Interestingly the Glomex RA109 is a stainless whip but it should work fine on both ch16 and on the AIS frequency:


Thanks GTom for this excellent graph! Do you have a source for these data?
(I’ve scoured the internet for curves like these - but neither “antenna testers” nor the websites for Shakespeare, Glomex or the others seem to publish them.

I’m currently looking for a new masttop combined AIS and VHF antenna using the Garmin 600 splitter/transponder. From this graph, the Glomex RA109 should be the obvious choice!

[Sportswagoneer]

Update: Found the source here:
https://media1.svb-media.de/media/sn...2011-03-16.pdf

[akopac]

Got a Vesper VM-VA159 and digital yacht B+ AIS & & splitter. Mounted 64 ft. Up on main mast. I can’t say enough about this setup. My range is over 65 lies on the vhf. To sure on AIS, but the satellites pic it up when they go over. SWR looks looked pretty good when I checked it as well. 1.3

[NYSail]

I went with a Garmin ais that has a built in splitter…… no issues. Works great and simple.

Greg

[goboatingnow]

Agree I have a AIS700 with integral splitter. Fantastic

[Shaneesprit]

I have an AIS 650 with an AIS 100 splitter
Not sure on the whip, but Whip installed 3 years ago. AIS installed this year including moving the VHF to be next to the AIS transponder.
I have never received an AIS communication, Splitter is working.
Have to decide to either change the Whip and associated cabling or to fit a dedicated AIS antenna and remove the AIS and put the VHF back wher it used to be.