VHF- AIS antenna placement. mast head or arch?

[skipmac]

Originally Posted by Jim Cate

FWIW: We have antennas at the masthead and on the arch. Haven't done any VHF comms comparison, but the masthead antenna gives consistent 35-50 mile reception of class A AIS while the arch mounted antenna peters out at around 20-25. No idea about changes in the received range of our AIS transmissions. The arch antenna is direct with a pretty short cable. The masthead antenna goes via a Vesper splitter and a long (~30 meter) RG8 cable. By c hoice we use the masthead antenna, keeping the other as a backup.

Jim

Hi Jim,

Have you noticed any downside to using the Vesper splitter?

For example, what happens to the AIS if you are in the middle of a long TX/RX session on the VHF? Do you get occasional dropouts on the VHF when the AIS needs the antenna or does the AIS not update until you stop using the VHF? Any idea how long the AIS needs to do its thing, like less than a second or several seconds or?

Thanks
Skip

[skipmac]

What about placing an AIS antenna on the spreaders?

Will give much better height than an arch, separate the AIS and VHF needs and you will also have a backup antenna for the VHF just in case.

I do understand that the proximity of the mast and upper shroud will have some impact on the AIS, mainly I think on the transmission. I think the main influence will be altering the radiation pattern of the antenna giving less power at certain angles but on a small boat at sea with normal yawing I this doesn't seem to be a fatal problem.

[TreblePlink]

Originally Posted by AlanR

At 160 MHz, RG58 loss is about 6 dB per 100 ft, say 3 dB to the top of your mast. That is a 50% power loss. Range is a square-law function of power, so the 2 dB increased loss over a 'good' co-ax might result in a decreased range of about 20 percent, 25% should your good coax be lossless (Ha!).

Alan

This is true in free-space, but with 50 foot antenna heights, propagation is limited primarily by line-of-sight realities.

[jmschmidt]

Usually the masthead whip VHF antennas are high-gain and have loaded bases (the little round can at the bottom) which compresses the signal beam to a very narrow disk shape of a few degrees. This will give increased range, not only because of its height but because of the compressed signal beam. This may be of limited use in rolling seas because the narrow signal beam oscillates up and down on the receiving antenna of the station being called and can produce and intermittent contact. Two boats, both in rolling seas, trying to call each other will compound the problem. In this case a standard-gain 6’ or longer deck mounted antennae not only mitigates the rolling moment of the boat, it projects a wider signal bean which may work better in such conditions but will not have the range. I have both types of antennas connected with a coaxial selector switch. This also provides a safety factor if the mast comes down. I may still have my deck mounted antenna, unless it get wiped out by the falling mast.

[J Clark H356]

I have an AIS DigitalYacht transmit only Class B. I have it primarily on the masthead and use my normal VHF antenna with a splitter and have consistent range of 30-35 miles. When my mast was down on a trip down the Tenn-Tom waterway, I used a dedicated AIS antenna with good cable mounted on a 10 foot piece of PVC pipe (2 5 ft long 1" pieces screwed together) which gave ma about 13 to 14 feet above the waterline and had about 8 miles.
Using my normal VHF antenna with the splitter I find no conflict with the AIS.

[IslandHopper]

Originally Posted by J Clark H356

I have an AIS DigitalYacht transmit only Class B. I have it primarily on the masthead and use my normal VHF antenna with a splitter and have consistent range of 30-35 miles. When my mast was down on a trip down the Tenn-Tom waterway, I used a dedicated AIS antenna with good cable mounted on a 10 foot piece of PVC pipe (2 5 ft long 1" pieces screwed together) which gave ma about 13 to 14 feet above the waterline and had about 8 miles.
Using my normal VHF antenna with the splitter I find no conflict with the AIS.

Are you sure?

The only "transmit only" AIS units i'm aware of are Type 1 AtoN'S, unless you are a lighthouse or some other Aid to Navigation these are of no use to you

[Paul Elliott]

Originally Posted by jmschmidt

Usually the masthead whip VHF antennas are high-gain and have loaded bases (the little round can at the bottom) which compresses the signal beam to a very narrow disk shape of a few degrees.

This isn't quite true. The vertical beamwith is really just dependent on the physical length of the whip.

My sailboat masthead antenna is a short half-wave whip that has a matching coil in the can, but this is just an impedance-matching circuit. This is a typical sailboat antenna and the radiation pattern is fairly broad. It is called a "3dB" gain antenna, but this is actually 3dBi (three Decibels more than a theoretical isotropic radiator). The high-gain whips (often 6dBi) seen on powerboats and catamarans have longer elements and a tighter pattern.

The matching element in that little can has essentially no effect on gain or pattern. It's just there to match the impedance of the antenna to your 50-Ohm coax cable.

[J Clark H356]

Originally Posted by J Clark H356

I have an AIS DigitalYacht transmit only Class B. I have it primarily on the masthead and use my normal VHF antenna with a splitter and have consistent range of 30-35 miles. When my mast was down on a trip down the Tenn-Tom waterway, I used a dedicated AIS antenna with good cable mounted on a 10 foot piece of PVC pipe (2 5 ft long 1" pieces screwed together) which gave ma about 13 to 14 feet above the waterline and had about 8 miles.

Using my normal VHF antenna with the splitter I find no conflict with the AIS.

OOPS! I said “transmit” when I really meant receive! When I transmit on my VHF with my masthead antenna, I see no degradation in my signal.

Thanks for pointing out my obvious goof. What I said was not at all what I meant to say!

[TreblePlink]

Originally Posted by Paul Elliott

This isn't quite true. The vertical beamwith is really just dependent on the physical length of the whip.

My sailboat masthead antenna is a short half-wave whip that has a matching coil in the can, but this is just an impedance-matching circuit. This is a typical sailboat antenna and the radiation pattern is fairly broad. It is called a "3dB" gain antenna, but this is actually 3dBi (three Decibels more than a theoretical isotropic radiator). The high-gain whips (often 6dBi) seen on powerboats and catamarans have longer elements and a tighter pattern.

The matching element in that little can has essentially no effect on gain or pattern. It's just there to match the impedance of the antenna to your 50-Ohm coax cable.

Exactly correct.

[continuouswave]

Regarding range of AIS signals:

A recreational boat will typically have a CLASS-B AIS transceiver, and a large commercial vessel will have a CLASS-A AIS transceiver. In assessing the radio range or distance between these two categories of vessels, consideration has to be given to the transmitter power. CLASS-A transmitters are 12.5-Watts, and CLASS-B transmitters are 2-Watts. This gives the CLASS-A signals an advantage of about 8-dB. The implication of this is as follows:

--assume everything else were the same, that is, the antenna height, the antenna gain, the transmission line loss, the receiver sensitivity;

--the CLASS-A vessel has an 8-dB stronger signal, which generally translates into an increase in radio range

The amount the radio range will increase by adding 8-dB to the signal power will depend on the nature of the path between the stations. For propagation over open water one can estimate the path loss will vary with distance by

dB = 40*log(d)

Solving this for d when db=8 gives 1.58

This means the path distance can be 1.58-times longer for a signal with 8-dB more power. In simple terms, a CLASS-B vessel is going to hear a CLASS-A vessel's AIS signal for a long time before the CLASS-A vessel hears the CLASS-B, when the two vessels are far enough apart to be at the very limit of the radio communication path. That last part is the important part. If the vessels are only a few miles apart, the path loss is not going to put the signals at the threshold of being received. While the extra gain helps when the vessels are far apart, it won't have as much effect when they are closer.

Another factor is how often the vessels transmit. A CLASS-B vessel underway at less than 14-knots only transmits every 30-seconds, while a CLASS-A vessel at the same speed range transmits every 3.3-seconds if changing course and every 10-seconds if on steady course. This means the CLASS-B vessel is transmitting something like three-times to almost ten-times more often than the CLASS-B. The more often you transmit data the more likely it is to be received, so, again, the basic methods of AIS tend to favor the CLASS-A vessel being received and detected by the CLASS-B vessel before the other way around.

If into this analysis we add the notion that the CLASS-B vessel has a much lower antenna height, probably a lower gain antenna, and is likely to have much more motion on the boat (causing the antenna radiation pattern to be skewed away from the horizon), we can find other influences that will tend to reduce the visibility of the CLASS-B vessel to the CLASS-A vessel.

There is a newer category of Class-B AIS transceivers called Self-Organizing, which will have increased power (5-Watts). This decreases the power difference to 4-dB, making the range factor 1.26-times.

[TreblePlink]

It is interesting to note that when using the standard 34 or so inch half-wave vertical at mast top, with 2 shrouds and two stays, you would have to heel well in excess of 45 degrees before reaching the extents of the elevation pattern. Below is an NEC (MiniNEC) model showing actual elevation pattern for a typical installation:

[Added] The elevation pattern is uneven. This is normal and quite common - an even pattern is very hard to achieve except in free space without the presence of other metal. In this case, due to the shrouds and stays carrying induced currents, the pattern is a bit lobed.

[continuouswave]

'PLINK--thanks for posting your calculated vertical radiation pattern of a typical mast top mounted antenna on a sailboat. That is an interesting plot in that the gain towards the horizon in down quite a bit from maximum gain. The main lobe is oriented skyward at 13-degrees, where it is wasted. The best range would come from radiation at the horizon or very close to it. But in your model the gain at the horizon appears to be down about 4 dB.

It is not intrinsic that all vertical antennas will fail to deliver their maximum gain at the horizon, and engineering and design of practical antennas is usually aimed at producing maximum gain at the horizon. In some cases of antennas with really high mounting height, say atop a tower of 1,000-feet, it is often desirable to aim some radiation below the horizon, which is known as beam tilt.

Here is a link to a practical vertical antenna of 0-dBd gain, a halfwave omni, and it has a bit of beam tilt. The main lobe peaks slightly below the horizon. The pattern is otherwise quite clean. Of course, this is probably a predicted pattern without allowance for nearby conducting surfaces. This pattern comes from the data page for the antenna. Cf.:

http://www.sinclairtechnologies.com/...-SF4SNM-DI.pdf

[TreblePlink]

A pattern like the Sinclair is very basic free-space model without nearby shrouds and stays. If I modeled a simple vertical dipole in free space it would look like that.

The power of NEC is that it can include the true interaction of nearby conductors and reveal patterns that closely agree with actual anechoic chamber measurements:

https://en.wikipedia.org/wiki/Numeri...magnetics_Code

[continuouswave]

Originally Posted by TreblePlink

...The power of NEC is that it can include the true interaction of nearby conductors and reveal patterns that closely agree with actual anechoic chamber measurements...

Yes, I am familiar with the method, but the outcome of the model depends on the input. Unless the input is carefully modeled after the actual, the output won't really reflect reality.

This is particularly true for end-fed half-wave antennas, as they really can be tricky to model.

For your model, you don't describe how the antenna was being fed, how the transmission line was decoupled from the antenna currents, and other influences. If you just modeled a half-wave vertical being fed at its center by a perfect exciting source, you may not have created an accurate model of any real world practical antenna. The real science of using NEC is what you put into it, not what it calculates from your input.

The Sinclair antenna is not a free-space model because it has beam tilt. Sinclair probably engineered that into the antenna. For VHF antennas intended for communication with local stations, it is far better to have a bit of radiation below the horizon than to have the peak far above the horizon.

[TreblePlink]

It is a typical end-fed half-wave vertical, with 4 very long sloping radials which represent stays & shrouds. The keyword here is "typical." It is representative of real world patterns.

Not every one will be exactly like this; the point is that with the most common mast-top antenna, the vertical beamwidth will be wide, and with lobes. Not to worry much about heeling.

If you get really lucky, you might have maximum radiation (and reception) at the horizon. But maybe not.