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VHF/AIS Antennas; Location, Types, Etc

Old 12-26-2017, 10:43 AM
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Default VHF/AIS Antennas; Location, Types, Etc

I never paid much attention to the antennas on our past boats. I have the opportunity to spec new gear (Furono).

The attached picture shows the mast of a different, but same boat I am getting. The horizontal gap between the two antennas shown is about 3- 4 ft. I know it should be closer to 6 ft. to avoid interference.

The Furono system builder is directing me to the GVA-100 GPS/VHF.

I have read through manuf. docs, and this website, but I still have some basic antenna questions:

1. I cant get the 6 ft. space, so I would like to use a combination AIS/VHF antenna. Does this exist? I dont see specific listings for combo antennas (Shakespeare for example).

2. In addition to the AIS, is the Furono GVA100 also for VHF radio comm's? IE, is this a combination VHF/AIS?

3. Looks like I need to buy the actual Whip (antenna) for the GVA100?
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Old 12-26-2017, 11:05 AM
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Re antennas that can be used for both AIS and VHF:

If you are trying to use one antenna for two transmitters, you have two fundamental problems:

--only one transmitter can use an antenna at a time; unless you have extraordinary money and resources to throw at the problem; and

--the single antenna must be suitable for the two transmitters, assuming they operate on different frequencies; that is, the VSWR of the antenna must be suitably low for BOTH frequencies.

Let's consider the second problem: a VHF Marine Band radio on a ship generally only transmits at the very low end of the band, around 156-MHz. The AIS transponder transmits at the very upper end of the band, 162-MHz. In order to use one antenna for both transmitters, you must get a specialized WIDE-BAND antenna that will have a VSWR that is acceptable (which usually means less than 2:1) at both ends of the band. Such antennas are available, but they are generally larger, heavier, and more expensive than antennas that are tuned for one use or the other. You can decide if you really want to spend the money on one wide-band antenna or two narrow-band antennas.

Now the real problem: EVEN IF you get a big, fat, wide-band antenna that can handle AIS and VHF at the same time, there is no way you can transmit simultaneously with two transmitters into one antenna in any sort of practical sense on a small boat. You can do this if you spend a huge amount of money designing some custom antenna system, and this is done in some instances when multiple transmitters must share a common antenna. But you won't want this on your boat. Too expensive, too complex, and won't work well.

The ONLY practical method for two transmitters to share one antenna is by a very fast electronic switch that lets one transmitter grab the antenna for transmit and stops the other from being connected to it when that happens. When the two radios are receiving, they can share the antenna without harm. As soon as one of them is to transmit, it has to get exclusive access to the antenna.

You can find such devices available now. An example is SRT's em-trak AIS S100 ANTENNA SPLITTER at about $260.

The antenna you mention--an AIS and GPS antenna--is not one antenna but two antennas in one housing. The GPS antenna works at 1575.42-MHz and the AIS antenna at 162-MHz. They are just combined into one mount, and they are not one antenna but two separate ones; that is why there are two transmission lines for them.

I doubt you can share the AIS antenna with your VHF Marine Band radio. It is probably only tuned for 162-MHz and would not work well at 156-MHz. Also, it may be just for receive, or just for Class-B 2-Watt AIS transmission.

If you can share the antenna for VHF and AIS, I don't recommend trying to combine your VHF Marine Band communication radio with an AIS transceiver into one antenna. First, a VHF Marine Band radio is a truly essential communication and safety device, and it should have its own dedicated antenna.

Second, if you install an AIS transponder, you should provide it with its own dedicated antenna. The specifications for the AIS system is the transponder is to have its own dedicated equipment, not integrated with other stuff. For example, that is why the AIS has its own GNSS receiver instead of sharing one with other navigation gear. It is specified to have a dedicated GNSS receiver all to itself. It should have its own transmitter antenna, too.
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Last edited by jhebert; 12-27-2017 at 07:22 AM.
Old 12-26-2017, 11:50 AM
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I have my AIS antenna mounted directly to the cabin roof and my primary voice VHF mounted on a 3.5' mast to provide separation. They are about 7' apart as well. While not ideal, better than if adjacent. I probably would do better if I could find a non-conductive mast and move the AIS antenna to just below the voice VHF antenna.

The GPS antennas are separate and on my radar mast.

I have Morad anntennas https://www.morad.com/collections/antennas Both in the HD versions
Old 12-26-2017, 12:13 PM
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I use an AMEC AIS/VHF antenna splitter, which seems to have worked well. It cost less than $200 when I bought it
Old 12-26-2017, 02:28 PM
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Thanks for the feedback.
Old 12-27-2017, 11:15 AM
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I had similar questions and started this thread:
Is 42" too close? AIS and VHF antennas

You can read through it but the bottom line is no one really knows for sure.

I went ahead with my installation:
- Vesper Marine AIS
- Icom 506 vhf
- Furuno TZT2 12” MFD
-8ft digital antenna VHF antenna
-4ft digital antenna AIS antenna

The antennas are only 42” apart. For me the performance has been terrific. I can pick up the big boys on AIS upto 35 miles away and I pick up other sport boats with AIS upto 15 miles away. That is pretty good considering my 4’ stick’s base is only about 8’ off the water.

I went with a 4’ AIS antenna since that would give me a bit of vertical separation from the VHF.

I am a die hard Furuno fan but for me the Furuno AIS gear is not competitively priced for the recreational market. I am pretty happy with the Vesper transponder and is was ~$300 less than the Furuno. You can also get a full transponder for the price of the Furuno reciever. It’s also interesting that many AIS transponders are all the same but just have different manufacturer’s labels on them. The value of the Vesper is further enhanced since it can also act as a NMEA gateway. However, I found that my heading sensor data was not translated quickly enough to use it for the radar overlay.

Let me know if you have any questions!

v/r
Griff
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Old 12-27-2017, 04:19 PM
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Originally Posted by Magpie View Post
I had similar questions and started this thread:
Is 42" too close? AIS and VHF antennas

You can read through it but the bottom line is no one really knows for sure.
Maybe no boaters know "for sure", but plenty of people familiar with antennas know how they interact with each other. See post #6

Is 42" too close? AIS and VHF antennas
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Last edited by jhebert; 12-27-2017 at 04:27 PM.
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Old 12-27-2017, 06:51 PM
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Fair enough but the question isn’t do the antennas affect each other, rather will the AIS and VHF still work with them less than 9’ apart.

At least for me, they do work (42” spread on a 4’ AIS and 8’ VHF sticks). Mileage for others may vary...
Old 12-28-2017, 09:43 AM
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It is nice to know that there is an anecdotal report about two antennas tuned for approximately the same frequency and mounted 42-inches apart (and I assume parallel to each other) providing sufficient function to satisfy one user--I think that is what your particular installation and your assessment of how it works can affirm.

The qualifying criterion that a system or device "still works" is a bit vague. It would be better to know more quantitative data, for example: what is the effect of one radio transmitting a signal on the other radio's receiver ability to receive signals? What is the effect on VSWR on the antennas' transmission lines?

A wavelength at 156-MHz is about 75-inches. A separation of 42-inches is then only a bit more than a half-wavelength. Typically two antennas that are closely resonant and only a half-wavelength apart and in parallel will have a significant effect on each other.

Vertical spacing is much more effective--where the antennas are in-line with each other (called collinear). For two vertically polarized antennas mounted so their ends are separated by 42-inches of vertical spacing and in-line (collinear), the interaction is less. Collinear mounting and vertical spacing is preferred over parallel mounting and horizontal separation.

Earlier BadgerS described his installation in which his two antennas have a vertical separation of 42-inches but are not collinear; the two antennas are 84-inches apart horizontally. This is different than being collinear. This arrangement is sometimes called parallel in echelon. One antenna is directly in the main field of the other, whereas when mounted collinear the antennas are each in the other's null or minimum field. Exactly how they will interact is difficult to predict for arrangements like this.

The general effects of having another closely-resonant antenna near the first antenna are:

--change in the radiation pattern

--change in the radiation resistance.

The change in radiation pattern is difficult to measure. The radiation pattern that results could have significant nulls created by the presence of the other antenna. Whether or not the change is noted by the operator in using the radio is hard to predict, as the orientation of the boat and the bearing to another station are entirely random occurrences.

The change in radiation resistance can be more easily measured; the VSWR on the transmission line will change when the second antenna is added to the boat.

To make things even more complicated, the effect of one antenna near another during transmitting is not the same as during receiving. This is because the source of the induced currents in the two antennas when receiving signals comes from far away, from a distance transmitter, while the effect of the two antennas and their induced currents during transmitting comes from one of the antennas.

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Last edited by jhebert; 12-28-2017 at 10:25 AM.
Old 12-28-2017, 10:01 AM
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Vesper makes a 3" whip antenna that is tuned in between DCS and AIS frequencies to try to work best as a single antenna solution. Their splitter also has a gain amp built into it to help compensate for signal loss.

If you are going to use a single antenna i recommend using either the vesper antenna or one tuned for VHF. VHF is the more critical safety feature. You only care about AIS at short range anyway so not seeing those boats 10 miles away doesn't matter.


If you do use separate antennas, be sure to mount them far apart as possible. I have had a customer who installed the antennas too close and the 25w coming out of his VHF damaged the AIS transponder, or so the manufacturer said.

Last edited by Amity83; 12-28-2017 at 10:10 AM.
Old 12-29-2017, 05:48 PM
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Originally Posted by Amity83 View Post
Vesper makes a 3" whip antenna...
I cannot imagine that an antenna that is only three inches long could be useful for any sort of transmission at 150-MHz. I think you must be mistaken about the antenna length.
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Old 12-30-2017, 09:26 AM
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Originally Posted by jhebert View Post
I cannot imagine that an antenna that is only three inches long could be useful for any sort of transmission at 150-MHz. I think you must be mistaken about the antenna length.
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Good catch. its 3'

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