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Old 10-10-2015, 19:28   #1
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Tuning VHF antenna for use with AIS

In recent times I have installed three AIS units that run through a splitter to share the antenna with the VHF radio.

On one of these jobs the antenna needed replacing and, when the job was done, the SWR reported by the AIS was 1:1 (that is, the antenna was perfectly tuned for the frequency being transmitted). On the other two installations where I used the existing antenna the SWR was quite a bit less favourable. When looking at an online AIS tracker, only the 'good' boat was visible. This tells me that the signal from the two 'not-so-good' antennas was not visible to the web-connected base station roughly 4 miles away. All boats were visible to other boats around them with different ranges (roughly) in proportion to the quality of the SWR.

It seems to me that when normal voice-only VHF comms takes place we don't pay too much attention to the SWR quality of the boats antenna. It either works or it doesn't, and most boaties would tend to blame the receiving station for not being on receiving watch when they can't get through.

When something like AIS is installed which gives real numbers/performance to quantify the antenna performance, it becomes obvious that something is not up to scratch.

I'm looking to this forum to ask "what next?". Should I just blindly change the antenna - which involves a trip up the mast - and hope that it comes good? Should I go prowling for connector loss and cable quality issues? Are there any more tests I can do to be a little more deterministic about the cause of the signal loss? What experiences do others have in this area?

TIA

Steve
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Old 10-10-2015, 20:34   #2
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Re: Tuning VHF antenna for use with AIS

Hi Steve,

the frequencies used for AIS are

AIS 1 - 161,975 MHz (Kanal 87B) and AIS 2 - 162,025 MHz (Kanal 88B) (as you might know)

For transmitting on VHF from the ship you use 156MHz - 157.4MHz. The 'normal VHF antenna' will cover this range with some sacrifice but there might be antennas out there that are more narrowband than others so an AIS antenna 'tuned for 162MHz' might help.

As you said you might also look for the fault in the cable and or connectors.

Regards,

Carsten
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Old 11-10-2015, 00:23   #3
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Re: Tuning VHF antenna for use with AIS

Can anyone tell me the typical range for an AIS transmitter/receiver. I know that this would be dependent on both the transmitting boat and the receiving boat but it seems to me that we have recently lost some distance in our ability to see other boats.

Presently we can see boats on the AIS at least up to 6 miles away (not to many boats around with AIS transmitters so hard to know if there is in fact anyone farther actually transmitting) and we can see the base station at approximately 11 miles away. Seem to remember being able to see boats up to 20 miles away previously. We definitely had a problem with our vhf antennae in April which we had repaired (couldn't find a similar mounting one in Darwin) but when we reinstalled it didn't seem any better. Problem is that trying to drag anyone out to the boat here to test the radio is like pulling teeth and we are planning on leaving on Thursday for the passage over the top to the east coast.

Would be thankful for any help that is received.
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Old 11-10-2015, 04:10   #4
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Re: Tuning VHF antenna for use with AIS

Originally I tried my standard VHF aerial with a splitter. With the Raymarine setup every time I transmitted the AIS was cut off and I got a failure alarm on the chart plotter . Both Raymarine and the splitter manufacturer told me it could not be disabled.

I tried AIS Satellite tracking. It was very poor. I then installed an AIS tuned aerial separate to my VHF radio. The strength was considerably better as demonstrated by Sat AIS tracking and i see ships at nearly 100nm. Aerial about 20m above the ocean.


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Old 11-10-2015, 05:14   #5
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Re: Tuning VHF antenna for use with AIS

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Originally Posted by DavefromNZ View Post
Originally I tried my standard VHF aerial with a splitter. With the Raymarine setup every time I transmitted the AIS was cut off and I got a failure alarm on the chart plotter . Both Raymarine and the splitter manufacturer told me it could not be disabled.

I tried AIS Satellite tracking. It was very poor. I then installed an AIS tuned aerial separate to my VHF radio. The strength was considerably better as demonstrated by Sat AIS tracking and i see ships at nearly 100nm. Aerial about 20m above the ocean.


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Wow, 100 miles!! It seems that I must still have a problem with the antennae as I thought. I was sure we used to register them 60 miles out before.
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Old 11-10-2015, 09:29   #6
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Re: Tuning VHF antenna for use with AIS

... 100 miles is simply impossible 'under normal conditions'. Will only happen if you have extraordinary radio wave propagation (e.g. ducting or tropospheric propagation - https://en.wikipedia.org/wiki/Tropospheric_propagation).

You can calculate how far AIS (or any other VHF, UHF, SHF wave) will travel (if you have the two antenna heights). The curvature of the earth is the limiting factor.

How to Calculate the Distance to the Horizon - BoatSafe.com

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Carsten
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Old 11-10-2015, 11:11   #7
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Re: Tuning VHF antenna for use with AIS

I'm dumb. I would look at the splitter and connections before getting to involved. Coax it a bitch at the connections.
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Old 11-10-2015, 12:13   #8
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Re: Tuning VHF antenna for use with AIS

From 30+ yrs in marine electronics business :

VHF antenna splitters seriously degrade performance-especially during transmission. Splitters are "tolerable" in receive only systems.

Vhf antennae are designed to provide peak performance/best swr at 156.800Mhz-Ch 16,but provide acceptable performance with little degradation over the simplex vhf band 156.025 -Ch 01 thru 156.450-Ch 88. Outside of this band,such as the duplex frequencies in 162.xxx (shore station) band,the swr is significant & degradation results. Standard boat antennae were not intended to Tx on these "shore station" frequencies,though they will,with poor performance on Tx.

Vhf Rx is not nearly as fussy regarding the antennae bandwidth,though will still Rx best on a "proper" antenna.

AIS Rx only should perform reasonably well on a std Vhf "boat" marine antenna.

AIS Tx @ 162.475+ should use an antenna tuned to that frequency range for peak performance.

Connecting a Tx/RX AIS to a standard marine Vhf antenna,tuned for 156.800 will work but with degraded performance,especially on Tx. Adding a splitter into the system would degrade it even more.

The theory that VHF signals are limited by line of sight is technically correct & basically true using anything less than a 6db antenna.(as rated by marine industry). My & many other's experience in commercial marine also shows reliable 50+ NM comms with 9 & 10db antennae -rolling boat to rolling boat-in poor weather, so I am always a bit concerned when someone posts this theoretical limit as an absolute limit.Same with radar.

I am referring to over water distances. Over land performance is closer to the theoretical. My personal rule of thumb-one mile of land propagation = 5 miles over water.

The above comments are from my own personal experience.

Cheers/ Len
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Old 11-10-2015, 12:24   #9
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Re: Tuning VHF antenna for use with AIS

If it's only 90 nm from Fla. to Bahamas,I am willing to bet that reliable marine vhf comms could be established over that stretch of water by using a Shakespeare 476 on a 50 ft above water tower at both ends.
Works in Bay of Fundy.
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Old 11-10-2015, 13:11   #10
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Re: Tuning VHF antenna for use with AIS

Quote:
Originally Posted by stevecody View Post
[...]I'm looking to this forum to ask "what next?". Should I just blindly change the antenna - which involves a trip up the mast - and hope that it comes good? Should I go prowling for connector loss and cable quality issues? Are there any more tests I can do to be a little more deterministic about the cause of the signal loss? What experiences do others have in this area?
First, I suspect there is quite a bit of variability in how well the individual antennas are tuned from the manufacturer. The only way to be sure you have a well tuned antenna is to use a VSWR meter. Some manufacturers, like GAM Electronics, will sell an antenna with a whip that can be cut to tune it to the desired center frequency.

The other things you mentioned, like bad connectors, coax cables, or sharp bends in cable runs, can also cause poor impedance match and reflections. In particular, I found that it can be pretty hard to get a good connection with the Vesper XB8000 UHF connector. Initially I was getting poor VSWR, but after several attempts I finally got the connector on the cable to seat properly and got VSWR down to the acceptable range.
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Old 11-10-2015, 13:41   #11
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Re: Tuning VHF antenna for use with AIS

For the most part I must agree with several others, answering the question, NNo you do not need to tune you VHF Anntenna. More critical is the height of the of your antenna and the other antenna, then you can factor in the transmitter power and the sensitivity of the receivers. However the more connections you have in your Co-ax the more sensitivity and power you will lose (I always used a loss of 3db for each connector). As for losing the signal when transmitting with a shared transmitter you either have a disabling relay in your RX or you are overloading the RX with the transmitted signal. So try individual antennas, or get a qualified technician.
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Old 11-10-2015, 14:10   #12
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Re: Tuning VHF antenna for use with AIS

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Originally Posted by CarstenWL View Post
... 100 miles is simply impossible 'under normal conditions'. Will only happen if you have extraordinary radio wave propagation (e.g. ducting or tropospheric propagation - https://en.wikipedia.org/wiki/Tropospheric_propagation).
Once I was about 5 miles off Cobourg (on Lake Ontario) and was trying to raise a sailing vessel near me on the VHF. Some folks in Oakville responded to my hail...they wanted to confirm my location because I broke squelch on their VHF (they were at the dock waiting for a VHF call from a friend). That distance was well over 100km, and we spoke clearly for several minutes exchanging pleasantries and discussing the distance. It was a bright clear sunshiny day with no obstacles between us. My 25 watt set had an antenna about 40 feet up (C&C27mk4).

I agree, it does not agree with the "line of sight" rule, but it proves that there are lots of exceptions to that rule if conditions and equipment are good.
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Old 11-10-2015, 14:31   #13
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Re: Tuning VHF antenna for use with AIS

Quote:
Originally Posted by deblen View Post
From 30+ yrs in marine electronics business :

VHF antenna splitters seriously degrade performance-especially during transmission. Splitters are "tolerable" in receive only systems.
That was probably true 30yrs ago, and some are definitely that bad today, but there are several quality splitters available that have <1dB insertion loss on transmit.

Check out the Vesper and Simrad splitters as examples.

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Old 11-10-2015, 14:46   #14
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Re: Tuning VHF antenna for use with AIS

It is all academic without the particulars. Mast height on both ends etc.. no two will be the same. My dumb ass guess on the AIS would be the coupling if the radio has worked.
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Old 11-10-2015, 16:19   #15
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Re: Tuning VHF antenna for use with AIS

I suppose it gets worse, since "SWR" really only measures how much of your transmitter power is being WASTED by being sent back to the transmitter instead of being consumed (another waste) or emitted by the antenna. And the last of those three is really what we want, and SWR doesn't really tell us anything about that.


Personally...I'd worry less about the tuning, since most antennas will do "well enough" to serve two masters. But I'd rather see one less piece of equipment that could break down (the splitter) and just run two separate optimized antennas. Having all your eggs in one basket in convenient, as long as the basket has no moving parts or adds no failure modes.
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