AIS Transponder and VHF Antennas on the Masthead?

[Wanderlust]

Originally Posted by chrisjs

We have 3 antennae mounted in close proximity (3 inches) at our mast head. 2 VHF and the AIS receiver. We purposely avoided installing a transmitting AIS because we understood that such close antenna proximity could cause feedback and damage to the VHF units when the AIS transmits. As a "receive only" antenna we have seen no problems. We understood that significantly greater separation would be required with the transmitting AIS.

3 inches apart. That's way closer than I had been planning. Don't forget that the VHF is a transmitter, so may be overloading the AIS receiver and you may miss out on AIS messages. A sequence of conversational VHF transmissions from you may just block out a series of AIS transmissions from another vessel. So you would see others and think all was OK, but be missing some.

[btrayfors]

Originally Posted by transmitterdan

I don't think these ratios are accurate. A loss of 1.5dB is only about 15% in terms of dB micro volts at the receiver. While any loss is not ideal -15% should not be noticeable on FM except for extremely weak signals.

Dan,

Thanks for catching that. I gotta watch my electrical math late at nite after a Mt. Gay or two :-)

3db represents a doubling of signal strength

-3db represents a 50% loss of signal strength

Therefore 1.5db represents a 25% gain or loss, not 50% as in my post #41 above.

Bill

[transmitterdan]

Originally Posted by btrayfors

Dan,

Thanks for catching that. I gotta watch my electrical math late at nite after a Mt. Gay or two :-)

3db represents a doubling of signal strength

-3db represents a 50% loss of signal strength

Therefore 1.5db represents a 25% gain or loss, not 50% as in my post #41 above.

Bill

-1.5dB is only 15% less in voltage which is what receivers care about. It's not significant enough to throw out the Vesper unit. Especially if it affords more height in the installation as that will help a lot in range.

I don't want to come across as being too AR but many times I see people worrying about small differences in signal level as if that somehow equates to range. Antenna height and multipath in harbors has a lot more effect on range than even 3dB in power. In most practical cases even 10dB less signal will still work.

[btrayfors]

Don't worry about how you come across: I can take it :-)

While I agree that small gains or losses in signal strength may not be noticed in many instances, larger ones may certainly cause problems.

A 25-watt VHF marine transmitter with a 10db signal loss would equate to an effective radiated power (ERP) of just 2.5 watts.....about half that which many handhelds can do.

Does FM signal strength make at difference at VHF frequencies? You betcha it does. Up to a point, anyway.

In any case, I'm not willing to compromise on VHF signal strength -- on transmission or reception -- since I believe the VHF to be a critical communications device on a boat.

Others, of course, may have different views about this....whatever the mathematical and $$$ implications.

Bill

[Dockhead]

Originally Posted by btrayfors

AIS transmissions use the upper part of the VHF marine band, i.e., 161.975 and 162.025 MHz (channels 87B and 88B).

The problem with locating the antennas close together is that transmissions from one may cause not only interference with the other, but could cause damage to the receiver's front end.

IMO, it is not a good idea to use one antenna with a splitter, not only because of the insertion loss but because of possible failures which, again, could be damaging.

The marine VHF is arguably the most important piece of electronic gear aboard in terms of safety. It does not make sense to do ANYTHING which could compromise its use.

Bottom line: put the VHF antenna at the truck of the mast. Locate an AIS transponder antenna elsewhere....on the spreaders, on a mizzen mast, on a radar arch, or even on the pushpit.

Bill

My AIS antenna is on my first spreader, so 33 feet up. It is connected with good, fat, low-loss cable - RG213. It works great - I see AIS targets from 100 miles away and more.

[chrisjs]

We have had the 3 mast head antennae set up that way for 4 years now. They are clustered around the mast head and for "3 inches" read 3-6 inches depending on which antenna. These are all Metz 36inch whip antennae. The AIS is a Comar receive only unit. We have experienced no problems of any type with either the radios or the AIS and typically have AIS out several miles or more. It was either put them all together at the masthead or risk "masking" by the mast with the AIS below the mast head, or "irradiating" by the radar lower down!!

[s/v Jedi]

Originally Posted by chrisjs

We have had the 3 mast head antennae set up that way for 4 years now. They are clustered around the mast head and for "3 inches" read 3-6 inches depending on which antenna. These are all Metz 36inch whip antennae. The AIS is a Comar receive only unit. We have experienced no problems of any type with either the radios or the AIS and typically have AIS out several miles or more. It was either put them all together at the masthead or risk "masking" by the mast with the AIS below the mast head, or "irradiating" by the radar lower down!!

I think you do have problems but don't realize it. Your AIS receiver with a masthead antenna should receive targets 50-100nm away, not just "several miles or more".

The range of your VHF radios will also be limited, and the noise figure well over normal levels. All 3 antennas will resonate when you key a radio.

[chrisjs]

I thought that AIS was "line of sight" and limited to about 20 miles, though we do receive VHF signals from the USCG quite often from locations that are a few hundred miles away. We keep our boat in the Narragansett and quite often pick up USCG from NJ or Chesapeake. As for AIS, we are a trawler with a 26ft mast, not a 65ft sail mast, and we are usually on a chart plotter setting showing that would limit us to closer targets except when offshore, so the several miles is probably a low ball figure. I will certainly look for longer distance targets next time we are outside. Again we get great VHF reception and transmit clarity and distance. I will check the AIS but am not aware of a problem. It has certainly proven to be a great capability, especially on overnights.

[chrisjs]

I am now thinking that the antennae are more like 9 inches apart but will check tomorrow .

[rebel heart]

I just hooked up a vesper splitter with great results. No noticeable loss in VHF range, I show up on marinetraffic.com (and plenty of other vessels broadcasting AIS around me do not), and I can't wait to get a cool antenna cable made up for our AM/FM radio so I can get some solid reception.

[s/v Beth]

I see I am in good company with my admiration of AIS. Just installed it Friday. My system works perfectly with good range (10 plus) and I still have the antenna in the drawer! Thats right folks, I am transmitting and receiving from an antenna that is not mounted, but is in a closed shelf in the cockpit up next to the hatch. Metal pretty much in all directions.Talked to a few tugs and they see me fine. Can see freighters out to sea and way up the Columbia.
Now I intend to mount it, probably on the push pit, but that it works surrounded by wench handles and crap aught to tell you a little about worrying about the blind spot (Hehe).
And yes, I am on the web sites too. (when it is turned on)

[s/v Beth]

BTW- on the masthead quiz. I would have my halyards in if I was raising the mast too. Why climb up there if you don't have too.

[boatseeker]

I have recently installed a Camino AIS transponder on my 38 Irwin. Milltech Marine supplied an elegant solution. Using a splitter it is possible to use the same VHF masthead antenna for both radio and AIS with no interference. Having completed a 3 day sail in busy Chesapeake Bay and received several calls from large ships and tugs, who could call by my vessel name, made communication much simpler. I also got the impression that the pilots and captains of the vessels calling us were very pleased to get immediate response. Yes there is a cost to buying the transponder and switch, but the respect and assistance the recreational sailor will gain from the professional is more than worth the outlay.

[ka4wja]

Okay, before I address (hopefully for the final time), the original issue of close-spaced, shared-band/freq antennas.....perhaps a few RF and mathematical facts would be helpful here...

And, while I'm more of a lemonade or iced tea guy, I understand the technical complications Mt. Gay rum may cause...




I agree here with Bill...

Originally Posted by btrayfors

While I agree that small gains or losses in signal strength may not be noticed in many instances, larger ones may certainly cause problems.

And, the small losses DO add up!!



And, for the rest of you, some facts for clarification here, might help...

1) When loss (or gain) figures, in decibels (db), are used in regards to antennas, coaxial cables, splitters, amplifiers, etc. these are power ratio figures, NOT voltage (nor current) ratio figures...
(the only exception is when measuring rf field strengths in uV/M, etc....but then it is known that you are using voltages....all other db figures used here abouts are assumed to be power ratios, and any that are not, will always be designated as such....)
Mixing these things up will cause you serious problems!!


2) Db vs. Power is of course is a logarithmic equation...
Db = 10log (p1 / p2)

RatiodB = 10·log10(P1 / P0)

In common usage (out to 2 decimal places) we use 3db as doubling (or halving) power, with 6db as 4 times (or 1/4), and 9db as 8 times (or 1/8), etc. etc....and unless I'm measuring multiple items in the lab, then that is good enough....
So, while some may quibble about 1/1000 error, in this discussion let's just round things off...



3) The Vesper SP160 specs state:
1.0db loss thru VHF radio Transmit path.. (21% loss)
1.0db loss thru AIS Transmit path... (21% loss)
1.5db loss thru VHF radio Receive path.. (29% loss)
(and a 12db gain in AIS Receive path, although that cannot be taken to literally quadruple its AIS receive range, I suspect that it does provide a slight receive improvement, of a few db, in AIS receive...)

Antenna Splitter for Class B AIS Transponders

If I get a chance to actually measure these figures in the real world, I'll let you know.....but I suspect that the transmit losses will be a bit less (better)....



4) For some common db figures, here are the approx. power values...
0.5 db = 11% loss
1.0 db = 21% loss
1.5db = 29% loss
2.0db = 37% loss
2.5db = 44% loss
3.0db = 50% loss

4db = 60% loss
5db = 68% loss
6db = 75% loss
7db = 80% loss
8db = 84% loss
9db = 87.5% loss
10db = 90% loss (1/10 of the power you started with)
13db = 95% loss
16db = 97.5% loss
20db = 99% loss
30db = 99.9% loss
40db = 99.99% loss
50db = 99.999% loss
60db = 99.9999% loss
70db = 99.99999% loss
80db = 99.999999% loss

Just in case anyone wonders why I've ran these figures up so high, the reason is the free-space path loss (line of sight, direct-wave) on VHF-hi-band might surprise some...
At 156mhz, at 1/4 mile the loss is 68.5db....and at 1 mile it is 12db higher at 80.5db!!!
Not to worry though, as our vhf-marine radios can receive signals that are VERY weak....as much as 140db weaker than what the transmitter produces....

FM SINAD specs of the receivers, FM capture ratios, FM signal-quieting once threshold is reached, etc. all enter into how things work....
But, unless we have some tropo enhancement, ducting, etc. typically our systems are pretty limited to line-of-sight plus 10%-15%....
So, when Bill writes that signal strengths make a difference, up to a point, he is VERY correct!!!

Originally Posted by btrayfors

Does FM signal strength make at difference at VHF frequencies? You betcha it does. Up to a point, anyway.

In any case, I'm not willing to compromise on VHF signal strength -- on transmission or reception -- since I believe the VHF to be a critical communications device on a boat.

So, here again Bill points out (without the math and $$$) something quite helpful....
You should all read what he wrote, and take it to heart!!



5) Now, back to the original issue, close spaced antennas on the same band/frequency...
a) None of us know the absolute "do NOT exceed" receiver input level of our particular receivers (AIS or VHF-marine)...

b) In years past, many assumed that +10dbm was a maximum, and while I still think this is a good target, in actual practice most rf receive and test equipment handle +20dbm....
My own analyzers spec +15dbm to +20dbm as their "max" do not exceed input....and while I have accidentally only subjected one of them to a signal a bit higher, once.....I have some friends who accidentally hit a VERY expensive (~ $75,000) R & S analyzer with a signal about +26 to +30dbm, and ended up with a bill of about $10,000 for repair and recalibration!!!
So, a lesson here...don't play with fire, if you can't afford it....afford both the $$$$ and the loss of use of the radio!!!

c) If you use +10dbm as a max receiver input, you'll be fine!!!
If you use +20dbm, you'll probably be okay, but you must pay attention to antenna spacing, cable losses, radio specs, etc....

d) Those with antennas in close proximity (as my old AIS rec-only antenna and my primary VHF antenna were), are playing with fire....and you need to understand that!!!



6) Bottom line:
Close-spacing (< 6', on VHF-marine / AIS freqs) of same-band / same-freq, is NOT good for AIS transponders and VHF-Marine Radios....and should NOT be done!!!
(although, IF you are willing to accept possible failure of an AIS receive-only system, then closer spacing can be done, but is not recommended....)

Do NOT place your AIS transmit/receive antenna near your VHF-Marine transmit/receive antenna....
Use the spacings and placements I mentioned in earlier posts....


Those who choose to discount these recommendations (provided by myself, Bill, and others of knowledge/experience), and the factual, mathematical information, etc. please take note and understand that the AIS transponder manufacturers, Vesper, Furuno, etc. (and marine vhf radio manufacturers) also give the same recommendations....

Just saying, if you were new to diesel engines, etc. and you got recommendations out of your Yanmar manual, and from an experienced diesel mechanic, and many who have used, maintained, and repaired their own Yanmar's for years....and they all gave you the same recommendations....
But someone down-the-dock says, "hey, no worries....I know someone who does things completely different, and he's never told me of any troubles."....
Which recommendation are you going to use???





To sum-up, I will not argue what is "right"....just providing the facts, and some learned advice (which was asked for)!!

I do hope this helps....

Fair winds..

John
s/v Annie Laurie

[transmitterdan]

A dB is a dB and it does not matter if it is used to describe a ratio of power or voltage. The result of the math will be the same. Example: A radio outputs 100V into a 50 ohm load. The power is 200 watts. If the same radio is reduced to 50V (-6dB) the power is now 50 watts (also -6dB). The -6dB ratio is the same whether comparing volts or power. This is just one part of the elegance of dB math.

32.5 miles has a free space path loss of about 110dB at 162MHz. 1dB additional loss reduces that 110dB path to a distance of about 29 miles. Few boaters can tell the difference because line of site is generally much less than 30 miles so the 1dB difference will be insignificant in practice. So my point is it's better to use the splitter than put 2 VHF antennas next to each other.