AIS Transponder and VHF Antennas on the Masthead?

[bazzer]

Originally Posted by transmitterdan

The tricolor is rotated 90 degrees away from the correct orientation.

I would rather have the AIS antenna somewhere other than on the mast. AIS does not need a lot of height and having it somewhere else gives redundancy in case of a dismasting.

Nav light is screwed. Also you wifi antenna will cause quite a blind spot which isn't al that Kosher. The pelicans around here would love the designer perch.

[jr_spyder]

Correct on the tri color light. I've always wondered how long the previous owner had it that way.

My new wifi antenna is in the same spot as that old cell antenna but much shorter. It doesn't have that extension tube under it. All kinds of upright things onboard can cause blind spots. Just have to minimize as best you can and the top of the mast has to be best for that.

[Jon Hacking]

Originally Posted by jr_spyder

... My new wifi antenna is in the same spot as that old cell antenna but much shorter...

What sort of coax are you using to feed the WiFi? At 2.5/5GHz, the line losses can be huge, even with RG8. Better is to put the whole WiFi transceiver right at the antenna & feed it digitally (USB, whatever).

[jr_spyder]

No coax at all. A CAT5 cable runs up the mast with two of it's wires providing 12vdc. The "antenna" actually has a powered radio in it's base. So there are no signal losses in the wire and no big fat coax in the mast. Works great.

Look here: Bullet | Ubiquiti Networks, Inc.

There is plenty of content in multiple CF threads on this type of setup.

[ka4wja]

Jd1, Wanderlust, Colemj, et al,
I've written about this quite extensively in the past....
The short answer here is easy....the long answer is a bit complicated...
So, the easy one first...

Originally Posted by Jd1

I am also thinking of adding an AIS transponder antenna to the masthead.
Seeing that the two antennas will be in somewhat close proximity, is this going to cause more problems than it's worth or will they be able to live happily together ?
I am thinking of mounting a stainless plate that extends out past the two sides of the mast maybe a foot on each side and stick one antenna on each side so they would be separated by maybe 2.5 ft. One thing I don't really want is for my VHF squelch to open up every 30 seconds or so when the transponder transmits.

1) Having two transmit/receive antennas, using the same VHF band, in close proximity (< 6' spacing) is going to be problematic and is NOT recommended....NOT at all...
And, this is NOT going to meet your design criteria...


2) Further, even moderate spacing, while it will keep damage from occurring to the receivers, is unlikely to provide you with the results you desire...

But, if "de-sense" and/or squelch breaking is acceptable to some, and they desire to mount an AIS transmit (transceiver) antenna up high...they can:

a) use the nulls above and below vertical antennas to give adequate antenna/rf isolation.....(you can attain 20-30db of isolation on VHF, in just a few feet of direct vertical separation...)
---or---
b) keep the antennas separated by at least 6' - 10' horizontally...(to attain 22 - 25db of isolation)




Okay, those are the quick and easy answers.....
Here are the looonnnggg answers...

3) You have three levels of isolation to consider...
a) adequate isolation to prevent receiver damage from the other's transmitter...
b) adequate isolation to prevent receiver "de-sense" from the other's transmitter (reduction of sensitivity when the other transmitter is used)
c) adequate isolation to prevent "de-sense", noise, and "squelch openings" caused by the wideband noise (oscillator phase noise and transmit IMD) from the other's transmitter...




4) While the formula for free-space path loss isn't too difficult...
20 log (4Pi x distance / wavelength) = Path Loss
For figuring things within a few tenths of a db to a 1/2 db, you can simplify things a bit....

Free-space path loss is approx. 22db at one wavelength spacing....with 6db increase for every doubling of distance....and 6db reduction for every halfing of the distance....

At 162mhz, one wavelength is about 6' / approx. 1.9m....

So, antennas horizontally spaced / separated by one wavelength (6' at 162mhz) would have an isolation of approx. 22db....
Those at about 9' - 10' horizontal spacing would be 25db...

Vertical antennas have distinct deep nulls (20-30db) immediately/directly above and below (within 5 degrees of directly above/below)....and as long as mutual coupling, feedline coupling is reduced/eliminated, spacing as close as a few feet vertically can attain 25db....
10' vertical spacing attains 35db to 45db isolation depending on antenna and mounting.....(reflections from mounting towers/masts limits this, and typical isolation for 10' separated antennas, closely mounted to the mast, such as 1/2-wavelength or so, is 30db - 35db....)




5) Solving issue "a" (preventing receiver damage), isn't hard....
Most modern VHF-FM receivers can handle inputs to +20dbm (1/10 of a watt).....some may handle more, but I'd not recommend getting too close to the maximum due to reliability issues, "fudge factors", etc....

So, if you have a 25-watt transmitter.....
+44dbm (25-watts) - coaxial cable losses (both VHF radio AND the AIS coax) + antenna gains for both antennas (real dbi, NOT "marketing hype gain) - 20dbm = absolute minimum antenna isolation required.....
(or for vertically separated antennas, removing antenna gains from the equation, and in their place you'd subtract the amount of the nulls in antennas)
{as an example 44 - 3 - 3 (two 100' lengths of LMR-240) + 2 + 2 - 20 = 22db....but I'd add a fudge factor of a few db....so figure about 25db as a minimum...}

This translates into a horizontal separation of 9' - 10' (~ 3m)....
---or---
a vertical separation of less than half that (maybe as close as a couple feet, directly above/below each other), but you'd need to keep the antennas away from the vessel's mast by at least 1/2 wavelength (~ 3')....
So, you could build a stand-off bracket to mount the antennas on, one pointing up and one pointing down..keeping their feed points a few feet apart, directly above/below each other...and not running the feedlines next to each other, etc.,
or...mounting an AIS antenna on the top spreader, 3' away from the mast...and a VHF antenna on a 3' stand-off arm at the masthead (keeping them directly above/below each other...)

You can see why "spreader mounting" or stern rail mounting of AIS transponder antennas is common....


{note that I used an AIS receiver-only, with antenna mounted only about one foot away from my primary VHF antenna, and it survived with no damage and very good performance for 5 years with only about 15-18db of antenna isolation hitting the AIS rec with quite a bit of transmitter energy, before I installed an AIS transponder and stern rail-mounted AIS antenna....}





6) Solving for issue "b", also isn't too hard...
De-sense or blocking levels for commercial VHF-fm radios, for 5mhz spacing (from ch. 16 to AIS channels is about 5mhz), has been the same for years, at about 0dbm....
AIS transponder receiver de-sense or blocking specs are not widely known (maybe the boys at Vesper can confirm their units are about the same as other VHF-fm receivers???), but I'm assuming they're about the same...

Use the same procedure/equation as above, except the last number in the equation is NOT +20dbm, but 0dbm....
Meaning about 42db - 45db of RF isolation is needed in order to keep the VHF from de-sensing the AIS....

Although with Class B AIS at only 2 watts transmitter power (+33dbm), if you could accept the VHF de-sensing the AIS, you could (possibly) survive with only 32-35db of isolation and still not have the AIS de-sense the VHF....
BUT...
But, go for more isolation...as more is better!!!
(not to mention, the fact that AIS transmitters are probably NOT as clean as the primary VHF's.....so, I'd NOT recommend trying with less than 42-45db of isolation, if de-sense is a consideration for you (as it IS for me)...


10' - 20' of vertical separation (immediately above/below each other), gives you 35db to 50db of isolation.....
And, 25' - 50' of horizontal separation will give you approx. 30 - 40db of isolation....

So, a spreader-mounted AIS antenna, 10' - 20' directly below the primary VHF antenna, or a rail-mounted AIS antenna 50' below and 20' - 30' aft of the primary VHF antenna will give enough rf isolation to reduce/eliminate de-sense....(the rail-mounted AIS antenna will give you 50 - 60db, or more isolation...)




7) Solving issue "c" is difficult without knowing the exact transmitter specs....but assumptions can be made that are pretty close...
Isolation to keep the wideband noise / phase noise from the transmitters, from effecting the other receiver is VERY dependent on the cleanliness of the transmitter.....
Most VHF-marine transmitters are pretty good...but I'm not sure about AIS transmitters....

In general, if the other transmitter is excellent the isolation required is about the same as that needed for "de-sense" elimination....
But, if the transmitter is just average, then an additional 15-20db of isolation (as much as 60db+) might be needed.....
And, by coincidence, the better AIS transponder / Primary VHF "splitters" (actually relays and splitters) have 60db or better isolation...

If the transmitter is poor (unlikely for most of our applications) there is nothing practical/cheap that you can do to isolate things, you just need better transmitters....




8) If you'd like to see what I've done and why I've done it....have a look here...
AIS Transponder


And, BTW, while a rail-mounted AIS antenna is cheaper than the excellent Vesper AIS transponder Splitter....
Antenna Splitter for Class B AIS Transponders
This Vesper AIS splitter ($250) is a good unit, and is the ONLY one I'd recommend!!



9) In case you are wondering, in addition to my 45 years of sailing/voyaging experiences, including using AIS rec-only on two Atlantic crossings in 2007, and my AIS transponder for coastal/regional cruising for the past year or so..... I have > 30 years experience with commercial, marine, and ham VHF radio / antenna isolation, in addition to all my other RF / radio experience over the past 40+ years....
So, that means that in addition to the facts, figures, equations, etc. I have many decades of real-world experience in this specific area....





I hope this helps....

Fair winds...

John
s/v Annie Laurie



P.S. I suspect you have:
a) a defective primary VHF radio (my first guess), or....
b) some interference (RFI) on-board, or...
c) have local RF sources interfering with your primary VHF radio....

Originally Posted by Jd1

As it is, I have issues with my VHF squelch opening up quite often while there is no signal on the air (squelch at max, Icom radio) - I will check the antenna connection to the VHF radio while the mast is down to see if there is anything I can see that might cause this noise issue.

I recommend solving this issue before installing an AIS transponder.....

[Wanderlust]

Thanks for the great post k4awja. There's a wealth of information in there.

I know you said dual masthead mounts were not recommended, and your post seems to give a pretty good explanation as to why. If I understand your points correctly, there should be no risk of damage as long as the antennas are not right next to each other .... yet your original installation (AIS receive only) had them a foot apart and you said it worked well.

However, you also said unless the antennas are several feet apart (impractical for horizontal mount at masthead), when one transmits, the other will "de-sense" which I assume is the receiver reducing gain, or perhaps actually overloading. In either case, some signals may be lost.

Your advice coincides with the AIS manufacturer (Vesper) while according to jr_spyder, only partly with the antenna manufacturer (Shakespeare) who recommend both at the masthead with 3 feet of separation.

I had a further discussion with the installer who will be doing mine (now scheduled for next week). He is a well respected mast specialist with many years' experience restoring and installing masts. His comments are based on what he sees, rather than on technical reasoning. He says nearly all the AIS antennas he sees or installs are mounted at the masthead, many quite close to the VHF antenna and they don't seem to be a problem.

Back to your original receive only installation ....

What I don't understand is why it worked well a foot apart, yet you suggest that is not recommended for an AIS transponder. VHF can transmit at 25 watts, while AIS is limited to 2 watts. So wouldn't any de-sensing be more likely on an AIS receiver with 25 watts of VHF transmission next to it (what you had), than a VHF receiver with only 2 watts of AIS transmission next to it? As for squelch, what did you mean by “squelch breaking”?

So is your advice not to masthead mount based on getting a perfect installation, or would around 3 feet of separation as recommended by Shakespeare be good enough for practical purposes? Perhaps you could translate some of those db figures into a practical example of how performance may be affected. For example, you said 6 feet of horizontal separation would result in an isolation of 22db … therefore I calculate 3 feet would be 16db. What do those figures mean in practical usage with average equipment?

Also, you mentioned a little about passive objects nearby - mast and mounting reflection. What about proximity to rigging wires if mounted at the spreaders? Vesper recommend against this location whereas you say it's OK.

Please don’t think I’m disagreeing with anything you have written. It's really helpful. I’m just trying to find a balance between opinions, experiences and the technical explanation.

[gegroves]

FWIW, we have a Comar class B AIS and a Comar VHF antenna splitter. They both work great. Since the AIS only broadcasts its info in short bursts, you are not really interrupting the vhf capability. We have conducted vhf radio transmissions whilst simultaneously broadcasting AIS without interference with either radio. Here in the Med, we encounter large freighter and high-speed ferry traffic. I want to "see" that traffic as far away as possible and alter my course accordingly. I don't trust the bridge to be watching visually, with radar, or with AIS. Bottom line, splitter works fine.

[ka4wja]

Steve,
You're welcome...

1) First off, please understand that in addition to having the knowledge, experience, etc. I also have the equipment to measure things precisely....
And, combining this with the fact that my old AIS receive-only unit was NOT a critical part of my vessel, I was able to take a few more "risks" than what I'd ever recommend to others...
So, what I chose to do (for my old AIS receive-only system), was just for MY application, based on MY antennas, MY measurements, and MY risk assessments...
And, this is NOT what I recommend....especially NOT for AIS transponders....
Please READ my article I posted a link to earlier, for details....
AIS Transponder



2) So, for the record, my quick and dirty recommendations (based on both calculation and measurements, and 30+ years of experience):

Do NOT place two transmit/receive VHF antennas in close proximity to each other, meaning NOT any closer than 6' horizontally separated, and usually never closer than 10'....or a few feet, (3' - 5' min) of direct vertical separation...
The risk of damage is too great, and I would NEVER recommend anyone risk damaging their primary communications device (their primary VHF radio)!!!

Also, understand that these recommendations are not opinion, but rather are based on factual (both measured and calculated) information....
(if I add opinion below, I will try to separate those from the facts, that way you'll be better able to make your decisions...)



3) Sorry about not explaining things better...

a) Keeping transmitter energy from damaging other receivers is THE most important issue here....no matter how much time I spend (and how much of your time is wasted trying to understand my ramblings) explaining the details, please understand the above is what is important!!!
The rest of this is just for clarification....


b) In layman's terms, De-sense is when your radio's sensitivity is reduced / blocked by another signal (or noise) on other channels/frequencies....(similar to "overloading", which occurs from signals on the same channel/freq)

In FM systems, many times this is not noticed even by "experienced radio techs", nor the radio's user.....and certainly not by the casual user....but in it IS detrimental to the system's performance...
{although, with the short burst transmissions of AIS, and the only occasional transmissions from your primary VHF, it is unlikely that many will notice de-sense, and even if they do understand it, they simply accept it....BUT...
But this is NOT my recommendation....and I would not accept this from anyone who is a "professional"... }



c) "Squelch-breaking" (particularly the short duration type I'm referring to here) is a common occurrence when another transmitters noise output (whether osc phase noise or IMD products) places a signal (a "noise signal") into the passband of another receiver and "breaks the squelch" or "opens the squelch"....but all the receiver hears is noise....
(note that this "noise" would also reduce or block any desired signal that you may be trying to receive...so, if another close-by transmitter is activated while you are trying to receive something, you may hear "noise" instead of the signal you are trying to hear...)

Although some transmitters can be truly horrible in regard to their transmitted noise (modern modified/"opened-up" ham radios come to mind here), and there may be little someone can do to reduce their detrimental effects on other receivers, most transmitters we use are clean enough to allow other receivers to be used, as long as we have enough isolation between the two....typically about the level needed to reduce/eliminate any de-sense...
(with 5mhz separation, that's not hard to do...as I wrote earlier...)




d) Regarding reflections or coupling to/from other objects (masts, rigging, lights, etc.).....in general, we do the best we can on our sailboats and accept whatever issues we have...
The rigging wire is unlikely to cause any significant issues, nor do small LED tri-color lights, etc....(but you must keep the antenna from touching the any of these things!!!)

Specifically, as long as the antenna is > 1/2 wave away from large items (such as the mast) there will be little/no coupling and only minor reflections....although there WILL be null in the direction of the mast (depending on the size of the mast and spacing of antenna from mast, this could be as much as 20* wide and 6-10db deep...or possibly only about half that...), and whether this is detrimental to YOUR system will depend on YOUR desires and application....
OPINION ALERT....
And, although this mounting arrangement (a spreader mount) is recommended by some it is not my favorite.....
I'll admit that with the typical rolling, pitching and yawing of our typical sailboats offshore whatever "null" is produced by this arrangement is moved around quite a bit, and may not be too detrimental to most users, it still is not my idea of the "best" approach....(useable and acceptable, yes....."best", no...)
This part here is my OPINION....as opposed to almost everything else I've posted here....
Whether to "accept" a spreader mount as "good", "okay", or "bad", is OPINION.....as it will work, and will work well....but it DOES have some issues (the null of the antenna pattern) and YOU must decide whether the OPINIONS are useful to you....


{BTW, as an aside, in addition to the recommendations you got from the Vesper guys (GREAT guys, BTW), the Furuno FA-150 Class A AIS installation instructions do allow for spreader mounting (actually closer to the mast than MY recommendations), but mounting in the clear is recommended as their "best" approach..

•

The AIS VHF antenna should be placed in an elevated position that is as free as

possible with a minimum of 0.5 meters in the horizontal direction from constructions

made of conductive materials. The antenna should not be installed close to any large
vertical obstruction. The objective for the AIS VHF antenna is to see the horizon
freely through 360 degrees.

•

The AIS VHF antenna should be installed safely away from interfering high-power

energy sources like radar and other transmitting radio antennas, preferably at least 3

meters away from and out of the transmitting beam.

•

There should not be more than one antenna on the same plane. The AIS VHF

antenna should be mounted directly above or below the ship’s primary VHF

radiotelephone antenna, with no horizontal separation and with a minimum of 2.8
meters vertical separation. If it is located on the same plane as other antennas, the

distance apart should be at least 10 meters.

Install the VHF whip antenna referring to the outline drawing at the back of this

manual. Separate this antenna from other VHF radiotelephone antennas as shown

on the next page to prevent interference to the FA-150.

Sorry, I could get the diagram to load in here....}

4) I don't wish to get into arguments about "who is right", particularly with 3rd hand info on the internet...
But, in general, you should take the radio (and AIS) manufacturer's recommendations to heart, and unless they can be PROVEN inaccurate and/or marketing hype, AND that you can accept any risks involved, heed these recommendations!!

(and, of course, heed my recommendations as well!! )

Originally Posted by Wanderlust

Your advice coincides with the AIS manufacturer (Vesper) while according to jr_spyder, only partly with the antenna manufacturer (Shakespeare) who recommend both at the masthead with 3 feet of separation.

Originally Posted by Wanderlust

I had a further discussion with the installer who will be doing mine (now scheduled for next week). He is a well respected mast specialist with many years' experience restoring and installing masts. His comments are based on what he sees, rather than on technical reasoning. He says nearly all the AIS antennas he sees or installs are mounted at the masthead, many quite close to the VHF antenna and they don't seem to be a problem.

OPINION ALERT....Steve, although I prefer to give recommendations based on facts, measurements, experience, etc. rather than just "advice" (as I don't know your installer), if you actually are looking for advice/opinion here, mine is to NOT take the advice of your "installer"....

Bottom line here is that while he may install a lot of AIS antennas at mastheads near the primary VHF antenna (say within a foot or so), I don't know if these are all for AIS transponders, but assuming they are, this is NOT good advice, and is NOT recommended by anyone I know, nor is supported by any facts nor measurements....
Understand that I don't doubt what your installer told you, but rather I'm saying that taking his "advice" to do what he seen others do is BAD advice....
VERY BAD ADVICE.....

Further, understand that while placing antennas in very close proximity may work in some applications, and not cause damage, it may NOT work in other applications / installations....(different equipment, different cables/connectors, even different cable/connector placement, different antennas, different antenna mounting, different masts, etc. etc..heck even variations in manufacturing tolerances can mean one may work and another
will not....)

And, finally, there is NO way you can get away from the de-sense and noise issues with antennas placed in such close proximity....just a fact of life....

So, bottom line here....
Just because I was able to design/install an AIS rec-only system that worked with antennas in close proximity, doesn't mean that others should do so....nor that this would work for AIS transponder and primary VHF radio antennas (it won't!)
Besides, with all my knowledge and experience in this area, do ya' think I'd waste my time/effort re-doing things (when installing my AIS transponder) if I didn't have to???
Read the details in my article I linked to....
AIS Transponder

I hope this helps clear things up....(just 'cause something works for someone, doesn't mean it is recommended...)

Fair winds...

John
s/v Annie Laurie

[DavefromNZ]

We fitted the Raymarine AIS650 with the Vesper VHF aerial splitter. We chose the Vesper splitter as it is supposed to boost the VHF signal whereas Raymarine doesn't. Three thoughts:

1. Once you have been out at night or in low viz weather with AIS you will never go back. I love AIS! Nice to watch on the chart plotter a big cruise ship alter course to pass behind us. And good to have the radio call signs available.

2. AIS with a VHF splitter has a problem that both Raymarine and Vesper tell me cannot be fixed. When I transmit on VHF I usually get a warning on the chart plotter that the AIS has failed. But it simply means that it has tried to use the aerial when VHF has preference. Not a big deal and I could put the AIS in silent mode when using AIS.

3. It is no longer a big ocean out there and we all need all the help we can get. Have a look at all of this oceanic ship traffic and realize we are never far from a ship!

Global ship traffic seen from space - FleetMon Satellite AIS and FleetMon Explorer - YouTube

[Wanderlust]

Thanks for the updated explanations ka4wja.

OK I won't get it mounted at the masthead ... still reviewing where to put it.

[btrayfors]

Dave,

Glad you like your new AIS/VHF setup.

You could avoid the problem you mentioned, of course, by installing a separate antenna for the AIS. Since the splitter you have incorporates an internal RF amplifier stage for AIS reception, you could still use it....just not the splitter function....and avoid the message problem on your chartplotter altogether.

By the way, the advertising hype for the Vesper splitter is misleading. It seems indeed to have some RF signal gain for AIS reception, but it has signal losses for your VHF and for your AIS transmit:

- 1.5db (or 50% loss on VHF receive)
- 1.0db (or 33% loss on VHF and AIS transmit)

See: http://www.vespermarine.com/antenna-...specifications

Don't know about you, but in my opinion the VHF is by far the most important communications tool aboard and I wouldn't want to do ANYTHING to compromise its performance.

Yes, there are lots of ships around and it's fun to watch them and even contact them. But, how many fishing boats, yachts, barges, buoys, and other floating obstructions are you seeing on the AIS screen? All those...and more....can sink you, too!

Bill

[bazzer]

A separate antenna will probably cost less than the splitter and has the advantage that it could be used as a backup for the VHF radio if your VHF antenna should fail

I have seen some very long ranges when at sea on receiving AIS targets. The USCG could "see" me about 400 miles down the Baja coast. I suspect that some kind of relaying is taking place rather than so called piping. My AIS antenna is mounted just above the Bimini.

[Paul Elliott]

Originally Posted by bazzer

I have seen some very long ranges when at sea on receiving AIS targets. The USCG could "see" me about 400 miles down the Baja coast. I suspect that some kind of relaying is taking place rather than so called piping. My AIS antenna is mounted just above the Bimini.

Piping (ducting) is a completely reasonable explanation for your long-range operation. I often see tropospheric ducting from northern California all the way down past the tip of Baja. The marine layer creates the conditions for strong ducting and there is often a solid marine layer just off the CA and Baja coast.

I don't believe that the USCG is doing any relaying.

[chrisjs]

Wanderlust:
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.

[transmitterdan]

Originally Posted by btrayfors

By the way, the advertising hype for the Vesper splitter is misleading. It seems indeed to have some RF signal gain for AIS reception, but it has signal losses for your VHF and for your AIS transmit:

- 1.5db (or 50% loss on VHF receive)
- 1.0db (or 33% loss on VHF and AIS transmit)

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.