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Old 05-07-2019, 13:11   #1
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VHF/AIS Antenna Separation

While horizontal antenna separation is clear, I am unsure about the vertical separation required, and how it is determined.
Does this mean that the antenna tips must be vertically separated? Must there be zero overlap?
Look at this illustration from Digital Yacht. It seems to indicate a meter separation between the antenna centers.



As the VHF signal is lobe shaped, does vertical separation actually decrease as the horizontal separation increases (to a degree)?
And does it matter that one antenna broadcasts at 25W, while the other is 2W?
And what about all the other crap nearby - GPS antenna(s), cellular, wifi, etc?
I have enough going on at the masthead, and now my pushpit is getting congested. What is best practices???
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Old 05-07-2019, 16:45   #2
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Re: VHF/AIS Antenna Separation

Simply put best practice is as much separation as possible between antennas (especially ones using the same band - like say marine VHF). For vertically polarised antennas (again marine VHF), a smaller vertical separation trumps a larger horizontal separation - as shown in the Digital Yacht illustration. Not so much separation is needed between antennas of different bands e.g. VHF and GPS.

Having said that, most often other physical considerations prevent achieving ideal separation so best practice is just that - achieving the best in your situation.

Regarding vertical separation in the VHF marine band, the antennas should ideally have the upper tip of the bottom one should below the base of the upper one. If you simply cannot achieve this, do the best you can although remember, the AIS doesn't really need to be masthead mounted. Many get by with the AIS much lower even rail mounted.

GPS antennas are sometimes suggested to have largish separation distances in installation manuals, but I have installed literally hundreds of them almost side by side (say 6" to 12" apart) without problems.

As an aside, I once attended some manufacturer's training on fitting some aviation antennas. After detailing the exacting antenna requirements for the particular piece of equipment, the lecturer remarked "I know you will not be able to meet all of our requirements regarding separation but really all we ask is you must try to meet as many as you can".
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Old 05-07-2019, 18:31   #3
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Re: VHF/AIS Antenna Separation

My concern is that there is no consensus. Among manufacturers(!), recommendations are 1 meter, 2 meters, 1.5 wavelengths, 9 feet, etc.
If they can't agree, how can one make an informed decision. One person stated 19", and I found anecdotal reports of 3"!!!

I have decided to mount VHF and AIS antennas on the pushpit for several reasons. As I am able, I will try to provide as much vertical separation as possible. Horizontal separation is determined by width of stern.

I think I am going to end up making the best of my situation, and check VSWR on the VHF sources/antennas. If too high, I will move items around until acceptable. I suspect that at some point I am going to reach critical mass in the electrons bouncing back and forth on my stern - flux capacitor??? And as long as GPS antennas/units still show a good constellation of satellites, is all well?


Ever optimistic.
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Old 05-07-2019, 19:33   #4
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Re: VHF/AIS Antenna Separation

Quote:
Originally Posted by redsky49 View Post
My concern is that there is no consensus. Among manufacturers(!), recommendations are 1 meter, 2 meters, 1.5 wavelengths, 9 feet, etc.
If they can't agree, how can one make an informed decision. One person stated 19", and I found anecdotal reports of 3"!!!

The differences are all about how much performance degradation you're willing to tolerate. To some extent they depend on antenna design. At 2 meters or less the antennas will affect each other in unpredictable ways.



Quote:

I have decided to mount VHF and AIS antennas on the pushpit for several reasons. As I am able, I will try to provide as much vertical separation as possible. Horizontal separation is determined by width of stern.

You want to provide one or the other. There are various antenna extensions out there you can use to achieve vertical separation as well as better line of site. For example, if you mount your VHF on a 4' extension, and your AIS with no extension, they wouldn't affect each other much..


If VHF performance (particularly range) mattered to you then you would put the VHF antenna atop the mast. Any other choice compromises range to a great degree. Putting the VHF on the pushpit means you've made a decision that VHF performance doesn't matter as much as other pragmatic considerations. That being the case, having your AIS further degrade your VHF is sort of a footnote to the whole thing, and doesn't matter as much.



Quote:

I think I am going to end up making the best of my situation, and check VSWR on the VHF sources/antennas. If too high, I will move items around until acceptable. I suspect that at some point I am going to reach critical mass in the electrons bouncing back and forth on my stern - flux capacitor??? And as long as GPS antennas/units still show a good constellation of satellites, is all well?

Ever optimistic.

Checking the VSWR is a good first step but by itself it is not a guarantee of a good installation.


A more comprehensive check would be to measure RF field strength using a transceiver at various angles from the boat (ahead, astern, port beam, starboard beam, quarters, etc).
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Old 05-07-2019, 20:01   #5
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Re: VHF/AIS Antenna Separation

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Originally Posted by Jammer View Post
The differences are all about how much performance degradation you're willing to tolerate. To some extent they depend on antenna design. At 2 meters or less the antennas will affect each other in unpredictable ways.

You want to provide one or the other. There are various antenna extensions out there you can use to achieve vertical separation as well as better line of site. For example, if you mount your VHF on a 4' extension, and your AIS with no extension, they wouldn't affect each other much..


I want to optimize AIS performance as best as possible. If cockpit VHF is (modestly) compromised I can deal with it. An extension is an approach I will investigate, with the AIS mounted higher.


If VHF performance (particularly range) mattered to you then you would put the VHF antenna atop the mast. Any other choice compromises range to a great degree. Putting the VHF on the pushpit means you've made a decision that VHF performance doesn't matter as much as other pragmatic considerations. That being the case, having your AIS further degrade your VHF is sort of a footnote to the whole thing, and doesn't matter as much.


Primary VHF radio is at nav station, using masthead antenna. Secondary VHF (though used more frequently) is in cockpit with 4' antenna on pushpit.



Checking the VSWR is a good first step but by itself it is not a guarantee of a good installation.


A more comprehensive check would be to measure RF field strength using a transceiver at various angles from the boat (ahead, astern, port beam, starboard beam, quarters, etc).
Not sure how to do that. I am more comfortable with wrenches and hammers, though I am a pretty quick study, usually.
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Old 05-07-2019, 20:20   #6
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Re: VHF/AIS Antenna Separation

You're right that there is no consensus to the necessary vertical/horizontal separation. And it's very hard to notice the difference between say 12" and 36" in the real world.

But as Jammer says, there is no doubt that antenna height dramatically improves the range of both VHF and AIS. A masthead antenna is also unbothered by the deck level metal of the mast, boom, shrouds, lifelines, rails, helm, chartplotters, speakers and so on.. A masthead antenna will run rings around anything on the pushpit.

So why not get a good splitter like the one from Vesper and use one antenna at the masthead? The oft repeated horror stories about splitters are a decade out of date. Signal loss is minimal with the ones you buy today (even less for the Vesper as it has its own internal amplifier).

The cost of the Vesper splitter is little more than the cost of a 2nd antenna, mount, and low noise coax. Less if you pay someone to install your 2nd antenna.
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Old 05-07-2019, 21:36   #7
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Re: VHF/AIS Antenna Separation

"So why not get a good splitter like the one from Vesper and use one antenna at the masthead? The oft repeated horror stories about splitters are a decade out of date. Signal loss is minimal with the ones you buy today (even less for the Vesper as it has its own internal amplifier).

The cost of the Vesper splitter is little more than the cost of a 2nd antenna, mount, and low noise coax. Less if you pay someone to install your 2nd antenna."


Excellent points, but I will tell you my reasons for the pushpit location. This is my current masthead antenna:


3 ft VHF Marine Antenna - 3dB Gain


It is tuned so that the center point is about 156.8 MHz. AIS is about 162 MHz. The performance plot of my current VHF antenna (see link) shows a drop off of signal strength at AIS frequency, and the manufacturer has confirmed to me that it is not a good choice for AIS application. For just VHF use it has been great. I have a continuous run of LMR-400 from masthead to radio and it has performed well. It's just not suitable for AIS and VHF.


So to use a masthead location would require a new dual-use AIS/VHF antenna up top, say $65-$95, plus me going up the mast again. Plus the cost of a splitter, say $240. Or for budget purposes, say $325.
Cost comparisons to a dedicated AIS antenna at the stern, $55- $70, plus mount, $20-$40, plus RG8-X cable i have on hand (and maybe an extension tube), favor the pushpit. Plus I don't have to get hoisted up to the masthead - I'll go if I have to, but I get real uncomfortable once I am too far above the spreader.


Now, I am not sure that absolute range is critical for me. When offshore at night on delivery boats, my concern is the 3-5 mile range as that is the minimum "action" zone where I feel evasive action can no longer be delayed and must be taken. Admittedly, my experience with AIS is limited to other's boats, and my ideas may change after more use and experience. I still do a 360 visual every few minutes, even f I have to leave the wheelhouse to do so. Sailboats are a bit easier in that regard.


I also don't want perfect to be the enemy of good enough. AIS is just one more tool in the pocket, and as a Luddite who just added a chartplotter four years ago (after over 50 years of sailing), I see this as very helpful, but certainly not critical to good seamanship. Just MHO.


By the way, I appreciate everyone's insights. Thank you.

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Old 05-07-2019, 21:58   #8
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Re: VHF/AIS Antenna Separation

I like the splitter but since you don't...
You might consider simply inverting the AIS antenna beneath the masthead antenna. The signals don't care if the antenna is upside down, both have best possible height at the top of the mast, minimal interference with the signal from each other and the metal of the mount in between, common cable routing, and it's pretty easy to get at least a meter between centers that way.
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Old 05-07-2019, 22:08   #9
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Re: VHF/AIS Antenna Separation

Quote:
Originally Posted by Checkswrecks View Post
I like the splitter but since you don't...
You might consider simply inverting the AIS antenna beneath the masthead antenna. The signals don't care if the antenna is upside down, both have best possible height at the top of the mast, minimal interference with the signal from each other and the metal of the mount in between, common cable routing, and it's pretty easy to get at least a meter between centers that way.

Is separation measured between antenna centers? That would infer the possibility of some overlap. Do antennas broadcast their signal equally their entire length or is it concentrated at the tip? The downward facing antenna would be blocked by the mast, no? My head is starting to hurt
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Old 05-07-2019, 22:09   #10
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Re: VHF/AIS Antenna Separation

Quote:
Originally Posted by redsky49 View Post
My concern is that there is no consensus. Among manufacturers(!), recommendations are 1 meter, 2 meters, 1.5 wavelengths, 9 feet, etc.
If they can't agree, how can one make an informed decision. One person stated 19", and I found anecdotal reports of 3"!!!

I have decided to mount VHF and AIS antennas on the pushpit for several reasons. As I am able, I will try to provide as much vertical separation as possible. Horizontal separation is determined by width of stern.

I think I am going to end up making the best of my situation, and check VSWR on the VHF sources/antennas. If too high, I will move items around until acceptable. I suspect that at some point I am going to reach critical mass in the electrons bouncing back and forth on my stern - flux capacitor??? And as long as GPS antennas/units still show a good constellation of satellites, is all well?


Ever optimistic.
The issue with antenna spacing is not VSWR. It is about swamping one radio when transmitting on the other. Dumping 25 watts of power into an adjacent antenna will at best deafen the receiver. At worst it will burn out the front end of the receiver. Dealing with GPS receivers is very different - you are not transmitting so one won't effect the other.

My $.02 - put your AIS antenna on a spreader and your VHF at the masthead. You really don't care is a vessel 20 miles away knows you are there but you might want to talk a long way in an emergency.
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Old 05-07-2019, 22:52   #11
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Re: VHF/AIS Antenna Separation

Quote:
Originally Posted by redsky49 View Post
Is separation measured between antenna centers? That would infer the possibility of some overlap. Do antennas broadcast their signal equally their entire length or is it concentrated at the tip? The downward facing antenna would be blocked by the mast, no? My head is starting to hurt
Depends on the antenna!

A rule of thumb for a vertically polarised 1/4 wave antenna (or similar), any vertical air gap between the elements is OK, any overlap is a compromise, the greater the overlap, the greater the compromise until you reach the point where the horizontal separation becomes the deciding issue.
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Old 07-07-2019, 20:50   #12
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Re: VHF/AIS Antenna Separation

I understand what you are trying to do and think that your best bet is to use an antenna extension on the AIS antenna. Shakespeare and others sell them. They vary in length and have varying requirements for the rigidity of the base; some very long ones require auxiliary supports at some point above the base.


From an RF perspective, longer is better, more separation, more height above the ocean. Mounting considerations, aesthetics, and windage will limit your choices.




Quote:
Originally Posted by redsky49 View Post
Is separation measured between antenna centers? That would infer the possibility of some overlap. Do antennas broadcast their signal equally their entire length or is it concentrated at the tip? The downward facing antenna would be blocked by the mast, no? My head is starting to hurt

The nearfield (interactions within a few wavelengths) behavior of antennas is extremely complex. There is modelling software but even that is difficult to use.


Any reasonable installation will work and it is unlikely that you will notice minor differences unless you make a concerted attempt to measure them. With VHF, it is rare that you are dealing with a weak signal; due to line of sight you either have a strong signal or nothing at all except in a very small fringe area. So, yes the mast affects performance of a spreader-mounted antenna, but no you won't notice. Be careful about water intrusion with an inverted mount.
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Old 13-07-2019, 19:29   #13
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Re: VHF/AIS Antenna

Redsky49,
The short answer to your original question (from last week) is:
the "vertical separation" is actually the separation between the antennas.....meaning this is the vertical distance between the top of the bottom antenna, and the bottom of the top antenna....

EDIT:
And, as I see that your primary VHF antenna is at masthead (that's good...leave it there), and this VHF radio is below at Nav Station (that's fine...but, you may at some point consider a remote cockpit mic / control for this primary radio?)

Since these antenna separation distances that you're discussing is for a secondary VHF (used in cockpit, and presumably only for short-range comms such as for bridge-to-bridge comms to/from other vessels in your immediate location / within line-of-sight), and for your AIS transponder....and since you have the room, your set-up should be fairly easy / straight-forward....

Further, use of a separate stern rail mounted antenna for your AIS transponder, is just fine!!
And, I cannot dispute what "Digital" says about their own antenna....but, fyi....with the feedline losses (up to and back from your masthead antenna) it is doubtful that your AIS transponder would see objectional VSWR (so, doubtful it would reduce power)....and further fyi, a poor VSWR in itself will not cause "loss of signal"....just mentioning this as it is an often misunderstood situation (myth).

Also, when looking for vertical separation, "inverting" antennas is done all the time....and as long as the antenna is designed for this (waterproof upside-down, AND still have a clean radiation pattern when inverted), this is good...BUT
But, assuming you have an alum mast, you cannot simply invert a VHF antenna and have it run in close proximity to the mast!! (it really needs to be at least 30" away from the mast, such as mounting out on a spreader would do....but even then this antenna's pattern will be effected / shadowed by the mast!)


But, here are the details / answers for the rest of your questions...

1) Actually there is engineering consensus among professionals...and while the exact distance recommended depends on exactly what you are trying to accomplish....and to a lessor extent your exact application...but, the consensus is quite clear...(and I wrote about this consensus and the details of "how" / "why" here years ago...in this thread :
AIS Transponder and VHF Antennas on the Masthead?

{as for what you are trying to accomplish....there are 3 different things...and what your "minimum distance" is, will depend on which one is your priority (and/or some particular manufacturer's priority / spec)...

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, etc.) from the other's transmitter...


Almost all of us would love to make all 3 of these priorities...and as such, we choose either, a lower mounted AIS antenna (spreader or stern-rail), or a well-made high-isolation VHF/AIS relay/"splitter" (such as the Vesper SP-160)...
But, everyone must at the least make "a" their number 1 priority! }

But...
But, the issue is that most of the marine electronics manufacturers do not employ RF engineers that actually have knowledge of this...most of the younger ones have no first hand experience (nor education) on VHF/UHF systems and the older guys have either retired or were "downsized" due to their salaries...

Bottom line: RF isolation of antennas, in any / many configuration, has been well engineered a decade or more before I was born, by the likes of AT&T / Bell Labs, RCA, Motorola, GE, etc...(and the calculations of free-space path loss have been understood for decades before that, and near-field antenna coupling / isolation understood quite well for more than 50 years...)
{see details below and in links provided...}


2) The short answer (before all the details) is:
As Wotname wrote above, the more separation - the better!

But, remember there are possibly 3 different things you are trying to accomplish here, and whichever one is your priority, will determine your "minimum distance" is...

And, in short, that is usually going to be a minimum 6' of horizontal separation or 2' - 3' of vertical separation...but, hopefully more...

And, since this isn't all that convenient on most of our boats without custom mounts / stand-offs...like most of us you'll chose either a lower mounted AIS antenna (spreader or stern-rail), or a well-made high-isolation VHF/AIS relay/"splitter" (such as the Vesper SP-160)...

In addition to what I write here, and the links to the details, please read what Wotname wrote here...
Quote:
Originally Posted by Wotname View Post
Simply put best practice is as much separation as possible between antennas (especially ones using the same band - like say marine VHF). For vertically polarised antennas (again marine VHF), a smaller vertical separation trumps a larger horizontal separation - as shown in the Digital Yacht illustration. Not so much separation is needed between antennas of different bands e.g. VHF and GPS.

Having said that, most often other physical considerations prevent achieving ideal separation so best practice is just that - achieving the best in your situation.

Regarding vertical separation in the VHF marine band, the antennas should ideally have the upper tip of the bottom one should below the base of the upper one. If you simply cannot achieve this, do the best you can although remember, the AIS doesn't really need to be masthead mounted. Many get by with the AIS much lower even rail mounted.

GPS antennas are sometimes suggested to have largish separation distances in installation manuals, but I have installed literally hundreds of them almost side by side (say 6" to 12" apart) without problems.
What this means for VHF Marine and AIS Class B transponder antennas is:
a) 6' to 10' horizontal separation is officially (calculated / engineered) the minimum separation between a Marine VHF antenna and a Marine AIS antenna! (and this is of course what I recommend)

b) a 2' -3' (or more) vertical separation (assuming these antennas are directly above / below each other) is officially (calculated / engineered) the minimum separation between a Marine VHF antenna and a Marine AIS antenna! (and this is of course what I recommend)

[Note that Furuno, in their installation guide for their FA-150 Class A AIS transponder, recommend a minimum of 10m / 32' horizontal separation, or a 2.8m / 10' vertical separation....and this corresponds to accomplishing all 3 of the things I highlighted above in #1...a, b, and c...]

c) Fyi, 3' horizontal separation is calculated to be acceptable to some sailors (and some manufactures), and in actual practice has not been reported to cause damage to either radio / device... but this barely accomplishes "#1 a"....(and, while I'm hesitant to recommend this, others do....and I accept this as a solution for some, but I prefer to recommend a separate antenna mounted lower such as on a spreader or stern rail, or the Vesper SP-160 relay/"splitter"...)


I explained all of this in great detail here in this thread...6 years ago...ironically titled: Re: AIS transponder and VHF antennas on the masthead? / Isolation needed... (see post #35...)
AIS Transponder and VHF Antennas on the Masthead?

{fyi, please note that there are other issues (coupling, etc.) with antennas in each others near-field, that I do not get into in the above discussion....mainly since it is of moot point here in our situations, 'cuz we just do the best we can...and most just use antennas with large separations (such as one on the masthead and one on the stern rail), or use a VHF/AIS relay/splitter...}


There are also other threads here that discuss this, as well as other similar / ancillary topics regarding antennas, antenna isolation, VHF radiowave propagation, and actual real test results of the Vesper SP-160 relay/"splitter"...(see below for these links)


3) Some specifics...
Quote:
Originally Posted by redsky49 View Post
My concern is that there is no consensus. Among manufacturers(!), recommendations are 1 meter, 2 meters, 1.5 wavelengths, 9 feet, etc.
If they can't agree, how can one make an informed decision. One person stated 19", and I found anecdotal reports of 3"!!!
AIS Transponder and VHF Antennas on the Masthead?
Please read the above referenced thread for details...(but be forewarned, there is a LOT of detail there!)
But, in short, keep the antennas a minimum of 6' horizontally separated...(and a few feet vertically would be good too...)


I have decided to mount VHF and AIS antennas on the pushpit for several reasons. As I am able, I will try to provide as much vertical separation as possible. Horizontal separation is determined by width of stern.
Please read the above referenced thread for details...(but be forewarned, there is a LOT of detail there!)
But, in short, keep the antennas a minimum of 6' horizontally separated...(and a few feet vertically would be good too...)


I think I am going to end up making the best of my situation, and check VSWR on the VHF sources/antennas. If too high, I will move items around until acceptable.
? ? ?
Your antennas would need to be really close (as in <18" - 24") in order to have adverse effect on VSWR....

What you are trying to accomplish here is not "low SWR", but rather:
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, etc.) from the other's transmitter...



I suspect that at some point I am going to reach critical mass in the electrons bouncing back and forth on my stern - flux capacitor??? And as long as GPS antennas/units still show a good constellation of satellites, is all well?

Ever optimistic.

4) These other threads have additional info that should be helpful:

VHF and AIS Radiowave Propagation and VHF and AIS Radio Range

Vesper AIS SP-160 "relay/splitter" test results, lab/real world


VHF / AIS Antenna Separation


And, for VHF-Marine antenna and VHF-Ham antenna separation...
Multiple antennas at mast head




I do hope this helps clarify things....without rambling on with too many details?

Fair winds.

John

P.S. Please allow me to repeat this for everyone's benefit, as there really is consensus!! It's just that you need to specify what you're trying to accomplish!!

Actually there is engineering consensus among professionals...and while the exact distance recommended depends on exactly what you are trying to accomplish....and to a lessor extent your exact application...but, the consensus is quite clear...

{as for what you are trying to accomplish....there are 3 different things...and what your "minimum distance" is, will depend on which one is your priority (and/or some particular manufacturer's priority / spec)...

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, etc.) from the other's transmitter...


Almost all of us would love to make all 3 of these priorities...and as such, we choose either, a lower mounted AIS antenna (spreader or stern-rail), or a well-made high-isolation VHF/AIS relay/"splitter" (such as the Vesper SP-160)...
But, everyone must at the least make "a" their number 1 priority! }

But...
But, the issue is that most of the marine electronics manufacturers do not employ RF engineers that actually have knowledge of this...most of the younger ones have no first hand experience (nor education) on VHF/UHF systems and the older guys have either retired or were "downsized" due to their salaries...

Bottom line: RF isolation of antennas, in any / many configuration, has been well engineered a decade or more before I was born, by the likes of AT&T / Bell Labs, RCA, Motorola, GE, etc...(and the calculations of free-space path loss have been understood for decades before that, and near-field antenna coupling / isolation understood quite well for more than 50 years...)

Also, be aware that when having BOTH vertical and horizontal separation, you do need to take into account the antenna patterns and virtual angles between these antennas....but, on our boats we just do "the best we can!"
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Old 14-07-2019, 06:49   #14
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Re: VHF/AIS Antenna Separation

John has given it to you best, Redsky49. Your AIS will be just fine if you use your 156 MHz tuned masthead VHF antenna with a splitter. The change in VSWR with a 6 MHz shift in frequency is not large and would not be a problem for your AIS, nor would it detectably affect your transmit/receive signal strength. The source you referenced above has given you poor advice about that.

As for a pushpit mounted AIS antenna, I have used that in the past and still had extremely impressive detection range of AIS targets. It really isn't a problem given the way we use AIS. But you certainly want this antenna a good distance away from a 25 watt radiating antenna on a nearby frequency, for the health of your AIS receiver.
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