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Old 07-04-2016, 07:41   #1
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VHF / AIS Antenna Separation

Through a little communication mix up, my new masthead was setup with my VHF antenna and AIS antenna about 9 inches apart. My goal was to avoid using a VHF / AIS antenna splitter, which seems to generally be accepted as the better approach. I understand that if the antennas are too close together the VHF transmitter can destroy the AIS receiver by overloading it with way too much power. While some sources recommend more space, some say 1/4 of a wavelength is enough space, including this article...

How Close Can I Mount Two VHF/UHF Antennas | KB9VBR J-Pole Antennas

I'm trying to decide if I can just separate antennas, perhaps with custom mounts, the 19" minimum, or I should pursue another solution. Or just fall back on the splitter approach.
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Old 08-04-2016, 09:22   #2
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Re: VHF / AIS Antenna Separation

Steve,
The short answer is:
9" is way too close for comfort!
19" is also way too close for comfort!
36" is about the minimum (36" horizontal separation)!



The long answer is:
Well, it is LONG!

Seriously, I've posted about all of this before....and will refer you to those discussions for all the details....
(but, take note that while you could get away with a 36" separation, it isn't recommended....at least not by me....and I've had 35+ years experience with VHF/UHF antennas/repeaters/duplexer/etc., and RF isolation....)
Quote:
Originally Posted by SteveInMD View Post
....my VHF antenna and AIS antenna about 9 inches apart.
Have a look here:

AIS Transponder and VHF Antennas on the Masthead?
(post #35)




And, for lots more details on VHF and AIS radiowave propagation, VHF/AIS antenna "splitters", etc....
Have a look here:
VHF and AIS Radiowave Propagation and VHF and AIS Radio Range

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

Vesper AIS SP-160 "relay/splitter" test results



And, some more details about AIS (and VHF) antenna issues...
Have a look here:
AIS Problems with Antenna

AIS Problems with Antenna





If you wish to avoid using a "splitter", use an antenna mounted on a pole on the stern rail, bimini top, arch, solar array, etc....or even one mounted on a spreader (making sure the antennas are DIRECTLY above and below each other)....
Quote:
Originally Posted by SteveInMD View Post
My goal was to avoid using a VHF / AIS antenna splitter, which seems to generally be accepted as the better approach. I understand that if the antennas are too close together the VHF transmitter can destroy the AIS receiver by overloading it with way too much power.
Yes, the part about possibly damaging the AIS receiver is true....but understand if you are adding an AIS transponder, the AIS's transmit signal could also damage your primary VHF's receiver!






Understand that there is a big difference between radios on different freq bands versus sharing the same freq band, AND big differences between adequate separation to prevent receiver damage versus receiver de-sense (receiver blanking)....and even bigger differences between antenna separation to maintain their independent patterns (keeping them out of each others' capture area)!!
If you wish to discuss all of these things, best to do it in person or on-the-air, 'cause I don't have the time to type all of that here!
Quote:
Originally Posted by SteveInMD View Post
....While some sources recommend more space, some say 1/4 of a wavelength is enough space, including this article...

How Close Can I Mount Two VHF/UHF Antennas | KB9VBR J-Pole Antennas

I'm trying to decide if I can just separate antennas, perhaps with custom mounts, the 19" minimum, or I should pursue another solution. Or just fall back on the splitter approach.
Bottom line, a 1/4-wave separation is too close for any in-band transmit/receive antennas....read the referenced threads above for all the details!
If you wish to do some calculations, start with +44dbm minus your coax loss (~ 3db) = ~ 41dbm.....then do a free space path loss calc for the distance you desire....and subtract that number from 41dbm, and subtract the coax loss in your AIS antenna....and that is the power that will be reaching your AIS' receiver....
If it is below +10dbm, you should have no issue with damage at all...(but you will still have some desense until it is well below -20dbm)



{btw, before installing an AIS transponder a few years ago, I had a dedicated AIS receiver on its own antenna on the masthead....I had a vhf/uhf antenna (144mhz/220mhz/440mhz ham antenna) on my masthead, that I have used as an AIS receive-only antenna for many years....without incident....but, I measured the isolation and calculated the input power exactly....so do not do what I did...and NEVER try this with two transceivers such as a VHF marine radio and an AIS transponder!}



I do hope this helps...

Fair winds...

John
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Old 08-04-2016, 10:10   #3
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Re: VHF / AIS Antenna Separation

Quote:
Originally Posted by ka4wja View Post
or even one mounted on a spreader (making sure the antennas are DIRECTLY above and below each other)
It's going to be difficult to have a spreader-mounted antenna be directly below a masthead antenna!

For what it's worth, my AIS transponder antenna is on the spreader, the VHF antenna is on the masthead, and there have been no problems with this configuration. Given the antenna patterns and the vertical separation distance, the potential interference is *much* less than you would have with the suggested 36-inch distance arrangement.

I probably have some minor pattern distortion with the spreader-mount caused by the mast and shrouds, and I haven't done a careful measurement, but I've not noticed any problems.

Quote:
then do a free space path loss calc for the distance you desire
I don't think the standard path-loss calculation holds up in the near-field, which is what we are discussing.
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Old 08-04-2016, 10:28   #4
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Re: VHF / AIS Antenna Separation

Paul,
Thanks for pointing out my omission!
Quote:
Originally Posted by Paul Elliott View Post
It's going to be difficult to have a spreader-mounted antenna be directly below a masthead antenna!
What I omitted was using an approx. 30" long stand-off at the masthead, placing the antenna above a spreader, and then mounting a VHF antenna on that spreader, approx. 30" away from the mast...

During my typing, I got busy with another matter and forgot to add this important point!






If you have the antennas offset (not directly above/below each other), but still separated by approx. 3' to 6' vertically, there is adequate isolation....it's just that placing the antennas directly above/below each other increases the isolation (as this places the antennas in each others' null)....
Quote:
Originally Posted by Paul Elliott View Post
For what it's worth, my AIS transponder antenna is on the spreader, the VHF antenna is on the masthead, and there have been no problems with this configuration. Given the antenna patterns and the vertical separation distance, the potential interference is *much* less than you would have with the suggested 36-inch distance arrangement.

I probably have some minor pattern distortion with the spreader-mount caused by the mast and shrouds, and I haven't done a careful measurement, but I've not noticed any problems.

I don't think the standard path-loss calculation holds up in the near-field, which is what we are discussing.

The vertical isolation is MUCH greater than that of horz!

Sorry, gotta go.

John
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Old 08-04-2016, 10:37   #5
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Re: VHF / AIS Antenna Separation

Paul and Paul,

I promise you a shorter post than either of John's :-)

1. I do not agree with the recommendation to put a 30" extender atop the mast for the VHF antenna. IMHO, that's entirely unnecessary and adds an extra complication/expense. But John is dead right about putting an AIS antenna atop the mast anywhere near the main VHF antenna: don't do that!

2. The stern-mounted AIS antenna is a viable option. Mine is mounted on the stern rail, with a short extension. I get what I consider to be plenty of coverage with that. From the dock at Capital Yacht Club in DC I can follow commercial vessels (such as the Spirit of Washington) all the way downriver to Mt. Vernon. That's 11.3 nm as the crow flies.

On a long trip this summer up the Atlantic Coast to Maine, I found no important deficit in my ability to see or be seen by other vessels. The antenna is a simple s/s whip type which I trimmed for 1:1 SWR as measured by an antenna analyzer at the AIS transceiver.

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Old 08-04-2016, 11:55   #6
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Re: VHF / AIS Antenna Separation

The way I see it I could either use a splitter or add a stern rail antenna. The splitter has some loss as does the longer coax run up the mast. However the masthead has the height advantage. Is it a 6 vs 1/2 dozen situation, or is the stern rail antenna with no splitter really much better?

Also, do I need to remove the extra masthead antenna, or can I leave it as a spare? Does it cause any problem (interference) if it's not connected to a radio at all? Is it okay to use for FM radio reception?

Thank you for all the info.

Steve
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Old 08-04-2016, 12:17   #7
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Re: VHF / AIS Antenna Separation

I probably should not be on here when I'm working....but I was bored!


Two things...


1) I wasn't recommending using a 30" stand-off....just saying that if you indeed desired to use a spreader-mounted antenna, placing the masthead antenna directly above the spreader antenna (by using a stand-off) would improve/increase the isolation between them....(although in our application, this is unnecessary...)



2) The argument of "which is better?", a stern-rail-mounted AIS antenna or a good "splitter"????
The answer is subjective....

I have both....and have used both....

Here is my opinion:
a) For most cruisers / sailors, the stern-rail mounted antenna (especially if about 10' off the water, such as on the bimini rail, arch, solar array, etc.), is probably best...

b) For those few venturing offshore in heavy weather, especially on long non-trade-wind routes (beating into winds/seas), where signals from a low-mounted antenna can be blanketed by wave tops (especially when heeled over in a stiff breeze), use of a masthead antenna for both AIS and VHF is good...



Okay....gotta go again.....(don't 'ya just hate it when work gets in the way!)

John
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Old 30-06-2016, 21:18   #8
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Re: VHF / AIS Antenna Separation

This is a related problem to this thread-subject

I have a Ray218 VHF and RayAIS250 going through a splitter to 61' masthead antenna. AIS usually works very well (after replacing the first defective unit). Radio has been spotty in TX and RX where suddenly is cannot be heard and I use my Std Horizon handheld VHF to communicate. It has always had trouble receiving the NOAA WX channels, and today it could not receive either of the local channels. I could receive both local WX channels full signal on my handheld inside the boat.

I pulled the squelch up and could receive one of the WX channels on my Ray218 but the noise makes it unusable. I verified that my Ray is on the correct freq for the two WX stations (Ch2 162.400 and Ch3 162.475). I tried this in both LOC and DISTANT settings.

I was able to TX and RX the Ray218 to and from my handheld, as well as getting a radio check from a commercial vessel 1 mile away.

any suggestions of where to start looking in this rig?

I have been more than disappointed with the Tech support of Raymarine, let alone the hassle caused me by receiving a bad AIS unit to begin with.

fair seas
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Old 01-07-2016, 06:48   #9
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Re: VHF / AIS Antenna Separation

Related to the thread on SART devices, it occurred to me that there may be times when you want your VHF antenna at the masthead, and there may be times when you want your AIS antenna at the masthead. Is there a relatively inexpensive way to switch back and forth?

That is, have an antenna at the masthead, and an antenna on the stern arch, and a switch so that you can easily go back and forth, between VHF up and AIS back, and AIS up and VHF back.

Make sense? Logic tells me that shouldn't be hard to do. The switches that let you use one antenna for both are rather expensive, so I'm hoping for a much less costly solution that will serve both needs. Probably just need to spend some time practicing my google-fu.
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Old 01-07-2016, 07:19   #10
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Re: VHF / AIS Antenna Separation

Quote:
Originally Posted by denverd0n View Post
Related to the thread on SART devices, it occurred to me that there may be times when you want your VHF antenna at the masthead, and there may be times when you want your AIS antenna at the masthead. Is there a relatively inexpensive way to switch back and forth?
.
Yes!

The box for my AIS is behind the panel of the Nav Station. The VHF is directly above. So both antennas end in the same place. So I can just unscrew the antenna connections and swap them. It takes about 30 seconds.

I designed it so at sea the AIS would be at mast top but in port when I want to talk to people I can put the VHf to the mast top. In reality, my Antenna on the davits is fine for VHf. So I dont really ever change anymore.

At sea you certainly want the AIS to have the higher range.

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Old 01-07-2016, 07:23   #11
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Re: VHF / AIS Antenna Separation

BTW, I think splitters are an expensive crock of poop.
They cost $250 and they attenuate the signal. Yes, they do. Check the user guide. They cut the signal lower.

Where an extra antenna is only $70 plus $30 for the coax.


So you get redundancy, better performance and easy installation for CHEAPER. Whats not to like?
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Old 01-07-2016, 07:29   #12
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Re: VHF / AIS Antenna Separation

Quote:
Originally Posted by MarkJ View Post
So I can just unscrew the antenna connections and swap them. It takes about 30 seconds.
Call me lazy. I want something even easier. $250 is way too much for a splitter, but I'm thinking that there must be a way to do a flip-flop switch for a lot less than that. I've seen antenna switches in the range of $20-$25. These don't have the "smarts" that the splitter has to keep from back-feeding your transceiver, and you definitely NEED those smarts if you are sharing one antenna between two transceivers. But just to flip-flop you should NOT need those smarts, so the switch should be a lot less.
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Old 01-07-2016, 11:04   #13
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Re: VHF / AIS Antenna Separation

Found this: MFJ Enterprises Inc.

$30 and it is exactly what I had in mind... except for one thing. It is for HF transmissions, not VHF. Have not been able to find the equivalent for VHF.
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Old 01-07-2016, 11:23   #14
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Re: VHF / AIS Antenna Separation

I don't think the Vesper splitter attenuates the signal, rather I gather it boosts the signal to noise ratio, higher signal, lower noise.
I know that sounds like a free lunch, but people smarter than me says it's true
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Old 01-07-2016, 13:09   #15
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Re: VHF / AIS Antenna Separation

Symphony,
1) First off, please forgive my bluntness, but the RAYAIS250 is probably one of the most troublesome items Raymarine ever made....there have been many failures and many frustrated sailors...
And, with the darn thing costing 2 - 3 times the price of other AIS receivers and simple "splitters", I can see why so many are so upset with Raymarine!

So, please understand going in, that this might just be a "lemon", and there might not be any rhyme or reason to its troubles!


2) Secondly, when read of intermittent operation ("spotty"), the first things to consider are:
a) bad connections (bad coaxial cable connections)

b) loose connections (loose coaxial cable connections)

c) bad coaxial cables (both the short jumper to/from the Ray 218, and/or the coax to the masthead antenna)

d) don't forget any coaxial connections in the bilge / at the mast step....(this is often forgotten about!)

e) don't forget the coaxial connections at the antenna, at the masthead...

Some will immediately discount some of these things, because they think "hey, the AIS is working...so, the antenna and cables are fine"...
BUT...
But, since AIS signals are digital (either there completely or not there at all), and you haven't mentioned any signal strengths....and since many AIS targets are Class A AIS signals (12 watt AIS signals from large ships, with antennas high above the water), and you're in Connecticut with many large vessels around...
Fact is, most times intermittents are reported it is almost always bad connections....

But, since you mention the Raymarine AIS 250, which is highly suspect itself....and the generic fact that the AIS receiver has a built-in splitter...both combine to give significant weight to the possibility of the AIS 250 and its built-in splitter as being the primary cause...
BUT..
But, please check all your coaxial cable connections, first!!!


3) Here are some specifics....
Quote:
Originally Posted by Symphony View Post
I have a Ray218 VHF and RayAIS250 going through a splitter
Assume this is the splitter built-in to the AIS 250??

to 61' masthead antenna. AIS usually works very well (after replacing the first defective unit). Radio has been spotty in TX and RX where suddenly is cannot be heard and I use my Std Horizon handheld VHF to communicate.
Have you tried using the Ray 218 radio, connected directly to your masthead antenna, bypassing the Ray AIS250???
(this would narrow the issue down, to either being the Ray 218 radio, OR the AIS 250's built-in splitter)

I assume you did this when you were waiting on the replacement AIS 250??
What was the result??



It has always had trouble receiving the NOAA WX channels,
This is NOT good!
I'm not sure where in CT you are....but just about anywhere in US coastal waters, with a sailboat masthead VHF antenna, you should be able to receive at least one or two clear NOAA WX broadcast, 24/7/365!!
{I get my two local ones, plus 3 others 24/7/365, here in flat S. FL...and on good days a total of 6 - 7 of them...}

If you have never been able to properly receive these NOAA WX broadcasts clearly, then you either have an antenna / cable problem, or a radio / AIS splitter problem...

Have you tried using the Ray 218 radio, connected directly to your masthead antenna, bypassing the Ray AIS250???
(this would narrow the issue down, to either being the Ray 218 radio, OR the AIS 250's built-in splitter)

I assume you did this when you were waiting on the replacement AIS 250??
What was the result??



and today it could not receive either of the local channels. I could receive both local WX channels full signal on my handheld inside the boat.
I pulled the squelch up and could receive one of the WX channels on my Ray218 but the noise makes it unusable.
This is NOT good!
As you see from using your handheld VHF radio, the NOAA WX broadcasts are very strong!

If cannot receive these NOAA WX broadcasts clearly, then you either have an antenna / cable problem, or a radio / AIS splitter problem...

Have you tried using the Ray 218 radio, connected directly to your masthead antenna, bypassing the Ray AIS250???
(this would narrow the issue down, to either being the Ray 218 radio, OR the AIS 250's built-in splitter)

I assume you did this when you were waiting on the replacement AIS 250??
What was the result??




I verified that my Ray is on the correct freq for the two WX stations (Ch2 162.400 and Ch3 162.475). I tried this in both LOC and DISTANT settings.
Always use the "DISTANT" setting, unless you are trying to use the radio in crowded harbor and only wish to communicate to other vessels very close to you (< a few miles)



I was able to TX and RX the Ray218 to and from my handheld, as well as getting a radio check from a commercial vessel 1 mile away.
While I applaud your doing this, and it does show that your RAY 218's microphone is working well, and it is probably transmitting okay....it doesn't tell us much...
You could communicate with the commercial vessel 1 mile away, even if you didn't have any antenna hooked-up at all...(which just might be the case!)


any suggestions of where to start looking in this rig?
In addition to my advice above regarding bad/loose/corroded coaxial cable connections (check them ALL), if you have a way of measuring your 12vdc current draw while going from receive to transmit on the Ray 218, please do that...
Observe how much current the Ray 218 is drawing on transmit versus receive...should be about 1 amp on receive and about 5 amps on transmit (when at 25-watts / hi-power)...

If the coaxial connections are check good....and you are seeing about 5 amps of current draw on transmit, then I suspect it is your AIS 250 at fault...specifically its built-in splitter!

You can also trying using the Sea Tow Automated Radio Check Network...



I have been more than disappointed with the Tech support of Raymarine, let alone the hassle caused me by receiving a bad AIS unit to begin with.

fair seas
In addition to the above, have a look here...

Sea Tow's Automated Radio Check Network
https://www.seatow.com/arc
https://www.seatow.com/service-locator/arc?location=CT


AIS Transponder and VHF Antennas on the Masthead?

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

Raymarine AIS 250




And, finally....again, please forgive my bluntness, but I'd scarp the Ray AIS 250 and buy a real AIS system (and a real "splitter" that works)....
This translates to: Toss the Ray AIS 250, and buy Vesper Watchmate and Vesper SP-160!


I hope this helps...

John
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