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Old 01-04-2023, 13:32   #1
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AIS/VHF antenna backstay mount

Is there any problem with mounting an ais/vhf antenna on a backstay like this kato bracket does?

https://www.katomarine.com/gpsmount.htm

I have one of these Gam ss2 antennas at the top of the mast, and I want to run my vhf to that one, and my ais to a second antenna on the stern. This gives me redundancy in case one antenna fails.

SS-2 Split | USA made VHF/UHF Antennas & Accessories

I feel like I could easily adapt this inexpensive bracket using some small ubolts to clamp to the backstay, the same way the much more expensive Kato bracket does.

SS-B Right Angle Bracket | USA made VHF/UHF Antennas & Accessories

Is this a problem? I think this will theoretically link the two antenna cable's shielding together through the rigging and brackets.
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Old 01-04-2023, 14:28   #2
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Re: AIS/VHF antenna backstay mount

Linking both antennas grounds isn’t a consideration in terms of locations so not sure why you mention that but regardless, I wouldn’t worry too much about the interaction between the backstay and antenna but if you are, it’s simple to determine the effect which would show up in an increase in SWR compared with the antenna in “free space”.
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Old 01-04-2023, 14:36   #3
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Re: AIS/VHF antenna backstay mount

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Originally Posted by S/V Illusion View Post
Linking both antennas grounds isnít a consideration in terms of locations so not sure why you mention that but regardless, I wouldnít worry too much about the interaction between the backstay and antenna but if you are, itís simple to determine the effect which would show up in an increase in SWR compared with the antenna in ďfree spaceĒ.
Haha, I mention it mostly because my knowledge of electronics is limited to very simple circuits. I don't know what SWR is. What would an increase in it mean? Overall it sounds like you don't think it's an issue to mount an antenna there?
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Old 01-04-2023, 15:46   #4
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Re: AIS/VHF antenna backstay mount

I would have no concern about the location from an interaction performance, but rather performance. VHF range is a huge function of height. Some people will be pedantic on here and insist it is not strictly line of sight, and it is not, but height above water is the single best thing you can do to increase range. I really don’t see the advantage of the backstay other a sternrail mount, both are greatly inferior to a masthead location.
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Old 01-04-2023, 15:48   #5
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Re: AIS/VHF antenna backstay mount

Donít even hesitate, get it
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Old 01-04-2023, 15:53   #6
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Re: AIS/VHF antenna backstay mount

Thanks. As I said earlier, I have an antenna on the masthead. I was previously sharing it with my vhf and ais through a splitter, but the splitter is on the fritz. Rather than spend the money on a new splitter, I want to use a second antenna on the stern to run the AIS, while my VHF goes to the masthead. They will be the same antenna, so I'll have some redundancy if needed, which will be nice.

The reason I'm thinking about the backstay is mostly to keep it out of the way. I have a lot on my stern rail already, and a whippy antenna might be annoying. Also, there is a deck gland at the base of the backstay already from the old SSB that the previous owner used to have.
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Old 01-04-2023, 15:54   #7
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Re: AIS/VHF antenna backstay mount

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Donít even hesitate, get it
Thanks! I'll give it a shot.
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Old 01-04-2023, 16:42   #8
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Re: AIS/VHF antenna backstay mount

Not sure this is totally relevant, but: Boat came with a helical antenna, tuned for AIS (i measured it with antenna analyzer ). It got me a crummy 2 mile range. I then installed a plane quarter wave whip, tuned, on top of the solar panel frame. About the same height. Now see the big guys 15 miles out.
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Old 01-04-2023, 16:48   #9
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Re: AIS/VHF antenna backstay mount

I know nothing about these things, so I'm open to suggestions. The Gam SS2 installed on my masthead was recommended by the marine electronics outfit next to my boatyard. They said it is a well built, good performance antenna for a reasonable price. Gam's website says that the VHF/AIS version is tuned for an optimal frequency that gives good performance for both VHF and AIS frequencies. Could you give me links to the antennas you're talking about?
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Old 01-04-2023, 17:47   #10
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Re: AIS/VHF antenna backstay mount

The photo shows the antenna mounted on the short bit of wire that is below the backstay. It might be okay on a such a short bit of wire. I usually see this kind of 'Y' shaped setup as part of a backstay tension adjustment mechanism.

I tried mounting an antenna on my backstay that runs from the stern of the boat to the top of the mast. In any significant sea state it became a torsion pendulum twisting the backstay first one way then the other, first winding and then unwinding the backstay over and over again. Thoughts of repeatedly bended and unbended coat hangers haunted my mind for hours as we sailed along far offshore. I have no idea of the expected fatigue failure in that situation, but my mind was riveted on the wire backstay as it twisted and untwisted with every bounce and twist of the boat. I knew for certain what would happen if the backstay broke. Also, as it twisted the backstay, the antenna moved from vertical to severely angled pointing its radiation pattern into the sea or into the sky rather than out toward the horizon. I watched and worried becoming increasingly paranoid as the time went by.

The bracket came off the backstay. I think it is in my attic. I'll look if you are interested.
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Old 01-04-2023, 17:55   #11
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Re: AIS/VHF antenna backstay mount

I think it's just one leg of a non-adjustable split backstay, but I get your point, it's not something I thought of. The split backstay leg obviously won't twist, so it's a much sturdier mount point for the antenna. That being said, this antenna is pretty small and my backstay is pretty large, but even then your point makes me nervous enough to give this a bit more thought.
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Old 01-04-2023, 18:07   #12
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Re: AIS/VHF antenna backstay mount

Don't let my off shore paranoia unduly influence you. While I am an engineer, I am a retired chemical engineer and am unable to evaluate the risk. It may be of no real worry at all. There are others on this forum who could properly evaluate the risk. Perhaps one or two will chime in.

You might take a pair of vice grips, attach them to your backstay at the intended attachment point, twist the backstay with the vice grips, see what it feels like, then let your gut evaluate the situation -- both the fatigue problem and the angle problem.
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Old 01-04-2023, 18:12   #13
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Re: AIS/VHF antenna backstay mount

Now that I've had a few minutes to think about it, I do think that the weight of the antenna might twist the backstay a little, but it wouldn't create any risk to the stay. The bigger problem, I think, is that when I adjust the backstay tension with my navtec ram the backstay twists and untwists as it's tensioned and untensioned. So anything clamped to it would twist with it, and my antenna would only be centered and upright at a very specific tension, which wouldn't work well. So I think I'm back to a rail mount idea.

Any advice about the antennas themselves would be greatly appreciated. I'm just going by the marine electronics guys recommendation. When I look at antennas on defender or something, I have no idea what I'm looking at, and even though the one they recommended is more budget friendly, cheaper doesn't always mean worse, and vice versa.

Thanks guys!
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Old 01-04-2023, 22:21   #14
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Re: AIS/VHF antenna backstay mount

If you want an antenna that does it all, you need a wideband antenna. I donít like the VHF/AIS combi antenna because they arenít good at either.

A good choice for either vhf or ais would be the little giant galaxy antenna. For wideband, I bought this one but didnít install it yet: https://www.milltechmarine.com/shake...f-antenna.html

For the standard marine mount you need the extra female-female ferrule.
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Old 02-04-2023, 04:26   #15
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Re: AIS/VHF antenna backstay mount

Thanks. Looking at that wideband antenna, it actually seems very similar to the Gam SS2. The SS2 actually has a wider frequency range, but the idea is that you trim the steel whip to specific length to "center tune" to an optimal frequency for your application. According to Gam the one I have is tuned betweeen marine vhf and ais frequencies to give good performance for both. What do you think?
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