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Old 03-12-2011, 14:46   #1
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VHF Coax

I am installing a AIS from raymarine, it will connect to a C80 and an Icom M504.

I am planning to install a second mast mount antenna, Shaskpere 5215. Mast is down and about 48 ft in lenght ( deck stepped)

Presently I have the OEM Beneteau RG8u cable from the radio to the top of the mast. One PL-259 at the mast

My question:

Should I use rg-213 for the second coax run, if so would I be better to use the larger cable for the VHF or for the AIS?

Any and all suggestions for the best coax cable combination is would be greatly appreciated.

Thanks a Million

Fletch
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Old 03-12-2011, 16:09   #2
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Re: VHF Coax

You should not intall two VHF antenna's right next to each other. Installation instructions for AIS antenna's are to install them either directly above or below the primary VHF antenna with no horizontal separation at a minimum of 2 meters and if at the same level as the primary, they should be separated by 10 meters. Of course, there is usually a compromise on smaller vessels but they should certainly not be right next to each other at the masthead. Is this an AIS transponder or receive only? Why are you connecting it to your M504? RG8 and RG213 are about the same size. Did you mean you currently have RG8x?

Eric
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Old 03-12-2011, 16:21   #3
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Re: VHF Coax

Class A receive only

AIS to the C80 and then the nema signal for position etc to the Icom 504 (lower baud rate) from the C80

Not sure on the siz of the exsisting cable smaller than the 213. about 1/4 to 5/16" ( I believe RG8X)

raymarine suggest a sperate antenna for the AIS.
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Old 03-12-2011, 16:35   #4
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Re: VHF Coax

I'm sure you meant class B? Yes, sounds like you have RG8x. Even with receive only AIS, the antenna's should be well separated. If in the same horizontal plane, by 3 meters at the very least.

Eric
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Old 03-12-2011, 16:56   #5
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Re: VHF Coax

I contemplated installing the separate antenna on a spreader. Now I have used the masthead antenna on both the VHS and the AIS and conclude the higher mounting is preferable for either unit. An active splitter using just the one antenna would optimise the height. When you're out at sea and unlikely to get a VHS call, reception on the AIS is primary, when you're navigating inshore, both need antenna height. An active splitter allows both to use the masthead antenna. If you install the second antenna on a spreader or bracket below the masthead antenna and cable it to your nav area as a spare, it would be available if the active splitter failed, or if the masthead antenna failed, as mine did when I hit a bridge.
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Old 03-12-2011, 16:59   #6
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Re: VHF Coax

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Originally Posted by CampDavid View Post

Should I use rg-213 for the second coax run, if so would I be better to use the larger cable for the VHF or for the AIS?

Any and all suggestions for the best coax cable combination is would be greatly appreciated.
Use the bigger coax for the VHF - its range/signal loss is more important.

Put the ais antenna on one of the spreaders rather than the masthead - that gets it up with good range but enough away from the vhf antenna.

Make sure your pl259 connects are as perfect as you can get them - everything cut just to the right length, solder joints excellent barrel screwed on tight, etc. Most antenna troubles come from the connections.

The separate antenna is the best way to do this, but honestly using a splitter and sharing the antenna between the vhf and AIS works quite well (if done properly) and is a ton less work to install.
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Old 03-12-2011, 17:37   #7
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Re: VHF Coax

Raymarine told (phone) that the system works a way better with two antenna's, the AIS has a built in splitter with a connection for the fm radio. No big issue to connect
.
I guess worth a try however with the mast down it would not be hard to install a second antenna on the upper spreader. Way harder with the mast up.

Perhaps somebody has experience with a Raymarine AIS 250

Fletcher
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Old 04-12-2011, 09:17   #8
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Re: VHF Coax

FWIW, we have two vhf antennas right next to each other on the masthead. One dedicated to the marine vhf and the other connected to a 2 meter ham shared with an AIS class B receiver through a splitter. We have experienced no problems with vhf transmission or AIS reception with this setup. We, of course, do not use both vhf's on the same frequency at the same time.

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Old 04-12-2011, 10:04   #9
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Re: VHF Coax

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FWIW, we have two vhf antennas right next to each other on the masthead. One dedicated to the marine vhf and the other connected to a 2 meter ham shared with an AIS class B receiver through a splitter. We have experienced no problems with vhf transmission or AIS reception with this setup. We, of course, do not use both vhf's on the same frequency at the same time.

Mark
how do you know there is no interaction?
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Old 04-12-2011, 10:26   #10
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Re: VHF Coax

I didn't say there wasn't any interaction, I said we have experienced no practical problems in usage.

I base that on being able to see all AIS targets equally in all directions and at equal line-of-site ranges. Also, receiving and transmitting on both vhf's very well in all directions and with equal ranges. And seeing no degradation of AIS reception when transmitting on the other antenna.

There may very well be sever interactions and signal pattern distortion and anomalies. I am just pointing out that, in our case, this is one of those theoretical, and possibly very real, technical issues that doesn't matter much in practice.

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Old 04-12-2011, 10:41   #11
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Re: VHF Coax

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Originally Posted by colemj View Post
I didn't say there wasn't any interaction, I said we have experienced no practical problems in usage.

I base that on being able to see all AIS targets equally in all directions and at equal line-of-site ranges. Also, receiving and transmitting on both vhf's very well in all directions and with equal ranges. And seeing no degradation of AIS reception when transmitting on the other antenna.

There may very well be sever interactions and signal pattern distortion and anomalies. I am just pointing out that, in our case, this is one of those theoretical, and possibly very real, technical issues that doesn't matter much in practice.

Mark
Thanks. I was actually asking if you had any objective measure of any signal degradation. What you observe and what is actually happening can be mutually exclusive things. Not theoretically but as an entirely practical matter, antennas in close proximity and certainly those in the same plane will interact always without exception and it is for that reason I asked.
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Old 04-12-2011, 10:53   #12
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Re: VHF Coax

Shooting off the cuff:

There are at least 2 factors involved in antenna placement to consider. In olden days when the selectivity of receivers was somewhat less than today and having less robust front ends having 2 antenna close would overload the front end and bleed into the other radio. In some cases blowing the front end. Then there is the interaction in the radiation pattern.

I'm in the process of restoring an older boat. THe current plan calls for the AIS antenna to go in the radar mast at the stern of the boat and the VHF radio at the mast head. It is an option to put the AIS antenna at the first spreaders next time I have the mast down.

On the subject of coax here is a link to a chart of cable losses. Coax Cable assemblies |RFID Cables |Custom coax assemblies

As you can see there are some cables that significantly out perform RG213. I do some work with research vessels and tend to have LMR-900 installed when possible. For my boat I will be using LMR-400 with type N connectors where I can tie the cable down and give it a proper turn radius. This includes the VHF, AIS and SSB. Of course I'll use more flexible cable pigtails to the radios which will do the type N to PL239 conversion. Insertion losses for type N are very good and can be weatherproofed to a great degree.

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Old 04-12-2011, 11:18   #13
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Re: VHF Coax

Placing antenna's very close to each other is poor installation practice. I can connect my antenna analyzer to one VHF antenna and see the effects of another antenna as soon as it gets within 3'. If they are only a few inches apart, you could end up degrading or damaging the receivers in the AIS unit or whatever else you are using the antenna for.

Eric
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Old 04-12-2011, 14:10   #14
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Re: VHF Coax

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Originally Posted by fairbank56 View Post
Placing antenna's very close to each other is poor installation practice. I can connect my antenna analyzer to one VHF antenna and see the effects of another antenna as soon as it gets within 3'. If they are only a few inches apart, you could end up degrading or damaging the receivers in the AIS unit or whatever else you are using the antenna for.

Eric
I don't disagree, but there are limitations to mounting on sailboats. 13 years with no problem leads me to believe that having them 12" apart isn't hurting anything and my output and reception hasn't been compromised enough to put the system in the lower half of subjective performance of the boats out here (quite the opposite). Everything is connected with 1/2" LDF450 heliax and L4TNMPS N connectors, so maybe I am preserving in application what I'm losing in design.

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Old 04-12-2011, 14:54   #15
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Re: VHF Coax

FWIW,

I recently installed an AIS receiver with its antenna on the radar arch. The reciever is an Icom 5000, and I routinely see ships at around 20 miles range, even with the low antenna mount. For me, this seems adequate... not optimal to be sure, but frankly seeing ships at ranges of greater than 20 miles isn't important.

Cheers,

Jim
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